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Developing restoration strategies for temperate forests using natural regeneration processes

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103
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Developing Restoration Strategies for Temperate
Forests Using Natural Regeneration Processes
Holger Fischer, Franka Huth, Ulrike Hagemann, and Sven Wagner
CONTENTS
6.1 Introduction ........................................................................................................................ 104
6.1.1 Forest Restoration as a Holistic Challenge ......................................................... 104
6.1.2 Tree Regeneration as a Driver in Forest Restoration ........................................105
6.2 The Natural Regeneration Process .................................................................................. 107
6.2.1 Flowering, Pollination and Fruiting as Prerequisites .......................................108
6.2.1.1 Flowering .................................................................................................108
6.2.1.2 Pollination ................................................................................................ 109
6.2.1.3 Fruiting ..................................................................................................... 110
6.2.2 Mechanisms of Seed Dispersal and Consequences for Regeneration ........... 111
6.2.2.1 Zoochory .................................................................................................. 112
6.2.2.2 Anemochory ............................................................................................ 113
6.2.2.3 Dispersal Distances ................................................................................ 113
6.2.3 Costs, Benets, and Implications of Seed Storage ............................................. 114
6.2.3.1 Orthodox and Recalcitrant Seeds ......................................................... 115
6.2.3.2 Seed Banks ............................................................................................... 115
6.2.3.3 Widespread Susceptibility ..................................................................... 116
6.2.3.4 Predation by Rodents ............................................................................. 116
6.2.3.5 Predation by other Vertebrates .............................................................. 116
6.2.3.6 Scatter Hoarding ..................................................................................... 117
6.2.3.7 Pathogens ................................................................................................. 117
6.2.4 Germination in Stressful Surroundings: A Narrow Bottleneck ..................... 117
6.2.4.1 Germination and Safe Site ..................................................................... 117
6.2.4.2 Light .......................................................................................................... 118
6.2.4.3 Litter and Soil Organic Matter .............................................................. 118
6.2.4.4 Interaction with Mycorrhizae ................................................................120
6.2.4.5 Physical Damage to Seedlings ..............................................................120
6.2.5 Seedling Survival and Establishment: The Fine Step to Maturity ................. 120
6.3 Overstory Restoration Strategies Oriented toward the Natural
DisturbanceRegime .......................................................................................................... 121
6.3.1 Emulating Overstory Conditions Originating from Large-Scale
Disturbances to Support Ecological Processes .................................................. 121
6.3.2 Manifold Harvesting as a Strategy for Managing Structural and Species
Diversity .................................................................................................................. 123
6.3.2.1 Clearcut System ....................................................................................... 124
6.3.2.2 Seed Tree System ..................................................................................... 125
104 Restoration of Boreal and Temperate Forests
6.1 Introduction
6.1.1 Forest Restoration as a Holistic Challenge
Forest restoration projects have become increasingly common around the world and many
studies have accumulated in this eld during the last decades. Forest restoration means
changing the forest landscape component toward a “more natural” situation (sensu Fischer
and Fischer 2012). But what is “natural”? Bradshaw (2002) called it the “original ecosystem”
and focused on two major attributes: ecosystem structure and ecosystem function, with
typical values for both attributes that are reduced by ecosystem “degradation.” Ecological
restoration is an activity that also ideally results in the return of an ecosystem to an undis-
turbed status (Palmer and Filoso 2009). On the path back to the original ecosystem state,
natural recovery as well as ecological restoration offer many developmental options. As
these denitions necessitate a holistic approach, the attention in recent restoration projects
has turned to the integration of ecosystem services. Similarly, restoration actions focused
on enhancing biodiversity should also support increased provision of ecosystem services
6.3.2.3 Retention Tree System ............................................................................125
6.3.2.4 Irregular Shelterwood ............................................................................ 126
6.3.2.5 Emulating Old-Growth Gap Conditions for Regeneration
Processes ..................................................................................................126
6.4 Restoration Strategies for the Active Manipulation of Below-Canopy Stand
Components ........................................................................................................................ 129
6.4.1 Emulating Large-Scale Disturbances to Support Ecological Below-
Canopy Processes .................................................................................................. 130
6.4.2 The Function of Ground Vegetation: Facilitation versus Competition .......... 132
6.4.2.1 The Role of Ground Vegetation in Natural Forests ............................133
6.4.2.2 Interactions between the Different Hierarchical Strata
ofForestEcosystems ............................................................................... 134
6.4.2.3 Using Ground Vegetation to Promote or Discriminate
AgainstSpecic Tree Regeneration ...................................................... 135
6.4.3 Direct Control of Species Composition by Establishing Articial
Regeneration ........................................................................................................... 136
6.4.4 Manipulating the Small-Scale Seedling Environment ..................................... 137
6.4.5 Utilizing Deadwood to Improve Regeneration Survival ................................. 138
6.4.5.1 How Deadwood Facilitates Regeneration ...........................................139
6.4.5.2 Seed Interception, Retention and Storage............................................ 139
6.4.5.3 Germination and Seedling Establishment .......................................... 139
6.4.5.4 Seedling and Sapling Survival .............................................................. 140
6.4.5.5 Protection from Browsing Damage ...................................................... 140
6.4.6 Effects of Deadwood Properties .......................................................................... 140
6.4.6.1 Abundance ............................................................................................... 140
6.4.6.2 Origin and Type ...................................................................................... 141
6.4.6.3 Size ............................................................................................................ 141
6.4.6.4 Decay Status ............................................................................................. 141
6.4.7 Deadwood and Forest Restoration ...................................................................... 141
References .....................................................................................................................................142
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105Developing Restoration Strategies for Temperate Forests
(Rey Benayas etal. 2009). This approach presents an opportunity for enhancing benets
to human livelihood and funding sources as well as generating public support for such
initiatives (Trabucchi etal. 2012).
Restoration ecology is “repairing” degraded, damaged, or destroyed ecosystems usually
adversely affected by human activity. Restoration activities are often aimed at increas-
ing the land-base of a particular ecosystem; its biodiversity, resilience, and resistance, the
provision of ecosystem services and ecosystem sustainability (see Aronson and Alexander
2013). The formulation of sustainability as an integral part of restoration practice is essen-
tial at the very least with respect to the status quo of many European forest areas, which
have a long history of human intervention. Historic human impacts in these forests range
from the elimination of predators, clearing for agricultural use and settlements, the intro-
duction of domestic grazing stock, management for timber production, and management-
induced change of tree species with the introduction of nonnative, partly invasive species,
which disturb the natural cycle of forest regeneration (Willoughby and Jinks 2009).
Restoration success in general, and regeneration success in particular, are controlled by
these historical occurrences and past silvicultural practices within a forest stand, includ-
ing past and present species composition, stand structure, wildlife, and potential man-
agement-related soil disturbance. Further, recent anthropogenic modications such as
atmospheric deposition (e.g., of nitrogen) or climate change (Fischer and Wagner 2009)
also modify forest regeneration in time and space.
6.1.2 Tree Regeneration as a Driver in Forest Restoration
Tree regeneration is the most relevant and effective initial step in the context of forest
restoration, as every silvicultural action at this stage affects the development at least of
the next stand generation, and potentially beyond. From an ecological perspective, discus-
sions about the forest life cycle focus on the processes involved in replacing mature trees
with the next tree generation as well as the colonization of new habitats. Out of all forest
developmental phases, the regeneration phase offers the best opportunity to manipulate
tree species and forest structure, making it a key for achieving restoration objectives. In
this regard, Löf etal. (2012) dened the practice of planting trees and shrubs as a key com-
ponent of forest restoration.
The majority of existing studies on forest regeneration emphasize reforestation follow-
ing timber harvest for industrial purposes (Oliet and Jacobs 2012). Even today, there are
land use practices, such as surface mining (Hüttl and Bradshaw 2001) or remediation of
forest soils (Kauppi etal. 2012), that create extremely harsh site conditions for restoration.
These sites require amelioration using direct sowing and planting (Josa etal. 2012), often
in combination with intensive mechanical site preparation. These activities are all active
management interventions (Holl and Aide 2011). However, in almost every area with arti-
cial regeneration, the natural invasion of trees, shrubs, and other autotrophs (Mueller-
Dombois and Ellenberg 1974) may also play a role. In forest restoration, some groups
advocate relying exclusively on natural regeneration and succession dynamics rather than
active regeneration and restoration activities (Hüning etal. 2008; Baasch etal. 2009).
The different regeneration methods, each feature advantages and disadvantages.
Although forest restoration and natural succession both lead to ecosystem change, they
are quite different with respect to the degree of intentionality and should therefore be
clearly differentiated. While forest restoration is the assisted, intentional, guided recon-
struction of forests, natural succession is regarded as unintended, neither prescribed nor
directed by humans (Ciccarese etal. 2012). Despite public perception, natural regeneration
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106 Restoration of Boreal and Temperate Forests
techniques are not innately superior or always more appropriate than articial techniques
for restoring forest ecosystems. Where management objectives require precise timing and
a particular tree species composition of the restored forest, active intervention at the regen-
eration stage is essential. Interventions should be based on management objectives, the
evaluation and interpretation of site conditions, and profound silvicultural knowledge and
skills. Appropriate regeneration choices must contribute to current management objec-
tives and be exible enough for retaining future options. As forests are typically long-lived
ecosystems, forest managers continuously face the risk that changing conditions in the
future will negate the assumptions on which they based their current decisions.
In traditional forest management, regeneration success is dened by a minimum stock-
ing density of genetically adapted, vigorous young trees of a dened species composi-
tion with adequate leader shoot growth and high competitive power (McWilliams etal.
1995, Ponder 1997, Wagner etal. 2010) and appropriate root development (Brunner etal.
2009; Bayer etal. 2013). These criteria also apply to regeneration for restoration purposes.
Although stocking levels may differ depending on management objectives, the nancial
return to the landowner is in many cases an important objective of forest restoration,
together with other objectives such as biodiversity conservation. Restoration often aims to
alter species composition, sometimes formulating additional requirements for a minimum
number of individuals of certain species, and stocking levels can thus deviate from values
required for maximum timber production or quality. Restoration objectives may also call
for changes in stand structure. When the target is an uneven-aged stand structure, den-
ing an appropriate stocking density is somewhat more complex, because the relationship
between seedling density at a given time and the recruitment of trees for the upper tree
layers is not necessarily straightforward (Lundqvist 1995).
The complex task of choosing regeneration measures can be approached in two steps,
analysis and decision. At several scales—from stand to microhabitat—environmental con-
ditions must be evaluated for different regeneration measures to properly predict options
and obstacles over the entire regeneration phase (Nyland 1996). This has to be done for
each stage of the life cycle in order to identify potential environmental hazards as well as
environmental prerequisites. Recommended regeneration measures can then be derived
from these options (Perala and Alm 1990; Jobidon 1994; Van Der Meer etal. 1999).
Analysis can be a daunting task, and practical considerations often necessitate compro-
mises and decision-making based on incomplete information. For example, site conditions
may be surveyed and analyzed at different scales and levels of intensity (Barnes etal. 1998;
Kimmins 1987; Gholz and Boring 1991; Smith etal. 2007). In practice, the quality and inten-
sity of site surveys depends on the available resources and may be constrained by factors
such as staff availability (both numbers of individuals and their competence) as well as the
willingness of the forest owner to invest in site mapping and analyses. However, the forest
manager’s knowledge and experience regarding local site conditions and the performance
of indicator species may offset the lack of complete information. Such experience may thus
be critical in accounting for changes that may occur after the regeneration intervention,
such as potential competing vegetation (Wagner etal. 2010). Once the objectives regarding
the future provenance, species composition, stand and age structure have been set, the
forest manager must decide on the appropriate regeneration method or combination of
methods, adapted management intensity, and adequate timing of all interventions.
Forest restoration is in some ways a new application of silvicultural expertise and a chal-
lenge for any silviculturist. Therefore, we must remain mindful that widely used regen-
eration techniques may be underlain by different objectives and assumptions. We may
carefully use existing knowledge, but should also be prepared to seek new information
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107Developing Restoration Strategies for Temperate Forests
or analyses. After all, it is a substantial challenge for forest research to create new and rel-
evant knowledge for forests with a high dynamic potential both in time and space.
The following sections give an overview of the inherent and external limitations of the
regeneration process, structured by stage, and the measures available to counteract them.
Our aim in this chapter is to point out the relevance of single life cycle stages within the
regeneration process in the context of restoration ecology, in particular by illustrating the
interactions between these stages, site factors, and the techniques available to intervene in
the regeneration process. We therefore restrict the scope of the chapter to ecological aspects.
6.2 The Natural Regeneration Process
The events associated with the regeneration process of different tree species are some
of the most striking phenomena within forests. The natural regeneration cycle includes
numerous relevant stages, starting with the owering of mature trees and ending with the
No
damages
Storage
No vector
Missing
Failing
Damages
Limited
resources
Seedling
development
Favorable
environmental
conditions
Rich
Mast
Fruiting
Flowering and
pollination
Effective
vectors
Artificial regeneration
:
planting
Successful
establishment
and growth of
juveniles
Dispersal
Natural regeneration
Unfavorable
environmental
conditions
Germination
Artificial regeneration
:
sowing
Safe sites
No safe sites
Resource-rich
FIGURE 6.1
The regeneration cycle as a cascade of ecological processes and their success and failure in interaction with
environmental conditions exemplarily for European beech.
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108 Restoration of Boreal and Temperate Forests
recruitment and establishment of juvenile trees. Figure 6.1 shows a cascade of these eco-
logical processes interacting with environmental conditions and their success and failure,
illustrated with European beech (Fagus sylvatica). Both in theory and practice, forest ecolo-
gists have debated for decades over the predominant limiting factors responsible for insuf-
cient natural regeneration within individual forest stands. A vast literature illustrates
the complex interactions between mature trees, environmental conditions, and seedling
response; but the majority of publications focus only on a few particular phases with high
relevance for regeneration success. Ammer etal. (2011), however, underline the need to
look at the regeneration cycle in its entirety and to disentangle the multiple factors that are
involved throughout the entire process.
Understanding the regeneration cycle is crucial for achieving sustainable forest manage-
ment and ensuring “acceptable” forest restoration. From a silvicultural point of view, an
acceptable result for each stage can be dened by a minimum density of seeds, seedlings,
or saplings required for regenerating a given site; and the limitations are by denition
factors causing lower densities than required. In this context, a biological phenomenon
will be called “effective” if adequate densities result. This is particularly important in seed
dispersal. A detailed analysis of every individual stage will make it easier to evaluate the
absence of acceptable natural regeneration density in a specic area, particularly because
the failure of any single stage within the complex generative cycle will compromise the
overall regeneration success.
6.2.1 Flowering, Pollination and Fruiting as Prerequisites
Reproductive phenology, starting with oral initiation, via full orescence through pol-
lination and seed or fruit maturity, is well known for temperate conifers, but less studied
for temperate hardwoods (Owens 1995). In order to manipulate the regeneration cycle for
specic restoration aims, the drawbacks and opportunities for natural regeneration should
be fully understood, because constraints to seed or fruit reproduction are manifold and
inuence all subsequent stages of the regeneration cycle.
6.2.1.1 Flowering
The majority of seasonally owering trees ower infrequently. The generative character is
caused by a range of developmental, physiological, and environmental conditions such as
species-specic pubescence, light intensity, minimum and maximum temperature, water
regime, and nutrients. Before the generative character of buds and subsequent owering
becomes visible, particular biochemical changes are measurable (e.g., gibberellins mediate
and promote growth and development within the owering processes of conifers; Bonnet-
Masimbert 1987). Many temperate forest trees exhibit so-called “indirect owering,” char-
acterized by a period of dormancy between oral initiation and pollination (Owens 1995).
In most cases, oral initiation occurs before the onset of winter dormancy. In some spe-
cies, the percentage of reproductive buds can be predicted from their external morphology
(Owens and Blake 1985).
In silvicultural management, it is a common practice to modify light conditions in order
to increase the production of reproductive buds and improve owering. Such methods
have been successfully applied in North American temperate conifers (Ross and Pharis
1985) and European beech stands (Holmsgaard and Olsen 1966), where special thinning
regimes, adequate tree spacing, and crown management are promising strategies when nat-
ural regeneration is missing. Although higher temperatures at the time of oral initiation
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109Developing Restoration Strategies for Temperate Forests
may enhance owering in temperate trees (Ross 1989; Owens 1995), in contrast low winter
temperatures enhance owering in some species such as Eucalyptus nitens (Moncur and
Hasan 1994). Water stress and associated aspects such as shallow soils, poorly drained
soils such as pseudogleys, soil compaction, and anoxia have also been said to enhance
owering in temperate hardwoods and conifers (see Owens 1995 and references therein).
Often successful owering and later stages are not differentiated because the frequency
of (sometimes inconspicuous) owering may be less obvious than the frequency of seed or
fruit production at the end of the reproductive cycle. This is not permissible, however, and
may lead to false conclusions and misinterpretations when evaluating natural regenera-
tion success.
6.2.1.2 Pollination
Successful pollination is the next step within the regeneration cycle (Figure 6.1). It tends to
be relatively constant from year to year. Although the majority of tree species in temperate
forests are wind-pollinated, a few species have other pollination strategies, for example,
relying on insects (Prunus avium, Robinia pseudoacacia, and Tilia spec.) or even water (Abies
spec., see Chandler and Owens 2004). Ashley (2010) demonstrated that pollination and
pollen dispersal are complex phenomena which are inuenced by many ecological pro-
cesses. She cited numerous parentage studies showing long-distance dispersal of pollen
to be common in both wind- and animal-pollinated tree species, with average pollination
distances being hundreds of meters.
Generally, forest trees are highly diverse organisms genetically, and pollen dispersal is
usually considered the main driver of genetic relatedness patterns (Sagnard etal. 2011).
Genetic variability depends on population size and the spatial distribution of tree indi-
viduals. For tree species characterized by a large and continuous range such as European
beech, the genetic diversity within populations is typically high while the genetic diversity
between populations is comparatively low (Thomsen and Kjær 2002; Buiteveld etal. 2007;
Jump and Penuelas 2007; Nyari 2010). Species with small population size in glacial refu-
gia and a low mixing rate between postglacial recolonization lineages typically feature a
strong genetic differentiation among but not within populations (e.g., Fraxinus excelsior;
see Rüdinger etal. 2008; Dobrowolska etal. 2011). However, even within populations or
forest stands, genetic diversity is rarely homogeneously distributed, with genetic similar-
ity between individuals often decreasing with increasing distance between them, such as
described for the genera Fagus (Jump and Penuelas 2007) and Quercus (Bacilieri etal. 1994).
The knowledge of spatial genetic patterns resulting from population history and limited
gene ow can be important for regeneration management, in order not to misrepresent
genetic diversity of species or populations in the progeny population and to avoid autog-
amy (self-fertilization, measured by the heterozygosity of the resulting juvenile genera-
tion) when distances between parental trees are high (Streiff etal. 1998). Due to the impact
of lethal regressive genes, germination percent, height growth, or resilience can be lower
in the case of self-pollination compared to cross-pollination (Hattemer 2005). Hence, pol-
len transfer is especially important for small populations whose habitat has been consider-
ably reduced (e.g., Silver r (Abies alba), see Wolf 2003) or fragmented (e.g., genus Ulmus, see
Goodall-Copestake etal. 2005).
The degree of gene transmission from the parent to the juvenile generation in forest stands
depends on the applied silvicultural harvesting and regeneration methods (Finkeldey and
Ziehe 2004; Hosius etal. 2006). For example, the removal of a few mature trees from a stand
prior to seed production in uneven-aged Plenterwald forests may have no measurable impact
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110 Restoration of Boreal and Temperate Forests
on the genetic structure and variability of the naturally regenerated understory (Lefèvre
2004). Similarly, other regeneration strategies such as “Femelschlag” have no negative effects
on the progeny generation—the longer the regeneration period, the more uneven-aged the
progeny population. In this method, tree cutt ing intensity is low and explicitly heterogeneous
with respect to horizontal structure; the next generation consists of different age classes.
This method of harvesting and regeneration is believed to be well-suited to accommodate
the variable mast years of different forest tree species (Geburek and Turok 2005). In contrast
to these silvicultural regeneration systems, shelterwood and seed tree retention systems are
expected to affect genetic diversity to a higher degree, because only a limited number of
trees are left for reproduction or are given a limited number of years to pollinate (Buchert
1992). This is especially important if the trees left in the stand are genetically inferior.
6.2.1.3 Fruiting
Although effective pollination tends to be relatively constant from year to year, many tree
species uctuate between years of high and low reproduction, with few “average” years.
Even when owering occurs, fruiting can be erratic. Studies have shown that in harsh
climates, single extreme weather events can invalidate supposedly promising background
conditions for mast years. Mast years have been variously dened as years where seed
production exceeds the long-term mean by some predetermined level (LaMontagne and
Boutin 2009 and references therein). Mast seeding is a phenomenon referring to individual
trees producing seeds synchronously at “superannual intervals” (Rapp etal. 2013), which
leads to large uctuations in seed production at the population level. Evolutionary ecolo-
gists hypothesize that mast seeding occurs because synchronous reproduction among
conspecics is associated with several tness benets, including enhanced rates of pol-
lination (Kelly and Sork 2002), increased attraction of seed dispersers (Li and Zhang 2007),
and reduced seed predation (Curran and Leigthon 2000; Crone etal. 2011).
Apart from single weather events, the intensity and frequency of fruiting depends on
the age and vitality of the individual tree, as well as on the presence or absence of pollina-
tors, or seed and ower predators. Light-demanding pioneer species such as birch (Betula
spp.), aspen (Populus tremula), rowan (Sorbus aucuparia), and willow (Salix spp.) naturally
ower and fruit with high abundance after reaching (early) pubescence. While abundant
birch seed crops are observed at 2–3 year intervals in Northern Europe (Hynynen etal.
2010), Central European birch (Betula pendula) produces seeds almost annually (Cameron
1996), although with varying quantities (Huth 2009).
Heavy-seeded species such as beech, oak, or chestnut (Juglans regia), feature pronounced
mast years that usually occur at intervals longer than 5 years (Watt 1923; Sork 1993; Röhrig etal.
2006). Recent observations, particularly from beech forests in Europe (Hilton and Packham
2003), indicate a positive trend in mast frequency: according to the general opinion among
foresters in Northern and Central Europe, it has been easier to regenerate common tree spe-
cies during the last 20 years than previously. An analysis by Övergaard etal. (2007) illustrates
the observed trend of generally increasing mast year frequency, associated with increases in
mast crops. They found in Sweden that the average interval between mast years of beech has
decreased from 4 to 6 years for the period ~1700–1960, to 2.5 years during the most recent
30years, and there were two consecutive mast years twice during the latter period.
Climatic changes, especially increasing temperatures, may be responsible for the higher
frequency of mast years, but increased atmospheric nitrogen deposition may also be a con-
tributing factor (Schmidt 2006). More frequent mast years will likely simplify planning of
forest regeneration measures (Övergaard etal. 2007), but it is a legitimate question whether
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111Developing Restoration Strategies for Temperate Forests
and how this phenomenon is interlinked with tree and stand vitality and thus with sus-
tainable stand development.
Fruiting of species often varies from one year to the next and is not synchronized bet ween
species, even though seed rain may be synchronized among cooccurring species of the
same genus (Shibata and Nakashizuka 1995). Alternating abundance and composition of
seed rain (Lässig etal. 1995) may favor certain species on disturbed sites. In forest stands
with advance regeneration or dense ground vegetation, for example, a heavy-seeded spe-
cies will be favored over a light-seeded species by sudden disturbances such as wind-
throw (natural), harvesting, and site preparation (anthropogenic), because heavy seeds are
not impeded by vegetation and their seedlings can thus occupy local regeneration niches
rst. As seeds are dispersed over limited distances, seed availability highly depends on
the distance and on the strength of the nearest seed source (Clark etal. 1998). Selective
removal of seed trees of undesired species prior to site preparation or other regeneration
measures can thus be used to promote regeneration of desirable species. For dioecious tree
species, removal cutting can be restricted to female individuals.
Once a tree reaches maturity, its contribution to seed production depends on its canopy
position, vigor, and genetics. Preparatory cuttings, which are commonly used to precondi-
tion overstory trees in shelterwood systems (e.g., for beech), and fertilization aimed to induce
owering and fruiting can improve the generative potential of individual trees. The seed
crop per area can be maximized by an optimal combination of individual tree vitality and
tree density (Nyland 1996). Due to poorly developed individual tree crowns, this optimum
is seldom present in stands of maximum density. Selective thinning aimed at the promotion
of trees with good phenotypes has therefore been suggested as a possibility for improving
the probability of transferring desirable characteristics to the new stand generation (Nyland
1996). However, losses in genetic variability in stands that were thinned accordingly are
barely detected over the course of a single stand generation (Müller-Stark etal. 2005).
6.2.2 Mechanisms of Seed Dispersal and Consequences for Regeneration
The number of seed dispersal studies has grown exponentially during the last two decades
(Bullock and Nathan 2008; Schupp etal. 2010). Indeed, dispersal models are indispensable
for ecological research, because seed dispersal links the adult reproductive cycle to the
seedling stage (Wang and Smith 2002; Rother etal. 2013) and strongly inuences the demo-
graphic process of plants (Harper 1977).
Forester managers need to know how many seeds trees can produce and how far these
seeds are typically dispersed, for example, when initiating natural regeneration by har-
vesting single trees. For many forest stands, these seemingly simple questions are rela-
tively difcult to answer, because it is challenging to quantify the number of seeds prior
to dispersal, and postdispersal dynamics may have an important effect on recruitment
and later fertility (Wenny 2000; Birkedal etal. 2009). Although fertility can be inferred
from postdispersal seed densities (Schurr etal. 2008), this requires knowledge about the
parental tree from which the seeds originate, which is difcult to obtain because the seed
rain of neighboring trees typically overlap. The study of such environmental effects seems
particularly important for understanding and predicting plant performance in heteroge-
neous environments and their response to environmental change.
With the help of “inverse modeling” (Ribbens etal. 1994; Clark etal. 1998), tree fertility
and seed or fruit dispersal originating from a single individual are estimated simultane-
ously. Inverse modeling takes advantage of the specic probability that—out of a larger
number of trees—a particular tree can be considered the source of seeds caught in single
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112 Restoration of Boreal and Temperate Forests
traps placed in dened positions within a forest stand (Stoyan and Wagner 2001). However,
this method cannot be simply used to estimate effects of spatially varying environments
on fertility and dispersal. The process of seed dispersal can be structured into three com-
ponents (modied from Millerón etal. (2013):
1. Primary dispersal, which is the movement of seeds and fruits from the tree to
the ground by gravity (barochory). Apart from the potential area of plant recruit-
ment, this term also subsumes interlinked processes relevant for spatial patterns
of adult plants such as germination, predation, and competition (Nathan and
Muller-Landau 2000).
2. Secondary dispersal, which is dened as the removal of seeds and fruits by more
or less effective vectors such as animals (zoochory), wind (anemochorie), water
(hydrochory), and topographical gradients, once the seed is on the ground (Forget
etal. 2005).
3. Effective dispersal, which is the combination of primary and secondary dispersal
plus establishment.
In plant populations, recruitment necessitates component 1 (or 2) and 3. For trees in the
temperate zone, gravity, wind, and animals (especially via birds and mammals) are the
most important dispersal vectors. The term “dispersal range” refers to the distance a seed
can move from an existing population or a single adult tree. Seed density on the ground
generally decreases monotonically with distance from a seed source (Clark etal. 1999),
resulting in very low densities at long distances (Bullock and Clarke 2000). Effective dis-
persal distances are seldom greater than a few times the height of the seed bearer (Nyland
1996), but differences among species are considerable (Ribbens etal. 1994).
6.2.2.1 Zoochory
For some species, invasion into stands of other species can be regularly observed, for
example, oak or beech invading pine stands via zoochory of specialized birds (Pons and
Pausas 2008; Sheffer etal. 2013). In this context, far-ying jays (Garrulus glandarius) are
particularly important (Bossema 1979). They are abundant in oak and pine stands of all
densities and clearly prefer acorns to beechnuts, whereas nuthatches (Sitta europaea) are
more abundant in beech stands and prefer beechnuts to acorns (Perea etal. 2011). There are
other birds such as great tits (Parus major) that remove acorns and beechnuts, especially in
stands dominated by oaks, but they do not store the seeds.
It has been shown that zoochorous seed removal is determined by the structure of the
dominant vegetation because some habitats are more suitable for the disperser animals.
Pure pine stands can be appropriate regeneration sites as early as in the pole stage, when
light availability on the ground steadily increases (Sonohat etal. 2006). Densities of more
than 2000 oak stems ha1 of acceptable quality can thus develop if deer browsing does not
stunt or kill oaks smaller than 1.3 m (Mosandl and Kleinert 1998; Schirmer et al. 1999;
Stimm and Knoke 2004). Fencing can further accelerate this process.
The European jay has regenerated several thousand ha of pine stands with oak in Lower
Saxony, Germany (Otto 1996). Some foresters deliberately offer acorns to jays in special
boxes to facilitate the establishment of oak. Similar observations of succession have been
made for rowan, well known in Central and Eastern Europe for its ability to invade pure
Norway spruce stands via dispersal by birds (Zywiec etal. 2013) or mammals (Guitián and
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113Developing Restoration Strategies for Temperate Forests
Munilla 2010). Carnivorous mammals such as the red fox (Vulpes vulpes) and the European
pine marten (Martes martes) are also the main mammalian frugivore-seed dispersers in
temperate Scots pine forests (González-Varo etal. 2013) for rowan. In contrast to Scots
pine, however, gaps of a minimum size are essential for rowan to succeed in spruce stands.
These ndings provide a key starting point for understanding and modeling tree succes-
sion in restoration processes that include mammal-mediated seed dispersal, such as con-
nectivity, home range expansion, and recolonization.
In addition to mammals, another effective zoochorous vector in secondary dispersal
and predation can be seed removal by rodents. While some rodents feed on acorns thus
leading to seed predation, others (mainly scatter-hoarders) can also act as effective dis-
persers (e.g., for sessile and common oak, see den Ouden etal. 2005; for ash (Fraxinus excel-
sior) and wych elm (Ulmus glabra): Hulme and Hunt 1999). The role of rodents in tree and
shrub seed removal varies depending on the species, but is affected mainly by seed size
and morphology (Jansen etal. 2004) as well as by seed encounter and exploitation (Hulme
and Hunt 1999). However, successive dispersal movement and distance to rodent shelter
(shrub cover) are more important factors than acorn weight to determine dispersal dis-
tance and acorn survival (Perea etal. 2011). The ecological balance between seed predation
and effective dispersal is still largely unknown.
Effective dispersal not only depends on the quantity of dispersed seeds but also on the
quality of the seed dispersal process. According to Schupp etal. (2010), “quantity” is the
number of visits of a dispersal agent multiplied by the number of seeds dispersed per visit,
while “quality” is the probability that a dispersed seed survives handling by the disper-
sal agent in a viable condition (quality of treatment in the mouth and gut) multiplied by
the probability that a viable dispersed seed will survive, germinate, and produce a new
adult (quality of deposition). These interactions are well analyzed by Gómez etal. (2003)
for holm-oak (Quercus ilex) and one of its main dispersers, the European jay in a heteroge-
neous Mediterranean landscape. Moreover, there are animals not only effective for disper-
sal but also germination (Paulsen and Högstedt 2002).
6.2.2.2 Anemochory
Although seed removal by birds has been demonstrated to play an important role in long-
distance dispersal in the context of forest restoration, wind is certainly of special relevance
as a widely available seed dispersal vector that can transport many seeds over long dis-
tances. Unlike water, wind can transport seeds in all directions and is therefore important
for dispersal to upstream wetlands (Soons 2006) and to areas not directly connected to a for-
est. Compared to animals, wind transports seeds to a wider range of sites, therefore reaching
more sites but with lower seed densities. In wind dispersal, the effective dispersal distance
is determined by wind speed and direction, which can be inuenced by the density and
height of the remaining trees (Greene and Johnson 1996; Karlsson 2001). Many pioneer tree
species with seeds of very low sinking velocity rely on wind as a seed dispersal vector. Apart
from open landscapes, wind-dispersed birch species are thus also able to regenerate in pure
conifer stands, particularly favored by heavy disturbances (Perala and Alm 1990) that create
large gaps favorable for the growth of pioneer tree species (Huth and Wagner 2006).
6.2.2.3 Dispersal Distances
Forest ecologists commonly differentiate between mean dispersal distance (MDD), long
dispersal distance (LDD), and sometimes maximal dispersal density (MAXDD), because
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114 Restoration of Boreal and Temperate Forests
these distance measures have different functional relevance in ecosystem development
and the spatial management of forests. While the MDD is relevant in conventional silvi-
culture to produce quality timber by means of high stand density, the MAXDD is impor-
tant for long-term dispersal and invasion scenarios, particularly in fragmented landscapes
(Malanson and Armstrong 1996) and for natural species recolonization (Pacala etal. 1996).
For example, habitat clustering is a frequent phenomenon within forest restoration, result-
ing from habitat loss and/or fragmentation processes. Both processes operate at different
resolution and with different intensity, for example, wind-throw, forest clearcutting, site
degradation, or the segregation of tree species in pure stands due to management. The
recurrence of trees at different spatial scales varies according to landscape structure and
species dispersal strategies. Disentangling the relative impact of habitat loss and fragmen-
tation on the long-term survival of certain species requires understanding of the interac-
tions of habitat cluster availability and dispersal distance, and how they affect dispersal
success. Cattarino etal. (2013) addressed this problem by quantifying the magnitude of
these interactions, and emphasized the relevance of long distance dispersal.
The relevance of LDD and MAXDD with respect to restoration is stressed in another
example: An understanding of dispersal processes is relevant in the context of preventing
and controlling invasive alien tree species or managing the distribution of such species. To
predict the seed dispersal potential of Fraxinus pennsylvanica, an invasive alien in Germany,
Schmiedel etal. (2013) used a stochastic model predicting the number of seeds for a single
tree individual. The results were used to calculate species-specic dispersal distances and
the effect of wind direction for different assumptions regarding dispersal directionality
(isotropic and anisotropic). The topic of invasive species clearly illustrates the need to dif-
ferentiate between MDD and LDD, because the latter is of paramount importance for inva-
sion dynamics and the rate of colonization; further exemplied by Pairon etal. (2006) for
black cherry, a highly invasive forest tree species in Europe.
Knowledge of dispersal distances is required to determine “effective” seed density,
which should not focus on the maximum observed distances. In mixed stands, the effec-
tive distance to seed trees of all species is of particular interest if aiming to maintain the
current species mixture. If effective dispersal is lacking or the rate of recolonization is too
slow, for example, at long distances from seed sources, silvicultural techniques such as
direct sowing or planting of later successional species with limited dispersal ability can be
used to facilitate the establishment of desired tree species.
6.2.3 Costs, Benefits, and Implications of Seed Storage
Spatial discordance between primary and effective seed dispersal in forest stands indicates
that postdispersal processes are responsible for differences between seed rain densities
and seedling recruitment patterns. Although seed rain is observed mostly below and near
canopy trees, saplings are often established far from parental trees (e.g., for Fagus sylvatica:
Milleron etal. 2013, for maple (Acer spp.): Wada and Ribbens 1997). This discordance pat-
tern may be the result of secondary dispersal by animals or density-dependent effects
such as the Janzen-Connell effect (Janzen 1970 and Connell 1971). The period between
dispersal and recruitment, the so-called storage phase, is characterized by high potential
mortality rates and is thus one of the most relevant phases of the regeneration cycle.
For some tree species, seeds are not stored but germinate within the current growing
season (e.g., Acer rubrum, and Ulmus laevis). But for most temperate and boreal tree species,
seeds are stored on or within the forest oor because of unfavorable conditions for germi-
nation and growth outside the vegetation period. Winter storage is thus an adaptation of
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115Developing Restoration Strategies for Temperate Forests
species to the annual climatic cycle at middle and high latitudes (Runkle 1989). Extended
periods of opportunistic dormancy (i.e., enforced and induced dormancy; see Harper 1977)
can be regarded as an alternative to dispersal in space, sometimes referred to as dispersal
in time (Willson 1993).
6.2.3.1 Orthodox and Recalcitrant Seeds
Seeds of many tree species are sensitive to desiccation, humidity, anoxic conditions, frost,
fungal decay, or predation, and are therefore difcult to store. Seed storage patterns and
conditions not only vary among species, but also within species and even provenances. The
response of seeds to storage conditions and duration is generally expressed by the terms
orthodox” (desiccation-tolerant) and “recalcitrant” (desiccation-sensitive; see Roberts
1973). Orthodox seeds can be stored in situ for years, and also articially under cool low-
moisture conditions. Many forest tree species of important genera belong to this group,
including all European coniferous trees. Sugars (e.g., sucrose and rafnose) play an impor-
tant role as storage substances, accumulated in cells during maturation and degraded
during the rst 10–20 h of germination (Downie and Bewley 2000). In seed banks, seeds
of species with dormancy adaptations (e.g., Robinia pseudoacacia, Prunus serotina) can sur-
vive dormancy periods of several years. In contrast, the seeds of oak (Quercus spp.), beech
(Fagus sylvatica), or chestnut (Castanea sativa) are recalcitrant; they are intolerant of desicca-
tion and cannot be stored in the soil for more than one winter without the loss of viability
(Doody and O’Reilly 2008).
6.2.3.2 Seed Banks
Seed densities can increase rapidly following a major seed production event (e.g., masting
in heavy-seeded oak and beech) or progressively by a combination of consecutive smaller
dispersal events and seed storage (dormancy), thus building up seedling (ash, maple,
and black cherry, Prunus serotina) or sapling banks (beech and r) under closed canopies
(Szwagrzyk etal. 2001). Most of these species produce fewer seeds per tree and lower seed
densities on the ground over time than more light-demanding species (Ribbens etal. 1994;
Clark etal. 1998). Most tree species in temperate forest ecosystems do not build up long-
term soil seed banks or sapling banks (Halpern etal. 1999; Bossuyt and Honnay 2008).
But existing seedling and sapling banks are strategies by which low seed production is
compensated for by prolonged storage time. Underlying mechanisms include architec-
tural adaptations such as the relative proportion of leaf and branch biomass (Kohyama
1987), high morphological plasticity such as opportunistic plagiotropy in beech (Brown
1951), and metabolic adaptations such as low respiration rates (Walters and Reich 2000).
Individual seedlings are often capable of vigorous responses to sudden improvements in
resource supply, and may thus occupy promising niches in advance of other less tolerant
species.
Seed banks often serve as reservoirs of tree species diversity, which buffer the com-
position of plant populations and inuence the postdisturbance dynamics of vegetation
succession (Royo and Ristau 2012). Aerial seed banks, where seeds are retained in tree
crowns, have been observed for some re-adapted species outside Europe such as Pinus
contorta (Richardson 1998). For forest restoration strategies, forest managers can use a
range of regeneration methods relying on seed banks, but they must be familiar with the
particular mechanisms needed to break the dormancy of the desired tree species. These
mechanisms can be activated by light or heat (i.e., radiation), leading to particular cutting
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116 Restoration of Boreal and Temperate Forests
regimes or prescribed burning, as, for example, applied in eucalypts or serotinous pine
and spruce species (Smith etal. 2007).
6.2.3.3 Widespread Susceptibility
Upon making ground contact (primary dispersal), seeds are exposed to new risks and
mortality factors, among them are pathogens and predation. Postdispersal seed loss can
cause an important bottleneck in the natural regeneration of many tree species. The rela-
tive importance of each mortality factor varies depending on seed quantity and microhab-
itat conditions, resulting in spatial discordance in the performance of each regeneration
stage. In order to minimize postdispersal loss, some tree species produce seeds in irregu-
lar mast years. The intermittent mass production of seeds, for example, in beech stands,
is often invoked as a strategy evolved by some plant taxa to overcome the capacity of seed
predators to consume all seed (predator satiation; see Kelly and Sork 2002).
6.2.3.4 Predation by Rodents
Seed loss due to mice is common (Madsen 1995) and partly determined by storage dura-
tion. The ability of a rodent population to affect seed reserves at the scale of a forest stand
depends on whether or not—and if so, how frequently—the overall rate of seed consump-
tion exceeds the rate of seed production (Ruscoe etal. 2005). Signicant effects of burial
and microhabitat have been published for coexisting Mediterranean oak species in Spain
(Pérez-Ramos and Marañón 2008). The highest predation rates occurred for acorns located
on the ground surface (unburied), and in the most densely vegetated microhabitats, where
rodents usually exhibit higher activity. The lowest predation rates were observed in years
and at forest sites where the estimated seed production—and consequently resource avail-
ability—was higher than average, thus supporting the predator satiation hypothesis. This
temporal pattern of higher seed predation in nonmast years has been also documented in
other studies (Hulme and Borelli 1999).
Concerning restoration practices, an important recommendation for sowing is there-
fore to bury seeds 1–3 cm deep in the mineral soil (Birkedal etal. 2010, 2009). According
to different studies (Pérez-Ramos and Marañón 2008), seed loss can thus be considerably
reduced and the establishment of seedlings favored, as the leaf litter layer and mineral Ah
horizon may improve seed performance via reduced soil temperature and water evapora-
tion and increased local humidity levels (Rother etal. 2013). Moreover, it would be better
for restoration sowing activities to focus on microhabitats with a low or moderate vegeta-
tion cover and to avoid microsites with dense shrub canopies, where rodent activity is
usually high regardless of region and forest type (Fedriani etal. 2004).
6.2.3.5 Predation by other Vertebrates
Seeds of common tree species (e.g., oak and beech) are affected by vertebrate predation
both pre and postdispersal, because they produce large seeds of high nutritional value
(Nopp-Mayr etal. 2012). In return, birds disperse acorns away from the parent tree and
create seed caches as food supply for the winter (Stimm and Böswald 1994). However,
birds (as well as rodents) also predate on seeds such as acorns and beechnuts. Other omni-
present acorn and beechnut predators are wild boar, roe deer, red deer, and many beetles
(e.g., weevil larvae), who can consume most of the primarily and secondarily dispersed
seeds (González-Rodríguez and Villar 2012).
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117Developing Restoration Strategies for Temperate Forests
6.2.3.6 Scatter Hoarding
Seed dispersal by zoochory often results in highly clumped seed deposition, with high
seed densities in some locations far away from the parent tree in areas used preferentially
by dispersers (Schupp etal. 2010). Jensen (1985) found beechnuts scatter-hoarded with a
mean number of ve seeds per cache. The typical burial site is usually located a few cm
below the soil surface close to the wall of a rodent runway. Predation on caches is gener-
ally high, but in experiments more caches survived if a surplus of seeds was offered or if
rodent numbers were reduced. Age structure of saplings revealed that most individuals
had germinated in the years following mast years. Thus, there is strong circumstantial evi-
dence that scatter-hoarding animals inuence the population biology and evolution of tree
species by predation and dispersal of seeds. In turn, the synchronous production of seeds
leads to prolonged reproduction periods in the rodent species, resulting in outbreaks.
Recent research results highlight the importance of clumped seed deposition for patterns
of seedling survivorship and recruitment. By using spatially explicit simulation models,
Beckman etal. (2012) showed that clumped seed deposition increased the probability of
seedling establishment under both insect predation (host specic bruchid beetles) and
pathogen attack (Pythium and Phytophthora), as it led to local satiation of insect seed preda-
tors and made it harder for pathogen distributions to track seeds.
6.2.3.7 Pathogens
High humidity levels at the forest oor generally favor micro-oral growth, which reduces
the aeration of seeds and results in the production of toxins (Knudsen etal. 2004), thus
causing seed rotting and reduced germination. Seeds are particularly susceptible to a vari-
ety of pathogens and suffer considerable loss of viability during storage.
The pathogen Ciboria batschiana, a fructicolous Discomycete, is a serious problem affect-
ing acorn storage, which in combination with other native fungi is responsible for poor
storability. C. batschiana can destroy up to 80% of an acorn crop and may also result in
severe losses in oak seedling populations in forests (Schröder etal. 2004). Thus, oaks bene-
t from seed dispersal, as seedling recruitment is facilitated due to a decrease in pathogen
infections following dispersal (Clark and Clark 1984). Beechnuts are affected by fungal
infection to a similar degree. Massive mortality in stored beechnuts both in Europe and
in the United States has been caused by several Phytophthora species. Mycological studies
conrm the susceptibility of beech seedlings to Phytophthora spp. (Orlikowski and Szkuta
2004). Another relevant fungus in beech storage is Rhizoctonia solani (Hietala etal. 2005)
displaying symptoms of cotyledon rot. The disease is characterized by decay resulting in
reduced failure to sprout or death after emergence.
Pythium is a genus with high pathogenicity and causes the damping-off disease of ger-
minating seeds and seedlings during storage, which is not restricted to a single species
(Augspurger and Wilkinson 2007). As numerous species of this oomycetes have been
described for nursery soils (Weiland etal. 2013), there is a potential risk of infecting resto-
ration sites in the course of planting.
6.2.4 Germination in Stressful Surroundings: A Narrow Bottleneck
6.2.4.1 Germination and Safe Site
Occurrence and timing of germination play essential roles in subsequent plant establish-
ment (Baskin and Baskin 2001; Manso etal. 2013) and are often discussed as a bottleneck
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118 Restoration of Boreal and Temperate Forests
within the demographic transition from seed to sapling (Rother etal. 2013). Seeds germi-
nate only if certain conditions are met; including the breaking of dormancy. The species-
specic maternal reserves within the seed endosperm must be mobilized to provide for
the two autotrophic cotyledons, the primary leaves, and the proliferating roots. As the
primary integrators of environmental signals (Farnsworth 2008), phytohormones enable
the nascent seedling to adapt to its surrounding conditions. For many tree species, germi-
nation itself does not depend on light conditions (Nicolini etal. 2000), and radiation is thus
not relevant until the reserves in the endosperm are depleted.
The results for seedling survivorship after germination have been widely studied
(reviewed in Goulet 1995); the seedling environment is often described as extreme (Grime
1979) and conditions can be stressful and chronic, even in forest restoration. Factors induc-
ing stressful surroundings for the initial seedling development can be abiotic (e.g., unfavor-
able light and temperature regime, drought, pH, and anoxia) or biotic (e.g., high intra- and
interspecic competition, absence of mycorrhizae, herbivores, and pathogens). The latter
often benet from the same environmental conditions that are favorable for germinating
seeds or succulent seedlings, and fungal attack and death by damping-off disease are thus
common fates.
The microsite where the germination process can be successfully completed is termed
a “safe site” (Harper 1977). Safe sites generally provide a seed with sufcient moisture,
warmth, oxygen (Baier etal. 2007), and light of appropriate quality (Smith etal. 2007).
Pathogens are absent or ineffective at safe sites. A lack of safe sites restricts germina-
tion even at many restoration sites, especially those with extensively exposed soils and,
at the other extreme, those with a dense cover of competing vegetation (Urbanska 1997;
Galatowitsch 2012).
6.2.4.2 Light
In many cases, seedling reserves are critical, especially under conditions of limited light
availability (Ammer etal. 2008b). Even small reductions in biomass accumulation may
lead to seedling mortality (Fenner and Thompson 2005). In some forest ecosystems, the
light intensity at the forest oor is close to the photosynthetic compensation point, that
is, the light intensity at which respiration is equal to photosynthesis (Modry etal. 2004;
Facelli 2008). The successful establishment in such a stressful situation is only possible if
the seedling achieves a sustained positive carbon balance; photosynthesis must exceed
respirational carbon loss (Kitajima and Myers 2008). Seedlings produced by larger seeds
(e.g.,beech) are often more shade-tolerant (Leishman and Westoby 1994) due to more abun-
dant reserves. Although seed size may not be an adaptation to limited light condition per
se (e.g., Abies alba), it may assist in avoiding mortality as a result of herbivory, desiccation,
or burial by litter.
The morphology of seedling organs is also affected by varying light availability. If
growing in shaded conditions, leaves are generally broader and thinner, because this opti-
mizes light capture (Bazzaz 1996). Most species also feature lower specic root length and
increased leaf area under these conditions (Reich etal. 1998).
6.2.4.3 Litter and Soil Organic Matter
As a boundary layer, that is, a transition zone between the atmosphere and the soil, soil
surface characteristics strongly determine microsite conditions. Over the range of a few cm,
the environmental conditions can change dramatically. The boundary layer is primarily
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119Developing Restoration Strategies for Temperate Forests
characterized by site, species-dependent litter quality and quantity, stand age, and stocking
density. Existing vegetation can alter the boundary layer conditions by intercepting radia-
tion, affecting moisture availability, and determining the thickness of the litter and humus
layers and thus the humus form (George and Bazzaz 1999; Fischer etal. 2002).
The humus form, humus content in the mineral soil, and the thickness of the litter hori-
zons are particularly crucial for overall moisture supply. The surface substrate and the
vertical depth of litter and humus accumulation in relation to the relative size of seeds
and germinants often determine whether the given site can serve as a safe site for a par-
ticular species or not. Exposed mineral soil is usually the best substrate for germination
and initial growth because of its favorable moisture supply. Although litter has no effect
on emergence (or even a positive one, see Facelli 2008), thick humus layers can consider-
ably impede seedling emergence and germination. Apart from reducing light availability
(Facelli and Pickett 1991), litter may release leachates with potentially allelopathic effects
on seedling establishment (Olson and Wallander 2002). For example, Chrimes etal. (2004)
and Mallik and Pellissier (2000) discussed these interactions based on allelopathy for Picea
abies and Vaccinium myrtillus and other dominant ericaceous understory plants.
The fate of seedlings is thus affected by seed location at the time of germination. Plants
are more successful when their seeds are near the mineral soil surface as roots grow-
ing in litter substrate can fail to obtain enough water for survival. Acorns and beechnuts
often germinate best when buried 2–3 cm in the soil, regardless of the covering material
(Millerón etal. 2013), whereas birch seeds require physical contact with ne humus mate-
rial or the uppermost mineral soil (Ah) horizon to germinate successfully (Carlton and
Bazzaz 1998; Karlsson 2001).
Thick humus layers (e.g., raw humus) may need to be reduced to promote survival
of seeds and to facilitate germination, which can be done directly by prescribed re or
indirectly by preparatory cutting. However, if the thinning intensity is too high, advance
regeneration of undesired species or competing ground vegetation may thus be promoted.
Undesired species are often more shade-tolerant than desired ones, for example, Norway
spruce (Picea abies) on wet soils where pedunculate oak (Quercus robur) is preferred or in
beech stands on sites with good nutrient status where more valuable broadleaf tree species
are preferred. In mixed stands, species composition can be altered by means of early thin-
ning that selectively removes undesirable species.
Direct treatments aiming to make the physical environment of a site more suitable for
germination are more common than preparatory cutting. These treatments are intended
to modify the microclimate, improve access to water supply by exposing mineral soil, or
eliminate competing vegetation (Morris etal. 1993). Although site preparation may inter-
fere with natural succession (Nyland 1996), its benets include the elimination of unde-
sired tree species (Gordon etal. 1995; Lautenschlager 1995) as well as of grass and herbs
which provide cover for seed predators such as mice. Measures are usually directed at
species that have established advance regeneration and where control by weeding or pre-
commercial thinning is not feasible.
Apart from mineral soil, the germination of many species is facilitated by downed
deadwood, which consists of woody debris, stumps, and overgrown deadwood (LePage
etal. 2000; Hagemann etal. 2009). This is particularly relevant at harsh sites, for example
in mountain forests or in forests with a continuous grass cover (e.g., Calamagrostis spec.,
Deschampsia exuosa, and Carex brizoides). The successful establishment of species such
as Norway spruce, silver r, sycamore, and rowan could be enhanced in high-elevation,
grass-covered areas by increasing deadwood abundance (Santiago 2000; Motta etal. 2006).
The role of downed deadwood is discussed in more detail in the next section.
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120 Restoration of Boreal and Temperate Forests
6.2.4.4 Interaction with Mycorrhizae
Many case studies, both from laboratory experiments and eld studies, show that spe-
cic mycorrhizal fungi promote seedling establishment through increased access to soil
resources (Nara 2006), drought tolerance, and resistance to pathogens, among other ben-
ets (St. John 1997). In most undisturbed European ecosystems, high root density and
the ubiquity of long-lived mycorrhizal plants lead to conditions where inoculum (both
spores and hyphae) is almost always available to newly germinated seedlings (e.g., Janos
1992; Börner etal. 1995). However, in restoration sites, especially where afforestation is
required, conditions can be different as unfavorable soil status and/or articial vegetation
structures often prevail. After a site and its vegetation have been disturbed, its soil is more
likely to have reduced and/or patchy mycorrhizal infectiveness (Janos 1992), especially for
fungi that colonize new roots predominantly via hyphae rather than spores (Borner etal.
1996). Soils of early successional forests therefore typically show a low abundance and
diversity of mycorrhizal fungi (Galatowitsch 2012).
Allen etal. (2002) noted that restoration measures typically attempt to establish late suc-
cessional vegetation by planting late-seral species in early successional soils. The benets
of mycorrhizal symbiosis are therefore generally more readily apparent in forest restora-
tion than in stands with high naturalness. Even so, plants intended for afforestation or
enrichment planting in restoration should be inoculated in the nursery with appropriate
mycorrhizal fungi, whereas an inoculation will be of little benet for plants to be estab-
lished in undisturbed forest soils.
6.2.4.5 Physical Damage to Seedlings
Even after emergence, seedlings and saplings can be subjected to substantial substrate
movement, especially during forest restoration. As the result of wind erosion, young trees
are often killed from partial root exposure, as often described for postmining landscapes
(Hüttl and Weber 2001). Yet another phenomenon also creates unstable substrates for seed-
lings: in freezing air and low surface temperatures without snow cover, seedlings can be
heaved out of the soil by ice crystals forming near the soil surface (Facelli 2008).
6.2.5 Seedling Survival and Establishment: The Fine Step to Maturity
As all autotrophic plants require the same resources (i.e., water, nutrients, and light), com-
petition is the most important process during the postemergent phase of the young tree.
Both overstory and ground vegetation reduce resource availability for seedlings, and mor-
tality is thus common during subsequent seedling development.
Competition between individual plants can be differentiated into the effect of biomass
production on resource availability and the response of individual tness to resource
limitation (Goldberg 1990). In recent years, knowledge of different strategies used by tree
seedlings to acquire and allocate resources has improved considerably, particularly for
photosynthetically active radiation (PAR; see von Lüpke 1987; Coates and Burton 1999;
Hertel etal. 2012) and nutrients (Johansson etal. 2012; Guo etal. 2013). In contrast, knowl-
edge of belowground mechanisms and their importance for seedling vitality is still rudi-
mentary (Havranek and Benecke 1978; Flaig and Mohr 1990; Ammer 2003). Belowground
resources in forests feature an extremely high variability at both micro- and macro-scales,
which is difcult to control and separate from variability in PAR (Huss and Stephani 1978;
Reed etal. 1994; Walters and Reich 1997; Finzi and Canham 2000). In particular, it is not
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121Developing Restoration Strategies for Temperate Forests
well known how inter- and intraspecic root competition inuence the survivorship of
individual roots (Rust and Savill 2000; Beyer et al. 2013). Nevertheless, species-specic
responses to the complex resource pool are the key to understand the coexistence of spe-
cies (Tilmann 1982).
Restoration measures controlling competing vegetation during the seedling estab-
lishment stage can increase soil temperature, PAR availability, and nutrient availability,
thereby improving seedling survival and growth particularly under unfavorable site con-
ditions (Brand 1991; Madsen 1995; Groot 1999). The impacts of limited PAR availability
due to competing vegetation are species specic (Küßner etal. 2000), but also depend on
interactions with other environmental resources (Lautenschlager 1999; Küßner etal. 2000).
Specic recommendations for vegetation control depend on the target tree species (Löf
2000), the dominant weed species (Lautenschlager 1995), and the site. Specifying appropri-
ate treatment intensities is not easy (Tappeiner and Wagner 1987; Cain 1991), and should
account for the difference between a competition threshold and a critical-period thresh-
old (Wagner 1999). The former refers to the vegetation density at which yield loss occurs
(Jobidon 1994), the latter to the time when vegetation control should begin to prevent yield
loss (Wagner etal. 2010, 2011).
The main aim of this chapter was to illustrate the individual stages on the path to suc-
cessful natural regeneration and to allow for a more accurate interpretation of the general
recruitment probabilities. A clear focus was set on the identication of critical environ-
mental and biological factors potentially affecting the recruitment of the progeny tree
population. Although the young stand can mirror the seed rain distribution originating
from the parental stand, its characteristics more likely will be modied by differential
seed dispersal, seed storage and germination, mortality or predation, and growth (see
Figure 6.1). This background information will help to understand the specic silvicul-
tural strategies for forest restoration presented in the following two sections, discuss-
ing restoration strategies considering the overstory structure and below-canopy stand
components.
6.3 Overstory Restoration Strategies Oriented
toward the Natural Disturbance Regime
Forest restoration measures that take place in degraded forests can take advantage of exist-
ing canopies, which are to some extent degraded, that is, altered with regard to species
composition, structure, density, and so on. The forest canopy also affects tree regeneration
below, however, and these effects may be deliberately modied by treatments. This sec-
tion describes some options of canopy treatments to facilitate forest restoration through
regeneration.
6.3.1 Emulating Overstory Conditions Originating from Large-
Scale Disturbances to Support Ecological Processes
Two main functions of large-scale disturbances (between 10 and 100 km²; Temperli
2012) in forest ecosystems have been identied as the gradual progression toward
natural forest development stages, and consequently the possibility for adaptation to
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122 Restoration of Boreal and Temperate Forests
environmental changes after the destruction of the current forest system (Mitchell etal.
2004; Walker etal. 2007). The postdisturbance, more or less time-consuming successional
process includes a broad range of subprocesses, among them mineralization, transpi-
ration, regeneration, and recolonization. The progression and quality of each subpro-
cess are inuenced by the disturbance origin and the degree of destruction of different
structural compartments. Possible abiotic causes for natural large-scale disturbances in
temperate forest ecosystems are storms, drought, res, oods, avalanches, and wet snow
(Frelich 2002), which all change the structure of the previous overstory tree layer. The
damage inicted upon overstory trees and single stems strongly differs between broken,
uprooted, and burned stands (Busing etal. 2009; Goldammer 2013; Mitchell 2013). The
same applies to large-scale biotic disturbances, caused by bark beetle attacks or fungal
infections (Seidl 2009; Netherer and Schopf 2010), because of differences in individual
tree or tree species resilience. Although the overstory tree layer is often disturbed over a
large, more or less contiguous area, the understory vegetation and the surface soil layer
are characterized by heterogeneous disturbance patches of different size (Walker etal.
2007; Busing etal. 2009; Jonášová etal. 2010). The resulting mosaic of the previous under-
story is inuenced by disturbances in a different way than the overstory, because natural
large-scale disturbances never feature homogenous damage intensities throughout the
entire disturbed area. This is mainly due to the heterogeneity of specic site conditions
such as topography, relief, soil horizonation, or groundwater level. Moreover, the dif-
ferent traits of the present ground vegetation also contribute to spatial heterogeneity
(Cater and Chapin 2000). Both categories of inuential factors increase the site specic
heterogeneity after large-scale disturbances, and thus support the formation of diverse
ecological niches (Hutchinson 1978; Honnay etal. 2002). For example, small-scale patches
and structures occur frequently following different large-scale disturbances, including
broken branches, crowns and stumps, fallen trees, uprooted and leaning trees, as well as
root plates, pits and mounds (Ulanova 2000; Brang 2005a,b). Overall, the original stand
conditions are crucial for postdisturbance ecosystem development. It can be assumed
that large-scale disturbances lead to more heterogeneous conditions in natural for-
ests than in plantation forests (Keidel etal. 2008), where the overall variability is low
due to mono-structured stand conditions and articially homogenized site conditions.
Moreover, extreme climatic conditions probably increase the frequency and risk of large-
scale storm, drought, or ood events (Rowell and Moore 2000; Dale etal. 2001). Even
though heterogeneity following large-scale disturbances will increase the diversity of
plantation forest stands; it can be assumed that the next stand generation will still be less
diverse than disturbed natural forest stands (Millar etal. 2007).
For large-scale restoration, efforts aimed at modifying disturbance regimes can result
in the support of successional processes. According to Walker etal. (2007), “succession and
restoration are intrinsically linked because succession comprises species and substrate change over
time and restoration is the purposeful manipulation of that change. These authors argue that
restoration strategies are usually geared toward short time scales, while natural succession
processes need longer time frames to successfully develop (Thomasius and Schmidt 2004).
Taking this into account, the practical implementation of natural processes and struc-
tures associated with large-scale disturbances can be divided into primary and secondary
manipulations aiming to restore the system. Primary large-scale manipulations are asso-
ciated with considerable technical efforts and create important structures for supporting
specic successional processes. Restoration measures in this manipulation category pri-
marily affect the natural regeneration indirectly via altering the overstory conditions as
summarized in Table 6.1.
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123Developing Restoration Strategies for Temperate Forests
6.3.2 Manifold Harvesting as a Strategy for Managing
Structural and Species Diversity
Two common restoration objectives—the development of natural conditions within pro-
tected areas without any further forest utilization, or the establishment of near-natural
conditions as an integral part of silvicultural management strategies—require a combi-
nation of various structures to enhance species diversity. Restoration strategies within
protected areas (e.g., reserves and national parks) are often stratied according to zoning
categories, for example, “core zones” where forests are left to develop freely, “develop-
ment zones” dened as protected areas managed mainly for ecosystem protection, and
recreation zones” subjected to near-natural management concepts (e.g., Federal Agency
for Nature Conservation 2012). For national parks or reserves, restoration measures gener-
ally have only initializing character (Wojczulanis 2002; McComb 2007). Continuous resto-
ration measures are therefore mainly required for application in managed forest areas, and
TABLE 6.1
Possible Primary Large-Scale Restoration Measures Associated with Altered Overstory Conditions
Primary Restoration Measures Effects on the Regeneration Process
Complete clearing
Cutting of overstory trees and
removal of all woody material
Striving for a complete change in tree species composition due to high
environmental risks and to prevent further seed dispersal and
regeneration of undesired tree species. This procedure is typically used
for controlling invasive/alien tree species (D’Antonio and Meyerson
2002). Usually, low amounts of deadwood are left, because of the
nontarget tree species within the previous overstory
Simulated windthrow
Pulling trees down or breaking
stems using winch systems or
excavators
Decoupling and reducing overstory tree competition in favor of tree
regeneration. As a result, light, water, and nutrient availability at the
forest oor increase signicantly (Kliejunas etal. 2005; Koizumi etal.
2007). Particularly newly established light-demanding tree species,
advance regeneration and early successional ground vegetation will
benet from this measure. Root plates create pits and mounds. High
accumulations of different deadwood categories are present
Prescribed burning
Implementation of low or high
intensity burns by means of
controlled re lines
Low-intensity res kill the existing ground vegetation and reduce the
litter layer, but most of the soil seed bank (Hille and den Ouden 2005;
Goldammer etal. 2013) and the overstory trees (Kozlowski 1974) will be
unaffected. Only some trees will suffer cambium damage and
subsequent die-back, but the degree of canopy closure is largely
unchanged. It should be noted that, some tree species will be present in
the next stand generation through their ability of resprouting. Therefore,
favorable conditions for the regeneration of intermediate tree species
exist. In contrast, crown res with higher intensity destroy the overstory
trees as well as the ground vegetation and soil seed bank. Thus, light
availability at the forest oor will signicantly increase. The following
succession dynamics depend on the seed dispersal rate from
surrounding tree species (Piha etal. 2013)
Water level regulation
Technical implementation of
different topographical structures
within oodplains such as
oodplain drainage, blocking of
channels, restoration of
meandering riverbeds, and
rewetting of swamp areas
The regeneration of broadleaved tree species depends on the availability
of seed trees which are well adapted to uctuating water levels (Leyer
etal. 2012). Seed production, seed dispersal adaptations, and stream
velocity decide about the presence of different tree species in the
regeneration layer (Peterken and Hughes 1995). The composition and
the distribution of tree regeneration can be controlled by small-scale
modication of soil conditions, elevation, water level, and duration of
ooding (Deiller etal. 2003)
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124 Restoration of Boreal and Temperate Forests
must be differentiated by spatial scale to successfully combine restoration measures with
harvesting procedures. At the landscape scale, forest restoration practices should combine
manifold harvesting systems instead of implementing homogenous harvesting methods
over large areas (Lindenmayer and Franklin 2002). One of the main decits of homogenous
harvesting regimes with respect to restoration efforts and naturalness is the loss of small-
scale diversity of species and structures. Species with a low activity density need transi-
tion zones or corridors across scales to maintain their populations (Hunter 1999; McComb
2007). At smaller scales, the selection of suitable harvesting systems for restoration activi-
ties aiming to establish complex natural processes is often restricted by existing stand
structures, site conditions, and the available space. Usually, the selection of management
strategies in forest enterprises is affected by numerous external factors that are not directly
linked to the topic of restoration (Nyland 2002; Messier etal. 2013). In order to deliberately
use specic harvesting systems in forest restoration, the resulting structures or processes
must be identied (Fries etal. 1997; Kerr 1999). This section analyzes “classic” harvesting
systems and their suitability to establish or maintain near-natural structures and regen-
eration conditions (Table 6.2).
6.3.2.1 Clearcut System
Although the use and the size of clearcuts have been restricted in most temperate for-
est regions, the system is still preferentially practiced in some forest regions because
of the high degree of mechanization, low short-term costs, and simple demands with
TABLE 6.2
Overview of Typical Stand Characteristics within (a) Plantations and (b) Near-natural Forests, and
Restoration Measures Suitable for Initiating Change from (a) to (b) (marked by arrows).
(a) Plantation Forests Suitable Restoration Measures (b) Natural/Near-natural Forests
Single-species overstory Promotion and revitalization of admixed tree
species via control of crown competition
Minimal tree species admixture
Single-layer overstory Establishment of vertical and horizontal stand
structures via single-tree selection systems and
combinations of uneven-aged tree species
admixture
Single to multilayered overstory
Homogeneous canopy
closure
Combination of various differently-sized
disturbances to implement heterogeneity in
light, nutrient and water availability and
inuence inter- and intraspecic competition
Heterogeneous canopy closure
Homogeneous soil
conditions
Establishment of variable site conditions to
promote the diversity of ecological niches
available for different tree and ground cover
species
Natural, unaffected soil
conditions
Large-scale management
blocks with
homogenous treatment
Increase of the variability of treatment sizes,
methods and techniques and decrease of the size
of management blocks
Natural mosaic of successional
patches
Few selected, often not
site-adapted tree species
Selection of vital trees, well adapted to specic
site conditions; approximation to natural tree
species admixture with variable vertical and
horizontal structure
Long-term site-adapted tree
species
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125Developing Restoration Strategies for Temperate Forests
respect to management. The removal of the entire overstory leads to extreme changes
of environmental conditions (i.e., light, temperature, water, and nutrient regimes). As
a result, clearcut sites are often completely restocked using articial regeneration, par-
ticularly plantations where stocking control is an objective. Clearcutting is also used to
naturally regenerate forests where desired species are shade-intolerant. With respect to
restoration objectives and near-natural processes, clearcuts only emulate those aspects
of natural disturbances that are linked to the complete loss of overstory trees (Priewasser
etal. 2013), while—in contrast to natural disturbance regimes—important structural ele-
ments such as deadwood, small-scale soil surface heterogeneity and islands of potential
seed trees are lacking. If advance regeneration is already present prior to clearcutting,
the tree species composition of the future overstory is determined by natural conditions,
likely ltered by climate and interaction processes. Thus, clearcutting can be used as
an initial event for forest restoration oriented toward establishing natural succession
processes, as for example, practiced when converting late successional forests into non-
forest ecosystems such as meadows or heathlands (Leuschner 1994; Härdtle etal. 2009).
One relevant difference between regular clearcuts and naturally established open areas
is the presence of deadwood at these sites (Priewasser etal. 2013). Moreover, if suitable
seed trees are missing, small-scale supplementation with appropriate seeds or seedlings
is possible.
6.3.2.2 Seed Tree System
The seed tree system can be described as a “soft version” of the clearcut system with respect
to the creation of extreme environmental conditions and suitable seed trees (Kuuluvainen
and Pukkala 1989; Nyland 2002). This system is mainly used to regenerate one light-
demanding tree species throughout the entire site, but more natural modications of the
system can be easily made. Parts of or entire overstory trees can be left on site to increase
deadwood stocks (Rosenvald etal. 2008). Areas without dense regeneration of target tree
species can be left to regenerate naturally by other surrounding tree species, which will
result in an admixture of different tree species with a broader range of age and develop-
ment stages. For the seed tree system, the change from closed stand conditions to open-site
conditions is less pronounced than for the clearcut system.
6.3.2.3 Retention Tree System
When both forest practitioners and forest scientists had realized the discontinuity of the
complete clearcut system, they developed the idea of retaining a number of selected trees,
so-called retention trees to compensate for the loss of key species important for forest
ecosystems (Mitchell etal. 2007; Rosenvald and Lõhmus 2008; Lindenmayer etal. 2012).
In the retention tree system, also called reserve tree system, retention trees can create a
permanent link to original forest-adapted oral and faunal species of soil-dwelling organ-
isms, insects, mycorrhizae, or specialized seed dispersers such as small mammals and
birds (Matveinen-Huju etal. 2006; Gustafsson et al. 2010; Lindenmayer etal. 2010). This
“link function” also applies to important processes associated with typical forest condi-
tions. The shelterwood system—a special retention tree system—creates homogenous
low-density canopy conditions, which promote the establishment of dense tree regenera-
tion throughout the entire site. The regeneration of shade-tolerant tree species such as
European beech using the shelterwood reserves system has thus led to large areas with
dense homogenous beech regeneration (Wagner etal. 2010).
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126 Restoration of Boreal and Temperate Forests
6.3.2.4 Irregular Shelterwood
Without supporting activities, it is therefore impossible to integrate admixtures of other
tree species. Hence, manipulations which create irregular canopy openings to increase
tree species heterogeneity within the regeneration layer are one approach to increase
biodiversity by means of restoration measures (Raymond etal. 2009). Similarly, small-
scale preparatory cuttings can be temporally staggered to stimulate seed production by
overstory trees and to increase vertical structure. Additional cuttings within the regen-
eration layer can be used to reduce the competitive pressure on admixed tree species
with lower shade tolerance and small admixture percentage. Without additional restora-
tion measures, all harvesting systems outlined above produce even-aged tree regeneration
(Fries etal. 1997; Nyland 2002). In contrast, the following silvicultural methods are ori-
ented toward the establishment of uneven-aged tree regeneration. These also include the
two general types of group-selection methods, which reduce the degree of canopy closure
by means of gap and group-sized shelter cuttings (Sagheb-Talebi and Schütz 2002). Light
demanding pioneer tree species receive greater support by the imitation of larger natu-
ral gaps. Small-scale heterogeneity within those gaps naturally results from the spatial
within-gap heterogeneity of environmental conditions (e.g., light availability) and from
edge effects created by the surrounding border trees. The stronger the environmental
gradients, the more ecological niches can become colonized by different ground vegeta-
tion and tree species (Sagheb-Talebi and Schütz 2002; Poorbabaei and Poor-Rostam 2009).
Therefore, the exclusive use of shelterwood-group selection cutting limits the establish-
ment and growth of tree species requiring higher light availability. The adaptation of
group selection to natural disturbance variability can increase the variability of small-
scale environmental conditions as well as the dynamics of the ensuing regeneration pro-
cesses. Although group-selection methods generally feature a high exibility for use
in forest restoration, single-tree selection is often considered the preferential uneven-
aged reproduction method (Nyland 2002). However, single-tree selection systems are
often criticized for excluding light-demanding tree species (Bauhus etal. 2013; Huth and
Wagner 2013). Summarizing the possibilities for choosing harvesting systems aimed at
promoting regeneration processes in the course of temperate forest ecosystem restora-
tion, the large-scale combination of different harvesting systems, which at smaller scales
are oriented toward emulating natural disturbance regimes, is a feasible compromise
(Schütz 2002).
6.3.2.5 Emulating Old-Growth Gap Conditions for Regeneration Processes
It has long been known that canopy gaps are essential for the natural regeneration of for-
ests (e.g., Watt 1925, 1947). Compared to those parts of the forest with a closed canopy, the
resource availability at the forest oor in gaps is altered, that is, improved. This improve-
ment in availability applies to radiation (Canham etal. 1990), soil water (Gray etal. 2002),
and soil nutrients (Bauhus 1996). All of these resources are essential to tree regeneration;
the high relevance of gaps to tree regeneration is hence understood as a response of the
regeneration to increased resource availability (e.g., Beckage and Clark 2003). The occur-
rence of canopy gaps is linked to canopy “disturbance,” that is, a canopy gap is “a patch
created by the removal of the canopy” (Connell 1989). In the majority of events, the removal
of parts of the canopy means that old trees or tree crowns are partly or completely dam-
aged or destroyed. Thus, the decay of old trees that create canopy gaps is linked to the
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127Developing Restoration Strategies for Temperate Forests
rejuvenation of the forest. This association between tree decay and regeneration is inte-
grated in the framework of the so-called “gap-theory.” A comprehensive overview about
the gap-theory of forest regeneration is given by Yamamoto (1992). According to this the-
ory, forests are perceived as spatial mosaics (e.g., Remmert 1991) of various compositional
and structural phases that are changing cyclically over time, implying that canopy gaps
are pivotal to tree regeneration. Many scientists working in natural forests have adopted
the gap-theory explicitly (Zeibig etal. 2005) or implicitly (Leibundgut 1982; Korpel 1995;
Christensen and Emborg 1996).
To date, the gap-theory has proved useful in analyzing a variety of forest ecosystems
ranging from temperate deciduous forests dominated by shade-tolerant species (European
beech: Wagner et al. 2010; mixed stands: Busing 1994) to subtropical forests (longleaf
pine, Pinus palustris: Brockway and Outcalt 1998), and to tropical moist evergreen forests
(Brokaw 1985).
Within natural forests, gap size is highly variable. Runkle (1982) noted that the gap size-
frequency distribution of a forest may be approximated by a lognormal distribution, that
is, that a forest features many small and few large gaps. In natural forests, the lognormal
or near-exponential shape of the gap size-frequency distribution seems to be very similar
regardless of forest type (Foster and Reiners 1986; Wagner etal. 2010). However, differ-
ences between forests have been observed regarding the total fraction of area in gaps as
well as the average gap size (Runkle 1982; Denslow 1987).
In addition to merely providing space for tree regeneration as such, gaps are also struc-
tural elements relevant for the nontree species diversity of natural forests, for example,
for butteries (Hill etal. 2001), birds (Levey 1988), and ground ora (Hahn and Thomsen
2007). Not surprisingly, research on the regeneration ecology of forest tree species has
also very intensively focused on the relevance of gaps for diversity, particularly in mixed
stands. Two hypotheses are worth mentioning as they may help to understand the rele-
vance of canopy gaps to tree species diversity in forests: (1) the “gap partitioning hypoth-
esis” (Ricklefs 1977); and (2) the “intermediate disturbance hypothesis” (Connell 1978).
The rst hypothesis considers both the variability of the individual gap area and the
gap size-frequency distribution. The variability in gap size may serve as a template for
the niche specialization of forest tree species, thus leading to gap partitioning among
species. The latter hypothesis claims that “diversity is highest when disturbances are
intermediate in intensity or size and lower when disturbances are at either extreme”
(Connell 1978). Over time, conrming evidence has been presented for both hypotheses:
the gap partitioning hypothesis (e.g., Brokaw 1985; Denslow 1987; Abe etal. 1995; Sipe
and Bazzaz 1995) and the intermediate disturbance hypothesis (Sheil 2001). However,
there is also a remaining debate about the validity of either hypothesis (e.g., Brokaw and
Busing 2000; Fox 2013).
While the gap-partitioning hypothesis seems to be most appropriate for temperate,
low-diversity climax forests (Yamamoto 1992), the intermediate disturbance hypothesis is
most suitable for transient successional forests at nonequilibrium (Sheil 2001). An impor-
tant differentiation between disturbance regimes leading to succession and the distur-
bance regime in climax forests can be made based on disturbance intensity. Following
Yamamoto (1995), we agree upon a somewhat articial upper limit of 1000 m² for small-
scale gap disturbances in climax (old-growth) forests. More importantly, “gap distur-
bance regimes in forest dynamics may not be important in the forest communities where
the large-scale disturbance occurs at intervals shorter than the longevity of the trees”
(Yamamoto 1995). Here, we will exclusively follow the small-scale disturbance regime
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128 Restoration of Boreal and Temperate Forests
and the gap-partitioning hypothesis. Moreover, we appreciate a differentiation of niches
within a single gap reported for gaps of medium to large (but still 1000 m²) size (Sipe and
Bazzaz 1994; Wagner etal. 2010).
The idea “to capitalize on natural forest dynamics and gap dependency” (Hartshorn
1989) in forest management may be especially appropriate in forest restoration efforts. In
forest science, the utilization of gaps—created by either nature or man—was rst advo-
cated by Gayer (1886) for the promotion of mixed stands; and as early as 1927, Wiedemann
reported about articial gaps used in experimental forest research. Recent approaches to
emulate the gap traits of natural forests in forest management have been described by
Coates and Burton (1997). Single-tree-selection, group-selection, and irregular shelter-
wood cutting come to mind as being appropriate silvicultural tools for implementing a
gap-based approach at the stand level.
Emulating gap dynamics for the purpose of forest restoration, however, means more
than simply cutting patches in forest canopies. This holds even more true if we agree on
the objective of restoration as being “to reinstate ecological processes, which accelerate
recovery of forest structure, ecological functioning, and biodiversity levels toward those
typical of climax forest” (Elliott etal. 2013). Following this deterministic perspective, the
emulation of gap dynamics is a measure which unavoidably becomes easier to successfully
implement when the forest state is closer to climax conditions. In particular, this refers to
the tree species and age distribution of the respective forest, because these are the struc-
tural components which would be most affected by the implementation or alteration of
gap dynamics. Both mentioned aspects are well understood by forest science: the trans-
formation of even-aged into uneven-aged forests aimed at prolonged and selective regen-
eration cuttings (Schütz 1999); and group-selection cuttings are designed for regenerating
mixed forests stands (Vanselow 1949; Nyland 2002; Wagner etal. 2010). The feasibility of
gap-based concepts in forestry is indicated by work of Hagemann etal. (2013), Trotsiuk
etal. (2012), and Tabaku and Meyer (1999) who reported about the similarity of the gap
size-frequency distributions in old-growth forests and in examples of practiced close-to-
nature forestry with the same species. Even in plantation-like forests, a lognormal shape
of the gap size-frequency distribution may be observed (e.g., for spruce: see Huth and
Wagner 2006).
In an attempt to conclude on gap-based regeneration approaches in forest restoration,
two critical aspects are especially important to be mentioned:
In stands made up of species not adapted to the particular site, for example, in
plantations of exotic species, small-scale gap-based approaches to convert by natu-
ral regeneration seem unlikely to add much to the naturalness of a stand and
are therefore questionable. However, combined with articial regeneration of
native site-adapted species, gap-based approaches may make sense to implement
uneven-aged structures.
Small-scale gap-based approaches become easier to successfully implement the
closer to climax conditions in terms of tree species assemblage and age structure.
Following large-scale disturbances, for example, in the preclimax seral stages of
forest succession, the gap approach is questionable because most preclimax spe-
cies feature a poor ability to reduce resource availability. Gap effects with respect
to altered resource availability may therefore be small in these communities, for
example, in early successional pine forests (Bolte and Bilke 1998). When emulating
gap conditions in such forests, a range of potential problems should be kept in
mind (Table 6.3).
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129Developing Restoration Strategies for Temperate Forests
6.4 Restoration Strategies for the Active Manipulation
of Below-Canopy Stand Components
Restoration measures altering the overstory layer of forest stands also inuence all lower
layers and associated ecological processes. In general, overstory changes caused by natu-
ral disturbances or harvesting increase the below-canopy availability of light, water, and
nutrients due to the removal of dominant trees. Below-canopy restoration measures can
be used to improve resource availability for target tree species and to directly promote
their establishment. But considering that restoration measures within forest ecosystems
strive to enhance diversity of different compartments, it is necessary to keep in mind that
TABLE 6.3
Questions, Problems and Recommendations When Emulating Gap Conditions
Questions Problems Recommendations
Are there risks in
implementing
selection cuts for
the rst time in a
stand with formerly
closed canopy?
As canopy openings increase air
turbulence and wind loading of the
remaining trees (Panferov and
Sogachev 2008), wind-throw
disturbance may result, which would
be quite different from natural events.
A high vitality and stability of the
individual trees of the respective stand is
required (e.g., Röhrig etal. 2006)
Are there differences
between man-made
and natural gaps
(Schliemann and
Bockheim 2011)?
Soil disturbance and biomass removal
differ between articial and natural
gaps. The latter refers especially to
‘gap-makers’ in natural forests, which
are those—often old and large—tree
individuals that create canopy gaps as
a result of damage or death. In
managed forests, gap-makers are cut
and harvested.
Gap-makers should not always be
removed from the forest during
restoration measures. Crown damage,
girdling and pulling down are
appreciable alternatives to true ‘cutting’.
Which consequences
for management
result from shifting
the cutting regime
to a gap-based
concept?
Periodic selection cuts may have to be
repeated for a very long period of time,
for example, for the next 100 years, and
this may pose management obstacles
(but will be cost-intensive).
Extreme reductions of canopy density in
a single cut tend to shift the forest to
preclimax condition and should thus be
reconsidered.
The intensity of a single cut, that is, the
removal of canopy trees, should be based
on the fraction of forest area in canopy
gaps under old-growth conditions. As gap
birth rates per year in old-growth forests
may vary (e.g., 0.5%–2% per year for
mesic forests of Eastern North-America;
Runkle 1982) and the interval between
consecutive cuts should be at least 10
years, the canopy fraction to be removed
in one cut may also vary (e.g., a gap birth
rate of 1% per year and a 10-year cutting
cycle would lead to approx. 10% of
canopy removal in a single cut).
Which trees should
be cut?
The diameter and the spatial pattern of
the cut trees determines the nancial
outcome of the restoration measures as
well as the ecological consequences and
may lead to undesired results.
Proposed selection criteria are: tree species
(predominantly undesired species),
vitality (preferentially dominant
individuals), and spatial pattern
(irregular distribution appreciated). In
addition, where advance regeneration of
native species occurs, canopy trees may
be cut to vitalize the regeneration.
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130 Restoration of Boreal and Temperate Forests
overstory manipulations indirectly affect tree regeneration. Particularly for the early tree
regeneration stages, ground vegetation and its interactions with tree seedlings are impor-
tant for successful establishment of tree regeneration. Further, the denition of target tree
species for a given forest stand needs to consider the site-specic options for articial
restoration measures, which might be used to enrich the current system, to expedite the
process of near-natural development, or simply to ensure regeneration success. For addi-
tionally supporting the regeneration of target tree species, individual measures at smaller
scales can be used and underpinned with specic knowledge drawn from the regeneration
cycle. Regeneration can also be facilitated through the availability of deadwood, a compo-
nent whose ecological role in natural forest ecosystems has been described by numerous
studies.
6.4.1 Emulating Large-Scale Disturbances to Support Ecological Below-Canopy Processes
Although primary and secondary large-scale restoration measures are interlinked, it is
helpful to clearly differentiate between them to identify the key ecological processes that
can be inuenced through specic treatments. Secondary large-scale manipulations use
existing below-canopy structures established by primary overstory manipulations and try
to increase the quality or rate of specic processes. In this context, the recolonization of
ground vegetation or tree species depends on the quality of the soil seed bank, which is
strongly inuenced by primary manipulations. While changes in the existing soil seed
bank are low after wind-throws, the effect of wildre depends on factors including rate
of spread, intensity, and duration (Piha etal. 2013). Ground res of low intensity can favor
the regeneration process by reducing litter layer depth and competing ground vegetation
(Hille and Ouden 2004; Balandier etal. 2006). This gives rise to an accelerated mineraliza-
tion and exposes mineral soil, thus facilitating the establishment of pioneer species. But as
Ryan (2002) pointed out, res vary greatly in space and time, which makes their outcomes
hard to predict. Further, in areas as densely populated as Europe, the use of large-scale re
restoration is problematic with respect to economic efciency and social acceptance (Hille
and Ouden 2004; Král etal. 2012). Moreover, a re will likely destroy any desired advance
regeneration—in contrast to simulated wind-throws—even if the resprouting potential of
many tree species is often underestimated. On restoration sites where a wind-throw has
occurred, only slight changes of former ground vegetation structures and regenerated tree
species can be expected. The enrichment with pioneer tree species (such as birch, willow,
poplar, and larch) and thus the succession process itself will be slow.
The restoration of temperate oodplain forest ecosystems calls for very specic mea-
sures aiming to reestablish natural water level uctuations over large areas along rivers
and streams (Leyer etal. 2012). According to Peterken and Huges (1995) restoration mea-
sures are connected with uvial processes and geomorphologic features and linked to the
different vegetation types of particular forest ecosystems. Combinations of natural repro-
ductive and vegetative regeneration processes are also described as suitable restoration
methods for such areas (Deiller etal. 2003).
Numerous different combinations of the primary and secondary restoration measures
listed in Table 6.4 are possible and applied in restoration practice including typical features
of possible secondary large-scale measures and their effects on the regeneration process.
The use of planting and sowing is well established in forest management and has also
been an important restoration practice for several decades. Both techniques are character-
ized by a high degree of exibility with respect to site conditions, tree species admixture,
spatial heterogeneity, and growth potential. Depending on the primary manipulation, the
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131Developing Restoration Strategies for Temperate Forests
TABLE 6.4
Possible Secondary Large-Scale Restoration Measures Manipulating Below-Canopy Conditions
Secondary Restoration Measures Effects on the Regeneration Process
Planting
Introduction of site adapted and
natural tree species. Creation of
near-natural admixtures by the
use of successful plant
assortments and densities
Planting is possible after all primary restoration measures, but requires
different efforts (Zerbe and Wiegleb 2009). Planting in wind-thrown sites
among broken trunks is more complicated than in drained sites. Future
tree species composition can be directly determined (Olsthoorn etal.
1999). The remaining overstory trees and their condition decide about the
growth increment and the survival rate of the planted tree species.
Small-scaled patches (e.g., pits and mounds) can be considered.
Direct sowing
Introduction of site-adapted and
natural tree species
For successful sowing, soil preparation is generally required to reduce
above- and below-ground competitive pressure. The planning of species
admixtures is possible, but hardly predictable as germination depends
strongly on soil surface and microclimate conditions (Cole etal. 2011).
The realization of species-specic growth edges is sometimes more
complicated than for planted species (Löf etal. 2004; Birkedal etal. 2010).
Sowing densities should be adapted to the surrounding site conditions
(Kutscher etal. 2009).
Nurse plants
Promoting plants that have
positive effects for target tree
species establishment and
recovery
Following the stress-gradient hypothesis, the use of nurse plants after
large-scale disturbances increases the survival, germination and growth
rate of target tree species (Siles etal. 2010). The establishment of specic
nurse plants (e.g., shrubs, grasses, or mosses) can buffer unfavorable
postdisturbance climate or site conditions. It is important to note that
benets for tree species regeneration from nurse plants depend on site
characteristics, species-specic interactions, individual distance from
nurse plants and the regeneration stage of target tree species (Padilla and
Pugnaire 2006).
Soil preparation
Establishment of humus and
mineral soil mixtures to reduce
the inhibitory effect of ground
vegetation
Large-scale soil disturbances diminish the competitive pressure of ground
vegetation. The loosening of the surface layer also increases nutrient
availability, water, and root permeability for tree seedlings (Löf etal.
2006). Large-scale soil preparation also inuences the surface climate of
the disturbed sites (Kubin and Kemppainen 1994). Following soil
preparation (e.g., plowing), natural regeneration, sowing, and planting
are options for establishing target tree species. The techniques are
manifold: Strips, bands, and spots in some cases are preferred to full-areal
treatment to minimize the surface area affected by the treatment. The
degree of mechanization depends on overstory conditions and the
amount of deadwood.
Mowing
Cutting the above-ground
biomass of competing ground
vegetation
Mowing interrupts the above-ground competition of ground vegetation
and thus increases the probability of successful tree species establishment
by natural regeneration, sowing or planting. As root competition is
retained, mowing measures usually have to be repeated (Willoughby
etal. 2009). The degree of mechanization depends on overstory
conditions and the amount of deadwood.
Grazing
Reducing the above-ground
biomass of ground vegetation
The reduction of above-ground competing biomass can lead to better
seedling establishment, if the regenerated tree species are not preferentially
browsed by the grazing animals. Low intensity grazing supports also the
resprouting and owering of specic ground vegetation species (Adams
1975; Kuiters etal. 1996), which makes this procedure suitable for the
restoration of open areas within forests or for orchard meadows.
Mulching
Application of organic materials
at the soil surface
Forest restoration methods use different mulch materials originating from
plants (e.g., leaves, wood, and bark chips). The mulch layer decreases the
competition by ground vegetation, the loss of nutrients, and protects the
soil surface layer against drought or erosion. A loose mulched soil surface
can promote seed germination and the primary growth of tree seedlings
(Haywood 2000).
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132 Restoration of Boreal and Temperate Forests
applicability of planting or sowing must be weighed against the technical and economi-
cal effort (e.g., in the case of high deadwood abundance). Planting and direct sowing can
be implemented as an important part of large-scale restoration even without overstory
manipulation (Zerbe 2002; Birkedal etal. 2010). However, intense primary manipulations
within the overstory tree layer increase light and water availability for planted or sown
tree species as well as for ground vegetation (Ammer etal. 2002; Table 6.4). Taking this into
account, planting can be adapted accordingly with respect to plant spacing and choice of
assortment. Primary manipulations such as prescribed burning lead to a distinct tempo-
ral head start for the planted tree species and generally decrease competitive pressure. In
most cases, successful direct sowing requires additional surface manipulation, for exam-
ple, mowing or plowing, but seedlings may be better adapted to specic site conditions
(Kutscher etal. 2009; Cole etal. 2011).
The degree of mechanization of large-scale planting or sowing depends on the remain-
ing overstory density as well as on the amount and distribution of downed deadwood.
This also applies to mowing or soil preparation (Kubin and Kemppainen 1994; Löf etal.
2006; Willoughby and Jinks 2009). Although this approach results in lower restoration costs
for primary establishment, the renewed homogenization of site conditions is problematic.
Intemperate forest ecosystems, intensive soil preparation was often used for the restoration
of extremely air-polluted forest sites (Kozlov etal. 2000).
The use of grazing as a continuous large-scale disturbance is mostly connected with
the conversion of a forest ecosystem into nonforested ecosystems (Bengtsson etal. 2000),
while periodic grazing under a light shelter of tree species favors owering ground veg-
etation and can be used for restoring orchard meadows as a specic element of forest
ecosystems (Adams 1975; Putman 1996; Tasker and Bradstock 2006). Mulching is also typi-
cally practiced in open landscapes, but successful examples for the establishment of tree
regeneration are given by Haywood (2000) and Blanco-Garcia and Lindig-Cisneros (2005).
A specic mulching strategy applying an admixture of mulch material and seeds was
described by Krautzer and Klug (2009).
The use of nurse plants to restore forest ecosystems after large-scale disturbances is
frequently used in harsh climates (Callaway 1995; Castro etal. 2002). However, even in
the temperate zone, large-scale disturbances (and restoration measures such as complete
clearings) can lead to extreme site conditions (e.g., slopes and dry sites), where the facilita-
tive function of specic ground vegetation can be used to accelerate succession processes
and tree species establishment (Padilla and Pugnaire 2006). For this purpose, it is neces-
sary to have detailed information about species-specic interactions (Siles etal. 2010) and
potential nurse plants must be established or exist within the soil seed bank. Such nurse
plants need to have a higher natural recovery potential (e.g., vegetative or generative devel-
opment strategies) than tree species regeneration (Rent etal. 2010).
6.4.2 The Function of Ground Vegetation: Facilitation Versus Competition
Compared to boreal and tropical forest ecosystems, the overall diversity of most temper-
ate forests and their oral and faunal elements are described as medium and inherently
limited by nature (Ricklefs 1977; Thomas and MacLellan 2004). Although the terms used
to describe the lower layer of forest ecosystems vary (e.g., ground vegetation, ground
cover, understory vegetation, herbaceous layer, ground layer, etc.), only two fundamental
approaches have been used to describe and quantify its functions and processes in for-
est ecosystems. Both classications are based on the hierarchy of vertical layers or strata
(Gillam 2007; Owens 2007). The rst approach involves all vascular plants with an absolute
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133Developing Restoration Strategies for Temperate Forests
height of 0.5–2.0 m (Gilliam and Roberts 2003), with the moss layer mostly separated (e.g.,
Barbier etal. 2008). The second approach combines the vertical hierarchy with a classica-
tion regarding future development. “Resident species” will never outgrow (i.e., become
higher than) the dened lower stratum (see Figure 6.2). In comparison, “transient species”
(i.e., tree regeneration) have a high potential to grow into the overstory layer (Gilliam and
Roberts 2003). Even though this approach seems to be plausible within a development-
oriented analysis of forest ecosystems, the term “resident species” can be confused with
the way it is used in the context of invasive species (Pyšek etal. 2012).
6.4.2.1 The Role of Ground Vegetation in Natural Forests
Ground vegetation is inuenced by local climate and abiotic conditions (e.g., temperature
and precipitation, soil nutrient, and water availability). Without human impact, typical
ground vegetation communities and the abundance of specic species are primarily deter-
mined by the site potential (Gracia etal. 2007). The biodiversity of ground vegetation in
temperate forests is much higher than found in the overstory tree layer (Gillam 2007).
Restoration measures aimed at the ground vegetation layer are frequently associated with
the protection of a particular rare species, for example, orchids (Hermy etal. 1999; Honnay
etal. 2002; Dorland and Willems 2006), which is deemed possible by means of modied
forest management or manual regulation of ground vegetation. However, the preserva-
tion of complete plant communities requires complex measures at larger spatial scales
(Honnay etal. 2002). As a result of increasing ground vegetation diversity in entire eco-
systems, ecological niches for soil dwelling organisms, insects, and herbivores will also
become more abundant (Hutchinson 1978; Gilbert and Lechowicz 2004; Silvertown 2004).
The ground vegetation can provide information about the potential natural vegeta-
tion (pnV) of a site and thus the degree of naturalness. Often, the locally dened pnV
is used to determine the theoretical framework for restoration objectives and guide res-
toration activities (Zerbe 1998). Ground vegetation species can thus be regarded as the
“historical memory of ecosystems” (Oheimb etal. 1999; Härdtle etal. 2003; Verheyen etal.
2003). Furthermore, ground vegetation responds quickly to changes in site conditions, for
Intra-hierarchical
competition
Facilitation
t
t
r
r
r
t
t
t
t
r
Inter-specific competitionIntra-specific competition
Inter-hierarchical competition
FIGURE 6.2
Overview of the hierarchical interaction levels in temperate forest ecosystems; and the differentiation between
transient (t) and resident (r) plants. (From Gilliam, F.S. and M.R. Roberts. 2003. The herbaceous layer in forests of
eastern North America. New York: Oxford University Press.)
Downloaded by [Ulrike Hagemann] at 00:24 04 April 2016
134 Restoration of Boreal and Temperate Forests
example, atmospheric depositions, habitat fragmentation, edge effects, and disturbances
(Nabuurs 1996; Hermy etal. 1999; Bossuyt and Hermy 2000; Gaudio etal. 2008), causing
large shifts in dominance of ground vegetation species as well as the abundance of alien
species (Zerbe and Kreyer 2007).
In order to interpret ground vegetation characteristics and to distinguish the environmen-
tal factors driving its vitality, growth, and abundance; different theoretical indicators (e.g.,
light or soil humidity described by indicator values (Ellenberg 1996)) have been developed.
Ground vegetation has been used as an indicator for classifying forest ecosystem status and
guide for management based on decisions (Carignan and Villard 2002; LaPaix etal. 2009).
Ferris and Humohreyn (1999) divided this indicator function into three main categories,
differentiating (1) compositional diversity (number of plants within a dened area), (2) struc-
tural diversity, and (3) functional diversity. Applying these categories, numerous studies in
natural and near-natural temperate forests have shown that site and stand structures can
be described as spatial and temporal heterogeneous patches or mosaics (Korpel 1995; Bobiec
etal. 2000; Emborg etal. 2000). A higher diversity of the overstory trees does not automati-
cally lead to a higher diversity within the ground vegetation (Gilliam and Roberts 2003;
Gillam 2007; Mölder etal. 2008). Ground vegetation has the important role of quickly recolo-
nizing, covering, and protecting the exposed mineral soil (Pyšek 1993; Honnay etal. 2002;
Royo and Carson 2006). Manifold combinations of ground vegetation and tree regeneration
result from this early successional function (Bazzaz 1996; Brang 2005a; Walker etal. 2007).
6.4.2.2 Interactions between the Different Hierarchical Strata of Forest Ecosystems
For a long time, natural forests without any human impact have been rare or almost non-
existent in the temperate climate zone. Therefore, species composition and abundance
within the ground vegetation layer has also been shaped by human alteration of the tree
layer (Økland etal. 2003; Paquette 2006; Denner 2007). Plant–plant interactions can have
competing or facilitating characteristics (Bazzaz 1996; Wagner etal. 2011). Competition and
facilitation effects across different hierarchical layers within one ecosystem are termed as
inter-hierarchical (see Figure 6.2); those within the same hierarchical layer as intra-hierarchical
(Goldberg and Landa 1991; Casper and Jackson 1997). Competition between individuals of
the same species is dened as intraspecic; competition between individuals of different
species as interspecic (Bazzaz 1996). These interactions between different strata and devel-
opment stages lead to a complex system of competing and facilitating plant actors within
forest ecosystems (Casper and Jackson 1997).
The dominance of overstory trees is obvious for those resources and environmental
factors which are dependent on vertical gradients and growth space (Table 6.2). Under
continuous forest cover, overstory trees thus determine the environmental conditions
and resource availability for ground vegetation as well as for tree regeneration (Sydes
and Grime 1981; Augusto et al. 2003; Økland etal. 2003; Penne etal. 2010). Impacts of
overstory trees are mostly associated with light transmittance (van Oijen et al. 2005;
van Couwenberghe etal. 2010, 2011; Wagner etal. 2011), precipitation interception, and
throughfall (Bredemeier etal. 2011), litter accumulation (Carli and Drescher 2002; Augusto
etal. 2003), ne root density in the humus layer (Ammer and Wagner 2005; Meinen etal.
2009), and mycorrhization (Hunter and Aarssen 1988; Luoma etal. 2006). Despite strongly
reduced resource availability at the forest oor, ground vegetation species show special
adaptations to these conditions, for example, underneath dense beech canopies (Nagaike
etal. 1999; Gilbert and Lechowicz 2004; Silvertown 2004; Gaudio etal. 2008). According to
Härdtle etal. (2003) and Nagaike etal. (1999), the diversity of ground vegetation in such
Downloaded by [Ulrike Hagemann] at 00:24 04 April 2016
135Developing Restoration Strategies for Temperate Forests
systems is not hindered by low light availability. Besides the unfavorable effects of com-
petition, some positive effects of overstory trees are also known, for example, the shelter
function which mitigates extreme climate conditions such as late spring frosts (Hunter and
Aarssen 1988; Nyland 2002). These positive inuences caused by tree species or ground
vegetation are dened as facilitation effects (Padilla and Pugnaire 2006; see Figure 6.2).
The importance of facilitation effects generally increases with increasingly unfavorable or
harsh environmental conditions (Freestone 2006; Paquette and Messier 2011). Within tem-
perate forest ecosystems, facilitation has been particularly observed as a part of early tree
development stages (Jensen 2011).
6.4.2.3 Using Ground Vegetation to Promote or Discriminate
Against Specic Tree Regeneration
The previous sections have shown a complex system of linkages between overstory tree
composition, ground vegetation, and site conditions. In order to integrate ground vegeta-
tion and tree species regeneration into restoration strategies, concise objectives need to be
dened, such as the increase of ground vegetation diversity, specic ground vegetation
assemblages (distributions), or the promotion of ground vegetation that facilitates tree spe-
cies regeneration (Bakker etal. 2000). The dominance of certain ground vegetation groups
is a typical phenomenon of intensively managed forest ecosystems (Paquette etal. 2006;
Gaudio etal. 2008). It can be assumed that in managed forests or forest plantations (even-
aged, single-layered stands; see Table 6.2), the homogenization of site and stand conditions
also results in the homogenization of ground vegetation (Zerbe and Brande 2003; Bauhus
and Schmerbeck 2010). Restrictions on the use of herbicides for competition control limit
the number of active measures to manipulate ground vegetation. Willoughby and Jinks
(2009) compiled a list of ground vegetation types (i.e., grasses or monocotyledons, dicotyle-
dons, pteridophytes, and woody species) that are disproportionally abundant in managed
European forests (i.e., Deschampsia sp., Calamagrostis sp., Carex sp., Pteridium sp., Epilobium
sp., Senecio sp., Urtica sp., and Rubus agg.). All of these are considered potential inhibitors
of successful tree regeneration (Löf and Welander 2004).
Local management strategies to reduce ground vegetation dominance mostly include the
use of mechanical tools such as plows, mulchers, or cultivators (Löf etal. 2012; Ammer etal.
2009; Ammer etal. 2011). These activities primarily inuence soil conditions and microto-
pography and reduce the above- and belowground competitive pressure on tree regenera-
tion (Balandier etal. 2006; Metlen and Fiedler 2006). Other restoration activities focusing on
aboveground conditions are mowing, trampling, grazing, and prescribed burning (Adams
1975; Vanha-Majamaa and Tuittila 1996; Hille and den Ouden 2005; Vandenberghe etal.
2006; Král etal. 2012). Subsequent sowing with near-natural herbaceous plant or tree spe-
cies are other possible restoration methods (Roovers etal. 2005). Light-demanding seed-
lings (e.g., Quercus spp.) need a growth edge compared to the competing ground vegetation
(Lorimer etal. 1994; Davis etal. 1998; Jensen 2011), while shade-tolerant tree species (e.g.,
Abies alba Mill.) can survive high aboveground competitive pressure by dominating ground
vegetation for longer time periods (Leibundgut 1984; Grassi and Giannini 2005).
Compared to aboveground competition (Harmer etal. 2012), the regulation of below-
ground competition between tree species regeneration and ground vegetation by means
of direct belowground measures or overstory management is more complicated. Tree spe-
cies with fast growing taproots and dense ne root networks are competitive under water
stress (Pallardy and Rhoads 1993; van Hees 1997) but most competitive ground vegetation
species readily resprout and survive aboveground damage (Mallik and Gimingham 1985;
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136 Restoration of Boreal and Temperate Forests
Fotelli et al. 2001; Klimešova and Klimeš 2007). Early successional pioneer tree species
show a similar potential of vegetative response if environmental conditions are suitable
(Koop 1987; Mallik 1997). However, even a short window post-cutting without compet-
ing ground vegetation can be sufcient to offer an advantage for tree regeneration. A
more integrated restoration strategy based on silvicultural management options includes
the utilization of pioneer nurse crops and the manipulation of overstory canopy closure
(Augusto etal. 2003; Wagner etal. 2011; Paquette and Messier 2011).
6.4.3 Direct Control of Species Composition by Establishing Artificial Regeneration
Natural regeneration can be augmented with articial regeneration when important species
are lacking or when density or spatial distribution of the natural regeneration is unsatis-
factory. These conditions are also of concern in managed forests and measures to remedy
the conditions are termed enrichment or sowing, or reinforcement planting (Nyland 2002).
There is plenty of experience with enrichment measures in managed forests as restoration
plantings in Europe have involved beech (Ammer etal. 2008a), silverr(Wiedemann 1927),
and oak species (Mortzfeldt 1896). This section will focus on the effects of the old stand on
the regeneration and the effects of the regeneration technique itself, that is, sowing versus
planting.
The ideal planting material is adapted to the soil and site, and suitable for the chosen
method and time of planting. Two types of nursery material are available: bareroot and
container seedlings. Either type may provide good results given appropriate handling and
cultural practices. In general, more intensive measures are necessary in more extreme situ-
ations, such as caused by competing vegetation, herbivores, drought, or high ground water
table. Special preparation methods for the exact planting location and a careful choice of
high-quality planting material might prove necessary.
The planting of wildlings is another way to establish desirable species. Wildlings may
have fewer establishment problems under the canopy of an old stand than standard nurs-
ery stock, as they are already adapted to shade by physiological conditioning under an
overstory (Nörr etal. 2002). Moreover, wildlings may be less prone to browsing damage as
deer often preferentially browse well-watered and fertilized nursery stock (Suchant etal.
2000). Wildlings need careful selection and handling, however, to protect leader shoots
and obtain a sufcient mass of ne roots.
In many cases, a moderate residual canopy can have positive effects on the height incre-
ment of the regeneration (Paquette etal. 2006; Balandier et al. 2007). Moderately dense
canopies may favor tree regeneration over aggressive shade-intolerant graminoids or
forbs, particularly for shade-tolerant and intermediate shade-tolerant tree species (Wagner
etal. 2011). Successful enrichment measures have been reported for irregular shelterwood
and gap-based approaches with predominantly small- to medium-sized canopy openings
(Parker etal. 2001; von Lüpke and Hauskeller-Bullerjahn 2004). The effects of canopy den-
sity on ground vegetation were discussed above.
Sowing has some advantages compared to planting for restoration as the latter does not
always guarantee site-adapted and near-natural root development (Ammer and Mosandl
2007). As successful sowing is also cheaper than planting, it may be worth trying. Sowing
by broadcast, strip, or spot techniques (Nyland 2002) is generally recommended at the
beginning of the growing season in order to reduce the risk of seed predation, fungal
infection, or unfavorable environmental conditions such as excess moisture or heavy frost.
The probability of success increases if seeds are pretreated (e.g., by stratication or incuba-
tion) to obtain prompt and vigorous germination (Stoehr and El-Kassaby 2011). Moreover,
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137Developing Restoration Strategies for Temperate Forests
seeds may be treated with repellents to control predation. Soil scarication can be used
to prepare a site for articial sowing, which may be mechanized, such as practiced for
beechnuts or acorns in drills or spots (Leder etal. 2003), as well as sowing of acorns in hoe
strips (Preuhsler and Pinto da Costa, 1994). However, machine-sowing can cause injuries
in shallow-rooted old stands, particularly in spruce, whereas this is rarely a problem in
pine stands. As the germinants and the seedlings of small-seeded conifers are particu-
larly tiny and prone to environmental stressors, a careful investigation of sowing tech-
niques and appropriate stand conditions in advance of sowing measures is recommended
(Hamm etal. 2014).
6.4.4 Manipulating the Small-Scale Seedling Environment
Almost all manipulation measures described above can be transferred to smaller spatial
scales, but they differ regarding the application of the restoration techniques. The degree
of mechanization decreases if manipulations aim for immediate environmental changes
in the vicinity of individual or small groups of seedlings. For natural or near-natural for-
est conditions, typical gap size distributions resulting from natural disturbance regimes
have been described (Runkle 1985; Mountford 2001) and have shown that small gaps
caused by the loss of single or small groups of overstory trees dominate unmanaged
forest conditions (Lorimer 1989). Manipulating the seedling environment at small scales
starts with changing overstory density by establishing variable numbers of gap creators
and gap shapes (Schütz 2002; Wagner etal. 2011). Particularly in the center of canopy
gaps, competition with an overstory for resources is reduced, resulting in improved
growth and vitality of tree seedlings. As light-demanding tree species will be dispro-
portionately favored with increasing gap size (Huth and Wagner 2006), small parts of
a forest stands can be transferred “back” to earlier successional stages, thus increasing
overall species diversity. In contrast, intermediate and shade-tolerant tree regeneration
shows a high competitive ability and potential for survival in smaller gaps or at the
edges of larger gaps.
Gap creation for restoration can enrich tree species diversity and vertical structures
at small scales (Brokaw and Busing 2000). However, periodical regulation is required to
preserve intensive seedling admixtures and prevent highly competitive tree species from
dominating over time (Petritan etal. 2007). The diversity of small-scale niches must there-
fore be continuously preserved by diverse restoration measures, if a maximum small-scale
structural diversity is the objective (Grubb 1977).
Small-scale manipulations can be based on different development stages throughout
the regeneration cycle (Figure 6.1). To begin with seed trees, seeds and the soil seed
bank, the opportunities to increase the degree of naturalness within these development
stages seem to be limited by natural processes. Promotion and revitalization of poten-
tially rare, native target seed trees in forest ecosystems can be realized with low addi-
tional effort by using release cuttings or coppicing (Karlsson 2000). Most tree species in
temperate forest ecosystems do not build up soil seed banks for long-term storage, but
this can be different for ground vegetation species (Leck et al. 2008). Nevertheless, the
overview of literature by Bossuyt and Honnay (2008) concluded that temperate forest
soil seed banks fulll the following functions with respect to restoration processes:
(1) they provide information on the existing regeneration potential if the overstory
and/or any competing ground vegetation were removed; (2) they give an impression
about the discrepancy between the current forest conditions and the natural potential,
including the dispersal input from surrounding areas; and (3) the spatial variability of
Downloaded by [Ulrike Hagemann] at 00:24 04 April 2016
138 Restoration of Boreal and Temperate Forests
the soil seed bank indicates the actual species-specic diversity and dominance poten-
tial without the inuence of site conditions and interaction processes (Augusto etal.
2001). Restorative manipulations can thus aim to activate the soil seed bank by means
of small-scale disturbances, for example, through the removal of ground vegetation,
partial mixing of the humus and upper mineral soil layers, or the interruption of over-
story canopy closure (Korb etal. 2005). Another option is to transfer humus and litter
samples from natural forest sites to stands that are to be enriched (Bakker etal. 2000;
Rodrigues etal. 2009). Direct sowing on small patches is also possible (Ren etal. 2012).
Such secondary restoration measures as sporadic grazing have been shown to conserve
the soil seed bank (Chaideftou etal. 2011).
Small-scale restoration measures focus on the creation of species-specic safe sites
(Schupp 1995; Smit etal. 2006; Leck etal. 2008) by reducing the germination-hampering lit-
ter layer or ground vegetation, or by limiting the inuence of overstory trees. Unfavorable
soil conditions such as stony surfaces, mineral soil and litter accumulations, or acidic soils
must be modied. Measures for soil improvement (e.g., fertilization or liming) are useful
techniques to facilitate tree germination and to improve overall site conditions (Pabian
etal. 2012). Further, small-scale surface heterogeneity can be manually established by cre-
ating specic microtopography. For example, microclimate within small pits is character-
ized by higher soil humidity and possibly lower light availability (Ulanova 2000), while
mounds dry out faster but provide better light conditions (Carlton and Bazzaz 1998; Du
etal. 2013). Germination on or near downed deadwood or stumps can be favorable for
some species.
Light manipulation via overstory regulation represents the classical approach to pro-
tect advance regeneration. Approaches to manipulate competitors surrounding individual
trees are trampling (manual) and cutting with trimmers or brushcutters; damage to indi-
vidual target trees, such as caused by herbivory, can be reduced by fences or browsing
repellents (Côté etal. 2004; Willoughby and Jinks 2009). Special nurse plants to directly
inuence the seedling environment and to buffer small-scale climate extremes have been
used (Padilla and Pugnaire 2006). Highly specic restoration methods such as the inocu-
lation with mycorrhizae (Allen 1991; Nara 2006; see Section 6.2.4) or the establishment of
legumes (Carpenter etal. 2004; Siddique 2008) are mainly found at very small scales or
used for single plant restoration activities.
6.4.5 Utilizing Deadwood to Improve Regeneration Survival
Deadwood has long been recognized as an integral component of many forest ecosystems
with particular relevance for biodiversity (Jonsson etal. 2005; Stokland etal. 2012), carbon
and nutrient cycling (Cornwell etal. 2009; Kahl etal. 2012), structural integrity (Franklin
etal. 1987; Debeljak 2006), and forest regeneration. Regeneration is particularly facilitated
by downed deadwood (DDW), which consists of woody debris, stumps and overgrown
deadwood (Hagemann et al. 2009), and provides microsites for tree seedling germina-
tion and growth. The role of DDW as “nurse logs” for regeneration is relevant in many
forest types (Figure 6.3a), including temperate (Harmon and Franklin 1989; McGee and
Birmingham 1997; Kuuluvainen and Kalmari 2003), boreal (Hofgaard 1993), and tropical
(Lack 1991; Sanchez etal. 2009) as well as submontane (Korpel 1995; Reif and Przybilla
1995; Zielonka and Niklasson 2001; Motta etal. 2006) and montane forests (Santiago 2000).
Microsites created by DDW are especially common in natural forests where stocks are con-
tinuously replenished by gap creation and stand-replacing disturbances (Debeljak 2006;
Stokland etal. 2012).
Downloaded by [Ulrike Hagemann] at 00:24 04 April 2016
139Developing Restoration Strategies for Temperate Forests
6.4.5.1 How Deadwood Facilitates Regeneration
In order for the tree regeneration to be successfully establish on or near DDW, seeds must
be intercepted and retained by logs providing favorable conditions for germination, estab-
lishment, and growth. By modifying the microclimate, water, and nutrient availability,
the presence of DDW thus affects several phases of the regeneration process (Figure 6.1),
including seed interception and storage (Harmon 1989a), germination (Iijima and Shibuya
2010), and seedling survival (Szewczyk and Szwagrzyk 1996).
6.4.5.2 Seed Interception, Retention and Storage
Decaying wood generally covers <10% of the forest oor (Harmon 1989a; Zielonka 2006).
Seed retention mainly depends on the size and characteristics of the log surface (Chambers
1991; Baier etal. 2007; Bače etal. 2012). Due to a smaller surface area, narrow logs trap sig-
nicantly fewer seeds than large logs (Iijima etal. 2007). Seed retention on DDW generally
increases with progressive decay (Bače etal. 2012), and is higher for moss- or litter-covered
logs compared to logs with smooth bark or without bark (Harmon 1989a; Iijima etal. 2007).
Tree species also inuence seed retention, with better retention on logs with rough bark
such as Picea or Pseudotsuga spp. (Harmon 1989a,b; Iijima and Shibuya 2010), and higher
retention likelihood for smaller seeds (Szewczyk and Szwagrzyk 1996; Iijima etal. 2007).
6.4.5.3 Germination and Seedling Establishment
Deadwood modies species-specic seed germination rates through its surface prop-
erties, decay status, and size. Germination rates increase with log size and progressive
decay, because larger logs feature more stable moisture conditions than smaller logs
(Stokland etal. 2012). Water retention capacity of DDW is generally higher than mineral
soil and increases as decomposition progresses (Sollins et al. 1987; Zielonka 2006). The
thickness of the moisture-retaining moss cover is higher on more decayed logs (Harmon
1989a; Dynesius etal. 2010). However, seedling establishment on DDW may be impeded
by very thick moss layers preventing the extension of radicals into the substrate below
(Harmon and Franklin 1989; LePage etal. 2000), potentially discriminating between spe-
cies with different germinant size, for example, favoring Abies over Picea (Szewczyk and
Szwagrzyk 1996).
FIGURE 6.3
Spruce regeneration on decaying log in the Harz mountain, Germany (a) and yellow birch regenerating on
stump in Quebec, Canada (b).
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140 Restoration of Boreal and Temperate Forests
6.4.5.4 Seedling and Sapling Survival
Particularly for conifers, the establishment and survival of seedlings on DDW is often
better than on soil (Knapp and Smith 1982; Harmon and Franklin 1989; Szewczyk and
Szwagrzyk 1996; Kupferschmid et al. 2006), and it is most pronounced for the genus
Picea (Svoboda etal. 2010; Bače etal. 2012). However, higher seedling densities have also
been observed for hardwoods such as yellow birch (Betula alleghaniensis; see McGee and
Birmingham 1997) and rowan (Stöckli 1995). The increased survival rates of tree seed-
lings on logs is mainly an effect of decreased competition with ground vegetation com-
pared to seedlings on the soil (Figure 6.3; see Ponge etal. 1998; Ran etal. 2010; Bacˇe etal.
2012). Fallen logs can serve as a regeneration substrate until the moss mat becomes thick
enough to smother seedlings (Harmon and Franklin 1989). However, differences in sur-
vival rates cannot be entirely explained by reduced competition, indicating that seedlings
elevated on logs additionally have a lower risk of mortality from soil-borne pathogens
(Harmon and Franklin 1989; Szewczyk and Szwagrzyk 1996; O’Hanlon-Manners and
Kotanen 2004). Especially in harsh environments, DDW also promotes seedling growth
and survival by offering better conditions for the growth of benecial microorganisms
and mycorrhizal fungi (Ponge etal. 1998; Zielonka 2006), increased nitrogen availabil-
ity due to microbial xation and transport from soil to DDW (Zimmerman etal. 1995;
Brunner and Kimmins 2003) and a more favorable moisture regime than soil (Sollins etal.
1987; Ran etal. 2010). In contrast, DDW provides microsites with improved aeration on
waterlogged soils (Santiago 2000).
6.4.5.5 Protection from Browsing Damage
In addition, DDW can protect tree regeneration against herbivore browsing via physi-
cal and visual obstruction (de Chantal and Granström 2007). Similar to other refuges,
deadwood piles originating from wildres, insect outbreaks, or harvesting can impede
herbivore access to regeneration (Forester etal. 2007), thus reducing browsing damage
particularly to hardwood saplings (Grisez 1960; Rumble etal. 1996; Ripple and Larsen 2001;
de Chantal and Granström 2007; Smit etal. 2012). However, DDW refuges are not always
efcient (Fredericksen etal. 1998; Kupferschmid and Bugmann 2005; Forester etal. 2007).
The quality of protection likely depends on the dominating herbivore species as well as
on log pile characteristics such as size or branchiness (de Chantal and Granström 2007),
which is associated with disturbance type (Peterson and Pickett 1995).
6.4.6 Effects of Deadwood Properties
6.4.6.1 Abundance
As a result of climate and tree species, DDW abundance naturally differs among forest
types (Harmon etal. 2004; Christensen etal. 2005). Regardless of forest type, DDW abun-
dance in managed forests is typically drastically reduced compared to natural stands
(Hodge and Peterken 1998; Debeljak 2006; Motta etal. 2006; Meyer and Schmidt 2011), as
harvesting often removes most senescent and dead trees. Forest management also modi-
es DDW decay and size distribution (Stokland etal. 2012). As restoration aims to rehabili-
tate natural structures, processes, and species composition in modied forest ecosystems
(Bradshaw 1997), restoration measures should aim at increasing DDW abundance particu-
larly in coniferous and mixed forests where DDW provides an important regeneration
substrate (Bače etal. 2012). In addition to DDW abundance, factors such as origin, type,
Downloaded by [Ulrike Hagemann] at 00:24 04 April 2016
141Developing Restoration Strategies for Temperate Forests
size, and decay status also inuence the relevance of DDW for tree regeneration and other
ecological forest functions such as biodiversity (Harmon etal. 2004).
6.4.6.2 Origin and Type
Natural stand-level disturbances create large amounts of DDW immediately (storm)
or several decades (insects, wildre) following disturbance (Jonášová and Prach 2004;
Hagemann etal. 2009; Jonášová etal. 2010), while small-scale tree mortality results in the
more or less continuous creation of all sizes and types of DDW (Harmon etal. 2004). In
contrast, harvesting leaves mostly crown material and stumps (Hagemann et al. 2009;
Stokland et al. 2012). Like logs, stumps also offer microsites with reduced competition
and favorable growing conditions (Motta etal. 2006; Bače etal. 2011). Regardless of ori-
gin, stumps may even be superior to logs as seedling establishment occurs earlier and at
higher densities, likely due to better seed retention in surface depressions and faster decay
(Nakagawa etal. 2001; Bače etal. 2011). Even standing deadwood can indirectly affect tree
regeneration by accommodating birds which disperse the seeds of many tree species, such
as spruce, rowan or beech (Jonášová and Prach 2004). The cause of tree death can also
inuence regeneration density, with higher seedling densities observed on logs uprooted
or broken by wind compared to logs originating from bark beetle attacks (Bače etal. 2012).
Bark beetles facilitate the entry of brown-rot fungi (Stokland etal. 2012), and DDW decayed
by brown-rot fungi is less suitable for seedling establishment (Bače etal. 2012).
6.4.6.3 Size
It is well known that large-diameter DDW is particularly important for biodiversity, espe-
cially for saproxylic beetles, fungi and lichen (Jonsson etal. 2005; Müller and Bütler 2010;
Stokland etal. 2012). As larger logs offer more surface area for seed retention and seedling
establishment (Iijima etal. 2007; Bače etal. 2012), an increased abundance of large-diam-
eter DDW also favors the natural regeneration of conifers. Moreover, the retention and
creation of large DDW piles are recommended for protecting regeneration in areas with
high browsing pressure (de Chantal and Granström 2007).
6.4.6.4 Decay Status
The suitability of DDW as a substrate for regeneration changes over time as decompo-
sition alters its physical and chemical properties. This is reected in changing seedling
abundance, survival rates, and even physiological traits such as photosynthetic capac-
ity (Ran etal. 2010; Bače etal. 2012). Seedling density generally increases as decay with
progresses (Szewczyk and Szwagrzyk 1996; Zielonka 2006), but may slightly decreases in
the last decay stages due to increased competition among seedlings and other vegetation
(Zielonka and Niklasson 2001; Zielonka and Piatek 2004). For example, the highest number
of spruce seedlings is typically found on 30–60-year-old logs, but seedling establishment
may start as early 10 years after tree death (Zielonka 2006).
6.4.7 Deadwood and Forest Restoration
Although DDW is not the most important factor in forest regeneration (Szewczyk and
Szwagrzyk 1996), it plays an extremely large role in coniferous forests where trees regen-
erate predominantly on logs (Franklin etal. 1987; Svoboda etal. 2010). In order to facili-
tate natural regeneration in these forest types, DDW abundance could be increased by a
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142 Restoration of Boreal and Temperate Forests
range of passive or active restoration measures (Table 6.5; Stöckli 1995; Ježek 2004; Vanha-
Majamaa etal. 2007; Svoboda etal. 2010; Meyer and Schmidt 2011; Stokland etal. 2012).
However, the effects of such measures on tree regeneration will only become visible in
the medium- or long-term because DDW accumulation following abandonment of for-
est activities is a very slow process (Bobiec 2002; Vandekerkhove etal. 2009; Meyer and
Schmidt 2011) and it takes decades for newly generated DDW to become a suitable regen-
eration substrate (Zielonka 2006). Increased DDW abundance can also contribute to more
natural stand structures, as the strong association between DDW and seedling establish-
ment for some tree species results in nonrandom (i.e., linear or clumped) spatial patterns
of regeneration (Svoboda etal. 2010). However, measures to increase DDW availability
need to be supplemented by measures regulating canopy closure to ensure the successful
growth of seedlings established on decaying wood (Zielonka 2006; Svoboda etal. 2010).
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TABLE 6.5
Restoration Measures for Increasing Deadwood Abundance
Restoration Measures Effects on Deadwood Abundance and Quality
a. Passive measures
Abandonment of forest activities Gradual creation of snags and DDW by natural
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Abandonment of salvage logging Retention of disturbance-generated snags and DDW
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... Forest conversion in Central Europe is intended primarily to modify the uniform structure and tree species composition of pure and even-aged coniferous stands Dieler et al. 2017;Hilmers et al. 2020). Conversion is a long-term process that is typically initiated by selectively felling individual trees of species-specific target diameters to create small to moderate canopy gaps to facilitate planted, sown, or natural regeneration (Zerbe Introduction 4 landscape scale, forestry usually does not apply a one-size-fits-all conversion approach but implements different silvicultural options in accordance with the principles of adaptive forest management Seidl et al. 2011;Yousefpour et al. 2012 Besides stand structure, changes in overstorey composition and density, as modified during conversion, alter abiotic site conditions and resource availability of light, water, and nutrients for understorey vegetation and tree regeneration (Augusto et al. 2003;Fischer et al. 2016;Depauw et al. 2021). For instance, the diversity of overstorey trees is regarded as a key driver of understorey species richness Brunet et al. 2010;Lelli et al. 2019). ...
... In contrast, small to moderate canopy gaps may not inevitably influence the composition and species richness of understorey vegetation but only the abundance of pre-existing plant species (Kirchner et al. 2011;Kaufmann et al. 2018;. Less vigorous reduction in overstorey density has also been found to promote shade-tolerant species such as European beech (Wagner et al. 2010;Fischer et al. 2016). ...
... Compared to the mostly evenaged and single-layered coniferous stands of the 1990s, I detected the emergence of uneven-152 aged and multi-layered stands that indicate an increase in stand structural heterogeneity (Pretzsch et al. 2016;Oettel and Lapin 2021). This development was most likely promoted by significant reductions in overstorey density that increased resource availability (e.g., light, water, nutrients) in the understorey, stimulating artificial or natural tree regeneration Fischer et al. 2016;. As canopy cover is an important element of forest management, particularly in coniferous forests (Coote et al. 2013), I consider silviculture a major driver of the detected dynamics. ...
Thesis
Full-text available
Globally, forests provide essential ecosystem services to society, but their functionality is increasingly impaired by abiotic and biotic disturbances that are expected to further increase with predicted climate change. Since the 1990s, forest management in Central Europe has been converting pure and even-aged coniferous stands towards more diverse and uneven-aged mixed broad-leaved forests. Compared to monocultures, mixed forests are expected to provide multiple benefits such as a greater resistance and resilience to intensified disturbance regimes. Forest conversion is a multi-decadal and context-dependent process driven by forest management and accompanied by several natural ecological processes that commonly shape forest development. The detailed assessment of current conversion progress is essential to derive accurate management options to achieve silvicultural objectives. This doctoral thesis is a case study of a typical Central European lower montane forest landscape currently covered by coniferous monocultures but which has been under conversion since the 1990s, i.e., the Bavarian Spessart. This thesis will contribute to the assessment of conversion efforts by a) studying temporal changes in forest structure, understorey vegetation, tree species composition and diversity, and by b) identifying the most important drivers of tree regeneration. Thus, the development of pure coniferous stands since the 1990s can be quantitatively evaluated, the status quo of forest conversion can be objectively assessed, and respective management options can be derived. The first aim of this thesis was to elucidate temporal dynamics of forest structure and tree communities in coniferous monocultures to evaluate the status quo of forest development under conversion since the 1990s (Chapter 2). This was done by resurveying 108 semi-permanent sampling plots from four coniferous stand types: Norway spruce (Picea abies (L.) Karst), Scots pine (Pinus sylvestris L.), European larch (Larix decidua Mill.), and Douglas fir (Pseudotsuga menziesii (Mirbel) Franco) about 30 years after an initial assessment. I found an increase in stratification that indicated the development of multi-layered, more heterogeneous, and uneven-aged stands. Although mean species richness of the overstorey remained constant, regrowing tree communities in the shrub and lower canopy layers exhibited significant diversification of tree species. The “winner” species included late-successional broad-leaved (e.g., European beech [Fagus sylvatica L.], sessile oak [Quercus petraea (Matt.) Liebl.]), broad-leaved pioneer (e.g., silver birch [Betula pendula Roth.], European rowan [Sorbus aucuparia L.]), and shade-tolerant coniferous (e.g., silver fir [Abies alba Mill.], Douglas fir) tree species. Spruce was substantially reduced in the overstorey, but it regenerated extensively in the understorey. Despite the currently transitional stage of forest development, I conclude that forest conversion has, to date, resulted in diversifying forest structure and tree communities. Forest management may further include active interventions to guide the tree community towards desired stand diversity at maturity. This thesis also aimed to identify dynamics of understorey vegetation and the concomitant changes in abiotic site conditions since the 1990s’ initiation of conversion (Chapter 3). Therefore, vascular plant and epigeal bryophyte communities in the forest understorey were resurveyed on the same 108 sampling plots as in Chapter 2. I found temporal changes that indicated a decrease in soil acidity and a “thermophilization” of forest understory communities. Despite the constancy of mean species richness, Shannon and Simpson diversity indices of understorey species increased. I did not find significant evidence for overall floristic homogenization but the forest understorey experienced a decrease in typical coniferous and an increase in typical broad-leaved forest understorey species. The detected increase in specialist species (closed forests, open sites) most likely compensated for the decrease in generalist species. I conclude that understorey dynamics are closely linked to observed temporal changes towards mixed broad-leaved forests, and that conversion processes may have masked a trend of understorey floristic homogenization by facilitating more structurally heterogeneous and tree species-diverse forests. Tree regeneration is the essential process that determines both structure and species composition of future forests. Therefore, a third study focused on assessing regenerating trees and identifying the most important drivers of the observed regeneration patterns (Chapter 4). This was done by recording the density, species diversity, and structural diversity of tree regeneration together with a variety of potentially influencing variables. Tree saplings with different life-history strategies were sampled in the majority of sampling plots and species identities mirrored both silvicultural promotion and natural regeneration. Although in total 22 tree species were sampled, overall tree regeneration was dominated by two species, Norway spruce and European beech. I identified understorey light availability, stand structure, diaspore source abundance, and browsing pressure as the most important drivers of tree regeneration density and diversity. These drivers and their relative importance for sapling density were interspecific (i.e., between Norway spruce and European beech) as well as intraspecific (i.e., between the different developmental stages of each species). For instance, the density of Norway spruce regeneration increased with increasing light availability, while the density of European beech regeneration increased with decreasing light availability or increasing overstorey density. Tree species and structural diversity especially benefitted from increasing light availability, decreasing stand basal area, and low to moderate browsing pressure. I conclude that careful forest management may be able to balance the regulation of overstorey density, stand basal area, and browsing pressure to achieve silvicultural conversion objectives concerning tree regeneration. Based on the results of the three presented studies, general recommendations for forest management strategies to support silvicultural decision-making for further conversion of coniferous monocultures to more diverse and structurally heterogeneous mixed forests were derived. To safeguard the current structural diversity and tree species composition long-term, management interventions should be selected that favour tree species with different life-history strategies. Forest managers are particularly advised to control for expansive natural re-growth of Norway spruce or monospecific dominance of European beech; either could lead to tree species-poor stands in the future. To achieve a high level of vertical and horizonal heterogeneity, forest management can generate canopy gaps varying in shape and size to diversify growth conditions for tree regeneration and understorey vegetation. Besides planting or direct seeding of target tree species, the potential of natural regeneration should be utilised wherever possible and reasonable. Due to the detected negative impact of high browsing pressure on the density, species diversity, and structural diversity of tree regeneration, forest management is advised to adapt current hunting regimes or to intensify measures of silvicultural protection such as fencing. Otherwise, the success of more, and particularly browsing-prone tree species, will be repressed and will limit the effective development of mixed forests. Finally, I emphasise the conversion of pure coniferous stands to mixed forests as a promising silvicultural strategy to cope with the uncertainties associated with global environmental change, which increasingly impair forest ecosystems. To compensate for these expected negative impacts on forests, I suggest that current efforts to convert even-aged coniferous monocultures to more diverse and structurally heterogeneous forests be intensified.
... Of particular interest for these objectives is the density of established Douglas-fir regeneration. Here "established" means that the regeneration has successfully passed through the various cascades of the regeneration cycle that affects natural regeneration from flowering and pollination to seedling establishment (Fischer et al., 2016;Harper, 2010). ...
... Because of these numerous influencing factors, in this study a quantile regression approach for the dispersal of established Douglas-fir regeneration was applied for the first time, i.e., effective dispersal was quantified. The density of established regeneration is the result of several dispersal events as well as further ecological processes such as seed storage, germination, and establishment (Fischer et al., 2016). We interpret the highest seedling densities as a proxy for the potential of the species in the given environment. ...
Article
Full-text available
Recent extreme weather conditions in Europe have led to widespread destruction of Norway spruce by storms and bark beetles, creating large clearings that need replanting. The shortage of planting material has shifted focus to natural regeneration processes, with Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) emerging as a potential substitute due to its growth performance and drought tolerance. This study introduces and applies methods for investigating the regeneration ecology of Douglas-fir, focusing on the potential density of established regeneration and its dependence on the distance to the nearest seed source. This dependence is modelled with various classical spatial dispersal kernels, the parameters of which are estimated with a quantile regression approach implemented in a new R package quaxnat. Regeneration data from 44,257 sample plots in the state forest of Lower Saxony, Germany, are combined with remote sensing-based positions of potential seed trees to illustrate these methods. Among the standard dispersal kernels provided by quaxnat, the spatial t distribution proves to be the most suitable. Here, for the .999th quantile, the estimated potential regeneration density reaches almost 11,000 trees per hectare in the immediate vicinity of the seed trees and decreases sharply with increasing distance. A simple simulation model that takes dispersal and establishment into account illustrates how these results can be linked to management scenarios. The study provides valuable information for nature conservation and silviculture, suggesting buffer zones around sensitive habitats and guiding forest management decisions regarding natural regeneration options.
... Forest conversion aims at modifying the homogenous forest structure and tree species composition of even-aged coniferous plantations (von Lüpke et al. 2004;Schall and Ammer 2013;Dieler et al. 2017). These modifications change abiotic site conditions and thus resource availability of e.g., light, water, nutrients, and space for species regeneration (Augusto et al. 2003; Barbier et al. 2008;Fischer et al. 2016;Kremer and Bauhus 2020). Depending on the dimension of canopy cover reduction or gap size increase, specific forest structures and tree species are promoted. ...
... For instance, pioneer tree species such as rowan (Sorbus aucuparia L.) or silver birch (Betula pendula Roth.) benefit from higher light availability due to more vigorous canopy reduction (Yamamoto 2000;Huth and Wagner 2006;Dobrowolska 2008). In contrast, shade-tolerant species like European beech (Fagus sylvatica L.) profit from less vigorous canopy reduction (Wagner et al. 2010;Fischer et al. 2016). According to the "heterogeneity-diversity relationship", a high structural diversity is often accompanied by high levels of biodiversity, particularly regarding vascular plants (Stein et al. 2014;Heidrich et al. 2020;Oettel and Lapin 2021). ...
Article
Full-text available
Planted monocultures of even-aged coniferous tree species are abundant worldwide but increasingly damaged by biotic and abiotic stressors and disturbances. In Central Europe, a fundamental goal of ecologically oriented forest management is thus the conversion of pure and often even-aged coniferous stands into structurally more diverse and mixed broad-leaved forests. This conversion is often achieved by single-tree selection resulting in small canopy openings that promote artificial or natural regeneration. Consequently, forest conversion aims at altering stand structure and tree communities. In order to describe the status quo of forest conversion and derive implications for forest management, we investigated changes of tree composition and forest structure in the Bavarian Spessart mountains in southwest Germany. We conducted a resurvey of 108 semi-permanent plots in four different coniferous stand types of Norway spruce, Scots pine, Douglas fir, and European larch about 30 years after the initial survey. We found significant differences in the stratification and cover of respective forest layers between the two sampling periods that indicated an increase in stand structural heterogeneity. While species richness of the overstorey remained constant, species richness and diversity of the shrub and lower canopy layer increased significantly. Regenerating "winner" species included late-successional broad-leaved (e.g., European beech, sessile oak), pioneer broad-leaved (e.g., silver birch, rowan) and shade-tolerant coniferous (e.g., silver fir, Douglas fir) species. Although Norway spruce was significantly reduced in the overstorey, it regenerated in parts extensively in the understorey. We conclude that the forest conversion in the Spessart mountains was overall successful in terms of diversifying forest structure and tree species. Its effects are, though, still emerging and the stands are in a transitional phase. Besides the preferred natural regeneration of target tree species, forest management may consider active measures to guide the facilitated diverse tree community of previously pure and even-aged coniferous stands towards stand maturity.
... Accordingly, in beech old-growth forests, a balance is established between seedling mortality and the occurrence of new seedlings, which ensures the continuity of the regeneration process, allowing beech to use any additional influx of light and suppress other herbaceous and tree species. Successful regeneration results from the joint action of multiple factors such as flowering, seed production, seed dispersal, storage, germination, seedling development, competition with other species and successful establishment of juvenile trees (Fischer et al., 2016;Axer et al., 2021). It is imperative to recognize the key drivers that influence the spatial heterogeneity of the occurrence and presence of seedlings in old-growth beech forests. ...
Article
Full-text available
Understanding the processes occurring in old-growth forests and identifying their key aspects can significantly enrich modern forestry practices with innovative ideas and concepts. The natural regeneration process in beech old-growth forests exhibits distinct spatial heterogeneity and temporal variability. To define the key drivers that influence the spatial heterogeneity of regeneration processes and their effects, research was conducted in three beech old-growth forests situated in Serbia, Southeastern Europe: Felješana, Vinatovača, and Kukavica. In each old-growth forest, a network of circular sample plots with an area of 0.1 ha (totaling 45 plots) was established to gather data on structural characteristics and ecological conditions. Within each circular sample plot, data on the regeneration layer were collected on four square sample plots of 1 m 2 (180 in total). Using linear mixed models, the key drivers of spatial heterogeneity of regeneration processes in beech old-growth forests were analyzed. Based on the results, several key factors contribute to the highly heterogeneous distribution of seedlings, including the canopy, the presence of a middle layer comprising young trees, ground vegetation, and soil stoniness, while a significant influence of the combined effect of the canopy and the presence of a middle layer of young trees is also defined. The spatial heterogeneity of the regeneration process is also represented through the assessment of the ratio between the abundance of one-year-old and older seedlings. The dominance of one-year-old seedlings intensifies with increased canopy density (in instances of very dense canopy (1.0), the ratio of one-year-old and older seedlings is 70:30%). Seedling growth characteristics are shaped by multiple factors, including the influence of the canopy, the presence of the middle layer of young trees, slope, and soil stoniness, with a substantial combined influence of the canopy and the middle layer of young trees. This indicates that the spatial variability of the regeneration process in beech old-growth forests is primarily driven by factors with a substantial individual influence, which may also act combined. It is of paramount importance to understand these factors and determine their influence on the regeneration process in managed beech forests.
... Based on these results, we assume that NR will not only better withstand water limitation induced by weather extremes in the stand context, but also show higher growth performance at increased solar radiation availability, compared to AR. However, it should be noted that NR is tied to the success of additional regeneration phases compared to AR, namely masting, overwintering, and germination (Fischer et al., 2016;Wagner et al., 2010). These phases are strongly affected by environmental conditions and represent potential bottlenecks under climate change (e.g. ...
Article
Successful European beech (Fagus sylvatica L.) regeneration is both of great ecological and economical importance in European forest ecosystems and severely threatened by climate change impacts. To increase our knowledge of beech regeneration dynamics under climate change and the potential for controlling it through forest management, we studied interactive effects of solar radiation (PHAR), water and nutrient availability on the height growth of artificially (AR) and naturally regenerated (NR) beech seedlings. The study was conducted in the framework of experimental canopy gaps, under the influence of the 2018/19 drought and heatwaves. We measured PHAR by means of hemispherical photography, approximated water availability based on the inverse of modeled fine root density distributions of overstorey beech (BGRB) and oak (BGRO) and approximated nutrient availability based on soil fertility (SF), derived from forest site mapping. Results indicate that seedling resource availability and resulting growth responses increase with canopy gap size and vary among locations within the gap. Multiplicative non-linear mixed models suggest that AR and NR relative height growth (RI) was best explained by interactive effects of PHAR, BGRB, BGRO and SF, which reflect complementary resource use patterns of beech seedlings. At optimal resource availability, AR reached a potential RI of 174%, which is about 20% higher compared to NR. While the low light growth responses of AR and NR both reflect saturation at 5 to 15% PHAR, depending on individual size and the availability of the remaining resources, NR showed a higher RI than AR at intermediate and high PHAR levels in cases of limited BGR and SF. In contrast to AR, NR growth was affected to a lesser extent by SF and BGRB and not significantly affected by BGRO. These results suggest that overstorey oaks have a lower effect on water availability of beech seedlings than overstorey beeches. Additionally, NR showed higher tolerance to water and nutrient limitation than AR, probably due to better root system development. In conclusion, site-specific potential for mitigating the effects of climate change on beech regeneration through forest management lies in the adaptation of silvicultural systems, i.e., the creation of canopy gaps larger than 200 m², thus significantly exceeding the average gap size of the natural disturbance regime, and the choice of the regeneration method.
... quality and quantity) of seed production is fundamental for plant reproduction and spread. In the case of a commercially important woody species planted outside its native range, an abundant yield of seeds is desirable since it favors species establishment and lesscostly forest restoration (Richardson, 1998;Haysom and Murphy, 2003;Fischer et al., 2015). Continuous supply of numerous viable seeds linked with effective dispersal also allows spontaneous colonization of new sites (Bucharova and van Kleunen, 2009;Steele, 2021). ...
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Recognition of the seed crop size and the periodicity of abundant seed production is essential for the management and control of introduced tree species. Here we studied acorn production of the North American northern red oak, Quercus rubra-the most common commercially important and invasive alien tree in European forests. A four-year (2017-2020) study conducted in even-aged Q. rubra stands, planted 55-60 years ago in sites of coniferous, mixed, and deciduous forests in central Poland, revealed a great variation in size of premature and mature acorn yields both among stands within the same year and among years for the same stand. However, each year numerous acorns of the introduced oak entered existing ecosystems, and the supply of mature propagules was more stable in the studied forests than in the Q. rubra native range. The total acorn crop was correlated significantly with the forest site, however, the forest site had a weaker effect on Q. rubra masting than the weather. Increases in abortion of acorns and decreases in mature acorn crops were preceded by reproduction-inhibiting weather events, and lower crops of mature acorns were correlated with abundant premature acorn abscission. In reaction to the weather, the phenology of premature and mature acorn shedding was very variable in subsequent years, but it was synchronized in time among all forest sites studied. The size of seed crop at the stand level was shaped by acorn crops of the highly-productive trees, and their identification within stands can be crucial for both effective management and control of Q. rubra in European forests.