Article

Tree-Related Microhabitats Follow Similar Patterns but are More Diverse in Primary Compared to Managed Temperate Mountain Forests

Abstract and Figures

The impact of forest management on biodiversity is difficult to scrutinize along gradients of management. A step towards analyzing the impact of forest management on biodiversity is comparisons between managed and primary forests. The standardized typology of tree-related microhabitats (TreMs) is a multi-taxon indicator used to quantify forest biodiversity. We aim to analyze the influence of environmental factors on the occurrence of groups of TreMs by comparing primary and managed forests. We collected data for the managed forests in the Black Forest (Germany) and for the primary forests in the Western (Slovakia) and Southern Carpathians (Romania). To model the richness and the different groups of TreMs per tree, we used generalized linear mixed models with diameter at breast height (DBH), altitude, slope and aspect as predictors for European beech ( Fagus sylvatica (L.)) , Norway spruce ( Picea abies (L.) ) and silver fir ( Abies alba (Mill.) ) in primary and managed temperate mountain forests. We found congruent results for overall richness and the vast majority of TreM groups. Trees in primary forests hosted a greater richness of all and specific types of TreMs than individuals in managed forests. The main drivers of TreMs are DBH and altitude, while slope and aspect play a minor role. We recommend forest and nature conservation managers to focus: 1) on the conservation of remaining primary forests and 2) approaches of biodiversity-oriented forest management on the selection of high-quality habitat trees that already provide a high number of TreMs in managed forests based on the comparison with primary forests.
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Tree-Related Microhabitats Follow
Similar Patterns but are More
Diverse in Primary Compared
to Managed Temperate Mountain
Forests
Thomas Asbeck,
1
* Daniel Koza
´k,
2
Andreea P. Spı
ˆnu,
1
Martin Mikola
´s
ˇ,
2
Veronika Zemlerova
´,
2
and Miroslav Svoboda
2
1
Chair of Silviculture, University of Freiburg, Tennenbacherstr. 4, 79085 Freiburg, Germany;
2
Department of Forest Ecology, Faculty of
Forestry and Wood Sciences, Czech University of Life Sciences Prague, Kamy
´cka
´129, 165 21 Prague, Czech Republic
ABSTRACT
The impact of forest management on biodiversity is
difficult to scrutinize along gradients of management.
A step towards analyzing the impact of forest man-
agement on biodiversity is comparisons between
managed and primary forests. The standardized
typology of tree-related microhabitats (TreMs) is a
multi-taxon indicator used to quantify forest biodi-
versity. We aim to analyze the influence of environ-
mental factors on the occurrence of groups of TreMs
by comparing primary and managed forests. We col-
lected data for the managed forests in the Black Forest
(Germany) and for the primary forests in the Western
(Slovakia) and Southern Carpathians (Romania). To
model the richness and the different groups of TreMs
per tree, we used generalized linear mixed models
with diameter at breast height (DBH), altitude, slope
and aspect as predictors for European beech (Fagus
sylvatica (L.)),Norwayspruce(Picea abies (L.))andsil-
ver fir (Abies alba (Mill.)) in primary and managed
temperate mountain forests. We found congruent
results for overall richness and the vast majority of
TreM groups. Trees in primary forests hosted a greater
richness of all and specific types of TreMs than indi-
viduals in managed forests. The main drivers of TreMs
are DBH and altitude, while slope and aspect play a
minor role. We recommend forest and nature con-
servation managers to focus: 1) on the conservation
of remaining primary forests and 2) approaches of
biodiversity-oriented forest management on the
selection of high-quality habitat trees that already
provide a high number of TreMs in managed forests
based on the comparison with primary forests.
Key words: integrative conservation approaches;
old-growth elements; natural disturbances and
dynamics; black Forest; carpathians; habitat trees.
HIGHLIGHTS
Primary forests provide more tree-related micro-
habitats than managed ones
Received 14 September 2020; accepted 2 July 2021;
published online 11 August 2021
Supplementary Information: The online version contains supple-
mentary material available at https://doi.org/10.1007/s10021-021-0068
1-1.
Author Contributions T.A. performed the research, analyzed the data
and wrote the paper; D.K., M.S. performed the research and wrote the
paper; A.S., M.M. and V.S. contributed to writing the paper.
*Corresponding author; e-mail: Thomas.asbeck@waldbau.uni-freiburg.de
Ecosystems (2022) 25: 712–726
https://doi.org/10.1007/s10021-021-00681-1
2021 The Author(s)
712
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The conservation of primary forests is essential
for providing tree microhabitats
Selecting habitat trees that provide microhabitats
is crucial in managed forests
INTRODUCTION
To tackle the biodiversity and the climate crisis that
forests face (Schelhaas and others 2003; Hane-
winkel and others 2013; Seidl and others 2014,
2017), a great number of approaches including
adaptive strategies, retention forestry, close-to-na-
ture forest management or ecological forestry have
been developed (Bauhus and others 2009; Messier
and others 2019; Gustafsson and others 2020;C
ˇada
and others 2020). Yet, the impact of altered man-
agement approaches on biodiversity is difficult to
scrutinize on large spatial scales as well as along
gradients of management (Paillet and others 2010;
Bruelheide and others 2020; Asbeck and others
2021b). One major step towards analyzing the
impact of management on biodiversity is compar-
isons between types of managed and unmanaged
forests, including sites where management has
ceased relatively recently and to a lesser extent
primary forests (Paillet and others 2010; Schall and
others 2018). However, when it comes to summa-
rizing the margins of influence of management on
biodiversity, the results may vary greatly according
to the time of absence or type of management
(Paillet and others 2015b; Schall and others 2020).
Therefore, remnants of primary forests deliver a
unique source of information to address the ques-
tion of influence of management or natural dis-
turbances and dynamics on biodiversity (Koza
´k
and others 2018); however, these remnants are
rare and difficult to locate (Sabatini and others
2018; Mikola
´s
ˇand others 2019). Another challenge
in assessing differences between managed and pri-
mary forests is the comparability of datasets; for
instance, the collection of data on taxonomic
groups might not always be comparable across sites
(Bruelheide and others 2020). To overcome this
problem partially, a multi-taxon indicator beyond
single-species information has been widely imple-
mented and used for quantifying forest biodiver-
sity, namely the standardized, hierarchical typology
of tree-related microhabitats(Larrieu and others
2012,2018; Paillet and others 2018; Asbeck and
others 2020b; Basile and others 2020a; Jahed and
others 2020; Asbeck and others 2021a). The most
common definition for a tree-related microhabitat
(TreM) is ‘‘a distinct, well delineated structure
occurring on living or standing dead trees, that
constitutes a particular and essential substrate or
life site for species or species communities during at
least a part of their life cycle to develop, feed,
shelter or breed’’ (Larrieu and others 2018). The
hierarchical TreM typology distinguishes 15 groups
of TreMs in seven forms:
Cavities: woodpecker breeding cavities, rot holes,
concavities, insect galleries and bore holes;
Tree injuries and exposed wood: exposed sap-
wood and/or exposed heartwood;
Crown deadwood in different forms;
Excrescences: twig tangles (witches broom),
cankers and burrs;
Fruiting bodies of saproxylic fungi and slime
molds: perennial and ephemeral fungi fruiting
bodies;
Epiphytic, epixylic and parasitic structures: epi-
phytic crypto- and phanerogams, nests of verte-
brates and invertebrates, microsoils
Fresh exudates such as sap run and heavy
resinosis.
A variety of taxonomic groups have been linked to
the different levels of the hierarchical typology of
TreMs based on literature and empirical data and
include invertebrates such as insects, arachnids and
gastropods as well as vertebrates such as birds, ro-
dents and bats (Larrieu and others 2018; Paillet and
others 2018; Basile and others 2020a).
Based on this standardized typology, datasets
from primary (Koza
´k and others 2018) as well as
managed forests (Asbeck and others 2019) pro-
vided first analyses of driving factors of TreM
richness in temperate mountain forests of Europe.
The identification of these drivers of TreM abun-
dance and richness still deserves further attention
as most studies have only been able to identify that
DBH and tree species are most important for living
trees (Larrieu and Cabanettes 2012; Paillet and
others 2019; Asbeck and others 2021a). Other
studies have analyzed the influence of the living
status, ownership or the time since last harvest on
TreMs(Larrieu and others 2012; Johann and
Schaich 2016; Paillet and others 2017,2019). As
TreMs are considered a biodiversity indicator that
could guide the selection of retention elements in
managed forests, one major open question is the
impact of management and the natural life cycle of
trees in primary forests on the richness of TreMs
(Larrieu and others 2012,2014; Asbeck and others
2020b). Here, we aim to disentangle the influence
of environmental factors on the richness and
occurrence of specific groups of TreMs by directly
comparing primary and managed forests. This
might provide valuable information for the devel-
Tree-Related Microhabitats Follow Similar Patterns713
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opment of evidence-based management strategies
to provide old-growth elements and small-scale
retention elements throughout managed forests
and as well address the importance of primary
forests for the conservation of biodiversity (Bauhus
and others 2009; Asbeck and others 2020b; Basile
and others 2020a; Gustafsson and others 2020).
Old-growth elements are largely absent in man-
aged forests, but provide valuable and rare habitats
for the conservation of dependent species and in-
crease the connectivity and dispersal ability of these
species (Baguette and others 2013; Kraus and
Krumm 2013). We aim to provide a first compar-
ison of drivers of TreM richness between primary
and managed forests that 1) highlights the impor-
tance of the primary forests for the conservation of
biodiversity and 2) identifies focal points for forest
management to increase these habitats for the
conservation of forest dwelling species.
MATERIAL AND METHODS
Data Collection
In total, for both the managed (N= 3327) and
primary forests (N= 14,665) we recorded TreM
data for 17,992 trees in European temperate
mountain forests. We collected data for the man-
aged forests placed in one-hectare forest plots lo-
cated on state land in the Black Forest region
(latitude: 47.6–48.3N, longitude: 7.7–8.6E,
WGS 84, Figure 1). The plot selection followed a
landscape gradient of forest cover in the 25 km
2
surrounding the plots and a gradient of structural
complexity indicated by the number of standing
dead trees per plot (0, 1–9, >9 snags per ha); all
trees are located in stands older than 60 years and
exclude any infrastructure (for details, see Storch
and others 2020). We inventoried 161 plots of 1 ha
size, where a full inventory of all living trees and
their TreMs would have been beyond the capacity
of this project. Hence we pre-selected living trees
based on their crown radius from GIS in four
classes (<31m
2
,>31m
2
and <51m
2
,>51m
2
and <97m
2
,>97m
2
) and inventoried two trees
in each of the three lower classes and 15 trees in
the largest class per plot. We automatically delin-
eated individual tree crowns of all trees in all plots
using the TreeVis software (Weinacker and others
2004). The data basis was a digital surface model
(DSM) photogrammetrically generated from aerial
images (40 cm ground sampling distance) and a
digital terrain model (DTM) based on LIDAR
flights. We selected a subset of plots that were
managed for timber production and excluded strict-
protected ones reported in Asbeck and others 2019.
The plots are in continuous cover forests excluding
clear-felling and employing the principles of close-
to-nature forest management (CTNFM). Close-to-
nature forest management in the Black Forest is
characterized by common principles including: a)
use of site-adapted tree species, typically of the
natural forest vegetation, b) promotion of mixed
and structurally diverse forests, c) avoidance of
large canopy openings such as clear-cuts, d)
employment of natural processes such as natural
regeneration, self-thinning and self-pruning, and
(e) silvicultural focus on individual trees rather
than stands (Bauhus and others 2013; Brang and
others 2014). We recorded the position of all
inventoried trees, their diameter at breast height
(DBH), species identity and TreMs in the snow-free
and leaf-free period between fall 2016 and spring
2017. We collected all data, including altitude per
tree with the use of handheld tablets.
For the primary forests, we collected the data in
mixed forests of Western (Slovakia, 210 plots) and
Southern Carpathians (Romania, 190 plots). We
refer to ‘‘primary forest’’ as a forest developed un-
der a natural disturbance regime that showed little
or no evidence of past human activities. Our defi-
nition of primary forests is consistent with rela-
tively widely accepted criteria for European old-
growth forests (Wirth and others 2009; Burrascano
and others 2013; Knorn and others 2013). How-
ever, in addition to late-successional forests, we
also considered all developmental phases, including
early seral stages and young forests that originated
after natural disturbances and natural regenera-
tion, and without subsequent management, as with
primary forest mosaics (Mikola
´s
ˇand others 2019).
The main criteria of the primary forest inclusion
were the absence of the historical human impact
detected by the forest stand maps, historical maps
and field inventory. During the field inventory, all
forests were surveyed for various indicators of
naturalness (for example, coarse woody debris in
various stages of decay, pit-and-mound topogra-
phy, large trees, natural tree species composition).
Surveyed plots were selected from an existing
international network of permanent inventory
plots (REMOTE, www.remoteforests.org), encom-
passing primary forests in central and Eastern
Europe. All data were collected within 0.15 ha
circular plots randomly distributed across various
environmental gradients (but see Koza
´k and oth-
ers, 2018 for details of plot selection). Across the
primary forest plots, we recorded the positions of all
living, adult trees (6 cm DBH), their DBH, species
identity and TreM profile based on methodology by
714 T. Asbeck and others
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Larrieu and others (2018) during the vegetation
season in 2018 and 2019. Altitude was measured at
the center of the plot.
To decrease the observer effect (Paillet and others
2015a), inventories were carried out by the same
team within each location. Three observers in the
Black Forest and two in the Carpathian region
visually inspected the TreMs following the same
hierarchical typology (Larrieu and others 2018).
Statistical Analyses
In our analyses, we focused on a comparison of
living trees, since data for dead trees were not
available for the Black Forest and, as from a man-
agement perspective, the selection of high quality
living habitat trees is more complex than of
standing dead trees (Asbeck and others 2020b). We
calculated the overall richness as the sum of dif-
ferent TreM groups per living tree. To model the
richness as well as the groups of TreMs per living
tree, we used generalized linear mixed models
(GLMMs) as the number of TreMs per tree is count
data and to account for spatial autocorrelation be-
tween the plots, similar as in many previous studies
(Paillet and others 2015a,2018,2019; Asbeck and
others 2020a).
We tested the effects of the DBH, altitude, slope
and aspect on the richness and groups of TreMs on
individual living European beech (Fagus sylvatica
(L.)), Norway spruce (Picea abies (L.)) and silver fir
(Abies alba (Mill.)) trees for managed and primary
forests. These predictors have previously shown to
drive the richness and number of groups of TreMs
per tree (Larrieu and Cabanettes 2012; Koza
´k and
others 2018; Asbeck and others 2019,2021a; Paillet
and others 2019). We used the average slope of the
plot, and aspect was expressed as northness
according to the formula: northness = cosine [(as-
pect in degrees * p)/180)], similar as done earlier
(Janda 2019). Northness will take values close to 1
if the aspect is generally northward and close to -1
if the aspect is southward. We did not include
interactions in the models to prevent over-param-
eterization, and since in previous studies, these
predictors were less significant than the single
predictors (Asbeck and others 2019; Paillet and
others 2019). Prior to running the models, we
checked the predictors for correlations according to
a standardized procedure (Zuur and others 2010,SI
Table A1). We adjusted the continuous predictors
due to the different scales using the default setting
of the scale function in R, which calculates the
mean and the standard deviation (sd) of the pre-
dictor and then scales each element by those values
by subtraction of the mean and dividing by the sd.
The full models for European beech, Norway
spruce and silver fir in managed and primary
Figure 1. Map of the research area, indicating the location of the study sites in the primary forests (red squares) in
Slovakian and Romanian Carpathians and in the managed forests (blue squares) in the Black Forest, Germany.
Tree-Related Microhabitats Follow Similar Patterns715
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temperate mountain forests consisted of these
predictors:
DBH + altitude + slope + aspect + (1|PlotID)
To prevent autocorrelation of trees within the same
plot that might have more similar characteristics
than individuals in different plots (Dormann 2013),
we included plot identity as random factor. The
computation of models was performed in R (R Core
Team 2016). Since the richness data for TreMs were
of count type, we built models with the ‘‘glmmTMB
function’’ of the ‘‘glmmTMB package’’ (Brooks and
others 2017) with a negative binominal distribution
to solve overdispersion. To test for under- and
overdispersion as well as zero inflation in the mod-
els, we used the ‘‘DHARMa package’’ (Hartig 2018).
Obviously, with a large number of living trees that do
not bear TreMs, there were signs of zero inflation;
however, models did not improve when considering
this. We used the ‘‘ggpredict’’ function of the ggef-
fects package for plotting, which sets all other pre-
dictors, except the one for which the effect is shown,
to the same value (Lu
¨decke 2018).
RESULTS
Raw Inventory Data at Tree Level
in Managed and Primary Forests
We restricted the analyses to living individuals of the
three main tree species that were Norway spruce,
European beech and silver fir in both data sets.
Across all three species, the individuals in the pri-
mary forests provided a greater richness of TreMs per
living tree compared to the managed temperate
mountain forests (Table 1, Figures 2,3and 4).
Results of the Statistical Analyses
We found congruent results for TreM richness and
among the majority of TreM groups across the
studied forests. In the vast majority of models, trees
in primary forests provided higher levels of TreMs
(Figures 2,3and 4). The only case where managed
forests seem to provide a significantly greater
number of TreMs is crown deadwood in European
beech (Figure 2). The most important predictor for
overall TreM richness and most groups of TreMs is
the DBH of the trees (Table 2, Figures 2,3and 4).
Altitude as a proxy of the site conditions was an
important driving factor for TreM richness in beech
(Figure 2), rot holes in beech and fir (Figure 2and
4) as well as for epiphytes in spruce and fir (Fig-
ures 3and 4; Table 2). All groups increased sig-
nificantly with increasing DBH as well as with
increasing altitude apart from rot holes in fir (Ta-
ble 2). Crown deadwood in fir increased with
increasing slope (Figure 4and Table 2), while as-
pect was only significant for concavities in spruce
that increased on more northern slopes (Figure 3
and Table 2).
We found significant predictors in 71 of the total
96 models in both primary and managed forests
(Table 2, Figure A1). In 17 cases, only the TreMs in
primary forests were significantly related to one or
a combination of the tested predictors. Insect gal-
leries in all tree species (Table 2, Figure A1) were,
for instance, only related to predictors in the pri-
mary forests. In contrast, we found the opposite
pattern that TreMs are significantly predicted in
managed but not in primary forests only two times,
for nests in Norway spruce that increase signifi-
cantly with increasing DBH and slope in managed
forests (Table 2). In several cases, low number of
observations omitted the statistical analyses, for
instance for perennial and annual fungi in man-
aged forests (Table A2).
DISCUSSION
The increasing importance of primary forests as key
habitats for the conservation of biodiversity re-
cently inspired silvicultural approaches that
emphasize the role of old-growth attributes and
natural disturbance legacies in management activ-
ities (Keeton 2006; Lindenmayer and others 2006;
Bauhus and others 2009; Nagel and others 2014;
Thom and others 2019;C
ˇada and others 2020). Our
approach compared the tree-level TreM richness of
primary forests in the Carpathians with managed
forests in the Black Forest to analyze the role of
environmental factors on the provisioning of
specific habitats. Trees in primary temperate
mountain forests hosted a more diverse array of
TreMs in terms of overall richness and specific types
of TreMs compared to their counterparts in man-
aged forests.
The most prominent result is that we observed
similar patterns but higher numbers of TreMs on
living trees located in primary compared to those in
managed temperate mountain forests. Previous
studies were not able to extract this information as
clearly as we did; for instance, Vuidot and others
(2011) did not find this difference when comparing
managed and unmanaged forests on the tree level.
Our findings might be influenced by the time that
management is absent as in the mentioned study
the unmanaged forests were left without timber
extraction for a maximum of 150 years and logged
previously. Regarding the natural life cycle of the
inventoried tree species, the time span of 150 years
716 T. Asbeck and others
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might be too limited to find significant differences
in TreMs, as tree age of the individuals in the
inventoried primary forests can reach more than
300 years (Spıˆnu and others 2020). In contrast, we
are the first to analyze primary plots without any
traces of human activities due to difficulty of
accessibility in temperate forests, for boreal systems
first results show that old-growth patches provide a
higher number and diversity of TreMs (Martin and
others 2021). Despite the fact that CTNFM aims at
multi-functionality and incorporates other func-
tions than merely timber production, it creates
relatively uniform structures and productive stands
with limited longevity of the trees (Storch and
others 2018; Frey and others 2020; Asbeck and
Frey 2021). On the other hand, trees in primary
forests often grow slowly, competing with other
individuals under the vertically diverse canopy
with longevity multiple times higher than the trees
in managed stands (Bigler and Veblen 2009;Di
Filippo and others 2012). Suboptimal tree growing
conditions, such as poor soil conditions or sup-
pression, were connected to the formation of cer-
tain TreMs, such as cracks, bark lesions and rot
holes (Jo
¨nsson 2000; Fritz and Heilmann-Clausen
2010). Such conditions are more likely to be found
in primary forests because forest management is of-
ten avoided on nutrient poor and inaccessible sites.
In addition, suppressed trees are removed to some
extent in CTNFM, whenever they compete strongly
with high-quality trees for timber production. In this
context, tree senescence is considered to play an
important role for the occurrence of TreMs, but so far
has only been modeled in one cross-sectional
(Courbaud and others 2017) and one empirical
study (Puverel and others 2019). We assume that the
abundance and richness of TreMs increase with tree
senescence, which might be the main reason for
trees in primary forests bearing more TreMs, as they
could be older compared to individuals of similar
dimensions in managed forests.
Increased richness of specific TreM types such as
rot holes, concavities or crown deadwood on pri-
mary forest trees implies the importance of the
natural disturbances for the formation of certain
TreMs. The most important natural disturbances in
central and eastern European mountain forests are
wind, bark beetle outbreaks, snow and ice (Nagel
and others 2014; Svoboda and others 2014; Janda
and others 2017; Kulakowski and others 2017).
The importance of effects from large-scale cyclones
and convective instabilities on dynamics of these
mountain forests has recently been recognized
(Pettit and others 2021). Wind can cause damage
either directly by breaking the stem or limb of a
tree or indirectly through trees damaging each
other when breaking or uprooting. Forest man-
agement may substitute the role of wind as damage
caused during felling operations could create simi-
lar TreMs (Vuidot and others 2011). Such practices
may be effective in mimicking the natural creation
of TreMs in managed stands (Fritz and Heilmann-
Clausen 2010). Besides wind damage, galleries
from wood drilling insects resulting from insect
outbreaks of various severities are common in pri-
mary forests, but not significantly related to any of
the predictors in managed forests. Here, we cannot
fully distinguish whether the lack of significance is
directly related to management or if it is due to the
lower number of observations in managed forests.
We cannot distinguish these effects from our
models, but assume that this effect might be a direct
consequence of the removal of trees showing insect
galleries. Clearly, this group of TreMs is highly
unfavorable in managed forests because of its
negative impact on timber value combined with
Table 1. Comparison of the Main Attributes of the Inventoried Living Trees and Sites in the Inventoried
Managed and Primary Forests.
Tree species N of
trees
Share
(%)
DBH (cm) Mean (SD) Aspect
(northness)
TreM richness/
living tree
Min Max Mean (SD) Altitude (m) Slope ()
Managed forests
European beech 877 26.4 7 128 36 (21) 750 (202) 20.0 (7.7) -0.2 (0.6) 1.9 (1.1)
Norway spruce 1788 53.7 7.5 115 46.5 (15) 910 (178) 11.2 (7.9) 0.2 (0.7) 1.6 (0.7)
Silver fir 662 19.9 8 137 56 (20) 849 (140) 13.9 (9.3) 0.2 (0.7) 1.9 (0.9)
Total 3327 46 (19) 856 (190) 14.1 (8.9) 0.1 (0.7) 1.7 (0.9)
Primary forests
European beech 3938 26.9 6 129 36 (22) 1132 (112) 31.4 (6.2) 0.03 (0.7) 3.2 (1.5)
Norway spruce 9457 64.5 6 117 35 (17) 1442 (134) 32.8 (8.8) 0.3 (0.6) 3.0 (1.0)
Silver fir 1270 8.7 6 119 32 (24) 1154 (134) 33.4 (6.0) -0.01 (0.5) 2.8 (1.2)
Total 14,665 35 (19) 1334 (194) 32.4 (8.0) 0.18 (0.6) 3.0 (1.2)
Tree-Related Microhabitats Follow Similar Patterns717
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Figure 2. Effect plots of the significant influence of aDBH and baltitude for overall TreM richness and groups per tree
from the generalized linear mixed models of European beech in managed (blue) compared to primary (green) forest sites.
The light color bands of the predictor indicate the 95% confidence interval and the rug at the bottom the distribution of
the data of the inventoried trees.
718 T. Asbeck and others
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Figure 3. Effect plots of the significant influence of aDBH, baltitude and caspect for overall TreM richness and groups
per tree from the generalized linear mixed models of Norway spruce in managed (blue) compared to primary (green) forest
sites. The light color bands of the predictor indicate the 95% confidence interval and the rug at the bottom the distribution
of the data of the inventoried trees.
Tree-Related Microhabitats Follow Similar Patterns719
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Figure 4. Effect plots of the significant influence of aDBH, baltitude and cslope for overall TreM richness and groups per
tree from the generalized linear mixed models of silver fir in managed (blue) compared to primary (green) forest sites. The
light color bands of the predictor indicate the 95% confidence interval and the rug at the bottom the distribution of the
data of the inventoried trees.
720 T. Asbeck and others
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Table 2. Results of the Generalized Linear Mixed Models Indicating the Magnitude of Influence and the Significance
a
of the Scaled Predictors.
European beech Norway spruce Silver fir
Intercept DBH Altitude Slope Aspect Intercept DBH Altitude Slope Aspect Intercept DBH Altitude Slope Aspect
TreM richness Managed 0.76*** 0.32*** 0.08* 0.06 -0.00 0.72*** 0.15*** 0.21*** 0.04 -0.03 0.71*** 0.20*** 0.14* 0.05 -0.01
Primary 1.16*** 0.20*** 0.11* 0.02 0.01 1.01*** 0.11*** 0.04 0.05** 0.03** 1.05*** 0.16*** 0.08 -0.02 0.01
Woodpecker
cavities
Managed -4.23** 0.80* 0.35 0.87 -0.15 n.s -13.99* 0.32 3.34** -5.61 0.24
Primary -8.89*** 0.45* 0.60 0.13 -0.36 -11.92*** 1.08** 0.49 -0.05 -0.25 n.s
Rot holes Managed -1.34*** 0.90*** 0.46* 0.30 0.03 -3.89*** 0.75* 0.76 0.45 0.45 -2.73 0.96* 1.72* 1.10* -0.19
Primary -1.39*** 0.41*** 0.44* 0.20 0.02 -4.82*** 0.60*** -0.59* 0.14 -0.08 -4.79*** 0.82*** -0.99* 0.60 0.56*
Concavities Managed -1.83*** 0.90*** 0.19 0.62* -0.01 -1.24*** 0.90*** 0.43** 0.16 0.19* -1.73*** 0.93*** 0.56* 0.46** 0.17
Primary 0.07 0.62*** 0.06 0.14 0.11* 0.03 0.54*** -0.12 0.25*** 0.15*** -0.8*** 0.65*** 0.29* 0.08 -0.05
Exposed sapwood Managed -1.64*** 0.64*** 0.24 0.04 0.09 -1.99*** 0.15 -0.15 0.31** -0.09 n.s
Primary -1.45*** 0.12*** 0.22 -0.13 -0.08 -2.59*** -0.16** -0.45*** -0.09 -0.09 -2.08*** -0.07 -0.92*** -0.05 0.06
Cankers and burrs Managed n.s n.s -4.31*** 0.50** -0.44 0.13 -0.01
Primary -2.28*** 0.67*** 0.51 0.13 -0.12 -3.31*** 0.6*** -0.84*** 0.15 0.13 -3.42*** 0.50*** 0.38 -0.59 -0.1
Nests Managed n.s -10.19*** 1.06*** -0.67 -1.38*** 0.23 -4.59*** 0.64** -0.21 0.02 0.09
Primary -4.34*** 0.45*** -0.47 0.72 -0.06 n.s -3.35*** 0.54*** -0.27 0.01 0.04
Insect galleries Managed n.s n.s n.s
Primary -4.6*** 0.37*** -0.9** -0.02 -0.39** -6.87*** 0.63*** -0.54 -0.60* -0.03 -4.64*** 0.33** -1.37** 0.27 -0.12
Exposed sap-
heartwood
Managed -5.13*** 0.83** -0.23 -0.03 0.11 n.s n.s
Primary -2.94*** 0.33*** -0.36 0.00 0.12 -4.01*** -0.15* -0.00 0.15 0.00 n.s
Crown deadwood Managed -1.08** 1.31*** 0.67*** 0.33 -0.05 -3.35*** 0.69* 1.00* 0.57 -0.12 -2.86** 0.79** 0.62 1.16** -0.53*
Primary -1.13*** 0.22*** -0.26 0.03 0.01 -0.82*** 0.56*** 0.04 0.7*** -0.02 -0.96*** 0.11 -0.08 0.63* 0.07
Twig tangles Managed n.s n.s n.s
Primary n.s n.s -7.61*** -0.68* 1.57 -0.43 0.13
Perennial fungi Managed n.s n.s n.s
Primary -5.03*** 0.42*** -0.36 -0.63* -0.26 -10.5*** 0.59* -0.15 0.35 0.39 -5.68*** 0.79** 0.07 0.07 -0.22
Annual fungi Managed n.s n.s n.s
Primary -3.81*** 0.43*** 0.07 0.03 0.13 -10.8*** 1.04*** 0.99 -0.25 0.08 n.s
Epiphytes Managed -0.29 0.29* 0.24 0.12 -0.04 -0.08 0.29* 1.96*** -0.2 -0.35 0.32 0.52*** 0.89** -0.29 0.08
Primary 0.21 1.08*** 2.14*** -0.79** 0.27 -0.75*** 0.09** 1.87*** -0.35** 0.33** 0.7* 0.79*** 3.22*** -1.27** 0.08
Microsoils Managed -8.18*** 1.66*** -0.8 -0.65 0.62 n.s n.s
Primary -3.68*** 1.16*** 0.81 0.51 -0.4 -5.68*** 1.12*** -0.89** 0.48* 0.18 -6.81*** 1.23*** 0.23 -0.1 -0.12
Fresh exudates Managed n.s n.s n.s
Primary n.s -13.05*** 1.19*** 1.68 -0.36 0.01 n.s
Positive values show an increase in the group of tree-related microhabitats.
a
Significance codes: ‘‘***’’0.001; ‘‘**’’0.01; ‘‘*’’ 0.05; n.s. = applies to models where none of the predictors was significant.
Tree-Related Microhabitats Follow Similar Patterns721
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
imminent large-scale insect outbreaks that hit the
central European region and became more severe
recently (Schelhaas and others 2003; Seidl and
others 2017). This removal usually targets all trees
bearing TreMs that are considered ‘‘defects’’ in
forest management (Martin and Raymond
2019).However forest management strategies that
aim to tackle this constant removal are provided in
the context of retention forestry in CTNFM (Asbeck
and others 2020b; Gustafsson and others 2020).
These integrative strategies might apply to retain-
ing certain thresholds of deadwood volumes or
numbers of habitat trees in managed forests and are
increasingly implemented in temperate mountain
forests in Europe (Kraus and Krumm 2013;Mo
¨lder
and others 2020; Thorn and others 2020; Asbeck
and others 2021a).
We demonstrated a positive effect of tree diam-
eter on overall TreM richness and a consistent ef-
fect across the vast majority of the studied TreM
groups. As observed previously, the diameter of the
living trees is an important factor influencing the
presence of TreMs across tree species, forest types
and environmental conditions (Larrieu and Ca-
banettes 2012; Koza
´k and others 2018; Asbeck and
others 2019; Paillet and others 2019).
Our results are as well in line with observed
patterns of increased TreM numbers in broadleaves
(Larrieu and Cabanettes 2012; Regnery and others
2013; Paillet and others 2019), as we found the
highest overall TreM richness on European beech.
Beech trees can provide up to eight different TreMs
compared to less than five in Norway spruce and
silver fir of similar dimensions. Interestingly, beech
trees seem to host slightly more woodpecker cavi-
ties in managed compared to primary forests. This
might be related to the fact that woodpeckers are
opportunistic in their choice of suitable trees for
cavity establishment (Basile and others 2020b),
whenever snags are available in lower quantities,
which is the case in managed forests compared to
primary ones, and they select living trees to exca-
vate their cavities. This is the reason for this result
as we included only living trees in the analyses. We
observed a higher number of epiphytes in higher
elevations both in primary and managed forests. A
higher altitude was responsible for an increased
diversity of epiphytes (Ding and others 2016),
which is possibly due to an increased precipitation
or humidity in higher altitudes.
Uncertainties in our results might be caused by the
variation in sampling design, as we selected living
trees for the TreM survey based on different criteria
in managed forests (Asbeck and others 2019), and
the primary forest plots (Koza
´k and others 2018).
However, we included the diameter as predictor in
our models and thus took the difference in diameter
into account. Our results should be robust con-
cerning this difference in sampling methods since in
CTNFM as well as in primary forests, patches of
natural regeneration can be spatially close to ma-
ture, dominating trees resulting in a relatively wide
range of the DBH on small spatial scales (Gustafsson
and others 2020). Despite the different time of data
collection, we found quite similar results among the
drivers of TreMs in managed and primary forests; in
future studies, the time of data collection could be
further standardized. We also have to point out that
we mostly interpreted the models that showed sig-
nificances in both forest types as these allowed a
clearer interpretation compared to cases where only
the model for primary or managed forests was sig-
nificant. The fact that there were many cases in
which primary forest models were significant (17 out
of 96) and managed ones were not (2 out of 96)
implies that forest management may hinder some of
the processes leading to TreM formation and devel-
opment. On the other hand, nonsignificant results in
our models could be grounded in the fact that we had
too few observations for the respective TreM groups;
for instance, twig tangles were not found in beech or
Norway spruce (Table A2).
CONCLUSION
We were able to identify that the main drivers of
richness and occurrence of TreMs follow similar
patterns in managed and primary forests, but that
trees in primary forests bear a greater richness of
TreMs. Our study suggests that primary forests are
essential in providing habitats for forest-dwelling
species through a high richness of TreMs. However,
many complexes of primary forests are being lost
due to poor mapping and lack of protection status
(Knorn and others 2013; Mikola
´s
ˇand others 2019;
Sabatini and others 2018,2020). This allows sal-
vage logging operations, which can lead to extrac-
tion of trees with high potential to bear or develop
TreMs, representing a threat to the ecosystem itself
and the function it fulfills for biodiversity conser-
vation (Thorn and others 2018,2020). Hence, our
results highlight the importance of primary forests
for biodiversity conservation but have as well sev-
eral implications for forest management. First, the
constant removal of trees or parts of trees that show
‘‘defects,’’ such as exposed sap- and heartwood or
crown deadwood created by natural disturbances,
needs to be decreased to some extent in managed
forests to provide these important TreMs as re-
722 T. Asbeck and others
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
sources. This could be implemented by focusing the
selection of retention elements such as habitat trees
on individuals that provide these obvious and ea-
sily identifiable TreMs (Bu
¨tler and others 2013;
Gustafsson and others 2020). Secondly, an in-
creased provisioning of large-diameter trees as well
as beech and other broadleaf species will increase
the number of habitats available for forest dwelling
species in the studied managed forests. Overall, we
recommend forest and nature conservation man-
agers to focus their approaches on: 1) protecting
the remaining primary forests and 2) selecting
high-quality habitat trees that already provide a
high number of microhabitats in managed forests
based on the comparison with primary ones.
ACKNOWLEDGEMENTS
This study was partly funded by the German Re-
search Foundation (DFG), ConFoBi project number
GRK 2123. It was as well supported by the insti-
tutional project No.CZ.02.1.01/0.0/0.0/16_019/
0000803 and by the Czech University of Life Sci-
ences (Grant IGA No. A_19_33).
OPEN ACCESS
This article is licensed under a Creative Commons
Attribution 4.0 International License, which per-
mits use, sharing, adaptation, distribution and
reproduction in any medium or format, as long as
you give appropriate credit to the original author(s)
and the source, provide a link to the Creative
Commons licence, and indicate if changes were
made. The images or other third party material in
this article are included in the article’s Creative
Commons licence, unless indicated otherwise in a
credit line to the material. If material is not in-
cluded in the article’s Creative Commons licence
and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will
need to obtain permission directly from the copy-
right holder. To view a copy of this licence, visit h
ttp://creativecommons.org/licenses/by/4.0/.
FUNDING
Open Access funding enabled and organized by
Projekt DEAL.
DATA AVAILABILITY
All data used in this article are available and have
been published previously in Koza
´k and others
2018 and Asbeck and others 2019.
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... For this reason, TreMs have proven to be indicators of forest biodiversity Larrieu et al., 2019;Basile et al., 2020), although the direct links between TreMs and species occurrence at the stand scale are not always clear (Asbeck et al., 2021a). The richness and diversity of TreMs, as well as the occurrence of specific types, are also relevant indicators of naturalness or old-growthness (Winter and Möller, 2008;Michel and Winter, 2009;Vuidot et al., 2011;Paillet et al., 2017;Asbeck et al., 2021b). While some TreMs have been studied for a long time, such as dendrotelms (Kitching, 1971) or cavities (Wesołowski, 2007), the concept of a list of different microhabitats that represent a significant part of the forest biodiversity is more recent. ...
... The absence of a specific theme is probably because this subject is often mixed with those of TreMs as an indicator of biodiversity or of the impact of forest management on TreMs (48.4% of the articles identified as addressing the question of TreM presence; Table 3). Overall, larger and senescent or dead tree are more likely to bear many TreMs (e.g., Michel et al., 2011;Paillet et al., 2019;Asbeck et al., 2021b;Kõrkjas et al., 2021b;Martin et al., 2021b;). For a same diameter at breast height, hardwoods tend to present a higher number and diversity of TreMs Bouget et al., 2014a;Paillet et al., 2019;Jahed et al., 2020;Asbeck et al., 2021b;Marziliano et al., 2021). ...
... Overall, larger and senescent or dead tree are more likely to bear many TreMs (e.g., Michel et al., 2011;Paillet et al., 2019;Asbeck et al., 2021b;Kõrkjas et al., 2021b;Martin et al., 2021b;). For a same diameter at breast height, hardwoods tend to present a higher number and diversity of TreMs Bouget et al., 2014a;Paillet et al., 2019;Jahed et al., 2020;Asbeck et al., 2021b;Marziliano et al., 2021). At the stand scale, we generally observe the higher TreMs richness and diversity in old and "natural" (i.e., either old-growth, primary or intact) forests (9.9% of the corpus; Table 3) or formerly managed forests untouched over several decades (16.8% of the corpus; Table 3) compared to younger and managed forests. ...
Article
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Article
Full-text available
Tree-related microhabitats (TreM) and deadwood are two forest attributes providing essential resources for biodiversity conservation and ecosystem services. Old-growth forests are generally defined by a high abundance and diversity of TreM and deadwood, but little is known about TreM and deadwood dynamics once the old-growth stage is reached, in particular in the boreal biome. In this context, knowledge on TreM and deadwood dynamics in old-growth forest stands is necessary to better understand how these forests contribute to biodiversity and ecosystem services. The aim of this study is thus to determine how TreM, and deadwood abundance and diversity vary within boreal old-growth forests. To reach this objective, we surveyed TreM and deadwood attributes, as well as structural and abiotic attributes, in 71 boreal old-growth forests situated in Quebec, Canada. We used hierarchical clustering analysis to identify TreM and deadwood abundance and diversity patterns in the studied stands. We identified five clusters of TreM and deadwood characteristics, which corresponded to three stages of old-growth forest succession: canopy break-up (beginning of the old-growth stage), transition old-growth stage (replacement of the first cohort by old-growth cohorts) and true old-growth stage (first cohort all or almost all gone). The peak in TreM richness and diversity was reached at the transition old-growth stage, whereas the peak for deadwood richness and diversity was reached at the true old-growth stage. Overall, true old-growth forests were defined by a combination of moderate to high TreM density and high deadwood volume, but these values significantly varied among stands depending on past secondary disturbances, stand structure and its composition (black spruce [Picea mariana Mill.] dominated vs mixed black spruce – balsam fir [Abies balsamea (L.) Mill.]). These results therefore underscore the importance of considering old-growth forests as dynamic rather than static ecosystems, as the composition of tree- and deadwood-related microhabitats in the same old-growth stand may markedly change over time. At landscape scale, these results also imply that the mosaic of habitats present in old-growth forests can vary greatly from one location to another, highlighting the importance of maintaining a diversity of old-growth forest structure and composition.
Article
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Wind is the leading disturbance agent in European forests, and the magnitude of wind impacts on forest mortality has increased over recent decades. However, the atmospheric triggers behind severe winds in Western Europe (large‐scale cyclones) differ from those in Southeastern Europe (small‐scale convective instability). This geographic difference in wind drivers alters the spatial scale of resulting disturbances and potentially the sensitivity to climate change. Over the 20th century, the severity and prevalence of cyclone‐induced windstorms have increased while the prevalence of atmospheric instability has decreased and thus, the trajectory of Europe‐wide windthrow remains uncertain. To better predict forest sensitivity and trends of windthrow disturbance we used dendrochronological methods to reconstruct 140 years of disturbance history in beech‐dominated primary forests of Central and Eastern Europe. We compared generalized linear mixed models of these disturbance time series to determine whether large‐scale cyclones or small‐scale convective storms were more responsible for disturbance severity while also accounting for topography and stand character variables likely to influence windthrow susceptibility. More exposed forests, forests with a longer absence of disturbance, and forests lacking recent high severity disturbance showed increased sensitivity to both wind drivers. Large‐scale cyclone‐induced windstorms were the main driver of disturbance severity at both the plot and stand scale (0.1–∼100 ha) whereas convective instability effects were more localized (0.1 ha). Though the prevalence and severity of cyclone‐induced windstorms have increased over the 20 century, primary beech forests did not display an increase in the severity of windthrow observed over the same period.
Article
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Understanding the relationship of stand structural complexity and forest management is relevant to create desired stand structures by adapting management strategies under changing disturbance scenarios and climatic conditions. To overcome difficulties in differentiating between strict categories of silvicultural practices and to describe the impact of forest management more appropriate, we used a continuous indicator of forest management intensity (ForMI). The ForMI consists of three components including volumes of natural deadwood, non-native tree species and harvested trees. There are a great number of approaches to quantify stand structure; here we used the recently established stand structural complexity index (SSCI) which represents a density-dependent as well as vertical measure of complexity based on the distribution of points in 3D space inventoried by terrestrial laser scanning. The data collection took place in 135 one-hectare plots managed under close-to-nature forest management (CTNFM) located in the Black Forest, Germany. We build generalized additive models to test the relationship of the SSCI with the ForMI. The model results did not prove a significant relationship between the SSCI and the ForMI, but components of the ForMI showed significant relationships to the SSCI. Our results indicate that the relationship between stand structural complexity and forest management intensity is, while plausible, not trivial to demonstrate. We conclude that forest managers have a relatively wide range of choices in CTNFM to adapt forests within a similar range of management intensity as presented here to future challenges, since management intensity does not change the forest structure drastically.
Article
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Purpose of the Review The concept of tree-related microhabitats (TreMs) is an approach to assess and manage multi-taxon species richness in forest ecosystems. Owing to their provision of special habitat features, TreMs are of special interest as a surrogate biodiversity indicator. In particular, in retention forestry, TreMs have gained attention over the past decade as a selection criterion for retained structural elements such as habitat trees. This review seeks to (a) address the suitability of TreMs as biodiversity indicator in the context of retention forestry, (b) summarize drivers of TreM occurrence and the status quo of the implementation of TreM-based retention concepts in forest management, and (c) discuss current and future challenges to the use of TreMs as biodiversity indicator. Recent Findings The TreM concept originated in Europe where it is now increasingly implemented. Most studies of the quantity, quality, and diversity of TreMs are focused on tree species from this region, although it is increasingly applied in other contexts. In addition to tree species, tree dimensions and live status have been identified as the main drivers of TreM occurrence. One major remaining research challenge is to verify relationships between the occurrence and abundance of forest-dwelling species from different taxonomic groups and TreMs to improve the evidence basis of this concept and thus increase its integration in forest conservation approaches. Summary TreMs are not the “silver bullet” indicator to quantify biodiversity of forest dwelling species, but they provide an important tool for forest managers to guide the selection of habitat trees for the conservation of the associated biodiversity.
Article
Full-text available
Managed forests are a key component of strategies aimed at tackling the climate and biodiversity crises. Tapping this potential requires a better understanding of the complex, simultaneous effects of forest management on biodiversity, carbon stocks and productivity. Here, we used data of 135 one-hectare plots from southwestern Germany to disentangle the relative influence of gradients of management intensity, carbon stocks and forest productivity on different components of forest biodiversity (birds, bats, insects, plants) and tree-related microhabitats. We tested whether the composition of taxonomic groups varies gradually or abruptly along these gradients. The richness of taxonomic groups was rather insensitive to management intensity, carbon stocks and forest productivity. Despite the low explanatory power of the main predictor variables, forest management had the greatest relative influence on richness of insects and tree-related microhabitats, while carbon stocks influenced richness of bats, birds, vascular plants and pooled taxa. Species composition changed relatively abruptly along the management intensity gradient, while changes along carbon and productivity gradients were more gradual. We conclude that moderate increases in forest management intensity and carbon stocks, within the range of variation observed in our study system, might be compatible with biodiversity and climate mitigation objectives in managed forests.
Article
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Research Highlights: Past disturbances occurred naturally in primary forests in the Southern Carpathians. High-and moderate-severity disturbances shaped the present structure of these ecosystems, which regenerated successfully without forestry interventions. Background and Objectives: Windstorms and bark beetle outbreaks have recently affected large forest areas across the globe, causing concerns that these disturbances lie outside the range of natural variability of forest ecosystems. This often led to salvage logging inside protected areas, one of the main reasons for primary forest loss in Eastern Europe. Although more than two-thirds of temperate primary forests in Europe are located in the Carpathian region of Eastern Europe, knowledge about how natural disturbances shape the forest dynamics in this region is highly essential for future management decisions. Material and Methods: We established our study in a primary forest valley situated in the centre of the largest temperate primary forest landscape in Europe (Făgăras , Mountains). A dendrochronological investigation was carried out to reconstruct the natural disturbance history and relate it to the present forest structure. Results: The dendrochronological analysis revealed high temporal variability in the disturbance patterns both at the patch and stand level. Moderate severity disturbance events were most common (20-40% of canopy disturbed in 60% of the plots) but high severity events did also occur (33% of the plots). Regeneration was spruce-dominated and 71% of the seedlings were found on deadwood microsites. Conclusions: We conclude that the current structure of the studied area is a consequence of the past moderate-severity disturbances and sporadic high-severity events. The peak in disturbances (1880-1910) followed by reduced disturbance rates may contribute to a recent and future increase in disturbances in the Făgăras , Mts. Our findings show that these disturbance types are within the range of natural variability of mountain spruce forests in the Southern Carpathians and should not be a reason for salvage logging in primary forests from this area.
Article
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Forests are increasingly affected by natural disturbances. Subsequent salvage logging, a widespread management practice conducted predominantly to recover economic capital, produces further disturbance and impacts biodiversity worldwide. Hence, naturally disturbed forests are among the most threatened habitats in the world, with consequences for their associated biodiversity. However, there are no evidence-based benchmarks for the proportion of area of naturally disturbed forests to be excluded from salvage logging to conserve biodiversity. We apply a mixed rarefaction/extrapolation approach to a global multi-taxa dataset from disturbed forests, including birds, plants, insects and fungi, to close this gap. We find that 75 ± 7% (mean ± SD) of a naturally disturbed area of a forest needs to be left unlogged to maintain 90% richness of its unique species, whereas retaining 50% of a naturally disturbed forest unlogged maintains 73 ± 12% of its unique species richness. These values do not change with the time elapsed since disturbance but vary considerably among taxonomic groups.
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Forests are increasingly affected by natural disturbances. Subsequent salvage logging, a widespread management practice conducted predominantly to recover economic capital, produces further disturbance and impacts biodiversity worldwide. Hence, naturally disturbed forests are among the most threatened habitats in the world, with consequences for their associated biodiversity. However, there are no evidence-based benchmarks for the proportion of area of naturally disturbed forests to be excluded from salvage logging to conserve biodiversity. We apply a mixed rarefaction/extrapolation approach to a global multi-taxa dataset from disturbed forests, including birds, plants, insects and fungi, to close this gap. We find that 75 ± 7% (mean ± SD) of a naturally disturbed area of a forest needs to be left unlogged to maintain 90% richness of its unique species, whereas retaining 50% of a naturally disturbed forest unlogged maintains 73 ± 12% of its unique species richness. These values do not change with the time elapsed since disturbance but vary considerably among taxonomic groups.
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Key message Drivers of the abundance and richness of tree-related microhabitats are similar in mountain forests of Europe and North America and their occurrence may be explained by tree functional groups. Abstract A common approach to support forest-dwelling species in managed forests is to preserve valuable habitat trees. To assess the quality of habitat trees, a hierarchical typology of tree-related microhabitats (TreMs) is applied in the European context for inventory standardization. The first aim of this study was to evaluate whether it is possible to use this hierarchical typology as a standard protocol regardless of location, which is important for potentially standardizing future studies of TreMs, by testing whether the typology could be applied to the western North American mountain forests of Idaho. The second aim of the study was to analyse drivers that influence TreMs in forests of the region. Thirdly, we assessed whether the occurrence of TreMs could be explained by functional groups of trees across the western mountain forests of Idaho and Central European mountain forests, using TreM inventory data previously collected in the Black Forest, Germany. Abundance and richness of TreMs per tree were analyzed as a function of tree species, live status (dead vs. live trees), diameter at breast height (DBH), and site factors (latitude and altitude). Our results show that the TreM typology could be applied with slight modifications in the forests of Idaho. The abundance and richness of TreMs per tree increased with DBH. Snags offered more TreMs per tree than live trees. We were able to group tree species from the two continents in functional groups that were related to the occurrence of certain TreMs. Tree functional groups offer an opportunity to predict the role of certain tree species for habitat provision through TreMs. Combinations of trees from different functional groups could be used to optimize provisioning of TreMs within forest stands.