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Mixed-species versus monocultures in plantation forestry: Development, benefits, ecosystem services and perspectives for the future

Authors:

Abstract

Plantation forests are increasing rapidly in the world in order to alleviate deforestation and degradation of natural forests, along with providing various goods and services. While monoculture plantations have been the dominant type of plantation in practice and well-recorded in research, in the face of intensifying climate change and resource scarcity, there is a growing interest in mixed-species plantations. Agroforestry systems are also catching the attention of foresters, smallholders and landowners. However, there are relatively limited number of studies on successful species mixtures. This paper first reviews the progression of monocultures and mixed-species, followed by the comparisons of advantages, disadvantages and effects on the surrounding natural ecosystems between these two types of plantations. The paper further investigates combinations of species with complementary traits for efficient use of limiting resources associated with improvement in growth development and production of tree species, as well as examining some other challenges in mixed-species. In addition, it is helpful to select and combine tree/crop species in mixtures based on complementary traits that maximise positive and minimise negative interactions and using the advance molecular technologies for genetic analysis. With careful design and proper management, mixed-species plantations with two, three or four species can be more productive and have more advantages in biodiversity, economy and forest health over monocultures. Many researchers are still working on different projects to explore the potential benefits and to promote the applications of mixed-species plantations and agroforestry.
Review Paper
Mixed-species versus monocultures in plantation forestry:
Development, benets, ecosystem services and perspectives
for the future
Corsa Lok Ching Liu
a
, Oleksandra Kuchma
a
, Konstantin V. Krutovsky
a
,
b
,
c
,
d
,
*
a
Department of Forest Genetics and Forest Tree Breeding, Georg-August University of G
ottingen, Büsgenweg 2, 37077, G
ottingen,
Germany
b
Laboratory of Population Genetics, N. I. Vavilov Institute of General Genetics, Russian Academy of Sciences, 3 Gubkina Str., 119991,
Moscow, Russia
c
Laboratory of Forest Genomics, Genome Research and Education Center, Siberian Federal University, 50a/2 Akademgorodok, 660036,
Krasnoyarsk, Russia
d
Department of Ecosystem Science and Management, Texas A&M University, 305 Horticulture and Forest Science Building, College
Station, TX, 77843-2138, USA
article info
Article history:
Received 2 April 2018
Received in revised form 26 July 2018
Accepted 26 July 2018
Keywords:
Ecosystem services
Monoculture
Mixed-species
Species diversity
Complementary trait
Agroforestry
abstract
Plantation forests are increasing rapidly in the world in order to alleviate deforestation and
degradation of natural forests, along with providing various goods and services. While
monoculture plantations have been the dominant type of plantation in practice and well-
recorded in research, in the face of intensifying climate change and resource scarcity, there
is a growing interest in mixed-species plantations. Agroforestry systems are also catching
the attention of foresters, smallholders and landowners. However, there are relatively
limited number of studies on successful species mixtures. This paper rst reviews the
progression of monocultures and mixed-species, followed by the comparisons of advan-
tages, disadvantages and effects on the surrounding natural ecosystems between these
two types of plantations. The paper further investigates combinations of species with
complementary traits for efcient use of limiting resources associated with improvement
in growth development and production of tree species, as well as examining some other
challenges in mixed-species. In addition, it is helpful to select and combine tree/crop
species in mixtures based on complementary traits that maximise positive and minimise
negative interactions and using the advance molecular technologies for genetic analysis.
With careful design and proper management, mixed-species plantations with two, three
or four species can be more productive and have more advantages in biodiversity, economy
and forest health over monocultures. Many researchers are still working on different
projects to explore the potential benets and to promote the applications of mixed-species
plantations and agroforestry.
©2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC
BY license (http://creativecommons.org/licenses/by/4.0/).
*Corresponding author. Department of Forest Genetics and Forest Tree Breeding, Georg-August University of G
ottingen, Büsgenweg 2, 37077, G
ottingen,
Germany.
E-mail address: konstantin.krutovsky@forst.uni-goettingen.de (K.V. Krutovsky).
Contents lists available at ScienceDirect
Global Ecology and Conservation
journal homepage: http://www.elsevier.com/locate/gecco
https://doi.org/10.1016/j.gecco.2018.e00419
2351-9894/©2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.
0/).
Global Ecology and Conservation 15 (2018) e00419
1. Introduction
Plantation forests are expanding rapidly all over the world. Monocultures have been dominated in practice and well-
documented in forest research, but in the face of increasing climate change and resource scarcity, there is a growing inter-
est in mixed-species plantation systems (e.g., Bolte et al., 2004;Spiecker et al., 2004;Hein and Dh^
ote, 2006;Pretzsch et al.,
2010,2013;2014;Cavard et al., 2011;Hulvey et al., 2013;Bielak et al., 2014;Forrester, 2014;L
of et al., 2014;Pretzsch, 2014;
Neuner et al., 2015;Metz et al., 2016;Pretzsch and Rais, 2016;Pretzsch and Schütze, 2016;Zeller et al., 2017;Coll et al., 2018).
Higher diversity of tree species increases the number of ecological niches, which can further increase the number of asso-
ciated species, for example, plants in understory and animals by providing them with a better habitat (Larjavaara, 2008).
However, there are limited examples of successful mixed-species plantations, especially mixtures with indigenous tropical
tree species (but see Amazonas et al., 2018). The mechanisms of mixing effects in mixed-species plantations and optimal for
particular conditions species combination with complementary traits are largely unknown. In addition, another land use
management system, agroforestry, which also involves elements of mixed-species, is catching the attention of foresters,
smallholders and landowners. It is essential to study and understand these kinds of mixed-species systems and their potential
socio-economic and ecosystem benets that could be obtained.
In this review paper, the importance of species diversity to ecosystems and the positive and negative aspects of mixed-
species will be discussed rst, followed by discussion on the general plantation forestry trends. The history and current
development of monocultures and mixed-species in forest plantations will be reviewed, respectively. In addition, the ad-
vantages and disadvantages of monocultures and mixed-species plantations, along with the effects to the surrounding natural
ecosystems will be studied and compared with the support of several species examples.The paper will also examine whether
mixed-species plantations can obtain higher productivity than monocultures, as well as other challenges associated with
mixed-species. The paper will further focus on the reasons of fewer studies on species mixtures with native tropical tree
species and mixtures with non-nitrogen xing trees. Moreover, identication of complementary traits is difcult. Therefore,
in this review, combinations of species with complementary traits will be investigated for efcient use of limiting resources,
in association with improvement in growth development and production of tree species. It will also discuss different design
and management operations that are suitable for adopting species mixtures. Various ongoing projects and programs related
to mixed-species will be explored for the future of forestry and agriculture.
2. Importance of biodiversity
Biodiversity refers to the variety of organisms, including microorganisms, plants, and animals in different ecosystems,
such as deserts, forests, coral reefs, etc. (Altieri, 1999;Hamilton, 2005;Carnus et al., 2006;Gugerli et al., 2008). It could be
partitioned as diversity within species, between species and of ecosystems or ecological diversity including molecular,
population and genetic diversity (Convention on Biological Diversity United Nations, 1992;Swift et al., 2004;Srivastava and
Vellend, 2005;Mace et al., 2012). The most commonly used representation of ecological diversity is species diversity, which is
dened as the number of species and abundance of each species living within a certain location (Hamilton, 2005). However, as
it has been pointed out in many publications (e.g., Rajora, 1999;Rajora and Mosseler, 2001;Rajora and Pluhar, 2003), genetic
diversity is the most important component of biodiversity. Indeed, it is a basis of all biodiversity and foundation of ecosystem
sustainability and stability. More than one genotype is needed for forestry plantations in order to address the biodiversity and
climate change issues.
Many species are interconnected and dependent on one another for survival. They perform important ecosystem functions
and offer different ecosystem services to support life on Earth and humaneconomies, for instance, waterquantityand quality,
seed and pollen dispersal, soil formation, nutrient cycles, regulation of pests and human diseases, carbon storage and climate
regulation, waste management and cultural services (Balvanera et al., 2006,2013;Carnus et al., 2006;Mace et al., 2012;
Mergeay and Santamaria, 2012). Ecosystems with higher species diversitycan be more efcient and are generally more stable
and resistant to disaster than those with fewer species, as a substantial number of species consist of many different traits,
which can contribute to various functions (Lohbeck et al., 2016). Tropical rainforest is an ecosystem with the greatest
biodiversity on Earth. Lefcheck et al. (2015) demonstrated that species-rich communities support higher levels of ecosystem
functions. They also showed data that herbivore biodiversity had stronger effects on ecosystem multifunctionality than plant
biodiversity, and these effects were consistent in aquatic and terrestrial habitats. Communities with higher diversity of an-
imals also accumulate more biomass (Schneider et al., 2016). It is fundamental to have keystone species, which is either a
plant or animal that helps maintain species diversity and the health of ecosystems (Balun, 2017). Without keystone species,
the ecosystems would be dramatically altered and species would be adversely affected.
Nowadays, biodiversity is threatened by climate change, pollution, overexploitation of natural resources and habitat loss
(Pereira et al., 2012). Loss of biodiversity weakens species connections and impairs the ecosystems, leading to extinction of
species and local populations, which will disrupt ecological services. For instance, insects, birds, bats and other animals are
known as pollinators. Declines in honey bee (Apis mellifera) populations may result in a loss of pollination services with
negative impacts on ecology and economy for fruit crops and owers, which will eventually affect the maintenance of wild
plant diversity, wider ecosystem stability, agricultural production, human welfare and global food security (Potts et al., 2010).
Not only terrestrial but also regional marine ecosystems, including estuaries, coral reefs, coastal and oceanic sh communities
C.L.C. Liu et al. / Global Ecology and Conservation 15 (2018) e004192
are rapidly losing populations, species or the complete functional groups. Reducing marine diversity will lessen resource
availability and rapidly decrease in coastal water quality, ecosystem stability and recovery potential (Worm et al., 2006).
Many studies have shown that plant diversity increases productivity and stability (Tilman et al., 1996,2006;Weigelt et al.,
2009;Jing et al., 2017). Diverse habitats with various plant species can provide forage supporting a wide range of insects and
vertebrates (Yadav and Mishra, 2013). Weigelt et al. (2009) and Jing et al. (2017) proved the importance of increasing diversity
of plants and other organisms by selecting suitable species with compatible management to achieve both high yields and high
persistence in managed grasslands, as well as in other ecosystems. In forestry, species diversity plays a signicant role in tree
breeding, environmental adaptation and improvement of meeting demands for goods and services (Larjavaara, 2008;Iveti
c
et al., 2016;Cordonnier et al., 2018).
3. Plantation forestry trends
Due to rapid growth of the world human population and its economies, natural forests in the world are under increasing
pressure to meet consumption demands for wood and bre production, while they are continuously supplying a wide range
of social and environmental services (Brown and Ball, 2000). Each year, large areas of natural forests are cleared, degraded,
and converted to other land uses (Brown and Ball, 2000;West, 2014). From 2000 to 2010, the global forest area has decreased
with a rate of around 13 million ha per year (FAO, 2006,2010). As a result, plantation forestry is developed to mitigate future
wood shortage problems and produce a huge proportion of world industrial wood and other forest products (Sedjo, 1999;
Brown and Ball, 2000;West, 2014).
Plantation forestry refers to cultivated forest ecosystems established through planting or seeding of native or introduced
species under the process of afforestation or reforestation (FAO, 2001;Carnus et al., 2006;West, 2014;Nghiem and Tran,
2016). Diverse types of plantations have different purposes, and they are expanding steadily all around the world. Sedjo
(2001) reported that it has been common in European regions over the past 200 years; and recently, since the 1960s,
intensive forest plantations have also become increasingly ubiquitous in other continents, including North America, South
America, Oceania and parts of Asia. The total area of global forest plantations increased from 167.5 million ha in 1990 to 277.9
million ha in 2015, and the percentage increased from 4.1% to 7.0% over this period (Brockerhoff et al., 2013;Keenan et al.,
2015;Payn et al., 2015). Specically, plantation forests in temperate zones are the largest with the sharpest increase from
93.4 million ha in 1990 to 154.4 million ha in 2015 (see also Fig. 2 in Payn et al., 2015). According to FAO (2010), East Asia,
Europe and North America are the top three regions with the greatest area of forest plantations (see also Fig. 3 in Payn et al.,
2015).
Forest plantations have been supplying up to 33% of the total industrial roundwood in the world, and are projected to meet
50% of the global industrial roundwood production by 2040 (Kanninen, 2010;Jürgensen et al., 2014). Furthermore, plantation
forestry in general is very useful in economy, ecology and society. Planted forests have a vital role in conserving natural forests
by relieving deforestation, improving and restoring degraded lands, sequestering carbon dioxide and combating climate
change (Sedjo, 1999;Dyck, 2003;Bauhus et al., 2010;Paquette and Messier, 2010;Pawson et al., 2013). Plantations can also be
used for regulating the water cycle, reducing soil erosion and alleviating desertication (Bauhus et al., 2010). Economically,
planted forests can provide job opportunities and revenue to improve livelihoods of the local communities, as well as
strengthening regional and national economies in some countries, such as Brazil, Chile and New Zealand (Nambiar, 1999;
Dyck, 2003). Although the effects of plantation forestry on biodiversity are controversial (Braun et al., 2017), numerous
studies indicated that forest plantations with proper management can conserve biodiversity by increasing variety of habitats
for different plants and animals (Hartley, 2002;Humphrey, 2005;Bremer and Farley, 2010;Irwin et al., 2014;West, 2014;
Nghiem and Tran, 2016) and also by lessening the harvesting pressure on native forests (Williams, 2001;Bowyer, 2006).
Plantations are important for metapopulations, because they improve connectivity between forest patches and buffer edges
across natural forests and non-forest lands (Brockerhoff et al., 2008;Bauhus et al., 2010). Moreover, plantations have
contributed to part of the mixed activities on agricultural land, referring to agroforestrydthe combination of trees and crops
(West, 2014). Both plantations and agroforestry systems can offer different forest products (wood, rewood, mulch), as well as
several ecosystem services (Montagnini et al., 2004;Jose, 2009).
Forest plantations cannot and should not completely replace all natural forests. There are also a few problems associated
with them. They require a lot of fertile land, and they were one of the major causes of elimination of natural forests replaced
by plantations in such countries in South-East Asia as, for instance, Indonesia, Malaysia and India. Furthermore, many
plantations need extensive treatments, such as pesticides and fertilizes, which can harm ecosystems and pollute environ-
ment. In addition, replacing natural forests with plantations decreases carbon storage. In some areas, plantations could create
a potential risk of genetic pollution to native forests, for instance, eucalypt plantations in Australia. Pollen dispersal and
subsequent hybridization could lead to the contamination of native gene pools.
4. Monocultures
4.1. Development of monocultures
Many studies have identied that most of the world plantations are monocultures, consisting of a small number of
common tree genera, such as Eucalyptus,Pinus,Acacia,Tectona,Picea,Pseudotsuga,Swietenia and Gmelina (Kelty, 2006;Piotto,
C.L.C. Liu et al. / Global Ecology and Conservation 15 (2018) e00419 3
2008;Richards et al., 2010;Alem et al., 2015). Monocultures have been developed for a long time. According to Nichols et al.
(2006), the earliest monoculture was documented in 1368, when Pinus sylvestris was grown in the Lorenzer Forest near
Nuremberg to produce industrial timber. The Western concept of monocultures also developed in the 18th and 19th centuries
in Europe because of the scarcity of timber, and the goal was to simplify the structure and speed up the cycles of natural
ecosystems, together with producing large amount of wood within the shortest time (Baltodano, 2000;Griess and Knoke,
2011).
Most monoculture crop species plantations consist of a single particular variety representing the same genotype with
almost no variation. Similar in forestry, clonal plantations consist of trees that are genetically identical to each other because
of originating from the same parent material. Clonal plantations offer potential benets in commercial wood production
including higher plantation productivity and uniformity. Monoclonal poplar, willow and eucalypt are very common in clonal
plantation forestry. For instance, there has been a successful development in clonal plantation of poplars and eucalypts in
India (Lal, 2008;Mushtaq et al., 2017).
4.2. Positive aspects of monocultures
The advantages of monocultures are well understood and documented. They are used for treating wastewater and
improving water quality (Minhas et al., 2015), rehabilitating deforested watersheds and degraded landscapes (Parrotta, 1999).
Many different timber and other forest products can be grown in this kind of large-scale plantation system as well. Mono-
cultures for wood and bre products are dominating in the tropics (Kanninen, 2010). Fast-growing, exotic and low-density
wood species, such as Eucalyptus,Pinus and Acacia are largely used for timber, paper pulp, charcoal and fuel, because they
have short rotation period and have advantages in competing for light, nutrients and water resources over native plants (Li
et al., 2014;Nguyen et al., 2014;Chaudhary et al., 2016). In temperate and boreal zones, Populus is planted to provide shelter,
protect soil and water resources, and sometimes produce wood fuel. Salix species can be also used as potential bioenergy
crops (Brown, 2000). According to Chaudhary et al. (2016), non-timber monoculture plantations, particularly in tropical
regions, can supply palm oil, rubber, plantain or bamboo. Countries in South America, Asia and southern Africa are promoting
monocultures of pine and eucalyptus for paper pulp supply (Table 1). There is also a fast expansion of rubber and oil palm
monocultures in South-East Asia to meet the increasing world demand.
In monocultures, all the site resources are mainly focused on the growth of single species with the most desirable
characteristics, such as growth rate and wood quality (Kelty, 2006;Piotto, 2008;Moghaddam, 2014). Tree species in
monocultures are mostly even-aged and planted at a high density in accessible areas, which allow the plantations to have
easy management and high resilience; thus, higher yields per hectare and more efcient harvest resulting in uniform
products can be obtained (Baltodano, 2000;Kelty, 2006;Nichols et al., 2006;Piotto, 2008).
4.3. Negative aspects of monocultures
Research by various authors have criticised single-species monocultural plantations as supposedly having several negative
social and environmental impacts in spite of the recognised economic benets (Erskine et al., 2006;Alem et al., 2015).
Regarding the social impacts, the introduction of large-scale plantations often leads to the change in the ownership from local
communities to large private companies, hence, resulting into a loss of traditional goods and cultures, customary rights, and
livelihoods associated with forced resettlement and unequal distribution of resources (Baltodano, 2000;Colchester, 2006).
Moreover, effects on the environment include the loss of soil productivity and fertility, disruption of hydrological cycles, risks
associated with plantation forestry practices (e.g., introduction of exotic species), risks of promoting pests and diseases,
Table 1
Fast-growing plantations by species, countries and mean annual increment. Data source: (Kanninen, 2010, p. 10).
Species Mean annual
increment (m
3
/ha/
year)
Rotation
length (years)
Estimated extent as fast-growing
plantation only (1000 ha)
Main countries (in decreasing order of
importance)
Eucalyptus grandis and
various eucalypt hybrids
15e40 5e15 ±3700 Brazil, South Africa, Uruguay, India, Congo,
Zimbabwe
Other tropical eucalypts 10e20 5e10 ±1550 China, India, Thailand, Vietnam,
Madagascar, Myanmar
Temperate eucalypts 5e18 10e15 ±1900 Chile, Portugal, NW Spain, Argentina,
Uruguay, South Africa, Australia
Tropical acacias 15e30 7e10 ±1400 Indonesia, China, Malaysia, Vietnam, India,
Philippines, Thailand
Caribbean pines 8e20 10e18 ±300 Venezuela
Pinus patula and P. elliottii 15e25 15e18 ±100 Swaziland
Gmelina arborea 12e35 12e20 ±100 Costa Rica, Malaysia, Solomon Islands
Paraserianthes falcataria 15e35 12e20 ±200 Indonesia, Malaysia, Philippines
Poplars 11e30 7e15 ±900 China, India, USA, Central and Western
Europe, Turkey
C.L.C. Liu et al. / Global Ecology and Conservation 15 (2018) e004194
higher risks of adverse effects of storms and re, and negative impacts on biodiversity (Baltodano, 2000;Evans, 2001;Bowyer,
2006).
Monoculture plantations may deplete soil, causing soil erosion and degradation (Baltodano, 2000;Bowyer, 2006). Tree
harvesting by machines can promote soil compaction, which will adversely affect the growth of understory. Single-species
plantations are also not efcient in trapping nutrients, because fewer roots exist near the surface, which may further lead
to signicant loss of nutrients from the harvest sites. In addition, some species, such as Eucalyptus and Gmelina can acidify soil,
and in addition Gmelina can release specic substances that inhibit the growth of other plant species (Baltodano, 2000). There
are some concerns about depletion of soil moisture and reduced stream ow in plantations. Some researchers have observed
that particular species (e.g., Eucalyptus) consumes more water than the others in natural forests, which may draw down the
water table in some regions (Baltodano, 2000;Morris et al., 2004;Bowyer, 2006). However, this could be because eucalypts
grow faster, and fast growing native forests may use a similar amount of water to similarly fast growing eucalypt plantations.
Furthermore, monocultures are more susceptible to pests and diseases. Owing to the uniform genetic composition and
closeness of tree species in monocultures, they can provide a huge food source and ideal habitat for insects and pathogens,
which will consequently give rise to rapid colonisation and spread of infection (Hartley, 2002;Bowyer, 2006;Carnus et al.,
2006;Brockerhoff et al., 2013;Moghaddam, 2014). However, monocultures are sometimes no more susceptible than mix-
tures (see Brockerhoff et al., 2017 for a recent review).
The link between plantation forestry and biodiversity is still debatable as mentioned before, because many researchers
suggested that plantation monocultures have a potential to provide habitats for indigenous ora and fauna and enhance
biodiversity in degraded lands (Cuong et al., 2013). However, an increasing number of studies have discovered that mono-
culture plantations have lower levels of biodiversity than surrounding native forests, and some of them have considered
exotic monocultures as biological deserts(Bowyer, 2006;Bremer and Farley, 2010;Brockerhoff et al., 2013;Pawson et al.,
2013). Harvesting monoculture stands by clearcutting is one of the possible reasons explaining the dramatic alteration of
habitat. Furthermore, uniform rows of monoculture plantations are completely opposite to diversity, and they have been
found to be poor habitat for native birds (Subasinghe et al., 2014;Chaudhary et al., 2016;Dislich et al., 2017). Nevertheless,
Kanowski et al. (2005) noted that the effects of plantations on biodiversity vary from case to case in terms of design, including
tree species, stand density, the retention or restoration of native forests, as well as management, such as harvest regimes and
chemical applications, together with factors related to landscape context.
Felton et al. (2010) reviewed negative ecological and environmental impacts of monoculture plantations of spruce and
showed that these plantations have lower resistance to biotic and abiotic disturbances aggravated by changing climates.
Moreover, expanding spruce monocultures in southern Sweden has resulted in population declines and increased risks of
extinction for numerous forest dependent taxa. The soils in those plantations become more acidic as well, and subsequently
generate unfavorable outcomes for biodiversity and other land uses in the long term. However, potential risks can be
minimised with proper planning and good management practices of monocultures (Bowyer, 2006;Kelty, 2006).
5. Mixed-species
5.1. Advancement of mixed-species
Plantations which are diverse in genotypes, species, structures and functions, are acclaimed as more environmental
friendly and sustainable plantation systems over monocultures, especially in case of mixing indigenous species (Manson et al.,
2013). However, we are unaware of a single specicdenition for mixed-species or, in other words, mixed-species plantation
(Griess and Knoke, 2011;Felton et al., 2016). Mixtures can be arranged in many ways with variations in species composition
and dominance, spatial arrangement and age structure (Ashton and Ducey, 1997). Current studies on multi-species planta-
tions are relatively limited, and mixtures of native trees are relatively uncommon. It is interesting and important to establish
and explore this type of diversied plantation system (Ashton and Ducey, 1997;Forrester et al., 2005;Nichols et al., 2006).
There is also wealth of research on the growth interactions with non-nitrogen xing species in mixed-species plantations and
their effects on the regeneration of woody plants (Alem et al., 2015). It was shown that potential benets can be obtained from
carefully designed mixed-species plantations (Piotto, 2008;Griess and Knoke, 2011;Manson et al., 2013;Nguyen et al., 2014).
There are several cases of mixed-species plantations that have been successful and popular over the centuries. According
to Kelty (2006) and Nichols et al. (2006), mixing larch trees with pine in Europe had already been recorded in 1910s, and
mixtures with other species, such as alder, oak and beech, were continued to develop for early income stream. Since the
1980s, rigorous experimental research focusing on the comparison of mixtures and monocultures were set up with
comprehensive data collection (Piotto, 2008;Plath et al., 2011). The applications of suitable tree species in mixed-species
plantations have been mainly demonstrated in Europe and North America, but a few studies have been also documented
in the tropics with some notable examples of mixing Eucalyptus,Albizia and Acacia (Ashton and Ducey, 1997). Furthermore,
there has been a gradual decrease in the area of single-species plantations in Europe and a steady progression towards
mixtures of species with the objectives of increasing productivity, resistance and resilience or converting plantations from
conifers to broadleaved species (Forest Europe, UNECE &FAO, 2011;Bravo-Oviedo et al., 2014). Generally, mixed-species
plantations consist of two, three or even four species of plants, but it is possible to have more diverse and complex mix-
tures (Nguyen et al., 2014).
C.L.C. Liu et al. / Global Ecology and Conservation 15 (2018) e00419 5
Agroforestry also represents an important type of mixed-species system, where woody perennials (trees and shrubs) are
grown in association with agricultural crops and pastures on the same land and at the same time (Mal
ezieux et al., 2009).
Agroforestry systems have been practiced in both tropics and temperate zones for thousands of years until the Middle Ages.
Then, they started to decline while crop rotation was evolved for soil protection (Smith, 2010b). In the late 1970s, a new
concept of agroforestry was introduced again and encouraged by many European policies in the 1990s, aiming for diverse
productions coupled with conservation of resource and environment (Smith, 2010b;Nerlich et al., 2013). Silvoarable and
silvopasture are the current major agroforestry practices in Europe based on using a range of dominant tree species
(Mosquera-Losada et al., 2009).
5.2. Advantages of mixed-species
There are abundant evidences that planting multiple species can gain numerous economic, environmental and social
benets (Hartley, 2002;Forrester et al., 2005;Plath et al., 2011;Pawson et al., 2013;Carnol et al., 2014;Alem et al., 2015;
Dr
ossler et al., 2015;Nguyen et al., 2015). First of all, species mixtures can maximise the use of resources, and consequently
increase stand-level productivity and carbon sequestration. Several studies have found that mixed-species plantations are
more productive in comparison with monocultures (Kanowski et al., 2005;Petit and Montagnini, 2006;Richards et al., 2010;
Zhang et al., 2012;Pretzsch and Schütze, 2016). An example from Chomel et al. (2014) demonstrated that mixing hybrid
poplar and white spruce increased wood production of poplar and sequestrated more carbon than monocultures of either
poplar or white spruce. Mixtures with stratication can also enhance individual-tree growth rates and stem quality of species
in upper canopies, whilst minimising the proportion of taller species that can reach the highest production (Kelty, 2006;
Piotto, 2008). However, uncertainty remains about mixtures achieving greater productivity than monocultures (Carnus et al.,
2006;Erskine et al., 2006;Piotto, 2008;Griess and Knoke, 2011;Dr
ossler et al., 2015).
In addition, Forrester et al. (2006) reported that several examples of mixing eucalyptus and nitrogen-xing species
increased productivity and nutrient cycling rates, and they had better results than monocultures. The study from Forrester
et al. (2010) also found that mixtures of Eucalyptus and Acacia can enhance water-use efciency, but there are still cases of
reduced productivity in mixtures (Binkley et al., 2003;Kelty, 2006). Another advantage of mixed-species over monocultures is
the promotion of diversifying production under different rotation periods (Forrester et al., 2006). Mixed-species plantations
are more resistant to damage caused by storms, insects or diseases (Hartley, 2002;Nichols et al., 2006;Griess and Knoke,
2011). Some species can act as nurse to other tree species, and mixtures of fast-growing and slower-growing species can
produce timber and more valuable wood products while reducing risks of soil erosion and providing shelter and protection
against frost or pests (Montagnini et al., 2004;Petit and Montagnini, 2006). Taller species in mixtures can also provide
shading to shorter species, resulting in less branching of the smaller ones, which may eventually improve the wood quality
(West, 2014). Moreover, mixed-species plantations could be more efcient in ltering of atmospheric pollutants (e.g., sulphur
and chlorine) in the areas with heavy precipitation (Zhao et al., 2017). There is a potential of using more complex mixtures
with ve to seventy species for restoration of degraded lands (Kelty, 2006;Nguyen et al., 2014). For ecological benets, Felton
et al. (2016) proved that spruce-birch and spruce-pine polycultures did not only simply support aesthetic and recreational
values, but they also increased avian diversity with special composition of bird species.
Correspondingly, agroforestry systems have been well recognised as an improvement on monocultures and being closer to
native forests (Chaudhary et al., 2016). They can provide a wide variety of goods (e.g., rubber, coconut, coffee or cacao), reduce
poverty, increase carbon storage, enhance soil fertility and improve water and air quality (Alavalapati et al., 2004;Jose, 2009).
Growing trees with agricultural crops can also produce high-value wood products and bioenergy, minimise the risk of pest
outbreaks and enhance biodiversity (Nerlich et al., 2013). There are several successful agroforestry examples. For instance,
Pelleri et al. (2013) presented a mixed plantation of walnut, poplar and some other nurse trees (e.g., black alder and hazel),
which had favourable impacts on the growth of both walnut and poplar, farm economics, and landscape quality, as well as this
plantation was less prone to disturbances. Mutanal et al. (2007) showed that mixed-species of fast-growing tree species and
tamarind had higher yields and better growth performance in comparison with monocultures, as well as having the capability
to prevent soil erosion and increase biodiversity.
Table 2 demonstrates an evaluation of risks for six different forest management approaches with scores 1e3 (low to high).
The evaluated risk factors included climate impacts, stability, human impacts, insects, diseases, wild game and res. Mixed
Table 2
Levels of risks for several forest types due to different factors. Data source: (Dedrick et al., 2007, p. 80).
Factor Eucalyptus Poplar Short rotation pine Long rotation pine Spruce monocultures Mixed forests
Climatic 2e32e32 2e33 1e2
Static stability 2 2 1e21 3 1e2
Anthropogenic 1 2 1 1e23 1
Insects 3 2e32e32 3 2
Diseases 3 3 3 2 3 2
Wild game 1 1 1 3 3 3
Fires 2 1 3 3 1 1e3
Total 14e15 13e15 13e15 14e16 19 11e14
C.L.C. Liu et al. / Global Ecology and Conservation 15 (2018) e004196
forests had the lowest scores in total among all the management strategies, and contrastingly, spruce monocultures had the
highest. This implies that mixed-species plantations are certainly less susceptible to biotic and abiotic disturbances, and it is a
good evidence showing that species mixtures are preferable to monocultures.
5.3. Disadvantages of mixed-species forest plantations
There are some disadvantages in species mixtures. Mixtures in tropical regions may negatively affect biodiversity. For
example, mixed-species plantations have lower diversity than local rainforests in Australia, and they support fewer rainforest
bird species than monocultures (Kanowski et al., 2005). Species mixtures in some conditions will also reduce soil fertility and
productivity because of asymmetric competition (Forrester et al., 2005;Petit and Montagnini, 2006;Manson et al., 2013).
Furthermore, improper choice of tree species or crops for mixtures can create local conditions that increase the risk of disease
outbreaks (Gebru, 2015;Thomsen, 2016). For agroforestry systems, they have very few drawbacks, but setting up a successful
one is very challenging and time-consuming.
5.4. Combination of species with complementary traits
A great number of studies have indicated that it is important to select species in mixtures with complementary structural
and functional traits, such as shade tolerance, height growth rate, crown structure, foliar and root phenology and root depth
(DeBell and Harrington, 1993;Kelty, 2006;Nichols et al., 2006;Nguyen et al., 2015;Schuler et al., 2017). Therefore, a suc-
cessful mixed-species plantation may combine fast-growing with slow-growing species, short-lived with long-lived species,
light demanding with shade tolerant species, shallow with deep rooting species, nitrogen-xing with non-nitrogen-xing
species or slim-crowned and height oriented with wide-crowned and more laterally expanding species (Forrester et al.,
2005,2006;Yadav and Mishra, 2013;Pretzsch, 2014;Nguyen et al., 2015). In such cases, species can potentially increase
light interception, biomass production, biodiversity and resistance to disturbances (Kelty, 2006;Pretzsch, 2014). There is also
a useful online platform named TRY, which offers a database of global plant traits for researchers (Kattge et al., 2011).
Modern molecular technologies including marker-assisted selection (MAS) based on quantitative trait loci (QTLs), genomic
selection (GS) based on genome-wide single nucleotide polymorphism (SNPs) associations with important traits, and genetic
modication are widely used in tree breeding (Moose and Mumm, 2008). MAS can greatly increase the efciency and
effectiveness in plant breeding compared with traditional breeding methods. At rst, it requires the determination of DNA
markers that are tightly linked to important genes or QTLs of interest, and afterwards, breeders may use the specicDNA
marker alleles as a powerful diagnostic tool to identify plants which carry the necessary genes or QTLs (Collard et al., 2005).
This method has proven to be successful in breeding of various crop species (e.g., maize, rice, barley and soybean), and it has
the advantages of improving yields along with increasing abiotic and biotic stress resistance (Francia et al., 2005).
However, polygenic inheritance of traits is a major limitation of MAS. GS can overcome the limitation of MAS by using
whole genome molecular markers and high throughput genotyping (either by using high-density SNP genotyping assays,
such as Illumina Innium system, or genotyping by sequencing with next generation sequencing (NGS) platforms) to improve
quantitative traits with higher accuracy in large plant breeding populations (Desta and Ortiz, 2014;Lin et al., 2014;Iwata et al.,
2016;Grattapaglia, 2017;Jonas et al., 2018). Genetic values of selection candidates can be predicted based on the genomic
estimated breeding values (GEBVs) through this approach (Newell and Jannink, 2014). Additionally, agro-morphological traits
can be introgressed to well-adapted crop species by the integration of selected candidates with the highest GEBVs and other
breeding programs.
In addition, the use of genetic modication provides a unique opportunity to improve novel complementary traits of
plants and accelerate tree breeding, resulting in an increase and reliable wood production in future (West, 2014;H
aggman
et al., 2016). Examples of agronomic traits, including enhanced herbicide resistance, enhanced resistance to pests, diseases
and abiotic stresses, modied lignin content and improved wood quality, could be attained by altering the expression of
specic gene(s) and incorporating new genes into the plant genomes (Harfouche et al., 2011) or by genome editing (Songstad
et al., 2017). Nevertheless, these techniques are still relatively new in forestry, and there are some risks or issues related to
environmental, economic and social aspects. Therefore, more laboratorygenetic engineering studies and tests are required for
approving genetically modied trees and crops.
Recently, several projects have been aimed at the identication of genetic control of the complementary plant traits in the
mixed woody species plantations. For example, the IMPAC
3
project identies novel traits in mixed-species of poplar (Populus
sp.) and black locust (Robinia pseudoacacia). Activity and gene expression are currently studied in different environmental
conditions with the use of transcriptome analysis based on the NGS (IMPAC
3
,2014;Kuchma et al., 2017). Mixed-species
plantations of poplar and black locust used in this project is a typical example of mixing nitrogen-xing with non-
nitrogen-xing species assuming that poplar may gain nitrogen from black locust, and that regional ecosystem in associa-
tion with higher yields can also benet from mixed-species (Zhai et al., 2006).
In intercropping system, diverse germplasm can be used in trials for assessing genotypes with favourable yield or quality,
and it is also considerable to breed plants with traits that are benecial to a companion crop (Brooker et al., 2015).
Furthermore, plant breeding research and cultivar development are crucial for improvement of food production, thus, the
availability of diverse genetic sources can enhance food security together with agricultural sustainability (Govindaraj et al.,
2015).
C.L.C. Liu et al. / Global Ecology and Conservation 15 (2018) e00419 7
5.5. Challenges of mixed-species
However, several obstacles exist in the expansion of polyculture plantations. Firstly, species mixtures require complicated
forest management operations, and some foresters have the perception that mixed-species plantings reduce productivity,
which is supported by some studies (Pawson et al., 2013;Felton et al., 2016). Secondly, there are limited evidence and
knowledge for matching species to site conditions, as well as growth strategies of native species in mixed-species plantations
(Nichols et al., 2006;Manson et al., 2013;Nguyen et al., 2014). It is uncommon to have mixtures with more than four species
because of the difculty in matching suitable characteristics, except mixtures of tropical native species that rarely include less
than four species (Amazonas et al., 2018). In newly developed mixtures, there is a chance that tree species suffer from both
interspecic and intraspecic competition (Nguyen et al., 2014). There are also very few instructions for designing and
managing mixed-species systems (Fischer and Vasseur, 2002;Nguyen et al., 2015). Furthermore, the shortage of time,
awareness and training between farmers and landowners are the additional restrictions of applying agroforestry systems
(Smith, 2010a;Wilson and Lovell, 2016).
6. Future perspectives of mixed-species
To gain a better understanding and sustainable use of mixed-species, many researchers keep exploring the potential
benets for the future of forestry and agriculture. In forestry, KROOF project will use tree and stand-level growth reactions
induced by a drought experiment to examine whether spruce suffers more in mixtures (e.g., spruce with beech) than in
monocultures under limited water supply (Pretzsch et al., 2014). Additionally, the purposes of developing the CommuniTree
Carbon program are to improve livelihoods of small-scale farmer families, sequestrate carbon, and enhance biodiversity and
environment by planting mixtures with ve native tree species in Nicaragua (Baker et al., 2014). In agriculture, Iijima et al.
(2016) suggested a new concept of mixing wet and dryland crops (e.g., pearl millet and sorghum with rice), which will
strengthen the ood tolerance of upland crops. Another research from Thünen-Institut is studying the potential of mixing
maize with runner beans and expecting an increase in production together with improvementof protein and energy supply in
monogastrics and ruminants (Thünen-Institut, 2014;Hamburd
a et al., 2015). The agroforestry systems can produce more
bioenergy and replace the use of fossil fuels in future (Nerlich et al., 2013). Furthermore, a project named SidaTim will assess
and model the economic and ecological potentials of growing Sida hermaphrodita along with valuable timber trees (e.g.,
walnut and cherry) to promote a diversied agricultural system in different European countries (FACCE SURPLUS, 2016). This
type of agroforestry will provide extra income for farmers, reduce erosion, act as windbreaks and improve ecological and
aesthetic values.
There are two more large-scale agroforestry projects taking placedAGFORWARD and BREEDCAFS. The former one will
give an in-depth analysis about the agroforestry systems in Europe, develop and assess innovative agroforestry designs and
practices with favourable impacts, and encourage a wider adoption of suitable agroforestry in Europe through dissemination
and policy (AGFORWARD, 2014). The BREEDCAFS project is led by CIRAD and developed to address climate change through
coffee breeding (CIRAD, 2017). This project may help increase smallholder farmers' earnings, produce coffee varieties with
high quality for coffee industry, as well as obtaining understanding of coffee physiology through the combination of phe-
notyping and advanced DNA analysis (WORLD COFFEE RESEARCH, 2017).
We think that there is insufcient number of studies on nutrition in mixed-species plantations, especially with non-
nitrogen xing mixtures due to less attention to other nutrients. Therefore, more studies of that kind should be done in
the future (Nichols et al., 2006). Besides, a special type of mixed-species planting (i.e. rainforestation farming), which was rst
developed in the Philippines, should be widely promoted in tropical regions (Nguyen et al., 2014). It is a novel strategy that
combines the concepts of rural development, resource management, biodiversity conservation and landscape restoration by
mixing indigenous tree species with local agricultural crops. This system can provide sustainable income to smallholders,
create community forestry and save the endangered species, such as the tiniest ape, Tarsius syrichta (G
oltenboth and Hutter,
2004). Most importantly, there is a necessity for greater amount of evidence, education, funding, incentives, innovative
experiments with wider range of tree species and analyses for polyculture expansion (Nichols et al., 2006;Moghaddam,
2014). In agroforestry, it is necessary to raise awareness and demonstrate practical management skills to farmers and
landowners, and there is a potential of establishing more agroforestry systems in temperate regions (Smith, 2010a).
7. Conclusion
In summary, there is a global trend of increasing forest plantations to relieve the pressure of deforestation and degradation
of natural forests, in addition to meet demands of timber products and forest services. The majority of world plantation
forests are monocultures with certain dominant tree species, which are favoured for timber production due to the uniformity
of trees and easy management. Monocultures are still expanding in South-East Asia. Meanwhile, mixed-species plantations
are growing and becoming more popular, since they have been found to have more benets in biodiversity, economy, forest
health and occasionally in productivity compared with monospecic plantations.
Undoubtedly, there are also challenges in designing, planting and managing species mixtures. Mixed-species plantations
can have negative or positive effects on tree growth (Piotto, 2008). It is necessary to select and combine tree or crop species in
mixtures with complementary traits that maximise positive and minimise negative interactions using the advance molecular
C.L.C. Liu et al. / Global Ecology and Conservation 15 (2018) e004198
technologies (Brooker et al., 2015). With careful design and appropriate management, mixed-species plantations with three
or four species can be more productive and have more advantages over disadvantages. As a result, mixed-species plantations
and agroforestry should be broadly promoted and adopted as they can produce more economic and ecological gains, and
contribute to food security.
Although, the issue of negative or positive effects of mixed-species forest tree stands on tree growth and ecosystem
functions is still controversial, we believe that mixed-species forest tree stands are benecial for both trees and ecosystems in
many regions under right conditions and appropriate management, and that new genomic tools should help us more ef-
ciently analyse functional interactions between different species.
Acknowledgements
This work was supported by the IMPAC
3
project Novel genotypes for mixed-species allow for IMProved sustainable land
use ACross arable land, grassland and woodlandfunded by a research grant fromFederal Ministry of Education and Research
in Germany (BMBF) supporting the Innovative Plant Breeding within the Cultivation System (IPAS) funding measure in the
National Research Strategy BioEconomy 2030 framework program (Innovative Panzenzüchtung in Anbausystemen, IPAS),
FKZ 031A351A.
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... Due to their high pro tability (Cubbage et al. 2020), easy management, and straightforward ecological relationships, most plantations are managed as pure forest stands. However, a growing number of researchers have indicated the disadvantages of this type of stand structure maintained for the production of timber only (Ching Lui et al. 2018). They suggest that pure forest plantations are particularly susceptible to insect outbreaks, pests and forest res, provide low levels of biodiversity and reduce the range of ecosystem services provided (Bowyer 2006;Horak et al. 2019). ...
... This comparison indicated that the inclusion of carbon values only lengthens rotations when carbon values are su ciently high. Whether a higher value placed on carbon effects the order of the LEVs of mixtures, or whether white oak with its higher rate of carbon sequestration might become more viable, remains to be seen.For many forest scientists a major perceived advantage of mixed plantations derives from their lower susceptibility to risk, especially against insect infestation (ChingLui et al. 2018). For instance, Willis et al. ...
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... Results also showed an increase in the promotion of mixed forests as well as in the share of plantations over the study period. Considering that mixed-species approaches to forestry provide a broader range of ecosystem services relative to monocultures (e.g., Gamfeldt et al. 2013, Felton et al. 2016, Liu et al. 2018, Messier et al. 2021, increasing the cover of mixed forests is considered an advancement towards multifunctionality and agrees with the trend observed across Europe (Forest Europe 2020). Spain hosts one of the highest tree diversities in Europe (Ollero & de Dios 2011, Morales Valverde et al. 2011, with almost half of the forest area containing four or more tree species (Forest Europe 2020). ...
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... Tropical forest plantations are dominated by a handful of tree species grown in monocultures, including species of Eucalyptus spp., Acacia spp., Pinus spp., and Tectona grandis, while mixed-species plantations are restricted almost exclusively to forest plantations planted for ecological restoration or protective purposes (Gunter et al. 2013;Pancel and Köhl 2016). This is despite the fact, that researchers have repeatedly highlighted the possible silvicultural benefits of mixed-species plantations, such as increased productivity and reduced overall risk (Kelty 2006;Knoke et al. 2008;Liu et al. 2018;Messier et al. 2021). Several studies have shown that some tropical mixed-species plantations can potentially achieve higher stand productivity than their monoculture peers, if species are carefully selected (Ewel et al. 2015;Le et al. 2020;Mayoral et al. 2017;Piotto 2008;Schnabel et al. 2019). ...
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... They also observed that each species under investigation did not contribute equally to the functions of the ecosystem, and hence, through the conversion of a heterogeneous forest to a homogenous monoculture plantation, we forego the benefits of some species. A forest with a high diversity of tree species provides suitable conditions for ecological niches to develop and strengthens the existing species interactions while simultaneously facilitating the evolution of new species associations (Liu et al. 2018). ...
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The present study was carried out in the districts of Mandla and Hoshangabad, in Madhya Pradesh, India. These two districts have sizeable areas under deciduous forests. Given the high dependence of tribal communities on these forests, it is essential to characterize the land use and the status of vegetation in these districts. Land use and land cover (LULC) maps were prepared for landscape characterization for two different time periods (2000 and 2017). The area was classified into eight classes, i.e. dense forests, scrub forests, open forests, agricultural lands, water bodies, fallow lands, built-up areas, and open/sandy areas. The assess�ments show that dense forest areas have increased in both districts. However, this was not reflective of increases in true forests. For instance, forests in Mandla have been converted to monoculture stand of T. grandis (~83%). Since 2000 Mandla and Hoshangabad have lost 8.73% and 6.43% of their open forest areas, respectively. Expansion of agriculture and built-up areas is common at both sites, occurring at the cost of ecologically important land cover types such as scrub forests and open areas/ sandy areas primarily as riverbeds. Results from change detection in forest cover and other land use classes show that compactness of open and scrub forests has reduced whereas patchiness has increased. Patches in close proximity to the forest edges will be vulnerable to edge effects and encroachments. The areas under dense forests have increased, while the number of patches and edge density has decreased. However, this has occurred owing to the existing gaps inside the dense canopies deep inside the forests being replaced by plantations resulting in an increase in contiguity. Although some dense canopy areas were seen to be rich in moisture content based on high NDMI values, large forest areas were under moisture stress. Emphasis has to be given to maintaining heterogeneity in species composition within the forests as well as to avoid fragmentation. Keywords Deciduous forests · Landscape characterisation · Forest fragmentation · Central India
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Genome editing in organisms via random mutagenesis is a naturally occurring phenomenon. As a technology, genome editing has evolved from the use of chemical and physical mutagenic agents capable of altering DNA sequences to biological tools such as designed sequence-specific nucleases (SSN) to produce knock-out (KO) or knock-in (KI) edits and Oligonucleotide Directed Mutagenesis (ODM) where specific nucleotide changes are made in a directed manner resulting in custom single nucleotide polymorphisms (SNPs). Cibus' SU Canola™, which the US Department of Agriculture (USDA) views as non-genetically modified (non-GM), is Cibus' first commercial product arising from plant genome editing and had its test launch in 2014. Regulatory aspects of the various genome editing tools will be discussed.
Article
Key message A better transfer to managers of studies examining the functional role of tree species diversity would be achieved by explicitly addressing two missing links: the effect of management interventions on coexistence mechanisms and the relationships between coexistence mechanisms and ecosystem functions. Context Plant species diversity has been shown to promote a wide array of ecosystem functions and ecosystem services. However, scientific results concerning relationships between species diversity or species mixing and ecosystem functions have not been well transferred to management practices so far. Part of the problem lies in the difficulty of assessing whether interesting species mixtures can persist over the long term and how management influences ecosystem functions. Aims We argue that a better transfer of knowledge to managers would be achieved by addressing two missing links: (i) the effect of management interventions on coexistence mechanisms and (ii) the relationships between coexistence mechanisms and ecosystem functions. Methods To do so, we first provide a brief overview of the recent scientific results on relations between tree diversity (or two-species mixing) and ecosystem functions, focusing on studies dealing with productivity and stability in forests. We further introduce the key question of whether mixed stands are transient or permanent. We then briefly present key elements of modern coexistence theory and illustrate them with three examples in forest ecosystems. We finish by discussing how management interventions in forests can affect coexistence mechanisms and by addressing some methodological perspectives. Results We provide examples of management actions (e.g. gap-based silviculture, preferentialselection of the most frequent species, preferential selection of the most competitive species, plantingweakly competitive species) that may increase the strength of coexistence mechanisms. Conclusion Analysing long-term management impacts on species coexistence and ecosystem functions with a combination of long-term monitoring of large permanent plots and mechanistic dynamic model simulations will be useful to develop relevant practices favouring mixed forests in the long term.
Article
Despite the high diversity of trees in the tropics, very few native species have been used in plantations. In a scenario of high international demand for nature conservation, ecological restoration and for the provision of forest products, mixed species forestry in the tropics emerges as a promising option. In this study, we examine three large experiments in the Atlantic Forest region of Brazil that combine early Eucalyptus wood production with a high diversity (23–30 species) of native tree species. We tested the following hypotheses: (1) Eucalyptus growth and survival is higher in mixed plantations than in monocultures, while that of native species is lower when intercropped with Eucalyptus; (2) The diameter of target native trees is influenced by the size and by the identity of neighboring trees; (3) The negative effect of competition from Eucalyptus on native species is directly related to their growth rate. We compared mixtures of Eucalyptus and a high diversity of native tree species with Eucalyptus monocultures and with plots containing only native species, replacing Eucalyptus by ten native species. To test our hypotheses, we examined inventory data considering the stand- and the tree-levels. We calculated survival rate, diameter and height growth and basal area of whole stands and groups of species. We also used a neighborhood index analysis to separate the effect of total competition (i.e. stand density) and the influence of groups of species (intra- and inter-specific competition). The Eucalyptus trees in high diversity mixtures grew larger and yielded nearly 75% of the basal area produced by Eucalyptus monocultures even though this genus accounted for only 50% of seedlings in the mixtures. In the mixtures, Eucalyptus negatively affected the growth of native species proportionate to the native species’ growth rate. With some exceptions, the mixed plantations had no overall negative effect on tree survival or height growth. We conclude that mixtures of Eucalyptus and a high diversity of native tree species are feasible and represent a potential alternative for establishing multipurpose plantations, especially in the context of forest and landscape restoration.
Chapter
Prediction of phenotypes is not only used for selection and breeding in animal and plant populations but also for the assessment of specific phenotypes, especially predisposition to diseases and disorders in human populations. The use of genetic markers has been shown to be useful for prediction and selection for phenotypic traits. The concept of using genetic markers for prediction of breeding values or phenotypes was suggested many decades ago, but applications of marker-assisted selection were limited due to the low number of markers that could be genotyped and the low number of confirmed quantitative trait loci (QTL) that could be selected upon. Genomic selection, in contrast, utilizes dense genetic markers across the whole genome for the prediction of phenotypes as all QTL can be assumed to be in linkage disequilibrium with at least one marker. Genomic selection allows thereby choosing the genetically best individuals without the need to confirm QTL. The concept of genomic selection, proposed in 2001, has since been further developed and applied. Nowadays, genomic selection is widely applied in breeding populations of plants and animals for the selection of future breeding individuals. The chapter introduces the general concept of genomic selection. It further discusses relevant prerequisites for the application of genomic selection, including genotyping platforms and reference populations. Some of the methods applied today as well as suggested advancements of methods are introduced. The final part of the chapter describes briefly applications in animal, plant, and human populations (status when writing this chapter), before concluding with some general notes on genomic selection.
Article
Research into mixed-forests has increased substantially in the last decades but the extent to which the new knowledge generated meets practitioners’ concerns and is adequately transmitted to them is unknown. Here we provide the current state of knowledge and future research directions with regards to 10 questions about mixed-forest functioning and management identified and selected by a range of European forest managers during an extensive participatory process. The set of 10 questions were the highest ranked questions from an online prioritization exercise involving 168 managers from 22 different European countries. In general, the topics of major concern for forest managers coincided with the ones that are at the heart of most research projects. They covered important issues related to the management of mixed forests and the role of mixtures for the stability of forests faced with environmental changes and the provision of ecosystem services to society. Our analysis showed that the current scientific knowledge about these questions was rather variable and particularly low for those related to the management of mixed forests over time and the associated costs. We also found that whereas most research projects have sought to evaluate whether mixed forests are more stable or provide more goods and services than monocultures, there is still little information on the underlying mechanisms and trade-offs behind these effects. Similarly, we identified a lack of knowledge on the spatio-temporal scales at which the effects of mixtures on the resistance and adaptability to environmental changes are operating. Our analysis may help researchers to identify what knowledge needs to be better transferred and to better design future research initiatives meeting practitioner’s concerns.
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Trees have long life cycles and become reproductively active only after several years. The progress of tree breeding programs is therefore strongly dependent on the time needed to complete a breeding generation. Additionally, the uncertainties associated with conducting decade-long breeding programs can be high. The convergence of genomics and quantitative genetics has now established the paradigm of genomic selection as a way to accelerate breeding of complex traits. With the progressive accumulation of GS data for thousands of individuals across several unrelated populations, GS should also provide a potentially powerful framework to investigate the molecular underpinnings of complex traits. Genomic selection can increase the rate of genetic gain per unit time of a tree breeding program by radically reducing the generation interval and by increasing the selection intensity because many more young seedlings can be genotyped and their phenotypes predicted than the number of adult trees measured in field trials. Genomic selection has therefore become a hot topic in the tree genetics and breeding community worldwide in the last few years since the first perspectives based on simulations and experimental results were reported. In this chapter, a comprehensive discussion is presented, covering the main factors, both theoretical and practical, relevant to the application of GS to tree breeding, including those that have emerged from the recent flow of experimental studies in different forest tree species. Following a review of the basic insights and perspectives of GS, a detailed compilation is presented of all published experimental GS studies in forest trees to date, highlighting their main contributions to our current understanding of this new breeding approach. The conclusion summarizes the main lessons learned so far, condensed in a nine-point tentative roadmap for implementing GS in a tree breeding program.
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Mixed species stands are on the advance in Central Europe and many recently published studies have reported that they can overyield monocultures in terms of volume growth. However, as forest research has in the past been focused on monocultures, knowledge of how mixed-species stands and monocultures compare in terms of wood quality remains limited. Based on five triplets of fully stocked monocultures and mixed stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.), we analysed whether tree species mixing modifies wood quality and, more precisely, tree ring wood density.
Article
The area of land covered by forest and trees is an important indicator of environmental condition. This study presents and analyses results from the Global Forest Resources Assessment 2015 (FRA 2015) of the Food and Agriculture Organization of the United Nations. FRA 2015 was based on responses to surveys by individual countries using a common reporting framework, agreed definitions and reporting standards. Results indicated that total forest area declined by 3%, from 4128 M ha in 1990 to 3999 M ha in 2015. The annual rate of net forest loss halved from 7.3 M ha y−1 in the 1990s to 3.3 M ha y−1 between 2010 and 2015. Natural forest area declined from 3961 M ha to 3721 M ha between 1990 and 2015, while planted forest (including rubber plantations) increased from 168 M ha to 278 M ha. From 2010 to 2015, tropical forest area declined at a rate of 5.5 M ha y−1 – only 58% of the rate in the 1990s – while temperate forest area expanded at a rate of 2.2 M ha y−1. Boreal and sub-tropical forest areas showed little net change. Forest area expanded in Europe, North America, the Caribbean, East Asia, and Western-Central Asia, but declined in Central America, South America, South and Southeast Asia and all three regions in Africa. Analysis indicates that, between 1990 and 2015, 13 tropical countries may have either passed through their forest transitions from net forest loss to net forest expansion, or continued along the path of forest expansion that follows these transitions. Comparing FRA 2015 statistics with the findings of global and pan-tropical remote-sensing forest area surveys was challenging, due to differences in assessment periods, the definitions of forest and remote sensing methods. More investment in national and global forest monitoring is needed to provide better support for international initiatives to increase sustainable forest management and reduce forest loss, particularly in tropical countries.