Mixed-species versus monocultures in plantation forestry:
Development, beneﬁts, ecosystem services and perspectives
for the future
Corsa Lok Ching Liu
, Oleksandra Kuchma
, Konstantin V. Krutovsky
Department of Forest Genetics and Forest Tree Breeding, Georg-August University of G€
ottingen, Büsgenweg 2, 37077, G€
Laboratory of Population Genetics, N. I. Vavilov Institute of General Genetics, Russian Academy of Sciences, 3 Gubkina Str., 119991,
Laboratory of Forest Genomics, Genome Research and Education Center, Siberian Federal University, 50a/2 Akademgorodok, 660036,
Department of Ecosystem Science and Management, Texas A&M University, 305 Horticulture and Forest Science Building, College
Station, TX, 77843-2138, USA
Received 2 April 2018
Received in revised form 26 July 2018
Accepted 26 July 2018
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 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 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.
©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€
E-mail address: email@example.com (K.V. Krutovsky).
Contents lists available at ScienceDirect
Global Ecology and Conservation
journal homepage: http://www.elsevier.com/locate/gecco
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.
Global Ecology and Conservation 15 (2018) e00419
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 beneﬁts 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 ﬁrst, 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 ﬁxing trees. Moreover, identiﬁcation of complementary traits is difﬁcult. Therefore,
in this review, combinations of species with complementary traits will be investigated for efﬁcient 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
deﬁned 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 efﬁcient 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 ﬂowers, 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 ﬁsh 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 signiﬁcant role in tree
breeding, environmental adaptation and improvement of meeting demands for goods and services (Larjavaara, 2008;Iveti
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 ﬁbre 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). Speciﬁcally, 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.,
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 desertiﬁcation (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, ﬁrewood, 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.
4.1. Development of monocultures
Many studies have identiﬁed 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,
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 beneﬁts 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 ﬁbre 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 efﬁcient 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 beneﬁts (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,
Fast-growing plantations by species, countries and mean annual increment. Data source: (Kanninen, 2010, p. 10).
Species Mean annual
Estimated extent as fast-growing
plantation only (1000 ha)
Main countries (in decreasing order of
Eucalyptus grandis and
various eucalypt hybrids
15e40 5e15 ±3700 Brazil, South Africa, Uruguay, India, Congo,
Other tropical eucalypts 10e20 5e10 ±1550 China, India, Thailand, Vietnam,
Temperate eucalypts 5e18 10e15 ±1900 Chile, Portugal, NW Spain, Argentina,
Uruguay, South Africa, Australia
Tropical acacias 15e30 7e10 ±1400 Indonesia, China, Malaysia, Vietnam, India,
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
C.L.C. Liu et al. / Global Ecology and Conservation 15 (2018) e004194
higher risks of adverse effects of storms and ﬁre, and negative impacts on biodiversity (Baltodano, 2000;Evans, 2001;Bowyer,
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 efﬁcient in trapping nutrients, because fewer roots exist near the surface, which may further lead
to signiﬁcant 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 speciﬁc substances that inhibit the growth of other plant species (Baltodano, 2000). There
are some concerns about depletion of soil moisture and reduced stream ﬂow 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 ﬂora 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).
5.1. 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 speciﬁcdeﬁnition 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 diversiﬁed 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 ﬁxing species in mixed-species plantations and
their effects on the regeneration of woody plants (Alem et al., 2015). It was shown that potential beneﬁts 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
beneﬁts (Hartley, 2002;Forrester et al., 2005;Plath et al., 2011;Pawson et al., 2013;Carnol et al., 2014;Alem et al., 2015;
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 stratiﬁcation 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-ﬁxing 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 efﬁciency, 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 efﬁcient in ﬁltering 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 ﬁve to seventy species for restoration of degraded lands (Kelty, 2006;Nguyen et al., 2014). For ecological beneﬁts, 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 ﬁres. Mixed
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-ﬁxing with non-nitrogen-ﬁxing
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
modiﬁcation are widely used in tree breeding (Moose and Mumm, 2008). MAS can greatly increase the efﬁciency and
effectiveness in plant breeding compared with traditional breeding methods. At ﬁrst, it requires the determination of DNA
markers that are tightly linked to important genes or QTLs of interest, and afterwards, breeders may use the speciﬁcDNA
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 Inﬁnium 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
In addition, the use of genetic modiﬁcation 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€
et al., 2016). Examples of agronomic traits, including enhanced herbicide resistance, enhanced resistance to pests, diseases
and abiotic stresses, modiﬁed lignin content and improved wood quality, could be attained by altering the expression of
speciﬁc 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 modiﬁed trees and crops.
Recently, several projects have been aimed at the identiﬁcation of genetic control of the complementary plant traits in the
mixed woody species plantations. For example, the IMPAC
project identiﬁes 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
,2014;Kuchma et al., 2017). Mixed-species
plantations of poplar and black locust used in this project is a typical example of mixing nitrogen-ﬁxing with non-
nitrogen-ﬁxing species assuming that poplar may gain nitrogen from black locust, and that regional ecosystem in associa-
tion with higher yields can also beneﬁt 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 beneﬁcial 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.,
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 difﬁculty 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
interspeciﬁc and intraspeciﬁc 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
beneﬁts 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 ﬁve 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 ﬂood 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 diversiﬁed 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
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 insufﬁcient number of studies on nutrition in mixed-species plantations, especially with non-
nitrogen ﬁxing 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 ﬁrst
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).
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 beneﬁts in biodiversity, economy, forest
health and occasionally in productivity compared with monospeciﬁc 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 beneﬁcial for both trees and ecosystems in
many regions under right conditions and appropriate management, and that new genomic tools should help us more efﬁ-
ciently analyse functional interactions between different species.
This work was supported by the IMPAC
project „Novel genotypes for mixed-species allow for IMProved sustainable land
use ACross arable land, grassland and woodland“funded 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 Pﬂanzenzüchtung in Anbausystemen, IPAS),
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