Introduction to the special issue: Tree invasions: towards a better understanding of their complex evolutionary dynamics
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Introduction to the special issue: Tree invasions: towards a better understanding of their complex evolutionary dynamics
Heidi Hirsch 0
David M. Richardson 0
Johannes J. Le Roux 0
Associate Editor: J. Hall Cushman
0 Centre for Invasion Biology, Department of Botany & Zoology, Stellenbosch University , Matieland 7602 , South Africa
Many invasive plants show evidence of trait-based evolutionary change, but these remain largely unexplored for invasive trees. The increasing number of invasive trees and their tremendous impacts worldwide, however, illustrates the urgent need to bridge this knowledge gap to apply efficient management. Consequently, an interdisciplinary workshop, held in 2015 at Stellenbosch University in Stellenbosch, South Africa, brought together international researchers to discuss our understanding of evolutionary dynamics in invasive trees. The main outcome of this workshop is this Special Issue of AoB PLANTS. The collection of papers in this issue has helped to identify and assess the evolutionary mechanisms that are likely to influence tree invasions. It also facilitated expansion of the unified framework for biological invasions to incorporate key evolutionary processes. The papers cover a wide range of evolutionary mechanisms in tree genomes (adaptation), epigenomes (phenotypic plasticity) and their second genomes (mutualists), and show how such mechanisms can impact tree invasion processes and management. The special issue provides a comprehensive overview of the factors that promote and mitigate the invasive success of tree species in many parts of the world. It also shows that incorporating evolutionary concepts is crucial for understanding the complex drivers of tree invasions and has much potential to improve management. The contributions of the special issue also highlight many priorities for further work in the face of ever-increasing tree invasions; the complexity of this research needs calls for expanded interdisciplinary research collaborations.
Biological invasions; evolutionary mechanisms; rapid evolution; tree invasions
Plants introduced by humans to areas well outside their
native ranges must confront multiple selective barriers
that influence their capacity to become invasive
(Richardson et al. 2000b, 2011; Blackburn et al. 2011)
Many invasive plant species show evolutionary responses
to these selective pressures along the introduction–
naturalization–invasion (INI) continuum
, and this can result in phenotypic,
physiological or ecological divergence from source
populations in the native range
(Prentis et al. 2008;
Whitney and Gabler 2008; Turner et al. 2014)
invasion success of a species can further be influenced by
events in its pre-introduction evolutionary history which
have shaped its plasticity, adaptability, or which may
have led to pre-adaptations
. For example,
Guo et al. (2014)
illustrated that a combination of
preand post-introduction evolution has contributed to the
invasion success of Phragmites australis in North
(Guo et al. 2014)
Guo et al. (2014)
found that pre-adapted ecophysiological traits mediated
the invasion of introduced P. australis and that
postintroduction evolution in photosynthesis- and
growthrelated traits further benefitted its invasiveness. Most of
our knowledge about the role and dynamics of
evolutionary processes during plant invasions, however,
comes from studies on relatively short-lived species like
herbaceous annuals, while information on processes
affecting trees is much scarcer. This is attributed to the
long lifespans and generation times of trees which
makes inferences about evolutionary dynamics over
multiple generations difficult
(Zenni et al. 2017)
However, the rapidly increasing number of invasive tree
species and the escalation in the overall extent and types
of their impacts worldwide highlights their importance
as damaging invaders
(Richardson and Rejmanek 2011;
Rejmanek and Richardson 2013)
. There is increasing
evidence that invasive tree populations can exhibit
morphological or physiological traits, or ecological interactions
(e.g. symbiotic relationships), that differ substantially
from those in the native range
(Gundale et al. 2014;
Heberling et al. 2016)
. A long history of forestry studies
(e.g. provenance trials) has helped us to gain an initial
understanding of the potential evolutionary mechanisms
(e.g. standing genetic variation, hybridization,
adaptation) which can contribute to these changes
et al. 1985; Stettler and Bradshaw 1994; Hamrick 2004;
Zenni et al. 2017)
. However, forestry studies focus mainly
on tree improvement and not on the role of evolutionary
mechanisms that occur during progression along the INI
continuum when trees escape cultivation and potentially
become invasive. Consequently, it is crucial to expand
our knowledge of evolution in invasive trees, not only to
enhance our understanding of invasion ecology, but also
to guide management practices. Evolutionary studies on
tree invasions are also interesting to ecologists and
evolutionary biologists because they are unique natural
experiments that afford outstanding opportunities for
understanding evolution during invasions as well as
during general colonization processes
(e.g. see Richardson
et al. 2004; Gundale et al. 2014; Bock et al. 2015; Zenni
et al. 2016)
. Invasive trees have several key
characteristics that distinguish them from other invasive plants and
which can have implications for their evolutionary
• Their unique architecture, long life cycles and high
reproductive output can have a strong influence on
the mode and rate of evolutionary processes (Petit
and Hampe 2006).
• Introduction history. Most invasive tree species were
intentionally introduced for forestry, food production
and agroforestry purposes
(Richardson and Rejmanek
. Because of their varied uses, many species are
introduced in very large numbers and over large areas,
ensuring high propagule pressure and thus genetic
diversity; this often helps in overcoming selective
barriers and results in the fast-tracking of local adaptation
(Wilson et al. 2009; Donaldson et al. 2014)
• Artificial selection. Intentionally introduced tree
species are often subject to selection for desirable traits
like vigour, growth rates or high tolerance of certain
environmental conditions such as disease; these traits
are often associated with invasiveness
(Le Roux et al.
2011; Donaldson et al. 2014)
• Symbiotic interactions. Most tree species rely on
symbiotic interactions of which those involving roots and
rhizosphere soil microorganisms (e.g. nitrogen-fixing
rhizobia, mycorrhizal fungi) are especially important
(Richardson et al. 2000a)
. Such closely associated
microbial mutualists, also referred to as ‘second
(Zenni et al. 2017)
, can be crucial for plant
growth and health, and potentially act as drivers of
(Berg et al. 2014; Rosenberg and
• Introduced trees are often planted in monotypic
stands covering large geographical areas in their
nonnative ranges. On the one hand, this exposes them to
spatially heterogenic environmental conditions which
can promote local adaptation
(Richardson et al. 2004;
Zenni et al. 2014)
. On the other hand, such stands may
attract antagonists (i.e. pests and diseases) which may
induce natural selection
(Jactel et al. 2005)
Despite the unique circumstances and characteristics
outlined above that often underlie tree invasions, very
scant information is available in the primary literature on
their evolutionary dynamics compared with other taxa,
and generality is certainly lacking. A workshop held at
Stellenbosch University in Stellenbosch, South Africa, in
November 2015 brought together 25 researchers from
many parts of the world representing disciplines that are
rarely integrated: genomic evolution, plant–mutualist
interactions, pathology and invasion ecology. The aims
of the 2-day meeting were to synthesize and extend our
current knowledge on the evolutionary mechanisms in
tree genomes, epigenomes and their second genomes.
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The workshop resulted in a joint publication
(Zenni et al.
that emerged during the discussion sessions and
12 additional articles which cover a wide range of
evolutionary mechanisms in invasions of introduced trees.
Early versions of these articles were presented and
discussed during the workshop and now, after peer review
and revision, form this special issue of AoB PLANTS.
Key Insights from the Special Issue
Despite their significant impact on overcoming barriers
along the INI continuum, evolutionary mechanisms
were missing from frameworks used in conceptualizing
aspects of biological invasions.
Zenni et al. (2017)
to populate the unified framework for biological
invasions proposed by
Blackburn et al. (2011)
potential evolutionary mechanisms that can affect each of the
invasion stages and barriers for introduced trees. The
aim of this review was to evaluate how these
mechanisms impact, positively and/or negatively, tree invasions
along the INI continuum. The following evolutionary
mechanisms underlying tree invasions were identified
and integrated in the unified framework:
preintroduction evolutionary history; sampling effect; the
influence of founder effects, admixture, hybridization
and polyploidization on genetic diversity (hereafter
termed ‘standing genetic diversity’);
genotype-byenvironment interactions; rapid evolution; epigenetics
(phenotypic plasticity); and second genomes. Here, we
firstly summarized these evolutionary mechanisms and
their relative importance during different stages along
the INI continuum (Fig. 1), and secondly, introduce the
12 papers of the special issue by assigning them to the
applicable evolutionary mechanism identified and
described in detail by
Zenni et al. (2017)
Pre-introduction evolutionary history
Evolutionary history within its native range can
determine whether a species will become invasive when
introduced to a novel area
(Marsico et al. 2010)
Miller et al.
investigated such pre-introduction factors in tree
invasions by determining whether differences in the
introduction status (introduced, naturalized, invasive)
can be explained by relatedness (phylogeny) and
biogeography (native range size parameters). The authors
considered two genera of Australian trees, Acacia and
Eucalyptus (sensu lato)—groups which are interesting to
invasion ecologists for different reasons. For example,
almost all introduced acacia species that have been
widely planted have become highly invasive whereas
only a few eucalypt species are highly invasive. The
authors found that in both genera the introduction
status (introduced, naturalized and invasive) of species
showed no phylogenetic signal, except for introduced
acacias. Therefore, the placement of taxa along the INI
continuum appears to be phylogenetically random. On
the other hand, range size appears to increase in both
Acacia and Eucalyptus from introduced to naturalized to
invasive. Although the native range size for species of
both genera increases with invasiveness status
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with larger ranges are more likely to be invasive)
level of aggregation (in their native ranges), as measured
by the scale-occupancy curves, was different, decreasing
for Acacia (higher rate of spread), but increasing for
Eucalyptus (lower rate of spread) along the INI
Standing genetic diversity
Standing genetic diversity must be important during
the progression of non-native species along the INI
continuum as strong selection is expected under novel
(Bock et al. 2015)
Several processes can impact genetic diversity, either
by reducing it (i.e. founder effects) or by counteracting
reductions associated with introduction (i.e. admixture,
hybridization and polyploidization)
(Bock et al. 2015)
Two special issue contributions focused on such
evolutionary mechanisms that can rescue invasive tree
species from the potential negative effects of reduced
Besnard and Cuneo (2016)
reviewed the history and
ecology of invasive olives (Olea spp.). Two subspecies,
European olive (O. europaea ssp. europaea) and African
olive (O. europaea ssp. cuspidate), are a superb study
system for tree invasions due to their parallel invasions in
different climatic zones of Australia. The authors point
out that the two olive subspecies had different
introduction histories: multiple introductions for European olive
which helped to maintain high genetic diversity vs.
successive bottlenecks in the African olive which resulted in
reduced genetic diversity. Moreover, genetic admixture
between the subspecies occurred early after their
introduction into Australia. The review provides
comprehensive directions for future research on these invasive
olives and shows how such research can increase our
understanding of the genetic basis of tree invasions.
In his review,
assessed the prominence
and role of hybridization during invasion processes of
trees. He found evidence of 20 hybrid invasive tree taxa.
Importantly, in seven of these taxa researchers identified
phenotypes that make the hybrids better invaders than
their parental species. Also, all hybrid taxa involved
intentional introductions of either one or more parental
species, or the hybrid itself. This is the first review of
hybridization and its link with invasiveness to focus on
trees. It also highlights that hybridization events can
hamper management efforts because invasive hybrids
may lack co-evolved biological control agents. Such a
lack of control opportunities may lead to an unrestricted
spread of hybrid phenotypes which may be more
vigorous, and therefore of increased impact, than their
An extensive body of literature shows that non-native
plant species can undergo rapid evolutionary changes in
response to novel selection pressures in their new ranges
(Whitney and Gabler 2008; Buswell et al. 2010; Lee
. Numerous ecological hypotheses have been
postulated to underlie such contemporary evolution. For
example, the evolution of increased competitive ability
(EICA) hypothesis postulates that, due to the release of
co-evolved and specialist natural enemies during
introduction into the new range
(enemy release hypothesis;
Keane and Crawley 2002)
, introduced plants may
reallocate resources associated with enemy defence
towards higher investment in growth and reproduction
(Blossey and Notzold 1995; Felker-Quinn 2013)
Rapid evolutionary responses can consequently be
assumed to be most important for a non-native plant
during its establishment and spread phases, when new
selective pressures are most likely to be encountered
(Fig. 1). Three contributions to the special issue
addressed aspects of rapid evolution during the range
expansion of invasive trees by applying common
environment or in situ experiments.
Hirsch et al. (2016)
investigated shifts in seedling
growth performance between native and non-native
populations of the invasive elm, Ulmus pumila, under
different water and temperature conditions. Traits such as
seedling growth, associated with the early parts of a
plant’s life-cycle, can play an important role during the
colonization of new sites. Ulmus pumila was introduced
to several regions outside its native Asian range and has
become naturalized or invasive in most of these regions.
A greenhouse experiment simulated different
temperature and water stress treatments. Under all treatments,
non-native populations from Argentina and the western
United States produced more biomass and had
enhanced resource allocation to aboveground biomass
compared with native populations from Asia. The
authors conclude that the enhanced growth
performance may contribute to the invasion success of U. pumila,
at least in areas with low competition from native
species. The study shows that experimental approaches
focusing on life-history traits important in the early
stages of colonization (i.e. germination and seedling
growth) can enhance our understanding of tree invasion
A study by
Zenni et al. (2016)
demonstrated the role of
rapid evolution in the invasion success of the non-native
loblolly pine, Pinus taeda, in Brazil, where the species has
escaped from plantations. The authors found evidence
of rapid evolution in growth rates and a lack of trade-offs
between growth and defence traits. It can be assumed
that this contemporary evolution facilitated the ability of
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P. taeda populations to cope with the new environmental
Siemann et al. (2017)
tested the EICA hypothesis using
Triadica sebifera, a tree which is an aggressive invader of
different ecosystems in the southeastern United States.
The authors grew T. sebifera plants from native and
introduced populations with insect suppression treatments in
common gardens in three geographic venues that varied
in T. sebifera status (native, casual alien, invasive) and
insect herbivore communities. The results showed that
herbivore damage was highest in the native range.
Further, more rapid aboveground growth rates
contributed to T. sebifera’s success in both the invasive and native
ranges independent of aboveground herbivory. Together
with strong variation among sites this indicates that plants
from invasive populations may only have a strong
advantage in a subset of sites in their invasive ranges.
The loss, co-introduction or subsequent accumulation of
closely associated microbial communities can promote
or confine the invasion success of non-native plants
(Richardson et al. 2000a; Pringle et al. 2009; Gundale
et al. 2014)
. Dynamics in such symbiotic relations can
therefore be considered as most important during the
introduction stage (i.e. loss or co-introduction of
symbiotic partners) but also at later stages (i.e. accumulation
of new symbiotic partners) along the INI continuum
(Fig. 1). Five special issue contributions helped to
elucidate how such second-genome dynamics can impact
tree invasions. The first three contributions focus on the
role of mutualist associations during the invasion
process and the last two contributions highlight the role of
co-introduced or newly accumulated harmful
associations (i.e. pathogens and herbivores).
Dickie et al. (2016)
used two extensive databases of
plant-fungal sporocarp associations to understand how
the symbiotic ectomycorrhizal associations of native and
introduced trees in New Zealand and the United
Kingdom differed. These data indicated a restructuring
of interactions towards less modular networks in both
countries and that functional diversity in symbioses is
lower in introduced compared with native trees in New
Zealand. This extensive assessment of symbiotic
interaction network function and structure at national scales
shows how physiological function of trees may be
modified, not only by genomic shifts, but also by shifts in the
entire assembly of their associated second genomes.
Klock et al. (2016)
assessed whether the invasion of
Australian Acacia species in California was influenced by
their promiscuity with rhizobial symbionts. The authors
paired Acacia species with different introduction statuses
(introduced, naturalized, invasive) in California with soils
containing different rhizobial communities, and
examined whether invasive acacias could form more effective
symbioses with a greater diversity of rhizobial strains
than introduced and naturalized acacias. Contrary to
(Klock et al. 2015)
, the results showed
no differences in host-promiscuity among invasiveness
categories. A reason for this may be that all Acacia
species that were examined are invasive in at least one part
of the world (though they differ in invasiveness in
California) and are therefore promiscuous hosts.
In their study, also on Australian acacias,
et al. (2016
) examined the structure of native
legumerhizobium interaction networks and how these change in
response to invasions. For this the authors collected
acacias, native legumes, and their associated rhizobia along
a gradient of invasion in South Africa’s Cape Floristic
Region. Rhizobia were isolated from all legumes and
their identities determined using DNA barcoding.
Constructed interaction networks showed that invasive
acacias do not infiltrate existing native
legumerhizobium networks but that they form associations with
a unique subset of rhizobia that are not associated with
native legumes, i.e. strong modularity. These results are
in stark contrast to other types of mutualistic interaction
networks (e.g. seed dispersal and pollination) indicating
that invasive plants usually infiltrate networks through
Le Roux et al. (2016
that their findings may reflect co-invasion of legumes
and their rhizobia.
Burgess et al. (2016)
conducted a mini-review to
explore the different fungal associates which arrive with
non-native trees. The results show that invasive success
of trees can vary depending on the different categories
of fungal associates. Beneficial symbiotic fungi can assist
the establishment of their host trees, but may not form
novel associations with native trees in the new range. In
contrast, parasitic fungi could potentially reduce the
invasion success of their hosts, with the potential to
move onto new native hosts (hosts shifts). Although the
frequency of identified host shifts was low, and
depended upon the fungal guild, when such shifts occur
they can be devastating for native hosts.
To investigate the dimensions of the ecological
disequilibrium caused by the separation of non-native trees
and their natural enemies (pathogens and insects),
Crous et al. (2017)
conducted a review on
longestablished, non-native tree plantations in South Africa.
They assessed the accumulation of non-native
pathogens and insect pests onto planted Acacia, Eucalyptus
and Pinus species, but also examined native (South
African) pathogens and insects that utilized species in
these genera. Importantly, the phylogenetic relatedness
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between non-native and native floras appears to
influence the likelihood of pathogen shifts among them. This
was not the case for insects.
Crous et al. (2017)
concluded that when multiple tree species are introduced
one should expect considerable spatial and temporal
variation in ecological disequilibrium conditions among
taxa, a result that has implications for biosecurity and
other management practices.
Epigenetics and phenotypic plasticity
Epigenetics involves phenotypic variation due to variation in
gene expression that is not linked to variation in gene
sequences, but to other molecular mechanisms such as
DNA and RNA methylation and histone modification
. Epigenetic variation is thus tightly
associated with phenotypic plasticity and may be heritable over
(Duncan et al. 2014)
. For non-native
plants, such plastic responses can be crucial for overcoming
novel environmental conditions that are experienced,
especially when genetic diversity is low
(Rollins et al. 2013)
Along the INI continuum, epigenetic mechanisms (i.e.
phenotypic plasticity) are assumed to be most important
during establishment and range expansion when rapid
responses towards the new environmental conditions are
essential (Fig. 1). However, our knowledge of the role of
epigenetics mechanisms during plant invasion processes is still
very poor; in the case of tree invasions almost nothing is
known. Invasive tree species in which invasion success is
largely facilitated by phenotypic plasticity could provide
ideal study systems for bridging this knowledge gap.
Zimmermann et al. (2016) investigated phenotypic
responses in traits associated with the invasion of
Casuarina equisetifolia in sites with stressful environmental
conditions (i.e. high temperature, solar radiation, drought
and salinity) in Brazilian sandy coastal plains. They
postulated that a low level of phenotypic plasticity would be
beneficial in habitats with multiple stress factors. Considering
that the phenotype is the result of the integration of
characters in each environmental condition, phenotypic
integration (the pattern and magnitude of correlation among
different plant traits) may constrain phenotypic plasticity.
Casuarina equisetifolia exhibited high germination plasticity,
although plasticity in traits of seedlings was low. The
authors argue that the positive effect of phenotypic
integration on the plastic expression of morphological traits in
shade is a key factor that allows C. equisetifolia to invade in
the Brazilian sandy coastal plains.
Concluding Remarks and Future
The contributions in this Special Issue have helped us to
merge contemporary questions, experimental data, and
ideas from multiple perspectives to shed new light
on the evolutionary dynamics of invasive trees.
Moreover, the Special Issue highlights the multi-trophic
aspects of tree invasions, in that trees might not
arrive alone (e.g. endophytes), but even worse, in time
would attract an extensive array of non-native
organisms that may or may not become invasive. We have
learned that incorporating evolutionary concepts is
crucial for understanding the complex drivers of tree
invasions and that such information can improve
The papers summarized here have also identified
priorities for further work in the face of ever-increasing tree
invasions. For example, there is much scope to utilize
long-term forestry plantations which provide superb
study systems for improving our understanding of the
evolutionary mechanisms that enable many tree species
trees to escape cultivation and spread. Shorter-term
experiments, comparing native and non-native tree
populations under common environmental conditions, can
also shed light on potential evolutionary shifts in early
life-cycle traits that are important for the successful
colonization of new areas during the spread of an invasive
tree species. Several contributions to the Special Issue
show that second genomes are a crucial mediator of
invasive success in trees. Relatively few studies have
been conducted in this area, and such research definitely
needs to be intensified on several model systems to
obtain more empirical data on the role of second
genomes during tree invasions. We know of no studies
on epigenetic mechanisms during tree invasions
Bra€utigam et al. 2013; Guarino et al. 2015)
, and such
insights would greatly enhance our understanding of
phenotypic plasticity and why many non-native tree
species are such successful invaders despite having low
genetic diversity (the ‘genetic paradox’). The complexity
and diversity of the research needed to unravel such
issues calls for enhanced transversal collaborations
involving researchers from disciplines like invasion
ecology, microbiology, forestry, plant pathology, mycology
and plant genetics.
Sources of Funding
The workshop on ‘Evolutionary dynamics of tree
invasions’ was hosted by the DST-NRF Centre of Excellence
for Invasion Biology (C•I•B) in Stellenbosch, South Africa,
in November 2015. Funding was provided by the C•I•B,
Stellenbosch University (through the office of the Vice
Rector: Research, Innovation and Postgraduate Studies),
and the South African National Research Foundation
(grants 98182 to JLR and 85417 to DMR).
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Contribution by the Authors
H.H. led the writing but all authors contributed equal
editorial advice to this article.
Conflict of Interest Statement
We thank all the authors of papers in the Special Issue
for their collaboration and all the referees who reviewed
the manuscripts. Thanks are also due to the
Editor-inChief and editorial staff of AoB PLANTS for their support
during the completion of this Special Issue. We
acknowledge support from the DST-NRF Centre of Excellence for
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