AoB PLANTS Special Issue: Evolutionary Dynamics of Tree Invasions

Invasion changes the landscape, but it also changes the invaders too. But can you study that when the invaders are trees with long life spans?

In 2015 the Centre of Excellence for Invasion Biology at Stellenbosch University held a workshop Evolutionary dynamics of tree invasions: drivers, dimensions and implications for management. The result is the AoB PLANTS Special Issue: Evolutionary Dynamics of Tree Invasions, edited by Heidi Hirsch, Johannes J. LeRoux, and David M. Richardson.

At the core is a simple, but under-researched puzzle. How do invasive tree species evolve when they move into new areas?. The Open Access papers in this issue go some way to tackling that gap in knowledge.

The Introduction to the special issue: Tree invasions: towards a better understanding of their complex evolutionary dynamics by Heidi Hirsch, David M. Richardson, and Johannes J. Le Roux, opens the issue by explaining why there needs to be particular study of trees as a special case. They point out that studying herbaceous annuals is a lot simpler because of the lifespans of the plants. This is true but the longevity of trees, plus the fact they get introduced around the world for a variety of purposes, means that trees are a problem that need to be researched. This is especially important when trees escape from a foresty context into the wild. They identify evolutionary mechanisms that contribute to whether or not a species will become invasive that other papers in the issue explore.

Evolutionary mechanisms involved in tree invasions and their changing importance during key invasion stages
Evolutionary mechanisms involved in tree invasions and their changing importance during key invasion stages (the thickness of horizontal bars indicates relative importance). For full details see Hirsch et al. 2017.

Miller et al. ask: Is invasion success of Australian trees mediated by their native biogeography, phylogenetic history, or both? The reason the question is important is that the invasiveness of a tree could be dependent upon the environment it is coming from. Invasions could therefore be spotted before they happened by identifying some species as having specific evolutionary histories that pre-dispose them to become invasive outside their range.

Gaskin looks at The role of hybridization in facilitating tree invasion. Studying Eucalyptus and Acacias, he notes: What we do not know about hybrid invasive trees limits our ability to control the invasion as a whole. Hybridisation brings in a variety of genes that can increase the genetic plants diversity of the invading plants. With that diversity comes the adaptability needed for invasion.

It’s not just things coming out of Australian that are a problem. Besnard and Cuneo take An ecological and evolutionary perspective on the parallel invasion of two cross-compatible trees by looking at parallel invasions of olives into Australia. They conclude: In the Australian context, there is an intrinsic cultural value placed on landscape identity, which is largely dominated by the eucalypt in its myriad of forms, either as a tall forest, lone paddock tree or distinctive silhouettes on a ridgeline. Understanding the biology and achieving effective control of woody invasive species such as European and African olives is about retaining ecosystem function, but also about retaining eucalypt woodlands—a core element of the Australian landscape identity.

Three papers examine rapid evolution of trees. Hirsch et al. find Non-native populations of an invasive tree outperform their native conspecifics. They conclude a shift in post-germination traits has likely occurred in non-native Ulmus pumila populations, giving the invaders a better start in life. Rapid increase in growth and productivity can aid invasions by a non-native tree say Zenni et al. who find climate has a key role to play in rapid evolution. Siemann et al. conduct An experimental test of the EICA hypothesis in multiple ranges: invasive populations outperform those from the native range independent of insect herbivore suppression.

Five papers tackle the importance of a second genome, associated with the microbial partners of invaders.

Dickie et al. see a Loss of functional diversity and network modularity in introduced plant–fungal symbioses. The mycorrhizal symbionts of trees are also in competition for resources, and these organisms, being introduced to a location, can be less effective at providing nutrients for plants than the native species.

It might seem that it would benefit an invading tree species to welcome any willing symbionts, even natives. However, Klock et al. argue Differential plant invasiveness is not always driven by host promiscuity with bacterial symbionts. They find that Acacias in California have established themselves to differing degrees, but that all are welcoming hosts for fungi.

Le Roux et al. examine The structure of legume–rhizobium interaction networks and their response to tree invasions Using phylogenetic and ecological interaction network approaches they provide the first report of the structure of belowground legume–rhizobium interaction networks and how they change along a gradient of invasion (uninvaded, semi invaded and heavily invaded sites) by Australian Acacia species in South Africa’s Cape Floristic Region. In stark contrast to aboveground interaction networks (e.g. pollination and seed dispersal) they show that invasive legumes do not infiltrate existing native legume–rhizobium networks but rather form novel modules. This absence of mutualist overlap between native and invasive legumes suggests the importance of co-invading rhizobium–acacia species complexes for Acacia invasion success, and argues against a ubiquitous role for the formation and evolutionary refinement of novel interactions.

This idea of bringing a partner along is further explored by Burgess et al. in Tree invasions and biosecurity: eco-evolutionary dynamics of hitchhiking fungi. They find when an invader tree relies on a fungal mutualism to survive in the new environment, there is a fundamentally lower likelihood of either the tree, or the fungus, establishing novel associations. In contrast, parasitic hitchhikers could merely use their host plants to move through the landscape and to become established on new hosts (host shifts). They conclude understanding the potential for hidden non-native fungi to form novel host associations in a new environment is important for biodiversity conservation.

Crous et al. also look at how Ecological disequilibrium drives insect pest and pathogen accumulation in non-native trees. What happens when tree shave been separated from their natural enemies (i.e. insects and pathogens). They find that native insects appear far more likely to expand their feeding habits onto non-native tree hosts than are native pathogens, although they are generally less damaging.

The final factors examined by the special issue are Epigenetics and phenotypic plasticity in the paper by Zimmermann et al. Experimental assessment of factors mediating the naturalization of a globally invasive tree on sandy coastal plains: a case study from Brazil. This study sought to identify traits that enable the alien tree Casuarina equisetifolia to overcome barriers to survival and reproductive and to become naturalized on sandy coastal plains. Long-term seed persistence in the soil, broad germination requirements (temperature and light conditions) and the capacity to survive in a wide range of light intensity favours its naturalization. However, C. equisetifolia did not tolerate water stress and deep shade, which limit its potential to become naturalized on sandy coastal plain.

Zenni et al. pull together the various topics into one review for the issue: Evolutionary dynamics of tree invasions: complementing the unified framework for biological invasions. They note the importance of evolutionary dymanics as an extensive body of research suggests that invasive populations often undergo phenotypic and ecological divergence from their native sources. Evolution also operates at different and distinct stages during the invasion process. It is important to incorporate evolutionary change into frameworks of biological invasions because it allows us to conceptualize how these processes may facilitate or hinder invasion success.

All the articles in the issue are Open-access. You can pick up Evolutionary Dynamics of Tree Invasions from AoB PLANTS.

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