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Predicting when plant-soil feedbacks promote or prevent alien plant invasion.

Periodic Reporting for period 1 - PSF-2-PREDICT (Predicting when plant-soil feedbacks promote or prevent alien plant invasion.)

Reporting period: 2020-03-27 to 2022-03-26

Predicting which alien species will become invasive is key to mitigating their impact. It has been suggested that plant-soil feedbacks (PSF) between invasive plant species and soil microbial communities in the invaded range is one factor that can determine whether an alien plant species becomes invasive. Positive PSF (the alien plant species experiences enhanced performance due to soil microbial communities in the invaded range) could promote alien plant invasion but negative PSF (the alien plant species experienced reduced performance due to soil microbial communities in the invaded range) could help their control. However, a framework to predict which alien species will experience positive PSF, and under what conditions, is lacking. Such a framework is needed to provide a foundation for applied research on the impact of plant invasions on native plant communities, and how PSF knowledge might be used in ecosystem restoration worldwide (targeting EU Regulation 1143/2014 on Invasive Alien Species and Goal 15 of the UN Sustainable Development Goals).

I hypothesized that close relatedness of invasive with native plant species would be one factor that could help predict the outcome of PSF for invasions, but that whether these outcomes promoted or prevented invasion would depend on local context. Focusing on variation in nutrient availability (as context), I aimed to address three key research objectives: 1) quantify the strength of PSF under differing nutrient conditions; 2) characterise mutualistic associations in the roots; and 3) characterise the soil microbial communities they cultivate.

Due to COVID-19 and subsequent offer of a permanent position, the duration of the project was only 4 months and work was entirely desk-based. Consequently, I sought to examine the major themes of the project from a theoretical perspective and/or using existing data. Specifically, I aimed to:
1) Identify whether knowledge of species relatedness could help predict the outcome of PSF;
2) Explore other factors that influence the degree to which PSF outcomes could be predicted;
3) Examine the role of local context in influencing the outcome of PSF.
This work is still ongoing and conclusions so far are that:
1) Knowledge of species relatedness can only predict the outcome of PSF when species are either closely related or belong to key plant taxa known to show strong responses to soil mutualists.
2) PSF outcomes are highly variable within and among studies, and it remains unclear what drives this very high variability.
Revised Aims 1 and 2: In a desk-based study, colleagues and I examined the role of phylogenetic relatedness in determining whether PSF were positive or negative, and whether the outcome of PSF could be predicted from knowledge of plant species' relatedness. We found that: 1) PSF become increasingly unpredictable with increasing phylogenetic distance; 2) this unpredictability goes in both directions, with both more extreme positive and more extreme negative PSF experienced by distance relatives; and 3) this pattern was driven by a few plant families that appear strongly responsive to soil microbial communities.

These results highlight an important role of co-evolution between some plant families and soil microbes, and further emphasize the need to examine the role of context dependence in driving the outcome of PSF for plant species. This work was published in the journal New Phytologist (2020, 228: 1440–1449), presented in seminars at the Netherlands Institute for Ecology and the University of York, UK, tweeted about on twitter and written about on my personal website (

Revised Aim 3: In desk-based research I started during the fellowship and continue to work on, I am developing a review/ideas manuscript that dissects the different components of PSF and examines the context dependence of each of those components. To initiate discussion on these ideas I organised a short online meeting in 2020 with researchers in the Netherlands and New Zealand. These ideas will be used to predict how PSF might respond to extreme climatic events, and examine how knowledge of PSF could help plant community recovery from extreme events.
Research on plant-soil feedbacks has gained considerable momentum over recent years. Despite this, developing general rules as to how PSF might affect things such as biological invasions has been plagued by the apparent unpredictability of PSF outcomes for plant species and communities. Wandrag et al. 2020 drew attention to issues with existing PSF research focused on using phylogeny to predict the outcome of PSFs, and provided a potential explanation. We also demonstrated an analytical method to compare PSF results among studies. This work has the potential to enable further research exploring why PSF appear so unpredictable and synthesizing data across studies. These two things are vital for understanding the role of PSF in community ecology and ecosystem management.

By examining the role of context dependence in PSF, it is expected that the second manuscript, currently in preparation, will draw attention to the crucial role of PSF in managing ecosystem response to global change more generally. Together with Wandrag et al. 2020, these results will contribute to a road map for studying the role of PSFs in driving ecosystem change and managing ecosystem recovery from perturbation.