Andrew Eagar: mycorrhizal types influence local spatial structure in temperate forest tree communities

Andrew Eagar, a phd candidate at the Kent State University, presents his article “Dominant community mycorrhizal types influence local spatial structure between adult and juvenile temperate forest tree communities” where he studied plant-soil feedbacks at community level, and talks about his passion for research.

About the paper

At its core, our paper is about expanding our understanding of the drivers and consequences of plant-soil feedback in temperate hardwood forests. Throughout their life, trees influence their surrounding soil environment through mycorrhizal associations (a mutualism between plants roots and fungi) and leaf litter inputs. Sometimes, these interactions result in the build-up of pathogenic soil fungi that negatively affect offspring near their parent tree, creating negative feedback between trees and soil. This can prevent abundant species from competitively excluding rarer species and maintain diversity in forested ecosystems. Other times, these interactions create novel nutrient landscapes that benefit the offspring of trees and encourage recruitment near parent trees, creating positive feedback. This occurrence can cause dominant species to competitively exclude others and reduce diversity in forested ecosystems. It can even create forests dominated by a single species, as seen in some rainforests in the tropics.

While there is still much we don’t know about these complex interactions, recent work suggests that the mycorrhizal association of a given tree species – either arbuscular or ectomycorrhizal – may play an important role in structuring plant-soil feedback and might even be a useful predictor of feedback outcomes in hardwood forests. Our study takes a community level approach to testing this emerging framework of plant-soil feedback by using point pattern analyses to examine the spatial relationship between adult and juvenile trees in communities of varying mycorrhizal dominance. Our work here is unique in several ways. Plant-soil feedback is traditionally thought to act at the species level, with different species having little influence on one another. We expand on this by documenting what we refer to as “spill-over effects”; where dominant adult trees collectively influence juvenile recruitment throughout the entire community regardless of the species identity of those juveniles.  We’re also studying plant-soil feedback in the natural environment, outside of the greenhouse setting of many past plant-soil feedback experiments.

Our findings are pretty exciting. In our forest, communities dominated by ectomycorrhizal trees had juveniles growing in closer proximity to adults than we expected, while communities dominated by arbuscular mycorrhizal trees had fewer juveniles nearby adults. We did not see these patterns when we looked at our data at the level of individual species. In fact, we observed certain tree species alternating between positive and negative feedback depending on the mycorrhizal type of the surrounding community! These results indicate that feedback generated from dominant species can collectively spill over onto all species present in forested communities.

About the research

We hope our paper encourages others to expand beyond species-specific interactions and consider community interactions in plant-soil feedback research. Natural communities are complicated, messy, and fascinating. They present unique opportunities to study interactions and outcomes that broaden our knowledge about the natural world. Especially for trees, which can be difficult to study in greenhouse settings due to their size, longevity, and diversity. Questions about how plant-soil feedback can drive community turnover, and the consequences of this for ecosystem function, need to be explored in situ. For example, we know that soils underneath ectomycorrhizal tree species store more carbon and have lower rates of nitrification compared to soil underneath arbuscular mycorrhizal tree species. If negative plant-soil feedback drives community turnover, arbuscular mycorrhizal tree species may be replaced by ectomycorrhizal tree species. Turnover rates could then decrease due to the influence of positive plant-soil feedback among ectomycorrhizal species and potentially stabilize, ultimately increasing the amount of carbon a forest can store in its soil. It is my belief that studying the consequences of plant-soil feedback in communities across timescales longer than a handful of generations will be key to understanding the importance of these interactions.

About the Author

Right now, I’m working on several molecular sequencing projects exploring soil fungal communities and how mycorrhizal associations of nearby trees can affect the abundance of saprotrophs and pathogens in soil. I’m also finishing up a greenhouse experiment teasing apart the drivers of positive and negative plant-soil feedback, despite all my advocating for more community-based studies! This diversity of approaches we adopt in research is one of my favourite things about ecology. Being able to create something – be it an experiment, an idea about how the natural world works, or a paper someone else can read – is really fun. However, this need for creativity is also exhausting. Constantly creating the things we ecologists do takes a lot of time and effort. In the end, I think it is our love for science that keeps us creating.

Read the paper here

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