Dissertation Defence: Associations between host mycorrhizal type and plant community structure within forest ecosystems

Corrina Kudla, supervised by Dr. Jason Pither, will defend their dissertation titled “Associations between host mycorrhizal type and plant community structure within forest ecosystems” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Biology.

An abstract for Corrina Kudla’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. Registration is not required for in person defences.

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ABSTRACT

Most plants form symbioses with mycorrhizal fungi, predominantly arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) fungi, which generally improve plant nutrient acquisition. Through a range of mechanisms, mycorrhizal host plants may benefit other plants that associate with the same type of fungi (“conmycorrhizal”). This may result in positive plant-soil feedbacks (PSF) and clustering among conmycorrhizal hosts. EcM fungi in particular may also protect against root pathogens, and this could help plant species overcome conspecific negative density dependence (CNDD), leading to competitive exclusion and reduced plant diversity. However, most studies of these hypotheses have focused on trees, despite most plant diversity residing in the herbaceous and shrub layers of forests. Additionally, many of these studies do not conduct appropriate null models and sensitivity analyses to test the robustness of their findings.

My goal was to examine whether patterns of whole-forest community structure are consistent with PSF and reduced CNDD in EcM hosts. Specifically, I ask whether conmycorrhizal host cover is correlated between forest layers (Chapter 2), how it compares to abiotic predictors of mycorrhizal host cover (Chapter 3), and whether plots with high EcM cover have lower evenness (Chapter 4) or richness (Chapter 5). Using 2,513 forest plots across the United States, I observed a significant positive correlation between AM host cover in the canopy and ground layer, and likewise for EcM hosts. I also found that the potential effect of mycorrhizal PSFs was often comparable in strength to that of climatic and edaphic drivers of host mycorrhizal type distributions. Lastly, I found a weak, negative effect of EcM cover on richness but not evenness.

My secondary objective was to explore how decisions made during the analysis impacted the results through sensitivity analyses, and I found that the results varied substantially in magnitude and direction. This is highly concerning as studies in this field use a variety of analytical decisions and usually do not conduct null models or sensitivity analyses, calling into question the robustness of their findings. Therefore, I argue that sensitivity analyses are vital when researchers are working with complex workflows, which are becoming increasingly common in mycorrhizal ecology.