Mycorrhizal associations shape complex plant-soil interactions in the early seedling recruitment of Bornean rainforest trees
Keywords: Biology.

Biotic feedbacks between plants and soil fungi have been shown to influence seedling recruitment and plant community structure. This dissertation is principally concerned with how mycorrhizal associations influence such feedbacks as experienced by tropical tree seedlings, how soil fertility might change these interactions, and how the fungal communities that drive feedbacks assemble on seedling roots. While work in temperate forests suggests that there are consistent differences in plant-soil feedback between plants with arbuscular and ectomycorrhizal associations, it is unclear whether these differences exist in diverse tropical rainforests or how they may be impacted by heterogeneity in soil nutrients. In this dissertation, I address several current knowledge gaps by examining how plant-soil feedbacks operate in a diverse but ectomycorrhizal dominated rainforest in Northern Borneo. I conducted a shadehouse experiment (Chapters 1 & 2) using a design that allowed me to test whether the direction and strength of plant-soil feedback depends on a tree species' mycorrhizal association type, phylogenetically driven variation in the soil microbiota, and the fertility of the soil. I established a large scale multispecies shadehouse experiment in which the edaphic and biotic origins of soils were manipulated. I found that phylogenetically structured plant-soil feedbacks could influence seedling performance, but did so differently for ectomycorrhizal and arbuscular mycorrhizal seedlings (Chapter 1). Further, I found that soil resource availability changed the strength and direction of feedbacks for arbuscular mycorrhizal hosts but not ectomycorrhizal hosts. Using environmental DNA sequencing to characterize root-associated fungi in this experiment (Chapter 2), I showed that root-associated fungal communities differed at broad taxonomic scales across host mycorrhizal types but were similarly structured by host identity and soil type at finer scales. Further, I show that evolutionary relationships between seedlings and the overstory species that condition the soil can predictably affect root community structure, but that this effect differed depending on seedling mycorrhizal type, providing a possible mechanism by which plant-soil feedbacks is mediated. In my final chapter, I examine plant-soil feedbacks in a field experiment that focused on a single ectomycorrhizal seedling species. I explored how plant-soil feedbacks interact with density-dependent seedling mortality across the landscape in the field (Chapter 3). I found that seedlings of this species might weakly benefit from recruitment near conspecific adults and were likely not subject to conspecific negative density dependence typically thought to limit recruitment in tropical forests. I found that local scale density-dependence in seedling mortality did not vary with proximity to conspecific adults, suggesting that for this species the predominant effects on growth and mortality might be driven more by variation in the presence of beneficial soil biota than natural enemies. The results of this thesis collectively suggest that mycorrhizal associations of tropical trees play an important role in shaping how complex plant-soil interactions affect early seedling recruitment and, ultimately, tropical tree diversity. I found that species' mycorrhizal type shaped the feedbacks they experienced in a diverse Bornean rainforest, and also that mycorrhizal type could affect how the overstory canopy shapes root fungal communities. Further, the impact of overstory canopy composition on seedling growth can be dependent on both mycorrhizal type and edaphic heterogeneity. Keywords: tropical, seedling, mycorrhizal, ectomycorrhizal, arbuscular, fungal, plant-soil feedback, plant community, recruitment, diversity.