Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Forestry and Environmental Studies

First Advisor

Comita, Liza

Abstract

How do so many species of trees coexist in a forest? A century of research has discovered multiple mechanisms that could maintain local-scale diversity. Chief among them, at least at early life stages, is a pattern of stronger neighborhood competition from conspecifics than heterospecifics. This pattern is driven, at least in part, by specialized natural enemies such as insects and pathogens that damage host trees more than others. Unsurprisingly, the strength of neighborhood effects varies widely among species and a current avenue of research involves understanding what explains this variation. A major axis of variation that has emerged among trees in temperate and sub-tropical forests is between trees that host either of two different guilds of symbiotic root fungi. Trees that host ectomycorrhizae (EcM fungi) in temperate and sub-tropical forests experience weaker neighborhood competition than co-occurring trees that host arbuscular mycorrhizae (AM fungi). However, similar information is lacking for most tropical forests, at least in part, because the most well-studied tropical parts of the world – Central and South America – have few or no trees that host EcM fungi. In my dissertation, I focus on a mixed dipterocarp forest in Sri Lanka to ask whether EcM-host trees of the family Dipterocarpaceae (or dipterocarps) differ from co-occurring AM-host trees in their neighborhood effects, and speculate on the consequences of our findings on the maintenance or erosion of diversity. In the first chapter, I test the effect of soils trained by adult trees and the effect of adult tree neighborhoods on seedling performance, using a plant-soil feedback shade house experiment and a field study of seedlings within known adult neighborhoods, respectively. Across both datasets, I tested whether EcM-host dipterocarp seedlings experience weaker effects of conspecific-trained soils or conspecific adult neighborhoods than AM-host tree seedlings. We found that the response of seedling growth and survival to neighborhood (conspecific or heterospecific) depended on mycorrhizal symbiont type (AM or EcM) in the field, but not in the shade house, suggesting that above-ground enemies or input from leaf litter drive neighborhood effects. AM host seedlings had lower growth and survival in conspecific vs. heterospecific neighborhoods in the field, but the opposite pattern was observed for growth in the shade house. In contrast, growth and survival of EcM host seedlings did not vary with neighborhood type in the field or shade house. These results support mycorrhizae-mediated neighborhood effects in tropical mixed dipterocarp forests. The ability of EcM-host dipterocarp species to tolerate conspecific neighbors likely contributes to their dominance, whereas negative effects of conspecific neighbors may allow AM host seedlings to persist in enemy-free space beneath heterospecific adults. In the second chapter, I used the same plant-soil feedback experiment from chapter 1 alongside a second plant-soil feedback experiment in the field to examine coexistence outcomes from interactions of pairs of species. I predicted that pairwise plant-soil feedback would be negative, and therefore stabilizing, because the selected tree species coexist in nature. I found statistically insignificant pairwise estimates for all except three pairs across both datasets. Pairwise plant-soil feedback in the shade house were positive on average while pairwise plant-soil feedbacks in the field were negative on average. Seven of ten species pairs in the shade house showed positive pairwise plant-soil feedback on average, potentially due to species-specific soil affinities cancelling out any negative effects of soil microbes. Four out of six species pairs in the field showed negative pairwise plant-soil feedback on average, suggesting the importance of above-ground and continual below-ground negative feedback in promoting coexistence. Our study underscores the importance of complementing shade house with field experiments in pairwise plant-soil feedback studies. In the third chapter, I used an expanded dataset of the field observation study from the first chapter to test whether EcM-host tree seedlings experience weaker effects of conspecific adult and seedling densities on survival as compared to AM-host tree seedlings, and whether differences the effects of conspecific neighbors of EcM-host versus AM-host seedlings are stronger for large compared to small seedlings. I found that survival of AM-host seedlings declined in response to total but not conspecific seedling neighbor density, and conspecific but not total adult neighbor density. EcM-host dipterocarp seedlings differed from AM-host seedlings in their neighborhood response: survival of EcM-host dipterocarp seedlings did not vary with conspecific or total neighbor density of adults or seedlings. Model coefficients for neighborhood density effects on survival were slightly stronger for large compared to small seedlings. These results corroborate the difference in neighborhood effects seen between EcM-host and AM-host species in other mixed mycorrhizal forests of the world while highlighting the primacy of adult over seedling conspecific neighbor density in mediating seedling survival and contributing to local-scale diversity in this forest. To summarize, my dissertation uses three lines evidence, two experiments and an observational study, to understand multiple overlapping aspects of mycorrhizae-mediated neighborhood effects in a mixed dipterocarp forest of south-west Sri Lanka. I find evidence for weaker effects of conspecific adults on EcM-host dipterocarps than co-occurring AM-host species (Chapter 1 and 3), but weak or no evidence that the pattern is driven by below-ground soil effects alone (Chapter 1). Any below-ground effects that vary by mycorrhizal type do not translate to stabilizing or destabilizing pairwise interactions: rather, mechanisms that foster coexistence regardless of mycorrhizal type appear to act in the field (Chapter 2). My results suggest that future studies should focus on mycorrhizae-mediated neighborhood effects from adult rather than seedling neighbors (Chapter 3), explore these effects under field rather than shade house conditions (Chapter 1), and assess the role of species’ traits and habitat affinities on plant neighborhood interactions. My study also addresses an important gap in our understanding of drivers of seedling growth and survival from mixed dipterocarp forests in general, and South Asian forests in particular.

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