The Local Neighborhood Interactions Shaping Tree Communities Through Ecological Disturbance and Environmental Change

My dissertation consists of four chapters focused on understanding the factors maintaining local species diversity, addressing a central goal of ecology increasingly critical for preserving ecosystem services and human well-being in the face of climate change. Specifically, I investigate how local neighborhoods – that is, the spatial arrangement, density, and identity of
nearby competitors – contribute to patterns of growth and survival along gradients of ecological disturbance and environmental change using forest tree communities as a study system.
In my first chapter, I develop a conceptual framework that synthesizes why we may expect local neighborhood interactions to weaken under ecological disturbances that primarily affect competitive densities or nutrient availability. This framework provides a foundation for understanding how disturbance-altered neighborhood interactions may influence forest recovery trajectories and community assembly.
For my second and third chapters, I focus on examining neighborhood interactions of two dominant conifer species in Pacific Northwest: P. menziesii (Douglas-fir) and T. heterophylla (Western hemlock) across life-stage and environmental condition. First, I examine how wildfires influence local neighborhood interactions between seedlings and surviving adults, revealing how wildfire disrupts neighborhood interactions that are otherwise thought to stabilize local populations in undisturbed ecosystems. Then, I use dendrochronological techniques to analyze how neighborhood interactions influence 60 years of climate-growth relationships in large, established adult trees. I demonstrate that interannual climate variability alters the strength and
direction of neighborhood interactions, with species-specific responses to temperature and precipitation that may shift competitive dynamics under future climate scenarios.
Finally, my fourth chapter investigates an important underlying driver of neighborhood interactions: the soil microbiome. I demonstrate that pathogenic fungi are more associated with tree community composition at low elevations, which have stronger stabilizing neighborhood interactions, whereas ectomycorrhizal fungi are more associated with tree community
composition at high elevations, which have weaker stabilizing neighborhood interactions.
Collectively, this dissertation advances our understanding of how stressors associated with environmental change and ecological disturbance alter the local interactions thought to contribute to maintaining diverse communities, with implications for predicting forest community responses to climate change and climate-altered disturbance regimes.