Title: Patterns, mechanisms, and implications of spatial variability in the ecological processes regulating nutrient access by forest trees
Description: The processes that regulate nutrient access by forest trees exhibit substantial variability on both large and small spatial scales. Explicit study of this spatial variability promotes a better understanding of the structure and function of forests. While the importance of space in ecological processes is being increasingly appreciated, there are major gaps in our knowledge about how space influences plant nutrient supply, particularly within a forest stand. This dissertation consists of three chapters that examine the patterns, drivers, and implications of spatial variation in three main processes that make nutrients available to trees: throughfall nutrient deposition, soil nutrient mineralization, and root system development. In Chapter 1, I use data from a field experiment to examine the effect of fertilization on nutrient transfer from the canopy to the soil via throughfall and litterfall in a tropical rainforest. I demonstrate that at small spatial scales, canopy density controls the flux of nutrients in throughfall, while at large scales, soil fertility is an important control, especially for phosphorus.

I also show throughfall can be as important as litterfall in the recycling of certain essential nutrients like potassium, and depending on soil fertility, phosphorus. In Chapter 2, I investigate the small scale spatial patterning in soil nitrogen, a nutrient that frequently limits tree growth, in a temperate forest. By quantifying the degree of spatial inequality and autocorrelation in two plots characterized by different dominant tree species, I show that soil extractable nitrogen pools and net nitrogen mineralization fluxes exhibit a high degree of spatial patterning at scales less than 5 meters, with a majority of nitrogen availability contained within hotspots comprising a small proportion of soil area. I also demonstrate that this spatial patterning affects seedling access to soil nitrogen, which has consequences for seedling growth and survival. Chapter 3 examines how tree species and tree size affect the spatial distributions of root systems in two temperate tree species and explores how differences in root spatial coverage could affect tree nutrient access.

I find that the spatial distributions of tree root systems can exhibit dramatic differences between species, with a tradeoff between root spatial coverage and total root length. I also discover that the effect of root spatial coverage on soil nutrient access is highly dependent on the spatial patterning of the soil nutrient, such that tree access to patchy nutrients varies greatly based on tree location within the local soil environment, even for medium-size trees. Together, these chapters characterize important patterns and mechanisms of spatial variation in the processes that regulate tree nutrient access.