Abstract:

This thesis explores several aspects of forest dynamics, with an emphasis on the coterminous U.S. Classical ecological theory on species coexistence focuses on differences ('tradeoffs') between species, and typically ignores variation among individuals within species. Recently, it has been suggested that this intraspecific variation could have important impacts on community dynamics by blurring the differences between species. Chapter 1 examines this claim via a simple two-species competition model in which the performance (competitive ability) of each individual is considered to be a random draw from a species-specific probability distribution. Analysis of the model shows that while intraspecific variation can facilitate coexistence, this effect should be weak in most cases. Therefore, the emphasis of classical theory on interspecific tradeoffs appears justified. Understanding the nature of these tradeoffs at broad geographic scales requires novel approaches to data analysis. Forest inventories, such as the U.S. Forest Service's Forest Inventory and Analysis (FIA) program, provide a vast amount of data that could be used to study forest dynamics at broad scales. A limitation of inventory data, however, is that they lack measurements of individual-level resource availabilities (e.g., water, light, nutrients) that determine individual plant performances, and ultimately community dynamics. Chapter 2 develops novel methods to estimate parameters relating sapling growth to light for inventory plots with known growth rates but unknown light levels. These methods provide a way to link forest inventory data to forest dynamics models. One of the pressing questions related to forest dynamics in the context of climate change is the amount of carbon (C) stored in forests of different ages. Reviews in the forest ecology literature suggest that in many forests, biomass (and thus C storage) peaks and then declines as forests age. In Chapter 3, I use FIA data to show that for nearly all forest types in the U.S., biomass increases to an asymptote, rather than peaks and declines, as forests age. Therefore, there is no conflict between conservation goals aimed at protecting old-growth forests, and climate change policy aimed at C storage.