Forest landscape dynamics: Consequences of interspecific variation in drought and shade tolerance

This thesis examines the patterns and dynamics of forest landscapes in the transition oak-northern hardwoods regions of central New England. Forest landscape patterns have long drawn the attention of ecologists seeking to understand how environmental heterogeneity influences the distribution and abundance of species. One of the most striking and well studied patterns of forest landscapes is species zonation along soil moisture gradients. Yet the underlying processes that give rise to zonation patterns are not fully understood because the distribution and abundance of tree species is determined both by their response to abiotic heterogeneity and by their competitive interactions with one another.^ This thesis presents novel approaches to assessing species responses to soil moisture and modeling the community- and landscape-level consequences of species differences in drought and shade tolerance. Chapter 1 examines the response of transplanted saplings to natural variation in soil moisture and the use of transplant experiments to assess the limits of species distributions. Chapter 2 examines the growth and mortality of naturally occurring saplings to quantify interspecific variation in drought and shade tolerance. Chapter 3 develops an individual-based model to simulate the competitive dynamics and distribution of tree species along soil moisture gradients. The patterns predicted by the model are also compared to observed patterns of forest composition using TM satellite imagery to classify extant forest canopy types.^ The results indicate that differences in drought tolerance among the study species are determined largely by interspecific variation in mortality rather than growth. Furthermore, the results suggest that there is not a strict trade-off between shade tolerance and drought tolerance. Finally, the simulation results show that interspecific variation in drought and shade tolerance can in part explain observed patterns in the distribution and abundance of species across the landscape.