A Dendrochronological Approach for Analyzing the Geographic Range Structure of Tree Species

The purpose of this dissertation research was to investigate the spatial patterns of abundance, growth, and stand structure across the geographic ranges of tree species using dendroecological methods. I assessed whether the biogeographic paradigms of the abundant center hypothesis and the principle of ecological amplitude adequately characterize spatial patterns of tree abundance, climate response, and stand composition. The abundant center hypothesis is a longstanding, yet rarely tested assumption that the centers of geographic ranges represent ideal conditions where species can achieve their greatest abundance, and abundance declines with increasing distance from the range center. A corollary to the abundant center hypothesis is the concept of ecological amplitude, which predicts that species will be subject to greater environmental stress near range margins, and thus, will be more sensitive to environmental variability and occupy restricted sites in peripheral locations.

To investigate ecological amplitude predictions regarding tree species of North America, I analyzed: (1) the abundance of red fir to directly test the abundant center hypothesis, (2) the response of longleaf pine growth to monthly climate variables at peripheral and interior sites, (3) the spatial pattern of annual growth sensitivity to climate in networks of tree-ring data for two widely-distributed species, and (4) the composition and structure of pine-oak stands at a central and a peripheral location within the ranges of several dominant tree species.

The analyses presented here demonstrate that the abundant center hypothesis and ecological amplitude principle do not accurately characterize spatial patterns of abundance, growth, or stand composition among North American tree species. Lack of support for the abundant center/ecological amplitude paradigm suggests that current models of forest change and species’ range dynamics should be reconsidered, and new models should be developed based on empirical analysis of range structure and dynamics.