Understanding the role of biophysical setting in aspen persistence

Aspen communities are critically important for maintaining biodiversity, soil -quality, firebreaks, scenic beauty and nutrient cycling.However, widespread decline of aspen has been documented in much of the western United States.This loss of aspen has been attributed to fire exclusion, ungulate herbivory and climatic change.The role of biophysical factors in controlling aspen dynamics is poorly understood.In this study I quantify the relationship between aspen distribution, performance and landscape change with biophysical variables such as climate, topography, and soils.Specifically, I analyzed how aspen distribution, aboveground net primary productivity, and change in aspen cover over the past 50 years vary with respect to environmental gradients.I used classification and regression tree analysis to relate aspen distribution to biophysical variables.I collected aspen increment cores to calculate aspen aboveground net primary productivity.I interpreted aerial photographs over the past 50 years to determine landscape change in aspen cover.I used Akaike's Information Criterion to select multiple regression models relating aspen primary productivity and landscape change to biophysical variables.My findings show strong biophysical control over aspen distribution.I was able to explain 37% of the variation in aspen primary productivity across the Greater Yellowstone Ecosystem using biophysical variables.Additionally, I documented a 34% reduction in aspen cover between 1955 and 2001.I was able to explain between 13% and 43% of the variation in aspen change using biophysical setting.My models of aspen distribution predict aspen presence in more areas than it currently occupies.I propose that aspen distribution in the Greater Yellowstone Ecosystem is restricted such that it does not currently occupy the full range of its abiotic tolerances.Aspens' range may have been constricted by limited seedling recruitment since the last glaciation, past and present fire regimes, competition with conifer, and ungulate herbivory.Aspen in this region appears to be restricted to a small proportion of its abiotic niche such that primary productivity is not optimized in current locations.As a result, aspen in this region may be susceptible to factors such as fire exclusion, competition, and herbivory.