Forest stand structure, productivity, and age mediate climatic effects on aspen decline
Conspicuous and rapid quaking aspen (Populus tremuloides Michx.) mortality, sometimes called sudden aspen decline, is becoming increasingly common across western North America, impacting forest composition, function, and structure. Both changes in forest structure during forest development and large-scale episodic declines often related to climate can contribute to sudden aspen decline. While forest stand structure, age, and productivity may mediate the impacts of climate on quaking aspen mortality, the relative importance of these factors, and their interactions, is uncertain. Using the proportion of aspen trees that were dead as an index of recent mortality at 841 US Forest Service Forest Inventory and Analysis plots, we examined the relationship of this mortality index to forest structure and climate in the Rocky Mountains and Intermountain Western United States.
We found that forest structure explained most of the patterns in mortality indices, but that variation in growing season vapor pressure deficit and winter precipitation over the last 20 years was important. Mortality was most sensitive to precipitation in aspen stands with high density, high relative basal area, high quadratic mean diameter, and high productivity, while mortality was most sensitivity to vapor pressure deficit in young stands. These results indicate that the effects of drought on mortality may be mediated by forest stand development, competition with encroaching conifers, and physiological vulnerabilities of large trees to drought. By examining mortality index responses to both forest structure and climate, we show that forest succession cannot be ignored in studies attempting to understand the causes and consequences of sudden aspen decline.