Abstract
Because forest stand structure, age, and productivity can mediate the impacts of climate on quaking aspen (Populus tremuloides) mortality, ignoring stand-scale factors limits inference on the drivers of recent sudden aspen decline. Using the proportion of aspen trees that were dead as an index of recent mortality at 841 forest inventory 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 index sensitivity to precipitation was highest in forests where aspen exhibited high densities, relative basal areas, quadratic mean diameters, and productivities, whereas sensitivity to vapor pressure deficit was highest in young forest 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.
Highlights
Episodic forest and tree declines arising from climatic stress may increase greatly as climates change, potentially reshaping forest landscapes
To better understand the role of climate and successional dynamics in predisposing forest stands to recent aspen declines, we examined the relationship of aspen snag densities with forest structure and climate in forests of the Rocky Mountains and Intermountain West of the United States (Rocky Mountain West hereafter; Appendix A: Fig. A1)
As a result of these interactions, the sensitivity of mortality index to decreasing precipitation and increasing vapor pressure deficit indicated that the climate effects differed among plots with divergent forest structure, productivity, and age (Fig. 2)
Summary
Episodic forest and tree declines arising from climatic stress may increase greatly as climates change, potentially reshaping forest landscapes. Stand characteristics impact tree mortality, such as increasing mortality with tree density as a result of inter- and intraspecific competition (Peet and Christensen 1987). Tree mortality, and forest decline, could arise from extrinsic (e.g., climate change) as well as intrinsic (e.g., competition) forcings. Because forest development, but not climate, can be modified by forest management, determining how forest structure mediates climate-induced declines could guide forest management in the face of climate change (Kashian et al 2007). Conspicuous and rapid quaking aspen mortality, sometimes called sudden aspen decline, is becoming increasingly common across western North America, impacting forest composition, function, and structure
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
