Controls over tree mortality in the western United States: Untangling climate from stand structure
Severe droughts cause widespread tree mortality and decreased growth in forests across the globe. Forest managers are seeking strategies to increase forest resistance (minimizing negative impacts during the drought) and resilience (maximizing recovery rates following drought). Limited experimental evidence in managed forests suggests that forests with particular structural characteristics have greater capacity to resist change and or recover ecosystem function in the face of drought. However, the applicability of these results to practical forest conservation and management remains unclear. Here, we examined tree mortality in conifer forests across the western U.S. from USDA Forest Inventory and Analysis data and quantified the relative importance of stand structure (notably density) and weather fluctuations on mortality patterns. We examined mortality patterns in tree species that range from subalpine (Engelmann spruce, subalpine fir and lodgepole pine) to montane (Douglas-fir, ponderosa pine) to woodland (two-needle pinyon pine and Utah juniper).
Results indicate that both weather conditions and stand structure exert influence over mortality, and the relative importance of these two drivers changes from mesic to xeric forest types. Stand structure was the dominant influence for species in cool, wet, high elevation locations and weather dominated responses of species in hot, dry, low elevation areas. Placing these relationships in the context of climate change will help us understand vulnerabilities of tree species to increases in temperature and changing precipitation patterns. Additionally, these results illustrate several important interactions between weather variables and stand structure that suggesting that the effectiveness of management strategies for maximizing drought resistance will vary geographically. This work highlights the consistent and important control that stand structure exerts over drought impacts on tree mortality and suggests a framework for assessing the consequences of climate change on managed and unmanaged forests.