Over half of top tree species in decline in the western United States

Abstract

In recent decades, forests of the western US have experienced unprecedented change in climate and forest disturbance regimes, and widespread shifts in forest composition, structure, and function are expected in response. However, efforts to quantify the joint demographic response of tree populations to such stress have yet to be conducted across broad spatial domains in the region. Thus, uncertainty remains regarding the current status of tree populations (e.g., expanding, declining) and how multiple drivers may interact to influence their performance. Herein, we develop an index of tree population performance that is independent of forest composition, stand age, and ecological setting. We then apply this index to over 24,000 recently remeasured Forest Inventory and Analysis plots to quantify the population performance of the most abundant tree species across their ranges in the western US, identify large-scale shifts in the spatial distribution of each species, and determine the relative importance of forest disturbances and climate in shaping species population performance. Our results provide empirical evidence to suggest the most abundant tree species in the western US are exhibiting strong divergence in population performance. Range-wide population decline was evident in over half of species examined, with subalpine species exhibiting the highest rates of range-wide decline. Spatial variation in population performance was evident for each species, indicating large-scale shifts in species abundance distributions are already underway. However, we found such shifts to be generally inconsistent with upslope or northward migration expected due to recent warming. Our results further indicate that species decline can seldom be attributed to a single forest disturbance agent, though heightened severity of wildfire and insect outbreaks exert disproportionate control on the population performance of top western tree species. Overall, this work provides an early warning sign of broad-scale forest change and declining forest health across the western US.

Publication
Over half of top tree species in decline in the western United States