Decadal drivers of quaking aspen decline across the Western United States

Although forests serve critical roles maintaining biodiversity and provisioning ecosystem services, many tree species world‐wide have experienced population declines due to climate change and other related factors. One such species is quaking aspen ( Populus tremuloides ), the most widely distributed tree species in North America, which has undergone reductions in survival, growth, and/or recruitment in many parts of its range.

To understand the patterns in and drivers of aspen performance across its range in six western US states, we evaluated the importance of 44 climatic, topographic, and forest structure variables in explaining mortality, growth and adult recruitment rates in 263 plots over a 9‐to‐13‐year period.

Aspen experienced substantial mortality during this study, with 4% of trees on average dying each year. In addition, aspen performed differently from co‐occurring conifers, with their dominance declining during the study while conifers increased. Mortality rates for aspen increased with higher aspen canopy dieback and dead aspen in the initial sampling and decreased with increased initial dominance of aspen and conifers as well as increased precipitation and reduced aridity. Growth rates of aspen increased with higher temperatures and winter precipitation, in more upland and south‐facing locations, and decreased with warmer and drier conditions relative to the historic baseline, greater initial aspen canopy dieback, higher initial dead aspen and greater initial dominance of aspen and conifers. Increased adult recruitment was associated with higher temperatures and winter precipitation as well as more upland and south‐facing locations, whereas reduced recruitment occurred with higher initial aspen and conifer dominance, greater amounts of initial dead aspen, and hotter and drier conditions relative to the historic baseline.

Synthesis . Across the western United States, the health and dominance of aspen declined during a decade‐long study, with annual mortality being three times greater than the recruitment of new adults. We frequently detected long‐lasting effects, where areas that were challenging or optimal for aspen at the outset of our study continued to be so over the next decade. Intra‐ and interspecific competition as well as hotter and drier conditions associated with climate change adversely influenced aspen populations.