Region-wide patterns and drivers of demographic rates for a foundation tree species along its dry range margin

Forests are a dominant feature of many terrestrial landscapes throughout the world and have large influences on critical biogeochemical processes and a wide range of other ecosystems services. Unfortunately, many tree species have been reported to be in decline, and increasing temperatures and drought have been implicated as important drivers of this change. To gain a more comprehensive understanding of tree performance during a period of unprecedented anthropogenic climate change, we need to assemble and evaluate demographic data for tree species over longer time periods and across large expanses of their geographic ranges, which often vary substantially in climate and topography. Here, we focus on quaking aspen (Populus tremuloides), the most widespread tree species in North America, because it is recognized as a foundational species and has undergone decline in numerous parts of its range. We evaluated the patterns and drivers of recruitment, growth, and mortality of aspen along the more arid parts of its range within a network of 184 aspen-monitoring plots distributed across five states in the western U.S. during a 10–13-year study period. We found that the mortality rate for mature aspen stems was high (mean = 4.3% per year) across all four geographic regions in our study area: Sierra/Cascades, Great Basin, Middle Rockies, and Wasatch/Colorado Plateau. Consequently, the live basal area of aspen decreased by 2% per year while that of co-occurring conifer species collectively increased by 4.4% per year. We found aspen demographic rates did not vary significantly among the four geographic regions in our study area. Model comparisons revealed that initial stand structure was among the best-performing predictors of aspen performance. Stands composed of fewer, smaller mature stems were associated with faster growth rates, higher sapling density, and increased recruitment into the mature size class (≥ 12.7 cm diameter at breast height). Variables related to summer and annual water balance predicted several aspen response variables, indicating that warmer temperatures and/or drier conditions were associated with faster growth rates, higher sapling and immature aspen densities, and — surprisingly — less mortality. Finally, variables that are either influenced by or correlated with winter and early growing season temperatures were associated with less mortality, increased mature stem recruitment, and higher immature aspen densities. Collectively, our findings indicate that temperature influences demographic rates of all aspen size classes, warranting further research into how interactions among heat, water availability, and evaporative demand, as well as the annual timing of these factors, affect aspen performance. This study documents a decline of aspen populations over the past 10-13 years within its dry range margin that is pervasive across large spatial scales. Despite this result, aspen performance is highly variable at smaller spatial scales, indicating that much of the variation in aspen performance is driven by landscape-scale factors including stand structure, topography, and variations in local climate