Hotter Drought as a Disturbance at Upper Treeline in the Southern Rocky Mountains

As we progress into the Anthropocene, rising temperatures have amplified evaporative demand and rendered heat-induced drought stress, or hotter drought, as the hallmark of climate change moving forward. It remains unknown, however, whether upper treeline environments have been affected. For this study, we grouped previously published and unpublished data from study sites within the southern Rocky Mountains by slope aspect to provide a possible baseline for what we expect for the Anthropocene. We returned to and resampled some of these study sites in the summer of 2019 after twelve years of sharply rising temperatures to measure patterns of seedling establishment. We also returned to a high-elevation site after seventeen years of warming to perform repeat photography in an attempt to capture visual evidence of threshold changes since 2002 in a location where little change had occurred during the twentieth century. Results from this research can be summarized into two main findings: (1) trends in recruitment over the past thirty years suggest that north-facing slopes are increasingly hospitable for successful seedling establishment compared to south-facing slopes at upper treeline and (2) spruce beetle–induced mortality is evident at upper treeline. Conceptually, this means that hotter drought could be progressively enveloping upper treeline along topoclimatic gradients. Unless ongoing trends in temperature deviate from expectations or unless precipitation increases considerably, there is little reason to justify the idea that upper treeline will continue to respond positively to hotter drought conditions during the Anthropocene, especially on south-facing slopes in the Northern Hemisphere.