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 For this study, we grouped previously published and unpublished data from study sites within the southern Rocky Mountains by slope aspect to provide a possible baseline for what we expect for the Anthropocene.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.

随着我们进入人类世，温度升高加剧了蒸发需求，并导致了由热引起的干旱胁迫（或更热的干旱），这是气候变化的标志。但是，高层树线环境是否受到影响仍是未知的。在本研究中，我们根据坡度对落基山脉南部研究地点以前发布和未发布的数据进行了分组，从而为我们对人类世的期望提供了可能的基线。在经历了十二年急剧上升的温度以测量幼苗建立模式之后，我们于2019年夏季返回并重新采样了其中一些研究地点。经过十七年的变暖，我们还返回了一个高海拔地点，进行重复摄影，以期捕捉自2002年以来阈值变化的视觉证据，该位置在20世纪几乎没有变化。这项研究的结果可归纳为两个主要发现：（1）在过去三十年中的招聘趋势表明，与上林木线的南坡相比，北坡对成功建立苗木的热情越来越高；（2）云杉甲虫诱发的死亡率在上林分线上很明显。从概念上讲，这意味着更热的干旱可能会沿着地形气候梯度逐渐包围上层树木。除非持续的温度趋势偏离预期，或者除非降水量大大增加，否则，