Climate change and disturbance threaten forested ecosystems across the globe. Our ability to predict the future distribution of forests requires understanding the limiting factors for regeneration. Forest canopies buffer against near-surface air temperature and vapour pressure deficit extremes, and ongoing losses of forest canopy from disturbances such as wildfire can exacerbate climate constraints on natural regeneration. Here we combine experimental, empirical and simulation-based evidence to show that soil surface temperatures constrain the low-elevation extent of forests in the western United States. Simulated potential soil surface temperatures predict the position of the low-elevation forest treeline, exhibiting temperature thresholds consistent with field and laboratory studies. High-resolution historical and future surface temperature maps show that 107,000–238,000 km² (13–20%) of currently forested area exceeds the critical thermal threshold for forest regeneration and this area is projected to more than double by 2050. Soil surface temperature is an important physical control on seedling survival at low elevations that will likely be an increasing constraint on the extent of western United States forests as the climate warms.

气候变化和干扰威胁着全球的森林生态系统。我们预测森林未来分布的能力需要了解再生的限制因素。森林树冠可以缓冲近地表的空气温度和水汽压赤字的极端情况，而野火等干扰造成的森林树冠持续损失会加剧气候对自然再生的限制。在这里，我们结合了实验、经验和基于模拟的证据，表明土壤表面温度限制了美国西部森林的低海拔范围。模拟的潜在土壤表面温度可预测低海拔森林林线的位置，表现出与田间和实验室研究一致的温度阈值。高分辨率历史和未来地表温度地图显示，目前 107,000-238,000^平方公里 （13-20%） 的森林面积超过了森林再生的临界热阈值，预计到 2050 年，这一面积将增加一倍以上。土壤表面温度是低海拔幼苗存活的重要物理控制因素，随着气候变暖，土壤表面温度可能会对美国西部森林的范围产生越来越大的限制。