Low-elevation forest extent in the western United States constrained by soil surface temperatures

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 km 2 (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. Soil surface temperatures constrain the low-elevation extent of forests in the western United States through their direct effects on seedling mortality, according to analyses of the relationship between post-fire tree recruitment and soil surface temperature across this region.