Limits to upward movement of subalpine forests in a warming climate

Abstract

Rapid changes to the earth’s climate are well underway, already influencing biological systems across the globe. Among the key impacts are potential shifts in the geographic range of many species or ecosystems, often predicted to move either poleward or upward in response to warmer conditions (1, 2). In mountain biomes, as upper elevations become more climatically hospitable to tree establishment, upward shifts in tree cover could cause alpine ecosystems (nonforest vegetation above treeline) to become both reduced in extent and increasingly isolated into smaller “sky islands.” However, currently we know relatively little about the actual nature of such shifts in treeline, or to what extent a warming climate alone will generate them. In PNAS, Macias-Fauria and Johnson (3) make an important step in quantifying the relative influence of temperature regime and geomorphic factors in determining current and future treeline across a large Rocky Mountain landscape. The authors’ data suggest that upward treeline shifts in a warming climate may be heavily constrained by geologic factors that influence the availability of growing substrates at high elevations, leading to much less, or at least much slower, tree colonization into alpine areas than predicted by climate alone.