Geographic patterns of upward shifts in treeline vegetation across western North America, 1984–2017

Abstract. Previous research has shown that (1) treelines are shifting upward in elevation on high mountain peaks worldwide, and (2) the rate of the upward shift appears to have increased markedly in recent decades. Because treeline elevational shift is a process manifested over broad scales of space and time, a particular challenge has been that of obtaining a broad-enough view of patterns of treeline shift to permit inferences about geographic and environmental patterns. What is more, intensive studies of treelines have been concentrated in North Temperate regions, such that little information is available about treeline shift patterns at lower latitudes. We have attempted to address this challenge by analyzing a long time series of vegetation indices derived from Landsat imagery obtained and analyzed via Google Earth Engine from the 1980s to the present. We sampled vegetation indices at points spaced every 100 m along 100 km transects radiating out from 115 high peaks across western North America (Canada to Central America); considerable data preparation was necessary, including ending transects <2 km into closed forest, identifying current treelines via reference to Google Earth imagery, and consideration only of up to <1 km above treeline. Patterns that emerged were—as is well known—that treelines are generally higher at lower latitudes, but—previously unknown—that the magnitude of treeline shifts is nonrandomly distributed with respect to latitude, longitude, and their interaction. This analysis, via a broad-scale view of treeline shifts over almost 40 years and a geographic span of more than 40° of latitude, suggests that climate change effects are most dramatic in tropical regions where few or no detailed treeline studies have been and are being conducted.