Exploring drought controls on spring phenology

Abstract

The timing of spring phenology can be influenced by several drivers. Many studies have shown the effect of temperature on spring vegetation growth, but the role of moisture is complex and not as well researched. We explored drivers for aspen spring phenology in the mountains of the western U.S. While temperature exerted control over the timing of aspen green-up in the spring, snow moisture as measured by April 1 snow water equivalent (SWE), played a significant role especially in southern locations bordering the Colorado Plateau. Maximum spring temperatures (March-May) were significantly (p<0.01) correlated with the start of the growing season across the entire Wasatch and Uinta Mountains and most of the western and northern Southern Rocky Mountains. Spring SWE was significantly (p<0.01) correlated with the growing season start across all of the Wasatch and Uinta Mountains and more than half of the Southern Rockies. The locations that experienced a larger snow influence, given by linear regression and R2 values, were located adjacent to the drier Colorado Plateau and south of 40oN latitude. Historical spatial patterns of regional snow accumulations and anomalies in the western U.S. have been chiefly explained by decadal antiphasing patterns across a north-south dipole. Anomalously low SWE co-occurs with a Pacific/North American teleconnection winter circulation associated with strong high pressure over the Pacific Northwest. The pattern shown in aspen phenology in this study, where the timing of spring green-up in the southern half of the intermountain West (south of a zone from 40oN - 42oN and mainly west of the continental divide in Colorado) showed higher sensitivity to winter snow moisture, and this spatial pattern was supported by other studies. Although winter moisture was not as consistent a factor as temperature in driving the start of the season, this study shows evidence that possible future winter drought could shift the growing season earlier than temperature increases alone.