Spring 2013 Water Seminar Series - "Recent Variations in Low-Temperature and Moisture Constraints on Vegetation in the Southwestern U.S."

Starting in the late 1970s, warming in the Southwest has produced fewer cool season freezes, losses in regional snowpack, earlier spring flowering and leafout, and hotter summers, all of which should affect vegetation differently across the region’s diverse climatic and biotic zones. Another potential impact of the ongoing regional warming is changes in how recent and future droughts affect vegetation. One way to examine the effects of drought and a warming climate on vegetation is to compare climatic controls on photosynthesis and transpiration during the major regional droughts of the 1950s and 2000s, periods of unusually dry conditions before and during the recent decades of warming. Here, we examine indices that represent climatic constraints on foliar growth for both drought periods and evaluate these indices for areas that experienced tree mortality during the 2000s drought. Relative to the 1950s drought, warmer conditions during the 2000s drought brought about fewer occurrences of temperatures too low for foliar growth at lower elevations in winter and higher elevations in summer, as well as higher vapor pressure deficits that were more limiting from spring through summer at lower and middle elevations. At many locations where tree mortality occurred during the 2000s drought, low-temperature constraints on foliar growth were extremely unlimiting, whereas vapor pressure deficit constraints were extremely limiting from early spring through late autumn. In addition to discussing how these results demonstrate the importance of seasonality and elevational gradients for understanding the effects of drought and warming on vegetation in topographically complex regions like the Southwest, we also explore how projected changes in future regional climate may potentially further or alter these effects.