Recent Variations in Low-Temperature and Moisture Constraints on Vegetation in the Southwestern U.S.
Dr. Jeremy Weiss, a senior researcher with UA’s Environmental Studies Laboratory, will discuss the importance of seasonality and elevational gradients for understanding the effects of drought and warming on vegetation in topographically complex regions like the Southwest, and explain how projected changes in future regional climate may potentially further or alter these effects.
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.
Weiss and his colleagues examined indices that represent climatic constraints on foliar growth for both drought periods and evaluated these indices for areas that experienced tree mortality during the 2000s drought. They found that relative to the 1950s drought, warmer conditions during the 2000s drought reduced cold temperature constraints at lower elevations in winter and higher elevations in summer. Warmer conditions also increased aridity (as measured by vapor pressure deficit) from early spring through late autumn. Increased vapor pressure deficits are extremely limiting to foliar growth.
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