Pulse-drought atop press-drought: unexpected plant responses and implications for dryland ecosystems

In drylands, climate change is predicted to cause chronic reductions in water availability (press-droughts) through reduced precipitation and increased temperatures as well as increase the frequency and intensity of short-term extreme droughts (pulse-droughts). These changes in precipitation patterns may have profound ecosystem effects, depending on the sensitivities of the dominant plant functional types (PFTs). Here we present the responses of four Colorado Plateau PFTs to an experimentally imposed, 4-year, press-drought during which a natural pulse-drought occurred. Our objectives were to (1) identify the drought sensitivities of the PFTs, (2) assess the additive effects of the press- and pulse-drought, and (3) examine the interactive effects of soils and drought. Our results revealed that the C-3 grasses were the most sensitive PFT to drought, the C-3 shrubs were the most resistant, and the C-4 grasses and shrubs had intermediate drought sensitivities. Although we expected the C-3 grasses would have the greatest response to drought, the higher resistance of C-3 shrubs relative to the C-4 shrubs was contrary to our predictions based on the higher water use efficiency of C-4 photosynthesis. Also, the additive effects of press- and pulse-droughts caused high morality in C-3 grasses, which has large ecological and economic ramifications for this region. Furthermore, despite predictions based on the inverse texture hypothesis, we observed no interactive effects of soils with the drought treatment on cover or mortality. These results suggest that plant responses to droughts in drylands may differ from expectations and have large ecological effects if press- and pulse-droughts push species beyond physiological and mortality thresholds.