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 C3 grasses were the most sensitive PFT to drought, the C3 shrubs were the most resistant, and the C4 grasses and shrubs had intermediate drought sensitivities. Although we expected the C3 grasses would have the greatest response to drought, the higher resistance of C3 shrubs relative to the C4 shrubs was contrary to our predictions based on the higher water use efficiency of C4 photosynthesis. Also, the additive effects of press- and pulse-droughts caused high morality in C3 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.
Plant functional types Extreme drought Ecosystem Community Thresholds
We would like to thank the many people who helped in establishing this experiment, especially Sue Phillips, Ed Grote, Matt van Soyoc, and Phil Adams. We would also like to thank Erika Geiger and Kristina Young for their help supervising the experiment and all of the field technicians for collecting the data. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government. We thank the USGS Climate and Land Use and Ecosystems programs for funding this effort.
Author contribution statements
DLH, MCD, and JB wrote the manuscript; DLH collected field data in 2014, compiled data for analysis, and completed the statistical analysis with input from MCD and JB; JB acquired financial support and established and supervised the field project.
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