Increasing precipitation event size increases aboveground net primary productivity in a semi-arid grassland
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Water availability is the primary constraint to aboveground net primary productivity (ANPP) in many terrestrial biomes, and it is an ecosystem driver that will be strongly altered by future climate change. Global circulation models predict a shift in precipitation patterns to growing season rainfall events that are larger in size but fewer in number. This “repackaging” of rainfall into large events with long intervening dry intervals could be particularly important in semi-arid grasslands because it is in marked contrast to the frequent but small events that have historically defined this ecosystem. We investigated the effect of more extreme rainfall patterns on ANPP via the use of rainout shelters and paired this experimental manipulation with an investigation of long-term data for ANPP and precipitation. Experimental plots (n = 15) received the long-term (30-year) mean growing season precipitation quantity; however, this amount was distributed as 12, six, or four events applied manually according to seasonal patterns for May–September. The long-term mean (1940–2005) number of rain events in this shortgrass steppe was 14 events, with a minimum of nine events in years of average precipitation. Thus, our experimental treatments pushed this system beyond its recent historical range of variability. Plots receiving fewer, but larger rain events had the highest rates of ANPP (184 ± 38 g m−2), compared to plots receiving more frequent rainfall (105 ± 24 g m−2). ANPP in all experimental plots was greater than long-term mean ANPP for this system (97 g m−2), which may be explained in part by the more even distribution of applied rain events. Soil moisture data indicated that larger events led to greater soil water content and likely permitted moisture penetration to deeper in the soil profile. These results indicate that semi-arid grasslands are capable of responding immediately and substantially to forecast shifts to more extreme precipitation patterns.
KeywordsGrasslands Climate change Precipitation variability Rain event size Pulse-reserve paradigm
Funding for this research was provided by the Shortgrass Steppe Long Term Ecological Research Program, Konza Prairie Long Term Ecological Research Program, the USDA Managed Ecosystems Program (NRI CREES), and an EPA Science To Achieve Results (STAR) fellowship awarded to J. L. Heisler White. Data for the long-term precipitation analysis were provided by the USDA-ARS CPER. For their generous help and assistance in rainout shelter construction, fieldwork, and execution of the precipitation manipulation experiments in this study, the authors would like to thank Mary Ashby, the CPER support crew, Mark Lindquist, the Shortgrass Steppe Field Crew, Priscilla Baker, and Vanessa Beauchamp. This manuscript was greatly improved by the comments and suggestions of three anonymous reviewers. EPA has not officially endorsed this publication and the views expressed herein may not reflect the views of the EPA.
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