Climatic controls of aboveground net primary production in semi-arid grasslands along a latitudinal gradient portend low sensitivity to warming
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Although climate models forecast warmer temperatures with a high degree of certainty, precipitation is the primary driver of aboveground net primary production (ANPP) in most grasslands. Conversely, variations in temperature seldom are related to patterns of ANPP. Thus forecasting responses to warming is a challenge, and raises the question: how sensitive will grassland ANPP be to warming? We evaluated climate and multi-year ANPP data (67 years) from eight western US grasslands arrayed along mean annual temperature (MAT; ~7–14 °C) and mean annual precipitation (MAP; ~250–500 mm) gradients. We used regression and analysis of covariance to assess relationships between ANPP and temperature, as well as precipitation (annual and growing season) to evaluate temperature sensitivity of ANPP. We also related ANPP to the standardized precipitation evaporation index (SPEI), which combines precipitation and evapotranspiration to better represent moisture available for plant growth. Regression models indicated that variation in growing season temperature was negatively related to total and graminoid ANPP, but precipitation was a stronger predictor than temperature. Growing season temperature was also a significant parameter in more complex models, but again precipitation was consistently a stronger predictor of ANPP. Surprisingly, neither annual nor growing season SPEI were as strongly related to ANPP as precipitation. We conclude that forecasted warming likely will affect ANPP in these grasslands, but that predicting temperature effects from natural climatic gradients is difficult. This is because, unlike precipitation, warming effects can be positive or negative and moderated by shifts in the C3/C4 ratios of plant communities.
KeywordsClimate change Precipitation Temperature Standardized precipitation evaporation index Evapotranspiration
We thank Dr Ann Hess, Colorado State University, for her significant assistance with the statistical analysis. We also thank the technicians and scientists at the SGS and the Sevilleta LTER sites for collecting, managing and sharing ANPP data, and the scientists associated with the Prairie Heating and CO2 Enrichment (PHACE) project in Cheyenne, Wyoming. PHACE was supported by the US Department of Agriculture-Agricultural Research Service Climate Change, Soils and Emissions Program and the US National Science Foundation (NSF; DEB no. 1021559). Additional research support for this analysis came from the NSF Macrosystems Biology Program. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US government.
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