Climatic changes are altering Earth's hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity (NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes in NPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations between NPP and precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta-analytical techniques to search for generalities and asymmetries of aboveground NPP (ANPP) and belowground NPP (BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) of BNPP was similar under precipitation additions and reductions, but ANPP was more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven by ANPP responses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider how ANPP and BNPP responses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes.

Agricultural Food and Nutritional Science University of Alberta Edmonton AB Canada

Área de Biodiversidad y Conservación Departamento de Biología Geología Física y Química Inorgánica Escuela Superior de Ciencias Experimentales y Tecnología Universidad Rey Juan Carlos Móstoles Spain

College of Forestry Northwest A and F University Yangling China

CREAF Cerdanyola del Vallès Catalonia Spain

CSIC Global Ecology Unit CREAF CSIC UAB Bellaterra Catalonia Spain

Department of Biological Sciences University of Alberta Edmonton AB Canada

Department of Biology and Graduate Degree Program in Ecology Colorado State University Fort Collins CO USA

Department of Biology University of New Mexico Albuquerque NM USA

Department of Biology University of Texas Arlington TX USA

Department of BioSciences Rice University Houston TX USA

Department of Environmental Science and Management Humboldt State University Arcata CA USA

Department of Environmental Systems Science ETH Zurich Zurich Switzerland

Department of Integrative Biology Department of Microbiology and Molecular Genetics and Kellogg Biological Station Michigan State University Hickory Corners MI USA

Department of Integrative Biology University of South Florida Tampa FL USA

Department of Microbiology and Plant Biology University of Oklahoma Norman OK USA

Environment and Parks Government of Alberta Edmonton AB Canada

Facultad de Agronomía Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura Consejo Nacional de Investigaciones Científicas y Técnicas Universidad de Buenos Aires Buenos Aires Argentina

Global Change Research Institute Czech Academy of Sciences Brno Czech Republic

School of Life Sciences Arizona State University Tempe AZ USA

U S Department of Agriculture Agriculture Research Service Fort Collins CO USA

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