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.

Understanding drivers of aboveground net primary production (ANPP) has long been a goal of ecology. Decades of investigation have shown total annual precipitation to be an important determinant of ANPP within and across ecosystems. Recently a few studies at individual sites have shown precipitation during specific seasons of the year can more effectively predict ANPP. Here we determined whether seasonal or total precipitation better predicted ANPP across a range of terrestrial ecosystems, from deserts to forests, using long‐term data from 36 plant communities. We also determined whether ANPP responses were dependent on ecosystem type or plant functional group. We found that seasonal precipitation generally explained ANPP better than total precipitation. Precipitation in multiple parts of the growing season often correlated with ANPP, but rarely interacted with each other. Surprisingly, the amount of variation explained by seasonal precipitation was not correlated with ecosystem type or plant functional group. Overall, examining seasonal precipitation can significantly improve ANPP predictions across a broad range of ecosystems and plant types, with implications for understanding current and future ANPP variation. Further work examining precipitation timing relative to species phenology may further improve our ability to predict ANPP, especially in response to climate change.

1. Models predict that the duration and intensity of drought will increase under future climate regimes. Although semi-arid grasslands are adapted to moisture limitation, the sensitivity of semiarid grassland vegetation to extreme and sustained dry periods that may occur in the future has not been tested. 2. In this study, we analysed vegetation cover data from an 11-year drought manipulation in the semi-arid shortgrass steppe in Colorado, USA. We examined changes in total vegetation cover, dominant plant (Bouteloua gracilis) cover and ruderal cover in plots receiving 25% and 50% of ambient precipitation during the growing season. 3. We found that 11 years of drought resulted in large reductions in total cover and cover of the dominant species, but significant differences did not emerge until the 4th and 7th year of drought. Drought severity significantly influenced the degree to which ruderal cover increased in the later period of the drought, but reductions in B. gracilis cover under drought were not significantly different under different drought severities. Bouteloua gracilis reduction did correspond strongly with increases in ruderal cover and species richness, suggesting that the dominant species plays a strong role in maintaining the structure of communities in the shortgrass steppe. 4. Synthesis. Our results suggest that the shortgrass steppe, and most notably the dominant species B. gracilis, can be significantly perturbed by long-term droughts that may be more common in future decades. By quantifying the resistance-limit of a drought-tolerant system, our results can inform theories of stability and management decisions for grassland systems under new climate regimes and provide a new temporal context for previous and future manipulation studies.

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Question Anthropogenic climate change is altering temperature and precipitation in grasslands worldwide. As grasslands are primarily water‐limited, these changes in climate will likely have dramatic impacts on ecosystem function and community structure, yet the magnitude of change and the types of species favoured may differ among ecosystems or even among local communities within ecosystems. How much might plant community responses to altered precipitation vary at two sites within one grassland region? Location Semi‐arid and sub‐humid natural grasslands in the US Great Plains. Methods At two sites we used rainfall shelters and irrigation to create irrigation, drought and control treatments. We measured changes in species composition, richness, Shannon's diversity, evenness and cover by plant functional groups across 4 yr (2008–2011). Results The semi‐arid grassland community was relatively insensitive to precipitation manipulations, and in the few cases where there was a significant relationship between treatment precipitation and the response variable, the slope of the relationship was weak. In contrast, the sub‐humid grassland community was very sensitive to changes in treatment precipitation throughout the experiment, and responded more strongly, and negatively, to drought compared to irrigation. Conclusions The differing sensitivity of the dominant species to altered precipitation amount appears to largely determine the overall differences in community response at these sites. The variable responses we observed within a single grassland region highlight the challenge of forecasting community impacts of climate change.