In this century, increasing concentrations of carbon dioxide (CO2) and other greenhouse gases in the Earth's atmosphere are expected to cause warmer surface temperatures and changes in precipitation patterns. At the same time, reactive nitrogen is entering natural systems at unprecedented rates. These global environmental changes have consequences for the functioning of natural ecosystems, and responses of these systems may feed back to affect climate and atmospheric composition. Here, we report plant growth responses of an ecosystem exposed to factorial combinations of four expected global environmental changes. We exposed California grassland to elevated CO2, temperature, precipitation, and nitrogen deposition for five years. Root and shoot production did not respond to elevated CO2 or modest warming. Supplemental precipitation led to increases in shoot production and offsetting decreases in root production. Supplemental nitrate deposition increased total production by an average of 26%, primarily by stimulating shoot growth. Interactions among the main treatments were rare. Together, these results suggest that production in this grassland will respond minimally to changes in CO2 and winter precipitation, and to small amounts of warming. Increased nitrate deposition would have stronger effects on the grassland. Aside from this nitrate response, expectations that a changing atmosphere and climate would promote carbon storage by increasing plant growth appear unlikely to be realized in this system.

Global changes in climate, atmospheric composition, and pollutants are altering ecosystems and the goods and services they provide. Among approaches for predicting ecosystem responses, long-term observations and manipulative experiments can be powerful approaches for resolving single-factor and interactive effects of global changes on key metrics such as net primary production (NPP). Here we combine both approaches, developing multidimensional response surfaces for NPP based on the longest-running, best-replicated, most-multifactor global-change experiment at the ecosystem scale-a 17-y study of California grassland exposed to full-factorial warming, added precipitation, elevated CO 2 , and nitrogen deposition. Single-factor and interactive effects were not time-dependent, enabling us to analyze each year as a separate realization of the experiment and extract NPP as a continuous function of global-change factors. We found a ridge-shaped response surface in which NPP is humped (unimodal) in response to temperature and precipitation when CO 2 and nitrogen are ambient, with peak NPP rising under elevated CO 2 or nitrogen but also shifting to lower temperatures. Our results suggest that future climate change will push this ecosystem away from conditions that maximize NPP, but with large year-to-year variability.California grassland | climate change | ecosystem ecology | global change experiment | Jasper Ridge A cross the globe, terrestrial ecosystems are experiencing simultaneous changes in climate, atmospheric composition, pollutant deposition, and broad-scale changes in land use, biological invasions, and disturbance regimes, including wildfire. How ecosystems respond will have profound consequences for the goods and services they provide for humans, including feedbacks to atmospheric composition and climate. Predicting ecosystem responses to these global changes is challenging due to the number of factors and the possibility of interactive effects across physiological, ecological, and biogeochemical aspects of ecosystem function. Manipulative experiments, which can resolve singlefactor and interactive effects, can effectively address these challenges, especially when focused on metrics such as net primary production (NPP) that integrate across many levels of response. For example, experiments combining warming and elevated atmospheric CO 2 have mostly reported increases in NPP, with stimulatory effects of elevated CO 2 partially eroded by the effects of warming (1). Experiments combining either of these factors with other treatments have been diverse, with the magnitude of ecosystem responses generally decreasing as the number of global-change factors increases (2, 3). However, four difficulties complicate the interpretation of multifactor globalchange experiments: Few experiments incorporate a realistic range of interacting factors, making it difficult to connect experimental with real-world responses; most experiments involve only two levels of each factor (e.g., ambient and +2°C warming), making it difficult t...