Nonlinear, interacting responses to climate limit grassland production under global change

Abstract

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...

Microclimate–forage growth linkages across two strongly contrasting precipitation years in a Mediterranean catchment

“…Pitt and Heady (1978) found positive associations between air temperature indices and standing forage in both March and June at the relatively wet HREC site. However, these reported positive associations between annual rangeland growth and temperature are in direct contradiction to a climate change manipulation study in the San Francisco Bay Area Jasper Ridge site that includes warming, CO 2 enrichment, and N fertilization, where no effects of an approximate 1°C warming were observed (Dukes et al, 2005;Zhu, Chiariello, Tobeck, Fukami, & Field, 2016). Strong associations were found between counts of growing season degree days and standing forage at 11 range monitoring sites in California (R 2 from 0.75 to 0.95, George et al, 1988), in addition to associations with various seasonal precipitation totals and the lengths of midseason droughts (George et al, 1989).…”

Asymmetric responses of primary productivity to precipitation extremes: A synthesis of grassland precipitation manipulation experiments

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Ambient changes exceed treatment effects on plant species abundance in global change experiments

“…When analyzed individually, these experiments often yield idiosyncratic treatment effects (Zhu, Chiariello, Tobeck, Fukami, & Field, 2016) that can vary in space and though time. When analyzed individually, these experiments often yield idiosyncratic treatment effects (Zhu, Chiariello, Tobeck, Fukami, & Field, 2016) that can vary in space and though time.…”