Multi-factor long-term global change impacts on grassland

Background/Question/Methods

Global change is intrinsically multi-factor, critically interacting with changes in the composition of the atmosphere, land use, nitrogen deposition, and the abundance of invasives. Global change also occurs against a background of ecosystem dynamics over the long term. Ecosystem responses to the multiple global change factors depend on species and community structures. We followed the aboveground biomass responses of an annual grassland in California to all possible combinations of experimentally warming (+80 W/m²), added precipitation (+50%), elevated CO₂ (+300 ppm), and nitrogen deposition (+7 g/m²) over 17 years. We integrated both the temporal and experimental dimensions as a modulator of responses and as a way to transform treatments from categorical to continuous scales. We used a model-based approach to investigate these high-dimensional spatial-temporal data for five main functional groups: annual grass, perennial grass, annual forb, nitrogen fixer, and perennial forb.

Results/Conclusions

We found that biomass strongly responded to global change factors not in total but in different functional groups, compensating each other to buffer global change impacts. For example, elevated CO₂ had no significant effect on total biomass (consistent with previous findings), but strikingly different effects on all functional groups. Across the multiple factors, main global change effects were stronger than interactions. The dominant annual grasses responded negatively to warming, positively to elevated CO₂ and nitrogen deposition; perennial grasses responded positively to warming and elevated CO₂ (both likely to increase in the future), negatively to added precipitation; annual forbs responded positively to warming and nitrogen deposition, negatively to elevated CO₂; nitrogen fixers responded positively to added precipitation, elevated CO₂, negatively to nitrogen deposition (contrasting with other groups); and perennial forbs responded negatively to elevated CO₂ and nitrogen deposition. These functional group responses point to important impacts on ecosystem processes and services. Future predictions suggest that climate change might increase perennials, pushing California grasslands back to the pre-European composition. Our integrated approach presents a new framework to better understand the relationship between experimental and broader global changes.