Multi-factor long-term global change impacts on grassland

Abstract:

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. We followed ecosystem production of an annual grassland in California to all possible combinations of experimentally warming (+80 W/m²), added precipitation (+50%), elevated CO₂ (+300 ppm), nitrogen deposition (+7 g/m²), and fire disturbances over 17 years. We examined ecosystem-level net primary production (NPP) and its aboveground (ANPP) and belowground components (BNPP), by integrating both the temporal and experimental dimensions as a modulator of responses and as a way to transform treatments from categorical to continuous scales. We developed a model-based approach to investigate these high-dimensional spatial-temporal data.

Across the experiment, we found that main effects of the four long-term treatment factors were substantial, increasing or decreasing production by up to 20%. Temperature had negative effects; precipitation had a positive effect on BNPP, but a negative effect on ANPP; CO₂ had a positive effect on ANPP, a slight negative effect on BNPP; and nitrogen had positive effects. Relative to their main effects, most interactions among these four global change factors were small, particularly for ANPP and NPP, indicating that most responses to the global change factors were additive. As single factor effects, the 2003 wildfire and 2011 prescribed burn were similar, with strong positive effects on ecosystem productivity in the following growing season by up to 40%, especially for ANPP. All these multi-factor long-term ecosystem productivity results provide a starting point and a foundation to understand ecosystem performance in a wide range of future global change scenarios.