Additive effects of simulated climate changes, elevated CO2, and nitrogen deposition on grassland diversity

Abstract

Biodiversity responses to ongoing climate and atmospheric changes will affect both ecosystem processes and the delivery of ecosystem goods and services. Combined effects of co-occurring global changes on diversity, however, are poorly understood. We examined plant diversity responses in a California annual grassland to manipulations of four global environmental changes, singly and in combination: elevated CO2, warming, precipitation, and nitrogen deposition. After 3 years, elevated CO2 and nitrogen deposition each reduced plant diversity, whereas elevated precipitation increased it and warming had no significant effect. Diversity responses to both single and combined global change treatments were driven overwhelmingly by gains and losses of forb species, which make up most of the native plant diversity in California grasslands. Diversity responses across treatments also showed no consistent relationship to net primary production responses, illustrating that the diversity effects of these environmental changes could not be explained simply by changes in productivity. In two- to four-way combinations, simulated global changes did not interact in any of their effects on diversity. Our results show that climate and atmospheric changes can rapidly alter biological diversity, with combined effects that, at least in some settings, are simple, additive combinations of single-factor effects.

Fig. 2.

Predicted and observed effects of combined global change treatments on forbs (F), annual grasses (A), perennial grasses (P), and total diversity (D). Predicted values are the summed effects of individual global change factors that make up each selected treatment combination. Points on the 1:1 line indicate perfectly additive (no interaction) responses to combined global change treatments. T, elevated temperature; C, elevated CO₂; P, elevated precipitation; N, nitrogen deposition.

Fig. 1.

Changes in total and forb diversity under single and combined global change treatments. Values are percent differences between control and elevated levels of each treatment. T, elevated temperature; C, elevated CO₂; P, elevated precipitation; N, nitrogen deposition. Table 1 provides means and significance levels.