Climate variability is expected to increase with anthropogenic climate change. In grassland ecosystems, it is well known that annual net primary productivity (ANPP) is strongly controlled by annual precipitation. Patterns of species diversity can also be structured by interannual precipitation variability, with some species having higher relative abundances in dry years and others having higher relative abundances in wet years. In theory these compensatory dynamics may stabilize productivity over time, but little empirical evidence exists to indicate how species diversity shapes ecosystem responses to shifting precipitation regimes. To test this relationship, we compiled 10 datasets (10 years or longer) of species composition, rainfall and ANPP for grasslands sites across a US climate gradient. We calculated the variance ratio (community variance/ aggregated individual species variances) as a measure of compensatory dynamics. A variance ratio less than one indicates negative species covariance, suggesting compensatory dynamics. We hypothesized that: 1) compensatory dynamics are stronger in systems with high precipitation variability and 2) compensatory dynamics contribute to the stability of ANPP, particularly in sites with low annual precipitation.
Results/Conclusions
The degree of compensatory dynamics, as described by the variance ratio, varied widely across sites. Supporting our first hypothesis, greater interannual rainfall variability (the coefficient of variation (CV) of annual rainfall) was associated with greater compensatory dynamics (a lower variance ratio). Sites with a higher proportion of annual species exhibited greater compensatory dynamics, indicating that species life-histories constrain the degree to which community composition can respond to interannual variability. In keeping with prior studies, ANPP was strongly positively correlated with mean annual precipitation (MAP) and the variability of ANPP (the CV of ANPP ) decreased with MAP. Supporting our second hypothesis, the variance ratio explained significant residual variation in the relationship between CV of ANPP and MAP, and the relative importance of compensatory dynamics was higher in systems with variable ANPP and low rainfall (i.e. a negative interaction term between CV of ANPP and MAP with the variance ratio, respectively). Our findings indicate that compensatory dynamics may provide an important buffer for the stability of ANPP to increased climate variability, particularly in low-rainfall sites that contain a high proportion of annual species.