Precipitation-productivity relationships in desert grassland: a test of the double asymmetry hypothesis.

The purpose of this data package is to provide the derived data and R code for analyses presented in the manuscript by Collins et al. Knowing the relationship between precipitation (PPT) and aboveground net primary productivity (ANPP) is essential for understanding and modeling the global carbon cycle. Across grassland to forest gradients, the PPT-ANPP relationship is well-defined and non-linear. Temporal patterns within a site over time, however, are more variable than spatial patterns and nearly always linear. Linear relationships, however, are inconsistent with positive asymmetry occurring when the increase in ANPP in a wet year is greater than the decline in a dry year. The double asymmetry model predicts that concave down non-linearities will occur when extreme high and low PPT years are included in a time series. We used long-term ANPP data from ambient plots, plus rainfall addition and reduction experiments to test the predictions of the double asymmetry model. By combining experimental drought, plus water and nitrogen addition experiments we found some support for the double asymmetry model. However, the response was concave up not down under high precipitation coupled with nitrogen addition. By experimentally extending the range of monsoon precipitation we generated a significant although weak, non-linear PPT-ANPP relationship, but only when nutrient limitation was alleviated. Our results demonstrate that multiple interacting factors govern the PPT-ANPP relationship within a site over time.