On a continental scale, a strong spatial correlation between precipitation and aboveground net primary production (ANPP) gives us great confidence that precipitation is the primary driver of ANPP in arid and semi-arid ecosystems, which occupy nearly 40% of the Earth's land surface.
However, as a series of comparable long-term data sets have become available in recent years, the relationship has not held up in a temporal context; annual precipitation accounts for only 20-40% of the interannual variability of biomass production. This failure suggests that researchers are not accounting for key mechanisms that control the ability of arid ecosystems to track fluctuations in precipitation. Yet increased fluctuations are a prominent feature of climate predictions in arid regions.
The proposed project puts forward four hypotheses to explain observed lags in ecosystem response to changing precipitation and tests them by altering patterns of total precipitation and precipitation variability, with and without nitrogen manipulation. These manipulations, together with the model analysis, will help determine the cause and magnitude of lags and legacies in the ecosystem response to precipitation. A thorough understanding of the relationship between precipitation and aboveground net primary production in these ecosystems is critical to understanding the global carbon cycle, and predicting changes with expected changes in climate, which include altered precipitation amount, increased temperature, and increased interannual variability.
The central objective of the proposed study is to elucidate the mechanisms of the indirect effects of water availability and climatic variability on ANPP. We hypothesize that most of the indirect mechanisms occur via changes in: (1) the structural characteristics of ecosystems (meristem density), (2) influences of drought on N availability, and/or (3) an asynchrony between above- and belowground- primary production.