Divergent response of aboveground net primary production to increasing precipitation variability in global drylands

Enqing Hou, Lifen Jiang and Yiqi Luo, Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, Marcy Litvak, Jennifer A. Rudgers, Scott Collins and William T. Pockman, Department of Biology, University of New Mexico, Albuquerque, NM, Dafeng Hui, Department of Biological Sciences, Tennessee State University, Nashville, TN, Shuli Niu, Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China

Background/Question/Methods
Interannual variability in precipitation has increased globally as climate warming intensifies. Increased variability can affect both plant production and terrestrial carbon sequestration; however, the underlying mechanisms are largely unknown. We examined the response of dryland aboveground net primary production (ANPP) to interannual precipitation variability using a combined approach of data synthesis and modeling. For data synthesis, we compiled a database of long-term (≥ 10 years) ANPP measurements from 45 globally distributed drylands with MAP < 500 mm yr-1, to explore the empirical relationship between ANPP and precipitation variability. For modeling analysis, we used a process-based Terrestrial ECOsystem (TECO) model to simulate ANPP under 8330 different precipitation change scenarios including changing precipitation mean and interannual variability individually and together. We compared model simulated response of ANPP to altered precipitation variability with the empirical relationship. We also extrapolated both the observed and the simulated effects of precipitation variability on ANPP to global drylands, to reveal the effect of precipitation variability on aboveground carbon across global drylands.

Results/Conclusions
Data synthesis showed that increasing interannual precipitation variability enhanced ANPP at low mean annual precipitation (MAP) but reduced ANPP at high MAP with a threshold at ~300 mm yr-1. Modelling analysis revealed a similar threshold for the effect of interannual precipitation variability on ANPP along a MAP gradient, with a peak of enhancement at 140 mm yr-1. Enhanced ANPP by increased interannual precipitation variability at low MAP was mainly attributable to more water available for plant growth via increased partitioning of precipitation into the subsoil. Reduced ANPP by increased interannual precipitation variability at high MAP was mainly caused by more water loss via runoff and leaching. The magnitude of the enhancement or reduction varied with both ecosystem properties (e.g., vegetation type and soil type) and mean annual temperature. By extrapolating these results to global drylands with MAP less than 500 mm yr-1, we estimated that ANPP would increase by 11.0 ± 4.2 Tg C yr-1 in arid and hyper-arid lands, which would more than offset the decrease by 1.5 ± 0.3 Tg C yr-1 in dry sub-humid lands under future changes in interannual precipitation variability. Thus, increasing precipitation variability may have net positive effects on net primary production in global drylands.