Drought and disturbance in dryland ecosystems: Synergistic effects on productivity in a multi-site field experiment

Samuel Jordan and Osvaldo E. Sala, School of Life Sciences, Arizona State University, Tempe, AZ, Sasha Reed, Southwest Biological Science Center, U.S. Geological Survey, Moab, UT, Steven Lee, Department of Biology, New Mexico State University, Las Cruces, NM

Background/Question/Methods
Intense droughts and human disturbances occur in many terrestrial ecosystems, but their direct and interactive effects on individuals, communities, and ecosystems remain largely unknown. Research in both disturbance and climate change ecology has a rich history, but there is a paucity of studies that combine both drivers. Using experimental manipulations to isolate the effects of drought, disturbance, and their interaction, we conducted a factorial field experiment (n=10 per treatment/site) in three dryland ecosystems from New Mexico and Nevada to Idaho to answer our guiding question: What are the aboveground ecosystem productivity responses to drought and disturbance? Do these drivers interact synergistically or antagonistically? Using non-destructive methods, we recorded aboveground net primary productivity and species identity for all species within our experimental manipulations. We utilized both parametric and non-parametric statistical frameworks to fit experimental outcomes within our research question.
Results/Conclusions
Drought and disturbance together reduced aboveground productivity across all three study systems, and the greatest negative synergism between the two drivers occurred at our most water-limited site. At that site, the interaction of drought and disturbance reduced ecosystem productivity by 75%.
Our experimental disturbance treatment reduced ecosystem productivity across all sites in both ambient (-17%) and experimentally reduced (-23%) precipitation regimes. There was a negative synergism between disturbance and drought conditions in two of our three sites, reducing aboveground productivity in plant communities beyond simple additive effects- we recorded a 64% reduction in productivity, whereas additive reductions would conceptually only amount to a 54% reduction. Drought alone consistently reduced productivity of two of the three major plant functional types in these systems: grasses (-38%) and forbs (-60%). In contrast, the third main functional type in our experiment, shrubs, had their productivity reduced primarily by disturbance in two of the three sites (-47%).
Our results suggest there is potential for negative synergisms between drought and disturbance in dryland ecosystems, and that these synergisms may be linked to long-term patterns of water availability. Thus, our findings support current knowledge of the sensitivity of productivity to precipitation in drylands, and offer new perspective into dryland ecosystem sensitivity to disturbance.