Divergent drought responses in two cold desert shrublands

Abstract

The frequency and intensity of extreme droughts are projected to increase in the future, yet current research indicates that ecosystem sensitivity to extremes will vary. Despite hundreds of observational and experimental drought studies across ecosystems, the underlying mechanism explaining the inconsistent responses to drought remains poorly understood. Differences in the magnitude or duration of drought, land use history, existing plant communities, or unique site conditions might all influence the drought response of a site. Due to unresolved questions related to ecosystem sensitivity to drought, additional studies are needed to improve our predictions about the drought responses across ecosystems. Here, we minimized differences in drought intensity, climatic variation, and land use history by conducting a rainfall manipulation study in two adjacent (~60 m apart) plant communities with distinct dominant species (Artemisia arbuscula and Artemisia cana) in eastern Oregon, USA. We used rainfall shelters to create a 1-in-100-year drought at each site to understand how chronic (4-year) drought will impact common, but understudied sagebrush steppe plant communities. We found that above- and belowground net primary production at both sites was remarkably resistant to four years of drought. Interestingly, litter increased over time in drought plots at the more productive site, and we hypothesize that hydraulic lift and litter interacted to increase shallow soil water content under drought at that site. The site with harsher edaphic features and a soil duripan experienced fewer changes in plant community composition than the more productive site, potentially due to the presence of specialized plant species at the harsher site. Non-native plant cover increased (primarily from Ventenata dubia) and native forb cover and density decreased across our four sampling years. This suggests that management may be needed to conserve native forb diversity and limit species invasion, especially as the climate and historic fire regimes change.