LB 9-71 - Hierarchical responses to drought and drought recovery in two sagebrush steppe plant communities

The sagebrush steppe is one of the most extensive and diverse drylands in North America, yet much is still unknown about its response and recovery to drought. Moreover, the consequences of drought may not affect plant communities uniformly – shifts in community composition may differ among plant communities with distinct dominant species, even when exposed to similar drought conditions. The hierarchical-response framework suggests that ecological changes begin at an individual level, leading to species reorganization within the community, ending in species loss or immigration. Our study aimed to address how two plant community components – aboveground species and seed bank composition – respond to extended drought and drought recovery within two neighboring sagebrush steppe communities with differing dominant sagebrush species in OR, USA. We paired a 4-year drought manipulation experiment with greenhouse emergence studies to quantify species composition aboveground and within the seed bank. Specifically, we monitored aboveground species composition for the final two years of drought and first year of recovery and collected soil cores for greenhouse emergence studies after three years of drought and during the first year of recovery. We compared aboveground species composition from the last two years of drought to the first recovery year, seed bank species composition during the third year of drought to the first recovery year, and similarity of species composition between control and drought plots aboveground and in the seed bank during the third year of drought and first recovery year.Surprisingly, we found that drought did not alter aboveground species richness nor species diversity in either plant community. In contrast, drought decreased seed bank diversity at the site dominated by Artemisia arbuscula but didn't affect seed bank species richness nor total seed abundance at either site. Seed bank species richness and diversity didn't change at either site during recovery, but all plots had decreased seed abundance. Bray-Curtis similarity of aboveground species composition didn’t differ between control and drought treatments at either site during drought or recovery. Likewise, Bray-Curtis similarity of seed bank composition didn’t differ between treatments during drought, yet similarity decreased between treatments at both sites during recovery. These findings provide limited evidence that drought led to change at an individual scale (e.g., decreased number of germinable seeds and lasting shifts in similarity between drought and control seed banks) at both sites. Further research may reveal changes in species presence/absence or in plant functional groups during drought and recovery.