COS 250-1 - Sonoran Desert rangeland plant community dynamics in response to changes in warm- and cool-season precipitation regimes.

Abstract: Precipitation in the Sonoran Desert is characterized by two distinct rainy seasons, both of which are being modified by anthropogenic climate change. Summer monsoon seasonal totals are remaining stable but rains are trending to fall in fewer, larger events with longer dry intervals, while winter/spring seasonal totals are declining. Understanding how these changes in precipitation regimes influence rangeland plant community dynamics is critical to adapting land management strategies as climate change intensifies. We hypothesized that less frequent but larger summer rainfall events would increase soil moisture at depth, which would benefit deeply rooted perennial plants more so than annual plants with generally shallower root systems; more frequent but smaller summer rains would have the opposite effect. We also hypothesized that winter drought and deluge treatments would influence vegetation dynamics in the spring and in the subsequent summer growing season. We tested these hypotheses in hydrologically-isolated plots beneath rainout shelters (100% interception) situated in a semiarid grassland in southeast Arizona. From 2020-2022, July-September seasonal totals were kept constant but rainwater irrigation treatments were temporally repackaged into 4 levels of event size/dry interval between events; in November 2021-April 2022, seasonal totals were manipulated into drought, average, and deluge treatments. Estimates of functional type vegetation cover and plant community species diversity were monitored monthly during the summer months and at the end of the cool rainy season each year. By the end of summer 2022, total vegetation cover was marginally highest in the large/infrequent summer precipitation treatment, but this treatment had substantially higher perennial grass and perennial forb cover compared to treatments with smaller and more frequent rains. Annual forb cover dramatically declined while annual grass cover dramatically increased in all summer treatments, which may be due to consecutive years of predictable rainfall despite summer temporal repackaging. The winter deluge treatment had the highest total cover in the spring and led to higher perennial grass and perennial forb cover the subsequent summer. Species diversity was lowest in the winter drought treatment both in the spring and in the subsequent summer. Forthcoming analyses will (1) test the interactive effect of summer precipitation temporal repackaging and adjusting winter precipitation seasonal totals on vegetation patterns and (2) determine whether soil moisture content at various depths under these precipitation treatment combinations provides a mechanistic understanding of these dynamics.