IMPACTS OF EXPERIMENTAL AND OBSERVED DROUGHT ON UPPER COLORADO RIVER BASIN DRYLANDS

More frequent, intense, and longer lasting drought conditions are now common across much of the Upper Colorado River Basin (UCRB), presumably altering plant communities and soil conditions in grassland and shrubland systems. To better understand the impacts of drying conditions on dryland plant-soil systems across the UCRB, we present findings from over ten-years of observational and experimental warming studies looking at how both episodic and periodic drought conditions might impact these landscapes. Generally, we see declines in perennial grass cover after only a couple years of persistent drought, while shrub cover remains stable until 5+ years of persistent drying trends. Additionally, decreases in vegetation cover can initially increase available soil nitrogen in medium severity drought conditions as soil nitrogen pools accumulate due to declining vegetation uptake. However, during severe drought years, it appears that soil microbial activity becomes water-limited, resulting in declines in soil fertility. Our work also explores how seasonal drought conditions can have long-lasting impacts on soil conditions, where drier winter or summer seasons can have legacy impacts for plant phenology and reductions in plant biomass in subsequent seasons and years. Given ongoing warming trends, it is also not clear if UCRB drylands will be able to recover from multiple year drought events, and which species might be more successful in a warmer and drier world. Ultimately, our work strives to better understand how increases in aridity might be pushing UCRB drylands ecosystems over a threshold where vegetation decline and soil nutrient cycling disruptions could impact ecosystem stability and resiliency.