COS 250-6 - CANCELLED - Invasion, precipitation seasonality, and precipitation legacies impact plant production in western US rangelands

Abstract: In western US rangelands, precipitation strongly limits plant production. Precipitation seasonality and legacies of the previous year’s precipitation can alter the relationship between annual precipitation and plant production, especially when communities contain functional groups that vary in water use patterns. For instance, shrubs are often better able to exploit dormant-season moisture than grasses and forbs, which tend to be more responsive to growing season precipitation. Similarly, perennials may respond more to last year’s precipitation—these plants may be more productive in a dry year that followed a wet year and may be less productive in a wet year that followed a dry year—than annuals. Finally, the relationship between production and precipitation may change due to shifts in community composition, as when shrubs and perennial grasses are replaced by invasive annual grasses and forbs. To evaluate how precipitation variation within and between years impacts production as community composition changes, we used two datasets from a long-term grazing experiment in Great Basin salt desert communities (Desert Experimental Range, Millard County, UT) that were collected before (1938–47) and after (2007–12) the onset of widespread invasion by annual species, especially Bromus tectorum, Halogeton glomeratus, and Salsola iberica. We first examined the impact of precipitation seasonality and past year’s precipitation on production of the most common functional groups at this site separately by time period, then examined whether the responsiveness of communities to precipitation changed over time. The previous year’s precipitation marginally impacted overall production in the earlier dataset (1938–47; P = 0.08) but had no impact on production in the recent dataset (2007–12; P = 0.45). Across all functional groups, production was generally more responsive to precipitation in the earlier dataset, indicating that the earlier time period had higher inter-annual variability in biomass production. This was especially evident in responsiveness to summer precipitation. In the recent period, only annual forbs responded significantly to summer precipitation, while in the earlier period all functional groups did. Together, these results highlight changes to the functioning of western US rangelands, which may result from widespread invasion by annual grasses and forbs. As invasions and precipitation variability increase—both within and between years—ecosystem functioning may change considerably.