PS 6-53
Impacts of snow and rain change on native vs. invasive species and fire fuel properties in a sagebrush steppe ecosystem
Sagebrush steppe is one of the most widespread ecosystem types in the western U.S., yet also one of the most vulnerable to large-scale ecosystem conversion due to a positive feedback between the non-native species Bromus tectorum (cheatgrass) and fire. One of the most ubiquitous invasive species in the western U.S., B. tectorum rapidly colonizes bare ground amid native vegetation, completes an unusually early phenological cycle, and subsequently becomes a serious fire hazard. The continued rapid spread of B. tectorum throughout the West and into upper elevations is alarming and may be exacerbated by changing climatic conditions—especially altered precipitation regimes due to the strong influences of precipitation amount, type, and timing on semiarid ecosystem structure and function. To evaluate potential climate change impacts on the distribution of B. tectorum near its high-elevation range margin, this study examined effects of precipitation amount, type, and timing on B. tectorum and native herbaceous species. Experiments used snow fences to simulate changes in snowpack (increased, decreased, and ambient depth) and irrigation to simulate increased frequency and magnitude of spring and summer precipitation. Phenology, species composition, B. tectorum density, leaf area index, and instantaneous photosynthesis measurements were used to assess species responses to precipitation change.
Results/Conclusions
Winter precipitation (November 2012-March 2013) was ~53% of normal. In January 2013, snow fences increased snow depth by ~31% and decreased snow depth by ~7% relative to ambient snow depth, though this did not translate into significant differences in spring soil moisture content. Bromus tectorum phenology was advanced in the decreased snow depth zone relative to the increased snow depth zone, while native species had divergent phenological responses to snowpack manipulations: Elymus elymoides phenology did not differ by snow zone, while Achnatherum hymenoides showed an unexpected phenological advance in the increased snow depth zone. Species richness, percent cover, B. tectorum density, and leaf area index did not significantly differ by snow zone. Bromus tectorum photosynthetic responses (CO2 assimilation, stomatal conductance, quantum yield of Photosystem II, and electron transport rate) to spring rainfall simulations were more pronounced than those of native species, which exhibited fewer physiological responses to watering. Native species physiology did not significantly respond to summer rainfall simulations (after B. tectorum had senesced). Overall, results indicate that B. tectorum is more sensitive than native vegetation to precipitation change. Predicted shifts from snow to rain could facilitate B. tectorum expansion at high elevation depending on the timing of rainfall events.