COS 109-4 - Mixed effects of rainfall manipulations on invasive annual grasses at two grazed Northern mixed-grass prairie sites

Background/Question/Methods

Invasion threatens ecosystem integrity, harming biodiversity and critical ecosystem services such as ecosystem resilience and stability. Grazing lands of Northern mixed-grass prairies are threatened by increasing drought severity and invasion by annual grasses. However, it is unclear if climate change will amplify the negative effects of plant invasions. We tested the effect of two years of water manipulation and one year of drought recovery paired with livestock grazing on Bromus tectorum and B. arvensis (brome) invasion in Montana and Wyoming. At each site, we established 3 blocks, each containing 3 grazing strategy paddocks (destock, stable, and heavy). Within each paddock, 6, 2x4m² plots (n=54 plots/site) correspond to 6 drought treatments (control x2, 25%, 50%, 75%, 99% reduced precipitation). We measured three plant properties of the invasive brome species (percent cover, aboveground primary production, and percent greenness of B. arvensis) and soil moisture in 2019-2020 (drought years) and 2021 (drought recovery year 1) from each plot. Of note, Wyoming experienced extreme wet conditions in 2019 and natural drought in 2020, possibly leading to smaller soil moisture effects of our drought treatments. Similarly, Montana experienced a natural drought in 2021 (drought recovery year).

Results/Conclusions

Overall, we saw greater drought effects on bromes in Montana compared to Wyoming. In Montana, drought decreased absolute brome productivity, increased relative brome production and B. arvensis relative abundance, and accelerated brome senescence. This means bromes might have a competitive advantage following drought over the native community, but their forage value may decrease earlier in the growing season. Conversely, in Wyoming, drought had a smaller effect on bromes, still decreasing absolute productivity, but also decreasing relative production and increasing time to senescence of B. arvensis, meaning bromes fared worse than the native community and forage quality of bromes will be maintained longer. The differences in ambient precipitation at each site likely led to these differential responses. Interestingly, the grazing treatments had little effect on invasion metrics. This is likely due to brome phenology and the timing of our grazing events late in the growing season. As winter annuals, bromes mature early in the season (mid-June) and may therefore be less affected by late season grazing. Overall, our results suggest that understanding the compounding effects of climate change and invasion, as well as the nuanced effects of different drought magnitudes, will be essential to maintaining the integrity of grazing rangelands.