Consequences of rainfall manipulations for invasive annual grasses vary across grazed northern mixed-grass prairie sites

Authors: FROST, MORGAN - University Of North Carolina Greensboro; KOMATSU, KIMBERLY - Smithsonian Environmental Research Center; Porensky, Lauren; Reinhart, Kurt; WILCOX, KEVIN - University Of Wyoming; KOERNER, SALLY - University Of North Carolina Greensboro

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2023
Publication Date: 6/28/2023
Citation: Frost, M.D., Komatsu, K.J., Porensky, L.M., Reinhart, K.O., Wilcox, K.R., Koerner, S.E. 2023. Consequences of rainfall manipulations for invasive annual grasses vary across grazed northern mixed-grass prairie sites. Rangeland Ecology and Management. 90:1-12. https://doi.org/10.1016/j.rama.2023.05.007.
DOI: https://doi.org/10.1016/j.rama.2023.05.007

Interpretive Summary: Drought and annual grass invasion are two major threats to the social, economic and ecological integrity of northern mixed-grass prairie rangelands. We used a manipulative experiment to better understand interactions among drought, annual grass invasion, and grazing management strategies. Specifically, we tested separate and combined effects of livestock grazing and experimental droughts on invasion by two annual brome grasses - cheatgrass (Bromus tectorum L.) and field brome (B. arvensis L.) - in both eastern Montana and eastern Wyoming, USA. To make results management-relevant, we created five precipitation levels representing a gradient of experimental drought, and we implemented grazing strategies selected by stakeholders to represent realistic management choices – destock, stable, and heavy grazing scenarios. We measured soil moisture and three plant properties of the invasive bromes (cover, production, and greenness) during two water manipulation years (2019, 2020) and one recovery year with natural rainfall (2021). We found that experimental droughts decreased brome biomass and induced earlier brown-down, or senescence. Interestingly, during the recovery year of 2021, bromes stayed green for longer in the formerly droughted plots, compared to plots that were not droughted in 2019 and 2020. In the first year after ending water treatments, Montana experienced a natural drought, and under these conditions we found that the combination of prior extreme drought and prior heavy grazing led to higher brome production than other combinations of drought and grazing. Unlike Montana, summer grazing had no strong effects on brome cover, production, or phenology in Wyoming. In general, summer grazing may not strongly impact bromes because these annual grasses mature early in the growing season (mid-June). Many results were site dependent, which may be due to inherent vegetation differences between our sites as well as site-scale differences in natural precipitation patterns. Together, our results suggest brome abundance may increase in the context of multi-year drought combined with heavy grazing, a more likely scenario with continuing climate change.

Technical Abstract: Northern mixed-grass prairie rangelands are threatened by increasing drought severity and invasion by annual grasses. However, it is unclear whether climate change will amplify or dampen annual grass invasion. We tested separate and combined effects of livestock grazing and experimental rainfall manipulation on invasion by two annual brome grasses - cheatgrass (Bromus tectorum L.) and field brome (Bromus arvensis L.) - in two mixed-grass prairie sites (Montana and Wyoming, USA). To provide management-relevant results, we manipulated precipitation at five levels representing a gradient of precipitation reduction and implemented grazing strategies selected by stakeholders to represent realistic management choices – destock, stable, and heavy grazing scenarios. We measured soil moisture and three plant properties of the invasive brome species (percent cover, aboveground primary production, and percent greenness) during two water manipulation years (2019, 2020) and one recovery year of natural rainfall (2021). Imposed precipitation reduction generally decreased absolute brome biomass and induced earlier senescence. However, during the recovery year, we saw a prolonged time to senescence in the formerly droughted plots. In Wyoming, summer grazing had no appreciable effect on brome cover, production, or phenology, perhaps because bromes mature early in the growing season (mid-June) and may therefore be less affected by summer grazing. However, in the first year after ending water treatments during a natural drought in Montana, we found that under heavy grazing, brome production marginally increased from 32.4 - 10.6 kg ha-1 to 130.8 -111.8 kg ha-1 (mean +/- standard error) with prior severe precipitation reduction. The magnitude of responses tended to be site dependent, which may be due to inherent vegetation differences between our sites as well as site-scale differences in natural precipitation patterns. Together, these results suggest brome abundance may increase in the context of drought combined with heavy grazing, a more likely scenario with continuing climate change.