Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2025
Publication Date: 5/1/2025
Citation: Tribitt, A., Porensky, L.M., Koerner, S.E., Komatsu, K.J., Reinhart, K.O., Wilcox, K. 2025. Browsing promotes drought resistance of Wyoming big sagebrush in a working rangeland. Rangeland Ecology and Management. 100:27-37. https://doi.org/10.1016/j.rama.2025.01.006.
DOI: https://doi.org/10.1016/j.rama.2025.01.006

Interpretive Summary: Droughts will likely become more extreme and more frequent in the future. To maintain healthy rangeland ecosystems, we need to understand how drought and different kinds of herbivory (grazing of grasses and browsing of shrubs) affect rangeland vegetation. In northeast Wyoming, USA, we created experimental droughts that ranged from mild to extreme. We then subjected vegetation experiencing different levels of drought to multiple levels of grazing (moderate, heavy) and browsing (ambient, heavy). We measured the effects of drought, grazing, browsing, and their interactions on the dominant shrub, Artemisia tridentata ssp. wyomingensis (Wyoming big sagebrush). We found that under ambient browsing intensity, extreme drought caused Wyoming big sagebrush canopies to shrink. Similarly, heavy browsing reduced shrub canopy size and leader survival. Interestingly, however, for shrubs that experienced heavy browsing, drought did not reduce canopy size and leader growth was actually greater with more extreme drought. This drought-resistance is likely driven by less leaf-scale water stress in droughted, heavily browsed shrubs compared with droughted, ambiently browsed shrubs. In browsed shrubs, lower early-season leaf area may lead to lower leaf-level water loss during drought. Reduced water stress was associated with more leader growth and higher canopy volume. This built-in mechanism for drought resistance in sagebrush highlights the importance of maintaining intact food webs. To promote sustainability of sagebrush and other shrub-dominated ecosystems in the face of increasing drought, it may be necessary to prioritize conservation of native herbivores within ecosystems.

Technical Abstract: Droughts are projected to become more extreme and more frequent throughout the remainder of the 21st century. As such, our ability to sustain rangeland functioning relies on understanding the impacts of extreme drought events on vegetation, and how these impacts interact with herbivory. Here, we report on an experiment in northeast Wyoming, USA that simulated five levels of drought intercepting 0%, 25%, 50%, 75%, or 99% of ambient rainfall for two years. These treatments were crossed with grazing intensity (moderate, heavy) and browsing intensity (ambient, heavy: ambient + 50% winter removal of leaders) treatments. We measured canopy volume changes, growth of unbrowsed leaders, and leaf water potential on the dominant shrub, Artemisia tridentata ssp. wyomingensis (Beetle & Young), to test three major predictions: (1) canopy volumes and leader growth would decline with greater drought magnitude, (2) heavy grazing would reduce the effects of drought magnitude, and (3) heavy browsing would exacerbate effects of drought magnitude. A key finding of this study is that, under ambient browsing intensity, extreme drought caused Wyoming big sagebrush (Beetle & Young) canopies to shrink. Interestingly, this effect went away and leader growth actually increased with drought magnitude in our heavy browsing treatment, despite overall negative effects of heavy browsing on shrub canopies and leader survival. We show that this drought-buffering effect is likely driven by less leaf-scale water stress in droughted, heavily browsed shrubs compared with droughted, ambiently browsed shrubs; this may result from lower early-season leaf area leading to lower transpiration-related water loss. Reduced water stress was associated with more leader growth and higher canopy volume. This built-in mechanism for drought resistance in sagebrush steppe highlights the importance of maintaining intact food webs, despite perceived deleterious effects of herbivores for plant abundance and growth. To promote sustainability of sagebrush and other shrub-dominated ecosystems in face of extreme precipitation change, it may be necessary for land managers and policy makers to prioritize conservation of native herbivores within ecosystems.