Can targeted cattle grazing increase abundance of forbs or arthropods in sage-grouse brood-rearing habitat?

Abstract

Suboptimal brood-rearing habitat often limits greater sage-grouse (Centrocercus urophasianus) populations in western North America. In mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana [Rydb.] Beetle) brood-rearing habitat, dense sagebrush canopy cover (> 25% cover) may limit the understory forbs and arthropods that sage-grouse prefer to eat during summer. Targeted cattle grazing is a potential habitat management tool to reduce dense sagebrush. This study evaluated targeted cattle grazing on a landscape scale to increase forbs and arthropods within core sage-grouse brood-rearing habitat. Objectives of this study were to evaluate how well targeted cattle grazing reduced mountain big sagebrush canopy cover, increased forb abundance and diversity, and increased arthropod abundance. Targeted cattle grazing was applied within two large, contiguous pastures (625 ha each) in the Beaverhead Mountains of southwestern Montana. The two pastures were grazed simultaneously at a light stocking rate (6.25 ha/AUM) for two weeks each year in mid-October 2015 and 2016. Vegetation and arthropod responses were measured during the following June-early July (2016 and 2017, respectively) within 32 microsites where mountain big sagebrush canopy cover exceeded 30% pre-treatment. Microsites were 0.008 ha to match the spatial scale at which sage-grouse broods select fine-scale habitat. One of two treatments was randomly assigned to each microsite: a low moisture block protein supplement or no supplement (unsupplemented). In the first summer after treatment, supplemented microsites averaged 78% less sagebrush canopy cover (8% vs. 36%; P < 0.001) and 25% more forb canopy cover (15% vs 12%; P = 0.122). Sage-grouse forb canopy cover was 50% greater in supplemented microsites (3% vs. 2%; P = 0.003). Forbs also comprised a greater proportion of the herbaceous understory in the supplemented vs. unsupplemented sites (48% vs. 36%; P = 0.002). Arthropod abundances were greater in supplemented than unsupplemented microsites only during the wet year of 2017 when arthropods were 1.7-1.8x more abundant in supplemented microsites. Sagebrush canopy cover was reduced by cattle trampling, not browsing. My results demonstrate that targeted cattle grazing can increase abundance of forbs and arthropods in mesic, dense stands of mountain big sagebrush in sage-grouse brood-rearing habitat.