Grazing intensity effects on herbaceous community composition in burned sagebrush steppe

Abstract

There is limited knowledge of grazing impacts on longer term plant community dynamics following fire in sagebrush steppe. This study evaluated vegetation response to different intensities of deferred rotation cattle grazing over 16 years (2007–2022) on burned Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis (Beetle & Young) Welsh) steppe in eastern Oregon. Treatments were applied in a randomized complete block, which included no grazing on burned (nonuse, n = 5) and unburned (control, n = 5) steppe; and cattle grazing at low (low, n = 4), moderate (moderate, n = 4), and high (high, n = 4) intensities on burned steppe. Vegetation dynamics were evaluated by repeated measures analysis of canopy cover and density of shrub and herbaceous species and functional groups. Herbaceous functional groups were an early‐season bunchgrass (one species, Sandberg bluegrass [Poa secunda J. Presl]), tall perennial bunchgrasses, perennial forbs, annual grass (one species, cheatgrass [Bromus tectorum L.]), and annual forbs. Tall perennial bunchgrass, Sandberg bluegrass, and perennial forb cover and density did not differ among the treatments but did decrease over time in all treatments. The cover of several tall bunchgrass species was generally less in the high treatment, mainly, Idaho fescue (Festuca idahoensis Elmer) and Thurber's needlegrass (Achnatherum thurberianum (Piper) Barkworth). The cover of cheatgrass and annual forbs varied among years but was greater in the burned‐grazed and nonuse treatments than in the control. Native plant cover in the burned treatments (grazed and nonuse) represented 77%–85% of total herbaceous cover versus the control where native plants comprised 91% of the total. Annual weather variability appears to account for most of the compositional dynamics measured in the various grazed and ungrazed treatments.