Refining Robel Pole Applications for Grazed Idaho Fescue Grasslands in the Big Horn Mountains, Wyoming

Abstract

Administrative oversight of grazing on public land requires efficient and reliable rangeland monitoring. In a portion of the Bighorn National Forest of Wyoming, the US Forest Service uses the modified Robel pole as a monitoring tool to measure residual herbaceous biomass after grazing. Questions about the vegetation standards have persisted for decades. From 2018 to 2020 we quantified relationships between herbaceous biomass and visual obstruction across spatial, temporal, management, and topoedaphic gradients in 11 grazed pastures selected mutually by stakeholders. Initial standards were developed on the basis of 1 yr of research, and state visual obstruction readings (VORs) were correlated with biomass with an R2 = 0.81, yet our research resulted in R2 values of approximately half (ranging from 0.39 to 0.45) with significantly different slopes 2 of 3 yr. Moreover, the intercepts for each year individually, when averaged, or when combined were always greater than the relationship used to develop initial standards, resulting in 140−564 more kg per ha of biomass at the 5-band standard level, or situations where lower band levels (2.5−3.9) would yield similar biomass levels. In addition to VOR and biomass, slope, aspect, plant and ground cover, weather, and livestock management variables were assessed with information theory models for predicting residual biomass. The top model included VOR, slope, exotic grass, and temperature. This suggests that greater knowledge of the site will increase understanding of biomass production on the site and help to mitigate disagreements among stakeholders by tailoring expectations to variable sites. Finally, the higher intercepts in all of our models may explain the persistent skepticism about the application of the original standard and provide the opportunity to refine standards to better match postgrazing monitoring with resource objectives, particularly if functional group dominance on these sites have changed over time.