Monitoring standing herbage on granitic soils in the Big Horn Mountains, Wyoming, USA

Abstract

AbstractMonitoring annual herbage use in rangelands is desirable for scientific management but problematic due to the difficulty and expense of making direct measurements of sufficient precision. We used a Robel pole with 1.27 cm alternating white and gray bands to indirectly measure standing herbage. This technique enables a smooth transition in the basis of annual use monitoring from percent use to residual herbage and overcomes the difficulties alluded to above. Our study was conducted in the southern Bighorn National Forest, Wyoming on granitic soils and within the Idaho fescue (Festuca idahoensis Elmer) vegetation type. The objectives were to develop and validate the relationship between Robel pole visual obstruction readings and clipped standing herbage, and develop guidelines for monitoring and estimation of optimum sample size. Clipped standing herbage ranged from 134 to 3792 kg dry matter (DM) ha−1 with a mean of 1558 kg DM ha−1. Visually obstructed bands (band = 1.27 cm) averaged 6.1 and ranged from 0.2 to 19.5. Approximately 75% of the variation in standing herbage was explained by linear regression on visual obstruction measurements. Both intercept and slope of the regression were highly significant (P = 0.001). The standard error of the estimate for a single observation was 458 kg DM ha−1. Cluster analyses grouped the visual obstruction readings into four management categories. This monitoring tool provides pertinent information for managers to develop guidelines based on band and/or standing herbage for natural areas, wildlife and livestock use.