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Evaluation of the Reference Unit Method for Herbaceous Biomass Estimation in Native Grasslands of Southwestern South Dakota
The high costs associated with physically harvesting plant biomass may prevent sufficient data collection, which is necessary to account for the natural variability of vegetation at a landscape scale. A biomass estimation technique was previously developed using representative samples or "reference units", which eliminated the need to harvest biomass from all sample plots. Use of reference units increases the number of plots that can be sampled, allowing monitoring to capture the variability of rangeland. This technique was developed for use on shrub foliage with limited validation of the method in herbaceous grassland vegetation. The objectives of this study were to: 1) validate reference unit method for herbaceous grassland biomass estimation, 2) examine multi-species reference calibrations, 3) validate the use of season-long calibration equations, and 4) compare differences among observers using the reference unit method. The study was conducted in 2010 on prairie dog colonies of the Buffalo Gap National Grasslands in southwestern South Dakota. These prairie dog colonies and the surrounding areas provided a range of vegetation height and plant communities to use for testing the method. Research revealed that reference units provided accurate and precise estimates of the mean for herbaceous plants on grasslands, including multi-species functional groups. Twenty-five of the 26 double sampling calibrations were validated by time with no changes in observer estimation trends over the sampling season. Use of the reference unit method was consistent among observers. Additionally, a technique to create shortgrass reference units was developed. The reference unit method is recommended for grassland evaluation and monitoring projects.