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The Visual Obstruction Method, or more commonly referred to as the Robel Pole Method, can be used to estimate the amount of standing biomass remaining on an area after a use period (i.e., measuring residual forage after grazing). Using this method height and vertical density of standing vegetation can be monitored. It can be used in both upland and riparian areas where vegetation is less than 4 feet tall. Various modifications of the original Robel design have been developed and published, but all are similar and use the Visual Obstruction method.
Measuring aboveground biomass commonly involves harvesting vegetation quadrats to be dried and weighed. This approach although accurate can be destructive and time-consuming and typically requires large numbers of samples. The Robel Pole Method is faster and less destructive to vegetation.
The Robel Pole Method involves using a Robel Pole (go to: http://www.blm.gov/nstc/library/pdf/utilstudies.pdf for a description and illustration of a Robel Pole). Key areas are chosen within which transects with associated sample points are established. Observations are made on the pole from opposing directions at each sample. The actual reading on the pole consists of identifying the last band visible on the pole before the pole disappears in the vegetation and then assigning a cover class. Cover classes are developed locally for each ecological site or plant community.
When using this method for grazing assessments, it is generally not as reliable in areas with low productivity, where shrubs are abundant, for example, where sagebrush or similar-sized shrubs frequently obstruct the visibility of the pole in herbaceous vegetation, and in areas with a high bare-ground component. It is important when using this method as a grazing assessment tool to 1. calibrate the measurements to local vegetation, 2. properly select of key areas, and 3. develop site specific guidelines.
The Robel Pole Method is a rapid technique, allowing observers to monitor height and vertical density of standing vegetation over large areas quickly.
It is also simple and accurate; however, the accuracy of the data depends on the training and ability of the observers since they will rely on ocular estimations, which can be highly variable between observers. Therefore, observers must first receive adequate training in laying out transects, determining cover classes, and reading the Robel pole.
It is important when using this method as a grazing assessment tool to calibrating the measurements to local vegetation, properly select of key areas, and developing site specific guidelines.
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