Browsed-Class Method of Estimating Shrub Utilization

Abstract

The browsed-class method has been developed to measureshrub utilization based on total weight of the plant. It uses growth form to place grazed shrubs into 6 browsed-classes. The method is fast, statistically sound, relatively free from personal bias, easy to learn and use, and can be used in research or land manngement. In a IO-year ease study to determine proper use of hairy mountninmahogany, plants were clipped initially and then reclipped once each year in the fall or winterovera ‘I-yearperiod at 0,10,30, JO, 70 and 90 % levels based on total weight of the plant. This was followed by P 2-year recovery study. Parameters studied were numbers, length, and production of twigs; area of live and dead crown cover; and general vigor and seed production. All criteria, except area of live crown cover, indicated that SO% of total weight WPS proper use of hairy mountain-mahogany. Millions of hectares of rangeland are dominated by shrubs. Manyoftheseshrubsarelowquality species invadingorincreasing at the expense of valuable forage species. However, many shrubs are valuable forage species that can be increased in abundance by proper use, treatment, and management. A necessary tool for good management is a reliable method of evaluating proper use of shrubs. Most browse utilization methods are based on ocular estimates or measurements of twig length, numbers or weight to evaluate the percentage of current annual twig growth utilized (Dasmann 1948, NAS-NRC 1962, Smith and Urness 1962, Smith et al. 1963, Basile and Hutchings 1966, Stickney 1966, and Jensen and Scatter 1977). Methods using measurements are generally time consuming, which often results in inadequate sample size. Faster methods based on ocular estimates are generally subjective, require extensive training, and have an uncertain degree of accuracy. To belp solve these problems the browsed-class method, which uses photo guides based on total plant weight, was developed to estimate browse utilization. Also, since there were no data on proper use based on total shrub weight, a IO-year study was initiated to measure the effects of 6 degrees of clipping on mountain-mahogany (Cercocarpus rn~nfmw var. parrcidenratus (S. Watson) F.L. Martin). The method is discussed in Part I and the utilization case study in Part IL agencies to estimate past use and vigor or condition of browse species (Dasmann 1951, Pattonand Hall 1966, U.S. Forest Service 1981). The method uses photo guides of shrubs based on heightweight curves to place randomly selected plants in 6 browsed classes-O, IO, 30,50,70,and 90%use(Fig. 1). Sincetheclassesare based on height-weight curves, they evaluate percentage of weight utilized. Also, since estimates are based on growth form, variations in height due to wet or dry years, deep or shallow sites, fertile and unfertile soils, age, etc., are automatically adjusted for by the eye. The method is a departure from conventional shrub utilization methods in that estimates are based on total weight of the plant rather than on current twig growth. As a result it primarily measures trend in utilization (in comparison to previous years estimates) but also shows the level of current use (in comparison to ungrared plants). Since shrubs do not die back to the ground each year, trends in utilization are sufficient for shrub management. When trends show that utilization is approaching or exceeding proper use, adjustments can be made in game and/or livestock