DEVELOPMENT OF RESOURCE VALUE RATINGS AND ESTIMATION OF CARRYING CAPACITY OF SOUTHERN ARIZONA RANGELANDS.

Abstract

The objective of this research was development and testing of a method for estimating cattle carrying capacities. A series of studies were conducted in developing this method. Range site and vegetation production data were grouped by topographic position and multiple linear regression equations were calculated for predicting vegetation production as a site deviated from the average case of a given range site. Overstory-understory relationships from the literature were adapted into overstory canopy cover classes for predicting understory production and tested on a variety of range sites. Use of these classes produced understory biomass estimates within 13% of measured biomass. Range condition class and understory aspect dominance by forage vs. non-forage species were investigated as estimators of forage value of the understory vegetation. Both were significantly related to amount of forage in the understory. However, understory aspect proved to be a better estimator when individual comparisons were examined. The previous findings, along with Soil Conservation Service range site guides, were used to calculate resource value ratings. Adjustment factors to be applied to the resource value ratings were calculated, using data from the literature, to account for the effects of slope and distance from water on forage utilization by cattle. These resource value ratings and adjustment factors form the basis of the carrying capacity estimation method. Pastures identified as properly utilized were used in testing the method developed. Pastures were mapped for range site, vegetation, slope and water location. Maps were converted to digital form and analyzed using the Map Analysis Package (MAP) computer program (Tomlin, 1975). Construction of a final range site-vegetation-slope-distance from water map, assigning of resource value ratings and adjustment factors, and computation of final carrying capacity estimates were accomplished using MAP. Carrying capacity estimates from the developed method were well correlated to estimates from ocular reconnaissance and area allowable use methods, r = .87 and .97, respectively, and with the actual use (perceived proper use), r = .95. These estimates were accomplished without intensive field sampling. The only information required was range site designation, amount of overstory canopy cover, understory aspect class, percent slope and water location.