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Management, under the concepts of multiple-use and adaptive management, requires the assessment of potentials and limitations of the natural ecosystems to provide satisfaction to human needs, to protect long term productivity, and preserve biological diversity. Overstory-understory relationships were developed for ponderosa pine (Pinus ponderosa) ecosystems to help managers to evaluate herbage production potentials. Secondary data sources from the Beaver Creek and the Heber Watersheds were divided as follows: igneous soils, igneous clay loam, igneous loam-sandy loam, sedimentary soils, alluvium, and sandstone. Regression models were fitted to the raw data by the least squared method. The dependent variables were herbage production (lb/ac) by component; namely total, grass and grass like plants, forbs and half shrubs, and shrubs. The independent variables were total and ponderosa pine basal area (ft 2 /ac). Semilogarithmic models fitted the data from igneous soils, while logarithmic transformations of hyperbolic models fitted the data from sedimentary soils. For igneous soils, ponderosa pine basal area suffice for adequate predictions of herbage production. Significant differences were found between equations for alluvium and sandstone. The resulting equations for ponderosa pine integrate the core of the model UNDER. Mathematical functions developed elsewhere are included in UNDER to compute herbage production in pinyon-juniper and mixed conifer ecosystems. UNDER is linked to other simulators by MICROSIM. MICROSIM, a multiple-resource simulation model, is a tool to assist in the assessment of potentials of forest and woodlands of Arizona. MICROSIM is a menu driven program for IBM or compatibles it contains the module Flora, for estimating plant responses, and module Fauna, to evaluate impacts on animals. Further development of MICROSIM should include the linkage to more modules and models, and to Geographical Information Systems.

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