This prototype model consists of a temperature regime ordinate, a moisture regime ordinate, and a regression equation relating them to aspen site index in the Southern Rocky Mountains. Its construction required a close look at a number of problems and considerations, and some possible methods, in ecosystem modeling. Clonal variation in aspen hight growth prevented a good test of the model, however. The temperature regime ordinate is analogous to degree-days, and integrates elevation and latitude within subregions. The moisture regime ordinate integrates estimates of monthly precipitation, monthly mean temperatures, potential direct-beam insolation, water-holding capacity of the soil, and factors influencing runoff. Equations are provided for estimating mean monthly precipitation, based on topographic and other factors.