Tree height and diameter growth models for Northern California conifers

This paper presents growth models for predicting the components of height and diameter growth for six conifer species in northern California. Sample data came from the lands of the industry members of the Northern California Forest Yield Cooperative, and the coefficients were developed for use in CACTOS, the California Conifer Timber Output Simulator. The basic field data required for predictions are 50-year (at breast height) site indexes for each species in the stand as well as the following items for each tree: species, diameter at breast height, total height, height to the crown base, and number of trees per acre. The models express growth as a product of potential and competition components. For height growth the potential is based upon 50-year (breast height) site index curves while for diameter growth, potential coefficients come from an iterative procedure using “free to grow” trees as a starting point. The competition components rely primarily upon the relative position of the crown, expressed as the crown closure at 66 percent of the tree’s height. Data from stem analysis were used to fit the height growth coefficients, but both the stem analysis and increment core data were used to fit and test the diameter growth coefficients.