Developing a hybrid growth model for multiaged Sierra Nevada mixed-conifer forests

A growth model was developed for multiaged mixed-conifer forests in the Sierra Nevada of California. The model incorporates process-oriented components that predict tree growth as a function of leaf area and absorbed light, as well as empirical information on site quality. Leaf area prediction equations were developed from sapwood area---leaf area relationships for Pinus ponderosa Laws., Abies concolor Gordon and Glend., Calocedrus decurrens (Torrey) Florin, Pseudotsuga menzieii var. menziesii (Mirb.) Franco, Pinus lambertiana L., and Quercus kelloggii Newb. Inclusion of site index improved model predictions across the range of sites sampled in the Sierra Nevada. Leaf area was used to calibrate a spatially explicit light model to conditions in the Sierra Nevada. The light model was able to predict the variable light environment within complex canopy structures. Moreover, it showed little loss in accuracy when model parameters were simplified for use of the model with inventory data. The model calculates light at any point within the canopy and can estimate mean incident and absorbed light of larger tree crowns. Boundary line analysis showed that the maximum amount of leaf area a tree maintained depended upon the mean light received over the growing season and differed among species. Tree growth efficiency, defined as stem volume increment per unit of leaf area, was correlated with tree size and light environment. Growth efficiency increased with tree height across the range of light intensity levels. Prediction equations were developed for volume increment as a function of projected leaf area and leaf area weighted by absorbed light. These relationships demonstrate the varying importance of shading and self-shading on tree growth in vertically structured stands. Additional models for height, basal area, and crown radial increment were developed to project volume increment and stand structural development over periods of up to 20 years. The whole model was validated with independent permanent sample plot data from two multiaged mixed-conifer stands. It can be used to assess the effects of stand structure upon stand and component yield and structural development. The model is intended to be a tool for managers to design multiaged mixed-species stands of variable structure.