Individual-tree height-, diameter- and crown-width increment equations for young Douglas-fir plantations

This work presents the results from the initial model development of a simulator to predict vegetation dynamics in young plantations growing in a Mediterranean environment. The simulator can predict growth dynamics for coniferous crop trees as well as competing hardwoods and shrubs. Model specification included conifer, shrub, and hardwood competition expressed at the plot-level. The system employs water-holding capacity as an indicator of productivity. Growth data were obtained from 109 plantations, ranging in age from 3 to 25, in southern Oregon and northern California. Douglas-fir (Pseudotsuga menziesii Mirb. Franco), the most common conifer species, was observed in 80 of the sampled stands. These observations were used for model development of the primary driving functions, which forecast 2-year basal diameter increment, height increment, and crown width increment. Parameters for all three dynamic expressions for growth were estimated using weighted, nonlinear three-stage least squares. This estimation method provided a predictive model with slight improvements in standard errors for two of the three equations (an average of 3% for height and diameter growth) and no improvement for crown width, when compared with two-stage least squares. The system includes competition from shrubs and hardwoods in predictions of height growth, diameter growth and crown width increment. This allows individual-tree/distance-independent simulator architecture to be extended to young plantations in southern Oregon and northern California.