Revised site index models for western redcedar for Coastal British Columbia

Summary: The British Columbia Ministry of Forests, Lands and Natural Resource Operations is implementing a program to update site index models by refitting the models with new fitting techniqies. The existing site index model for coastal western redcedar (Thuja plicata) was developed by MacMillan Bloedel Limited in 1978. This model was in particular need of updating because it gave unreasonable height and site index predictions for old trees and because some abnormalities in the height predictions were fixed with ad hoc adjustments. Data for the refit were obtained from four sources: pseudo-data from existing model, stem anaysis data from a wood quality study, a foliar nutrient-growth study, and a project done by MacMillan Bloedel. The grounded-Generalized Algebraic Difference Approach was applied to a Chapman-Richards function to derive a base-age invariant site index model for coastal redcedar. This new site index model fit the data well and gave approximately the same height predictions as its predecessor over the range of data used to develop the forerunner model, but gave more reasonable height and site index predictions when extrapolated to older ages.