Relating site index to ecological factors in black spruce stands: Tests ofhypotheses

Modeling dominant height through site index, defined as dominant height at some fixed tree age, requires two equations. The site index must be estimat ed as the average height of dominant trees at base age, and dominant height must be modeled as a function of age and site index, each model having its own error term, This raises a problem when squared errors are minimized to estimate parameters of interest: the estimates are biased and inconsistent because the error terms are correlated with the explanatory variables. Two additional problems are the autocorrelation among error terms and variance heterogeneity within trees. The generalized method of moments (GMM), which simultaneously takes these problems into account, offers a tool to estimat e parameters of the site index and dominant height equations consistently a nd without bias. Allowing parameters of the height growth model to depend o n explanatory variables representing ecological region and drainage class p rovides useful insight into growth patterns, and improves prediction based on these models. A simulation study confirms that the GMM combined with pre whitening of the time series for each tree produces useful estimates of the parameters and valid statistical tests of hypotheses about their value.