Deriving site index equation and curves for plantation forest

Douglas-fir (pseudotsuga menziesii Mirb. Franco) ranks as the second most important plantation tree species after radiata pine in New Zealand. Site index equations to derive site index curves for Douglas-fir plantations have been developed based on mid-term repeated measurement data sets, and then site index curve (base age 40 years) was developed. The function was developed by algebraic difference equation method. Of the projection functions tested, Gompertz polymorphic equation showed the higher precision of the fitting. Ninety-five percent of the observations used to fit model could be predicted within ±1.7 m of the actual values. Polymorphic family of site index curves, hence, which reflect different shapes for the different site index classes, were derived from the Gompertz equation. This function represents measurement times T₁ and T₂ as T₂-T₁, while the Hossfeld and Schumacher functions, which had the second third lower MSE values, encapsulate T₁ and T₂ as T₂/T₁. The applicability of the Gompertz function, therefore, was enhanced with this study data set that contain medium measurement cycles.