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Effects of age, sample characteristics and equation form on three non-linear models for volume yields

Document Type

Article

Journal/Book Title/Conference

Forestry Chronicle

Volume

61

Issue

1

First Page

14

Last Page

18

Publication Date

1985

Abstract

Inventory data from the British Columbia Ministry of Forests for nine species groups and three site classes were analysed to determine the effect of including mature stand data with immature yields to age 135 to improve estimates of volumes of unmanaged natural stands by three nonlinear functions. Chapman-Richards (CH), modified Weibull (W) and Gompertz (G) were used. Functions were fitted to provide estimates of maximum yield for immature and all data groups by three site classes. Compilation was facilitated by programs to estimate initial values. Results were evaluated by comparison of maximum yields and ages at which mean annual increments culminate, by calculation of standard errors of estimate and by a statistical test of the hypothesis that effects of including mature stands are significant. The consistent change in curve shape with site suggested that gains could be made by harmonization of coefficients by the site index equivalents of each class. Effects of sample size were not consistent; large samples were best but did not guarantee good results. Including mature stand data changed the shape of the function in some cases. It was concluded that nonlinear models should not be applied routinely for estimation of growth and yield without consideration of potentially harmful effects of parameter estimation and sampling characteristics.

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