Variable-form stem profile models for loblolly pine

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Abstract
Stem profile models for loblolly pine (Pinus taeda L.) that allow for both taper and form changes were constructed and evaluated. In 1956, H.R. Gray defined form to be the basic shape of the tree (e.g., cone or paraboloid) and taper to be the rate of narrowing in diameter given a tree form. D.W. Ormerod''s stem profile model was selected as the basic model since its parameters were readily interpretable in terms of Gray''s taper and form definitions. Two-state modeling procedures were used to relate individual tree taper and form parameters to tree and stand characteristics. Two second-stage parameter estimation alternatives were evaluated. Parameter estimates for both techniques, ordinary least-squares and random function analysis, were similar. Characteristics used to predict stem form were total tree height, crown ratio, height to the live crown, site index, and tree age. The taper parameter was related to diameter at breast height, crown ratio, and site index. Error evaluations suggest that a 10-20% gain can be made in predicting stem diameters using the variable-taper and variable-form stem profile models.

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