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Three-Dimensional Representation of Douglas-Fir Volume Growth: Comparison of Growth and Yield Models with Stand Data

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Growth and yield estimates for unthinned stands from the Douglas-fir Stand Simulator (DFSIM; Curtis et al. 1981) and the Tree and Stand Simulator (TASS; Mitchell and Cameron 1985) were used to construct graphical three-dimensional representations of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) stand growth on site index 44 meters (50 year). The three-dimensional models used three variables: trees per hectare, breast height age, and either mean tree volume or stand volume. The TASS and DFSIM models were in agreement over most of their common range of age and number of trees. At wider spacings and older ages, however, the volumes predicted by the DFSIM model exceeded those predicted by the TASS model by as much as 25%. Comparisons of these three-dimensional models to unthinned and thinned stand data from a similar site quality found the models to be reasonably accurate representations of unthinned stand growth. The thinned stands, however, had greater mean tree and stand volumes than those indicated by the TASS model for unthinned stands at similar spacings. Complete comparisons were not possible with the DFSIM model because of its limited range of number of trees. These results suggest the TASS model, and to a lesser extent, the DFSIM model may be underestimating the growth of widely spaced stands, or thinning may actually increase the growth of thinned trees over that of trees which had always grown at the post-thinning spacing. For. Sci. 34(3):724-743.

Keywords: Pseudotsuga menziesii; competition-density effect; correlated curve trend; self-thinning; thinning

Document Type: Journal Article

Affiliations: College of Forest Resources, AR-10, University of Washington, Seattle, WA 98195

Publication date: September 1, 1988

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