Neighborhood predictors of interspecific competition in young Douglas-fir plantations

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Abstract
Neighborhood models describing the effect of interspecific competition on the height and stem diameter of 4- to 9-year-old saplings of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) were developed from site-preparation experiments in the Oregon Coast Range. The influence of abundance measures, height, distance, arid spatial arrangement of nonconiferous woody plants surrounding individual saplings was examined. Optimum neighborhood heights and radii were defined. The best interspecific competition index for predicting Douglas-fir height and stem diameter was total percent cover for all woody species within a 2.1-m radius. Visual estimates of neighbor cover were superior to objective measures of crown area. The cover of woody species equalling or exceeding the height of the tree provided the best prediction for tree height. Woody species cover equalling or exceeding one-half the height of the tree provided the best index for predicting stem diameter. Accounting for the spatial arrangement of neighboring woody plants did not improve the competition index. Interaction between the competition index and tree age indicated that the negative effect of interspecific competition on Douglas-fir size increased with time. The age-adjusted competition index accounted for 11% of the variation in height and 19% of the variation in stem diameter. Douglas-fir stem diameter was more sensitive to neighboring woody plants than was height.

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