Vertical trends in maximum branch diameter in two mixed-species spacing trials in the central Oregon Cascades

The influence of spacing and competitor species on vertical trends in maximum branch diameter, the thickest branch per whorl, was assessed in two central Oregon spacing studies. One study involved a mix of Pinus contorta Dougl. ex Loud. and Pinus ponderosa Dougl. ex Laws., the other a mix of Abies grandis (Dougl. ex D. Don) Lindl. and P. ponderosa. Impacts of autocorrelation became statistically insignificant after introduction of a single random tree effect. Although tree variables such as diameter, height, and crown length were able to account for most stand conditions, models with explicit treatment variables representing spacing and species composition were superior. All profiles of maximum branch diameter were curvilinear and widened with increasing spacing and tree relative height. For trees in mixtures, maximum branch diameter profiles of dominant and subordinate species were wider and thinner, respectively, than the same species in pure stands at the same spacing. However, as spacing increased, profiles of the subordinate species in mixtures had a greater response than those in adjacent pure plots and in the dominant species in the mixture. In contrast, the dominant species had a larger spacing response in the pure plots than in mixed plots.