Stand Productivity and Development in Two Mixed-Species Spacing Trials in the Central Oregon Cascades

Two mixed-species spacing trials were established in central Oregon to test the effects of species composition and initial spacing on stand growth, yield, and structural development. One trial was composed of Pinus contorta Dougl. ex Loud. and Pinus ponderosa Dougl. ex Laws. and the other Abies grandis (Dougl. ex D. Don) Lindl. and P. ponderosa. Spacing, species composition, and age all had large influences on standing volume and periodic annual growth. In both studies, standing volume and periodic annual increment decreased with increasing spacing. In mixed plots, the least shade tolerant species had the fastest early growth rates, while the more shade tolerant species had the slowest. Over time, initial differences in volume growth rates among plots of different species composition decreased. Relative yields in mixtures of P. contorta and P. ponderosa indicated no yield benefits. Mixtures of A. grandis and P. ponderosa, however, resulted in relative yield totals greater than one at all spacings. Spacing and species composition play an important role in stand production and development, and mixtures can yield similar if not more volume than pure stands of the higher yielding species at some spacings and stages of stand development. FOR. SCI. 50(1):92–105.