Growth and Nitrogen Relations of Coppiced Black Cottonwood and Red Alder in Pure and Mixed Plantings

Growth and nitrogen status were assessed in pure and mixed plantings of black cottonwood and red alder. Annual dry-matter production of 2-yr-old coppice (stump sprouting) in the mixed plots was higher than production in pure cultures of cottonwood and alder. Twigs of cottonwood from mixed plantings contained 18% more nitrogen than twigs from pure plots. Soil collected in the mixed cottonwood-alder and pure alder cultures contained, respectively, 9% and 23% more nitrogen than soil from the pure cottonwood planting. For the upper 15 cm of mineral soil, average annual nitrogen accretion under the mixed stand was estimated at 32 kg/ha and under pure alder, 80 kg/ha. The beneficial effects of red alder in pure and mixed stands, obtained in such a short time, may have application for silage cellulose production, other short-rotation forest systems, and for proposed "biomass farms for energy."