Phytomass production in young mixed plantations of Alnus rubra (Bong.) and cottonwood in western Washington

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Abstract
Use of red alder in mixture with 28 clones of cottonwood in close spaced, short-rotation culture for fiber and energy was investigated. Early growth was rapid with red alder averaging 7.2 m and cottonwood mixed with alder averaging 8.2 m in height after 3 years. Alder significantly increased nitrogen content of cottonwood foliage in the 1st and 2nd years but not in the 3rd. Mean height of cottonwood at 3 years was increased in mixture with red alder. However, heights of the shortest cottonwood clones were reduced. Dry weight production on mixed plots (both species combined) was generally less than for pure cottonwood. Only with the slowest growing cottonwood clones did mixed stands show increased dry weight production. The best cottonwood clone, a hybrid (Populus trichocarpa × P. deltoides), produced 20.3 t∙ha−1∙year−1 dry weight for 3 years in pure stands compared with 2.8 t∙ha−1∙year−1 for the poorest. Production by alder was inversely related to cottonwood production in mixed plantings. Nodulation and C2H3 reduction was evaluated during the third growing season. Results indicated severe decline in nitrogenase activity where overtopping and shading of red alder by adjacent cottonwood occurred. At this site, the success of this mixture appears to depend upon use of cottonwood clones that do not outgrow the alder. Since the highest yielding cottonwood clones are much more productive at this site than the alder that we used, there would seem to be little incentive for mixed plantings of these species under the conditions of this experiment.

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