Mixed, Short-Rotation Culture of Red Alder and Black Cottonwood: Growth, Coppicing, Nitrogen Fixation, and Allelopathy

Abstract Red alder ( Alnus rubra Bong.) seedlings from a single geographical source were grown in a 1:1 mixture at a spacing of 1.2 x 1.2 m with 28 Populus clones including 25 clones of black cottonwood ( Populus trichocarpa Torr. & Gray) at one location. The trees were harvested at 4 years. At harvest, average heights were: cottonwood in pure stands, 10.2 m; cottonwood in the mixture, 11.0 m, and alder, 8.4 m. Most cottonwood clones sprouted satisfactorily after harvest but alder sprouted poorly. Aboveground biomass production at harvest averaged 15.9 Mg/ha/yr from the mixed plantings compared to 16.7 Mg/ha/yr for pure cottonwood, yet at 2 years the mixture was more productive than pure cottonwood. Nitrogenase activity (nitrogen fixation as measured by acetylene reduction) of alder declined in the fourth season with competition the most important factor influencing activity. Soil nitrogen level (range 0.08-0.22 percent N) had no effect on activity. A pot study showed inhibitory effects of ground cottonwood leaf and litter material on red alder seedlings. On the other hand, alder seedlings grew normally in field soil collected from the cottonwood plots. This finding suggests that at most allelopathy is probably a minor factor under field conditions. Results do not eliminate the possibility of beneficial results from mixed stands with red alder under different cultural arrangements particularly where soils are more nitrogen deficient. Forest Sci. 31:607-616.