Effects of irrigation, pulp mill sludge, and repeated coppicing on growth and yield of black cottonwood and red alder

Growth and yield of black cottonwood (Populustrichocarpa Torr. and Gray) and red alder (Alnusrubra Bong.) were measured at four successive 2-year coppice harvests. Three levels of amended pulp mill sludge (450, 225, and 0 t•ha−1) were applied before planting, and one-half of the plots were irrigated during the 2-year establishment period prior to the first coppice cycle. Yields of black cottonwood were generally much higher than yields of red alder; maximum ovendry yields obtained were 13.8 t•ha−1•year−1 for black cottonwood and 7.1 t•ha−1•year−1 for red alder. The species differed in their responses to treatment. Growth and yield of black cottonwood was increased by the initial sludge applications, but the reverse was true for red alder. The initial irrigation treatment had carryover effects on the yields of subsequent growth cycles for both species. For all treatments, the greatest yields were usually obtained at the first or second coppice harvest. On the average, red alder yields declined substantially between the third and fourth harvests; black cottonwood yields were generally more consistent between these two harvests. Both species sprouted after all four harvests; however, rootstock mortality generally increased with each harvest. Mortality was much higher for red alder than for black cottonwood; after four coppice harvests, mortality on red alder treatments averaged 55–79%. Average mortality on black cottonwood treatments ranged from 12 to 38%. The management implications of the results are discussed.