EFFECTS OF INTRA-SPECIFIC AND INTER-SPECIFIC COMPETITION ON ROOT AND SHOOT BIOMASS OF YOUNG DOUGLAS-FIR AND RED ALDER

The effects of intra- and inter-specific competition on root and shoot biomass of 5-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and red alder (Alnus rubra Bong.) were studied in experimentally manipulated stands. The density of each species was varied systematically in a two-way density matrix composed of five monoculture densities and 25 mixtures of all possible pairwise combinations of monoculture densities. Roots and shoots were harvested after the fourth growing season. Response surfaces for root, shoot, and total biomass per tree were generated within the matrix. Regression analysis quantified the effect of each species' density on biomass components. Alder overtopped the Douglas-fir in all mixed stands. Alder density influenced the root and shoot biomass of both species more than Douglas-fir density. Douglas-fir density interacted with red alder density to influence all biomass components. While increasing the density of each species reduced root and shoot biomass per tree, allocation of biomass to roots and shoot was not affected by competition, nor were the allometric equations relating biomass to stem diameter and stem volume index.