differential sensitivity of red pine and quaking aspen to competition

Growth of red pine (Pinus resinosa Ait.) and quaking aspen (Populus tremuloides Michx.) target trees with varying degrees of competition by neighboring aspen was measured in a 7-year-old clearcut in northeastern Minnesota. Competitive conditions were measured using indices of resource availability (percent open sky) and neighborhood stand density. Three-year diameter growth of target trees was reduced by more than 50% in both species because of competition from neighboring quaking aspen. The last 2 year's height growth was also reduced by approximately 30% for both species. The relationship between growth and percent open sky was convex for red pine and concave for quaking aspen, potentially indicating a greater sensitivity in aspen to any reduction below optimal resource availability. Competition reduced crown size in both species, but self-pruning occurred only in quaking aspen. Needle and leaf size were not affected by competition, but red pine needle density was lower in highly competitive environments. Red pine had a higher specific gravity when growing under competition, partially offsetting the reduction in wood volume.