Effects of competition from young northern hardwoods on red pine seedling growth, nutrient use efficiency, and leaf morphology
The effects of competition from three northern hardwood tree species on red pine (Pinus resinosa Ait.) seedlings were examined on two clear-cut sites in western Maine. We examined how planted red pine seedlings altered their nutrient use efficiency and shoot morphology under changing environmental conditions and how these changes related to their ability to tolerate competition. A three-factor experimental design was used to determine the effects of species of competitors and their abundance as well as fertilization on red pine seedling growth, nutrient use efficiency, and leaf morphology. The competitors were striped maple (Acer pensylvanica L.), red maple (Acer rubrum L.), and pin cherry (Prunus pensylvanica L.) established at two densities (high and zero) with two levels of fertilization (0 and 224 g m-2 of 10-10-2010 NPK commercial fertilizer). Nitrogen and phosphorus use efficiencies were calculated as total aboveground biomass divided by total nutrient content. Specific leaf area (cm2 g-1), leaf area ratio (cm2 g-1), and total leaf area (cm2) were measured for all red pine seedlings. Plots were harvested at two time periods, when pine seedlings were 2 years old (1989) and 3 years old (1990). Total biomass, annual production, and leaf area index (m2 leaf area m-2 ground surface area) were calculated for competitors on each plot.
Red pine seedlings had higher specific leaf area, leaf area ratio, and nitrogen use efficiency on competitor plots than on plots free of competition, suggesting a phenotypic response to resource depletion of light and nutrients by competitors. Fertilization decreased growth of red pine seedlings and decreased nitrogen use efficiency. Red pine total leaf area and biomass were lower on the competitor plots. Higher competitor biomass, leaf area index, and nutrient uptake explained the lower growth of red pine seedlings on competition plots. Pin cherry was the most significant competitor with striped maple being intermediate; red maple had the least effect on pine seedling growth. The results of this study indicate that the growth of red pine seedlings was governed by the availability of resources as influenced by the competitor species, the efficiency of nutrient use, and the ability of red pine to adjust its growth pattern in response to resource availability.