Effects of Chronic Nitrogen Additions on Understory Species in a Red Pine Plantation

Two plots in a red pine stand at the Harvard Forest, Massachusetts, have been fertilized for 7 yr with 5 g·m22·yr21 or 15 g·m22·yr21 of N and compared to an unfertilized control to study the effects of chronic N additions on nutrient cycling, plants, and soil. Movement of added N into soils and plant biomass was tracked using additions of an 15N label to the 5 g·m22·yr21 and control plots for two of these years. We present data on changes in the understory plant community of these plots. We measured aboveground biomass, density, N and other elemental concentrations, and 15N tracer recoveries to catalogue the effects of the N additions on this community. Nitrogen contents increased while biomass and nutrient cation concentrations decreased in some species. Percent recoveries of 15N tracers were small but detectable. The natural abundances of 15N also increased in a fertilized treatment without tracer additions. Though this forest has not yet reached N saturation by some definitions, it is possible that the understory is already saturated with N. Understory species may be useful indicators for N saturation through their increasing N content, decreasing nutrient concentrations, and increasing d15N in tissues.