Above- and belowground response of baldcypress and water tupelo seedlings to variable rates of nitrogen loading: Mesocosm and field studies

•Studied impacts of nitrogen (0–400 g N m−2 yr−1) on baldcypress & water tupelo.•Aboveground biomass production increased steadily to 400 g N m−2 yr−1.•Belowground biomass production increased to 100 g N m−2 y−1 then decreased.•Wood density was largely unaffected by nitrogen loading rate.•Diameter increase for seedlings averaged from 1.1 to 2.5 cm yr−1. A nutrient-loading mesocosm study was conducted on baldcypress (Taxodium distichum) and water tupelo (Nyssa aquatica) seedlings and a field study was conducted on baldcypress seedling diameter increase at five assimilation wetlands. Nitrogen additions ranged from 0 to 400 g N m−2 yr−1. Dependent variables included above- and belowground biomass production, root to shoot ratio, and wood density. Aboveground biomass production increased steadily to the 400 g N m−2 yr−1 loading rate for both species, whereas belowground biomass production and root to shoot ratio increased to the 100 g N m−2 yr−1 loading rate then decreased slightly. Wood density for N. aquatica was higher than T. distichum and wood density for both species was largely unaffected by nutrient loading rate. Diameter increase for seedlings in the five assimilation wetlands averaged from 1.1 to 2.5 cm yr−1 and was about ten times higher at all sites than that of nearby natural swamps. In all, high rates of nutrient loading did not negatively affect growth of either species.