Canopy N and P dynamics of a southeastern US pine forest under elevated CO ₂

Forest production is strongly nutrient limited throughout the southeastern US. If nutrient limitations constrain plant acquisition of essential resources under elevated CO ₂, reductions in the mass or nutrient content of forest canopies could constrain C assimilation from the atmosphere. We tested this idea by quantifying canopy biomass, foliar concentrations of N and P, and the total quantity of N and P in a loblolly pine (Pinus taeda) canopy subject to 4 years of free-air CO ₂ enrichment. We also used N:P ratios to detect N versus P limitation to primary production under elevated CO ₂. Canopy biomass was significantly higher under elevated CO ₂ during the first 4 years of this experiment. Elevated CO ₂ significantly reduced the concentration of N in loblolly pine foliage (5% relative to ambient CO ₂) but not P. Despite the slight reduction foliage N concentrations, there were significant increases in canopy N and P contents under elevated CO ₂. Foliar N:P ratios were not altered by elevated CO ₂ and were within a range suggesting forest production is N limited not P limited. Despite the clear limitation of NPP by N under ambient and elevated CO ₂ at this site, there is no evidence that the mass of N or P in the canopy is declining through the first 4 years of CO ₂ fumigation. As a consequence, whole-canopy C assimilation is strongly stimulated by elevated CO ₂ making this forest a larger net C sink under elevated CO ₂ than under ambient CO ₂. We discuss the potential for future decreases in canopy nutrient content as a result of limited changes in the size of the plant-available pools of N under elevated CO ₂.