Canopy N and P dynamics of a southeastern US pine forest under elevated CO2

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Forest production is strongly nutrient limited throughout the southeastern US. If nutrient limitations constrain plant acquisition of essential resources under elevated CO2, 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 CO2 enrichment. We also used N:P ratios to detect N versus P limitation to primary production under elevated CO2. Canopy biomass was significantly higher under elevated CO2 during the first 4 years of this experiment. Elevated CO2 significantly reduced the concentration of N in loblolly pine foliage (5% relative to ambient CO2) but not P. Despite the slight reduction foliage N concentrations, there were significant increases in canopy N and P contents under elevated CO2. Foliar N:P ratios were not altered by elevated CO2 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 CO2 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 CO2 fumigation. As a consequence, whole-canopy C assimilation is strongly stimulated by elevated CO2 making this forest a larger net C sink under elevated CO2 than under ambient CO2. 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 CO2.

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