Reassessment of carbon accumulation at the Duke free air CO2 enrichment site: Interactions of atmospheric [CO2] with nitrogen and water availability and stand development

Elevated [CO₂] has the potential to change many aspects of forest ecosystem development and function. One of the most important questions is the magnitude of the effect of elevated [CO₂] on ecosystem carbon (C) storage, which could be modified by changes in plant production, allocation of production, or turnover of living and detrital matter. Utilizing ten years of data from the Duke free-air CO₂ enrichment (Duke FACE) site, we evaluated plant and ecosystem C pools and the distribution of C in these pools. We found no compelling evidence that elevated [CO₂] allowed an increase in the size of the average tree at a given stand density, which would have indicated an increased carrying capacity of the Duke FACE site. Nor were mortality rates significantly different under elevated [CO₂] (2.4% yr^-1 for pines, 1.2% yr^-1 for hardwoods). Furthermore we found that although elevated [CO₂] resulted in a sustained treatment-level increase in plant biomass production (average 273 g C m^-2 yr^-1 or 28% greater stand level NPP) and led to increases in live plant C (average 234 g m^-2 yr^-1 or 36% greater accumulation) and ecosystem C (average 90 g m^-2 yr^-1 or 10% greater accumulation), it did not change the relative distribution of C in these pools. In quantifying interactions between elevated [CO₂], available nitrogen (N) and water availability (represented as precipitation minus potential evapotranspiration (P-PET)), we demonstrate that [CO₂] enhancement of all species NPP is dependent on P-PET and particularly on N availability. Notably, elevated [CO₂] reduced the interaction between N and water availability, such that changes in water availability affected all N levels similarly.