Re-assessment of plant carbon dynamics at the Duke free-air CO2 enrichment site: interactions of atmospheric [CO2] with nitrogen and water availability over stand development

P>The potential for elevated [CO2]-induced changes to plant carbon (C) storage, through modifications in plant production and allocation of C among plant pools, is an important source of uncertainty when predicting future forest function. Utilizing 10 yr of data from the Duke free-air CO2 enrichment site, we evaluated the dynamics and distribution of plant C. Discrepancy between heights measured for this study and previously calculated heights required revision of earlier allometrically based biomass determinations, resulting in higher (up to 50%) estimates of standing biomass and net primary productivity than previous assessments. Generally, elevated [CO2] caused sustained increases in plant biomass production and in standing C, but did not affect the partitioning of C among plant biomass pools. Spatial variation in net primary productivity and its [CO2]-induced enhancement was controlled primarily by N availability, with the difference between precipitation and potential evapotranspiration explaining most interannual variability. Consequently, [CO2]-induced net primary productivity enhancement ranged from 22 to 30% in different plots and years. Through quantifying the effects of nutrient and water availability on the forest productivity response to elevated [CO2], we show that net primary productivity enhancement by elevated [CO2] is not uniform, but rather highly dependent on the availability of other growth resources.