The feedback from forest growth to atmospheric CO2 is a key issue in coupling carbon cycle models to climate models. A free-air CO2 enrichment experiment to examine forest responses to rising CO2 was established in 1997 in a 10-year-old deciduous forest in eastern
Results/Conclusions
The response of NPP to eCO2 has declined over time. The mean increase in NPP over 10 years of CO2 enrichment was 21%, but the NPP response has steadily declined from 20% in 2004 to 10% in 2007. NPP in eCO2 is closely correlated with NPP in aCO2, suggesting that the trees are responding to similar environmental influences. The relative effect of CO2 was greatest in years when NPP was greatest and declined as ambient conditions caused NPP to decline, as was the case from 2005-2007. We examined possible causes of the declining growth response under eCO2. Leaf area index has been relatively constant and there is no correlation between interannual variation in NPP and LAI. Soil moisture can explain some of the variation in fine-root production, which is the most responsive NPP component to eCO2, but it cannot explain the more gradual decline in NPP response to eCO2. The most likely explanation for that decline is reduced N availability, which is supported by a model of the stand's carbon-nitrogen-water economy. We have shown previously that tree growth at this site is N limited, and our 10-year record provides evidence that N limitation has been increasing throughout the duration of the experiment, based on the steady decline in foliar nitrogen concentration in both aCO2 and eCO2 plots. Increased fine-root proliferation under eCO2, especially at depth in the soil, has thus far enabled sustained production responses and increased N uptake, but this has not benefited aboveground wood production. Wood production has declined, and the effect of CO2 on wood production, which previously was small, has now disappeared. Plant and soil analyses from the remaining 2 years of the experiment will further elucidate the mechanism of NPP decline.