Allometric determination of tree growth in a CO2-enriched sweetgum stand

In a free-air CO2 enrichment (FACE) study, above-ground growth of Liquidambar styraciflua (sweetgum) trees was measured in a closed-canopy stand to test hypotheses about long-term effects of rising [CO2] in a deciduous forest. Allometric relationships were established between above-ground dry mass and basal area, height, taper and wood density for all trees in the 314-m(2) plots. Leaf area index was calculated from leaf litter mass. In the first year of exposure, elevated [CO2] stimulated stand-level aboveground dry matter increment by 33%, but in the second year the growth stimulation was reduced to 15% and was no longer statistically significant. Leaf area index was not affected by CO2 enrichment in either year. These results indicate that large trees that are no longer in an exponential growth phase retain the capacity to respond to elevated [CO2]. The combined 2-yr response of a 24% increase in growth per unit leaf area is consistent with predictions from earlier studies. A longer-term data set is needed to resolve the reason for different responses in the 2 years.