Abstract, Ecological Society of America Annual Meeting, Spokane, WA, July, 1999.

NORBY R.J. Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. Increased basal area increment of a closed-canopy sweetgum stand in the first year of CO₂ enrichment.

Increased growth of trees exposed to elevated concentrations of atmospheric CO₂ has been widely documented in experiments with tree seedlings and young saplings, but some researchers have questioned whether larger trees will respond similarly. Using free-air CO₂ enrichment technology, we are exposing 490-m² plots of 10-year-old, 15-m tall sweetgum (Liquidambar styraciflua) trees to 560 ppm CO₂. The stand has a closed canopy (LAI=4.1) and is in a linear growth phase. During the first growing season under CO₂ enrichment, stand-level basal area increment (BAI) was 19% higher in CO₂-enriched plots than in control or ambient plots. Leaf area index was not significantly different between plots, but BAI per unit leaf area was increased 26% by CO₂ enrichment, consistent with previous predictions. The increase in BAI occurred across all size classes of trees and could not be attributed to differences between plots in stand basal area, tree size distribution, or pretreatment BAI. The important questions concerning how growth of a mature forest will respond to gradually rising CO₂ should not addressed based on these first-year data from non-acclimated trees, but these results demonstrate that older, larger trees in a linear growth phase do not lose their capacity to respond to CO₂ enrichment.