Loblolly pIne (Pinus taeda) and sweetgum (Liquidambar styraciflua) were grown in miniature stands at 7.6-cm spacings outdoors in open-top chambers (4.6 m in diameter and 3.5 m tall) for 16 months. Treatments consisted of ambient- and elevated-CO₂ , drought-stressed and well-watered, and stand type (monoculture and 50:50 replacement mixture). Gas exchange was measured monthly, growth parameters bimonthly.

Loblolly pine carbon exchange rate (CER) was positive throughout the winter in all treatments and averaged 83% of summer rates. Between November 1994 and April 1995, relative crowding coefficient (RCC) of pine stem volumes increased regardless of CO₂ or water availability. RCC of pine biomass increased in droughted stands relative to well-watered stands, while RCC of sweetgum showed the opposite response. Based on these results increased atmospheric CO₂ concentrations will not affect the competitive outcomes of loblolly pine and sweetgum mixed stands: loblolly pine will continue to be more competitive on dry sites, sweetgum on wet sites.

CER of loblolly pine and sweetgum, as well as soil respiration, were consistently significantly greater in elevated-C02 stands. CER in upper-canopy foliage was significantly greater than that of lower-canopy foliage for sweetgum. Loblolly pine, but not sweetgum, demonstrated a significant canopy position x CO₂ interaction, with upper-canopy CER greater only in elevated-CO₂ conditions. No consistent acclimation of CER to elevated CO₂ was statistically significant for either species, although acclimation response was stronger in sweetgum than in loblolly pine.