Loblolly pine (Pinus taeda) occurs in mixed stands with hardwood species including red maple (Acer rubrum) and sweetgum (Liguidambar styraciflua) across much of the southern United States. Rising atmospheric CO₂ concentrations over the next several decades could influence competitive interactions between these species.

This research examined the effects of increased atmospheric CO₂ concentration in combination with the availability of other resources on growth and gas exchange characteristics of these species grown in direct competition in miniature stands and compared characteristics of miniature stands with field-grown stands of similar composition.

In a greenhouse study, loblolly pine and red maple were grown in monoculture and 50:50 replacement mixtures at a 2.54 x 2.54 cm spacing for two growing cycles under ambient (408 ppm) or elevated (806 ppm) CO₂ concentrations and well-watered or droughted (0.5x well-watered) conditions. Loblolly pine dominated all mixed stands and was proportionally larger while red maple was smaller in mixtures versus monocultures. This pattern was also observed in weed-free f ld-grown stands after five growing seasons.

Biomass data from loblolly pine and red maple grown in monocultures and 50:50 replacement mixtures at 2.54, 5.08 and 7.62 cm2 initial spacing suggest that relative dominance of species in miniature stands is not dependent on initial spacing. Dominance of red maple over loblolly pine in all mixed stands in this experiment suggests that soil medium is important in determining relative growth of these species.

Photosynthesis rates and response to elevated CO₂ differed between whole stands and individual seedlings from these stands. Individual seedlings tended to be more responsive to elevated CO₂ compared to whole stands suggesting reduced responsiveness of lower canopy leaves may dampen the effect of elevated CO₂ on canopy photosynthesis rates.

Growth under elevated CO₂, alone or in combination with water did not affect relative sizes of these species. These results suggest that competitive interactions will not change between these species in response to growth in high CO₂ environments.