Effects of CO2 concentration and water availability on growth and gas exchange in greenhouse-grown miniature stands of Loblolly Pine and Red Maple

1. The study assesses the effects of atmospheric CO2 concentration and water availability on stand development and photosynthetic characteristics of Loblolly Pine (Pinus taeda) and Red Maple (Acer rubrum). Miniature stands of these species were grown from seed in monoculture and in a 50:50 replacement mixture for two accelerated growing seasons. 2. Both species had greater biomass under the higher levels of CO2 and water availability. Biomass of Loblolly Pine seedlings in mixed stands exceeded that in monocultures, while the opposite was true for Red Maple. No significant treatment interactions were detected for total biomass. Significant main effects for water and stand type were detected for stem height of Loblolly Pine. CO2, water and stand type interactions were observed for height of Red Maple. 3. Net photosynthetic rates were measured on miniature stand canopies and constituent seedlings from these stands. Both species exhibited higher photosynthetic rates under elevated CO2. However, expression of photosynthesis on a leaf mass or soil area basis affected conclusions regarding the role of water availability on stand-level response to elevated CO2.