Elevated CO2 generally stimulates C3-type photosynthesis, but it is unclear how an increase in CO2 assimilation will interact with other factors that influence plant growth. In wetlands, the response of plants to elevated CO2 will interact with soil saturation, particularly in forested wetlands where soil saturation is a strong regulator of plant productivity. We performed a four-month experiment to determine whether elevated CO2 and flooding interact to influence the growth of a flood-tolerant tree (Taxodium distichum) and a flood-tolerant herbaceous emergent macrophyte (Orontium aquaticum). Seedlings were grown in glasshouses at two CO2 levels (350 and 700 μL L−1) crossed with two water depths (5 cm above and ≥5 cm below the soil surface). We hypothesized that elevated CO2 would increase photosynthesis regardless of water depth and species; however, we also expected flooding to prevent elevated CO2 from increasing the growth of the tree species due to O2 limitation or other physiological stresses associated with reduced soil environments. We found that elevated CO2 increased whole-plant photosynthesis in both species regardless of the flooding treatment. For T. distichum, this higher photosynthetic rate resulted in greater biomass only in the non-flooded treatment. This result suggests that some factor related to flooding constrained the biomass response of the flooded woody plants to elevated CO2. In contrast, elevated CO2 increased O. aquaticum biomass regardless of the flooding regime, perhaps because it occurs in wetter landscape positions than T. distichum and is less sensitive to flooding. We conclude that flooding may limit plant growth responses to elevated CO2, particularly in woody plant species.
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Vol. 25 • No. 2