Elevated CO differentially alters the responses of coocurring birch and maple seedlings to a moisture gradient.

To determine the effects of elevated CO and soil moisture status on growth and niche characteristics of birch and maple seedlings, gray birch (Betula populifolia) and red maple (Acer rubrum) were experimentally raised along a soil moisture gradient ranging from extreme drought to flooded conditions at both ambient and elevated atmospheric CO levels. The magnitude of growth enhancement due to CO was largely contingent on soil moisture conditions, but differently so for maple than for birch seedlings. Red maple showed greatest CO enhancements under moderately moist soil conditions, whereas gray birch showed greatest enhancements under moderately dry soil conditions. Additionally, CO had a relatively greater ameliorating effect in flooded conditions for red maple than for gray birch, whereas the reverse pattern was true for these species under extreme drought conditions. For both species, elevated CO resulted in a reduction in niche breadths on the moisture gradient; 5% for gray birch and 23% for red maple. Species niche overlap (proportional overall) was also lower at elevated CO (0.98 to: 0.88: 11%). This study highlights the utility of of experiments crossing CO levels with gradients of other resources as effective tools for elucidating the potential consequences of elevated CO on species distributions and potential interactions in natural communities.