Proposed causes and mechanisms for variation in photosynthesis of Quercus rubra along an urban to rural gradient

Forests in the northeastern United States are currently sequestering atmospheric carbon dioxide released by fossil fuels. The effects of accelerating urbanization and global warming on plant carbon uptake will thus have important consequences for future atmospheric carbon dioxide concentrations and climate change. The urban environment of New York City(NYC) has been shown to produce increased plant growth and carbon sequestration in Populus deltoides. Here we investigate the effects of NYC’s urban environment on native red oaks (Quercus rubra L.). We grew four-year-old oak seedlings at four sites along an urban to rural gradient from NYC to the Catskills for 1.5 years. We measured the short-term response of photosynthesis to [CO2] in three seedlings at each site, as well as environmental parameters. Photosynthesis was highest in NYC, second highest in the Catskills, and lowest at the two intermediate sites. Comparison of photosynthesis data with environmental variables suggests that the combined effect of diurnal temperature range, [CO2], and ozone account for most of the variation in photosynthesis and growth along the gradient. These results and interpretations differ from those of Gregg et al. (2003); we suggest that these differences are due to the species tested, and that Quercus rubra may provide a more accurate indicator of how urbanization and climate change will affect carbon cycling in forests in the eastern United States.