Urban development in the southern Great Plains: effects of atmospheric NO_x on the long-lived post oak tree (Quercus stellata)

Concentrated human activities such as the burning of fossil fuels have resulted in chronic nitrogen (N) additions to urban ecosystems. We predicted that urban development in North Texas (NTX; the largest “megapolitan” region in the Great Plains) would be positively correlated with atmospheric concentrations of nitrogen oxides (NO_x) and with the leaf tissue quality (lower C:N ratio) and herbivory of the long-lived native tree, post oak (Quercus stellata). Data from air monitoring stations were used to calculate distance-weighted estimates of atmospheric NO_x for 11 sites of differing urban development across NTX. Soil samples were collected at each site along with post oak leaves, estimates of herbivory, and measurements of tree size. Percent urban development was strongly positively correlated with atmospheric NO_x concentrations, though there was no correlation between atmospheric NO_x and soil N. There was a positive relationship between soil N and leaf tissue quality, but only where atmospheric NO_x was relatively low, possibly due to factors that covary with urban development. Herbivory was not significantly correlated with leaf tissue quality, but leaves from the two most urban sites had the greatest amount of insect herbivory. The NO_x concentrations in NTX were lower than other industrialized cities, which may be due to climate and topography differences or the relatively young age of this urban area. This study adds to the expanding body of literature examining how urban ecosystems are affected (or not) by N deposition and suggests that interactions among NO_x, soils, and plants are complex and sometimes, counterintuitive.