Foliar nitrogen characteristics of four tree species planted in New York City forest restoration sites

Urban forests provide important environmental benefits, leading many municipal governments to initiate citywide tree plantings. However, nutrient cycling in urban ecosystems is difficult to predict, and nitrogen (N) use in urban trees may be quite different from use in rural forests. To gain insight into these biogeochemical and physiological processes, we compared foliar N characteristics of several common northeastern deciduous tree species across four newly planted New York City afforestation sites as well as at the Black Rock Forest (BRF), a rural oak-dominated forest in the Hudson Highlands, New York. Foliage sampled at BRF was consistently depleted in ¹⁵N compared to urban foliage, and Amelanchier canadensis, Nyssa sylvatica, Prunus serotina, and Quercus rubra showed significant variation in foliar nitrogen isotope signatures (δ¹⁵N) among the four urban sites. A. canadensis and P. serotina showed significantly greater ability to assimilate nitrate at BRF compared to urban sites, as measured through nitrate reductase activity (NRA). There were no significant differences in NRA among tree species growing at the four urban sites. Only P. serotina and N. sylvatica showed significant variation in foliar N concentrations (%N) both among urban sites and compared to BRF. The isotopic and %N data suggest greater N availability but less available nitrate at the newly planted urban sites compared to BRF, possibly due to different anthropogenic inputs or higher rates of nitrification and nitrate leaching at the recently planted urban sites compared to likely lower rates of N cycling in the intact rural forest. In addition, the tree species varied in their response to N availability at the urban sites, with potential implications for growth and survival. Understanding N cycling in urban systems and the associated physiological changes in vegetation is critical to a comprehensive evaluation of urban forest restoration, and may have implications for carbon sequestration and water quality issues associated with nitrate export, two important areas of management concern.