Ecosystem services provided by urban trees are well documented, yet we still lack detailed knowledge of the ways in which the ecophysiology of trees in urban forest patches differs from trees in intact rural forests. Furthermore, few studies of urban tree physiology have included more than one city, making it impossible to know whether results are specific to one urban area or may be generalized to urban ecological theory. In this study, we examined ecophysiology of white oak (Quercus alba) and red maple (Acer rubrum) trees in urban forest patches and forested reference sites in New York City, Philadelphia, and Baltimore. Using chlorophyll fluorescence measurements, stomatal characteristics, foliar chemistry, and soil physical and chemical characteristics, we assessed whether differences in tree physiology and soil characteristics were similar across the three cities. An increased understanding of the effects of urbanization on tree growth rates and physiological functions will help ensure that these forest patches continue to provide critical ecosystem services.
During 2015 and 2016, urban forest patches had warmer daytime and nighttime temperatures than nearby rural reference forests in all three cities, with the largest differences observed in New York City, followed by Philadelphia and Baltimore. However, differences in thermal tolerance of photosynthesis (Tcrit) between urban and reference sites were only found in Baltimore white oak trees. Specific leaf area and chlorophyll fluorescence were also higher for white oaks in Baltimore suggesting that trees in these urban sites had higher photosynthetic capacity. In addition, stomatal characteristics indicate that white oaks in Baltimore may have been less drought stressed compared to trees at the reference forest site. Leaf nitrogen and phosphorus were also higher in urban white oaks relative to the reference forest site. Increased chlorophyll fluorescence was also found in urban red maples compared to trees at reference sites, but only in Philadelphia and New York City, possibly related to the higher availability of calcium. Urban soils were sandier and more acidic than reference forest soils in New York City, with less of a clear trend found in Philadelphia or Baltimore. These results demonstrate that differences in native tree physiology occur between urban and reference forest patches but they are site and species specific, making it difficult to draw general conclusions about effects of the urban environment on tree ecophysiology. Data on local site characteristics and tree species performance over time remain necessary to gain insight about urban woodland ecosystem function.