Many aspects of the urban environment have the potential to affect tree growth and physiology in forest patches, including elevated temperatures and modified soil biogeochemistry. However, changes in tree growth are likely to vary by species and across urban areas, reflecting the local environmental conditions associated with the idiosyncratic trajectory of development in a city. An increased understanding of the effects of urbanization on tree growth rates and physiological functions, such as photosynthesis, respiration, and nutrient use, will help ensure that urban forest patches continue to provide critical ecosystem services. In this study, we examined growth rates of a dominant native tree species (white oak (Quercus alba L.) across urban and reference forest sites of New York, NY; Philadelphia, PA; and Baltimore, MD as well as soil properties and air temperature around each tree. A growth chamber experiment was then used to directly examine the effects of field-collected urban and rural soils and air temperature regimes on white oak acorn germination, and seedling growth and physiology.
Results/Conclusions
Urban forest patch sites had consistently warmer daytime and nighttime temperatures than reference forest sites. Urban forest patch soils also had elevated calcium, magnesium, and heavy metal concentrations compared to reference forest soils. Urban vs. reference tree growth rates varied by city and over time. Over the 145-year tree ring record analyzed, white oak basal area increment was significantly higher in urban trees compared to reference trees. Changes in the relative production of earlywood and latewood between site types in each city and over time may relate to precipitation patterns. In the growth chamber experiment, urban soils supported significantly higher total seedling biomass, but had no effect on root:shoot ratio. Soil type also had a significant effect on leaf-level physiological parameters, with seedlings grown in urban soils having greater Anet, Vcmax, ETRmax, Jmax, PNUE, gs, and Performance Index (PIabs; integrated chlorophyll fluorescence parameter). PIabs measurements taken throughout the experiment reveal a significant time * temperature interaction effect revealing potential heat stress. Perhaps due to their ability to persist in a wide variety of environmental conditions, mature white oak trees appear to be acclimating to urban forest patch conditions of the eastern U.S. and in some cases are experiencing enhanced growth rates compared to trees in nearby reference forests. Urban forest patch soils are also able to support robust white oak seedling growth, but continued warming air temperatures may cause seedling stress and reduced growth.