Urbanization and plant invasion are potential threats to plant growth and long-term adaptations subsequently impeding urban greenspace ability to effectively mitigate harmful environmental conditions. Our current knowledge of urbanization and plant invasion effects on tree adaptation responses has focused mainly on comparing tree growth and mortality. The goal of our investigation was to assess how urban stressors with respect to differing city size and sub-canopy multiflora rose invasion influence native Acer rubrum physiology and biochemistry. We hypothesized that foliar pigment and nitrogen concentrations will be greater within Philadelphia forests than in Newark forests, and in the absence of sub-canopy plant invasion due to higher soil nitrogen, higher mineralization rates, and less competition for resources. To test our hypothesis, we collected lower canopy leaves from A. rubrum trees (DBH: 7.5 – 23 cm) and sub-canopy soils in five forests within Newark, DE and Philadelphia, PA. We sampled five trees experiencing sub-canopy multiflora rose invasion presence within three forests and five trees experiencing no sub-canopy invasion within all five forests. We quantified total chlorophyll (TCC) and carotenoid (TCar) concentrations to assess foliar pigment concentrations. Additionally, foliar/soil nitrogen and foliar polyamines will be analyzed to assess biochemical responses to urbanization and plant invasion.
Preliminary results supported our hypothesis. TCC and TCar contents were greater in Philadelphia, PA than in Newark, DE forests (p<0.0001, F=8.84; p<0.0001, F=14.1, respectively). Greater pigment concentrations in A. rubrum leaves may due to nitrogen rich soils supplying greater bioavailable nitrogen to trees, which correlate with increase pigment synthesis. Additionally, leaf adaptation responses due to greater canopy-light penetration and higher local temperatures in larger cities could further pack plastids and concentrate pigments. Our results show an important role of city size on A. rubrum pigments adaptation response possibly due to greater resources with increasing city size. Sub-canopy rose invasion did not influence A. rubrum pigments significantly (p>0.05) in either city however, across city, Newark sub-canopy invasion effects on A. rubrum TCC and TCar concentrations were significant (p=0.0023; p<0.0001, respectively) compared to Philadelphia uninvaded sub-canopy trees. The difference between Philadelphia and Newark seems to be a stronger driver of forest tree pigment concentrations than sub-canopy invasion possibly due to greater canopy light penetration and higher nitrogen levels in warmer Philadelphia forests. We will analyze leaf and soil nitrogen, and foliar polyamines to provide further evidence to support potential mechanisms for differences observed between the two cities.