Anthropogenic stressors, such as altered climate conditions, altered biogeochemical cycling, and biotic invasions, are driving fundamental shifts in the abiotic and biotic characteristics of urban forest ecosystems. Urban forests are ideal surrogates for investigating the simultaneous-cumulative effects of global change factors on urban tree physiology and adaptive plasticity. Urban tree physiological bio-indicators of environmental stressors or stimulators have received inadequate attention in urban-plant ecology research. The objective of this study was to determine red maple (A. rubrum L.) foliar physiological and biochemical patterns in response to city size and sub-canopy Rosa multiflora invasion. We hypothesized that red maple foliar physiology and biochemistry patterns will be greater in Philadelphia, PA than Newark, DE forests due to evolved physiological plasticity to greater urban pressures while sub-canopy invasion will have direct positive/negative influence. This research utilizes a long-term urban forest ecology study, the FRAME (Forest Fragments in Managed Ecosystems) to test our hypotheses. We sampled mid-upper canopy sun leaves from 79 adult red maple trees (experiencing sub-canopy R. multiflora invasion presence/absence) and sub-canopy soils in 10 forests across Newark, DE and Philadelphia, PA. We analyzed leaves for pigments, polyamines, amino acids, nitrogen, and heavy metals, and soils for nutrients, bulk density and heavy metals.
Results/Conclusions
Foliar total chlorophyll (p<0.0001), carotenoids (p<0.003), nitrogen (p<0.003) and natural abundance nitrogen isotopic content (δ15N) (p=0.011) were greater in red maple trees within Philadelphia compared to Newark forests. The observed higher foliar chlorophyll concentration of red maple trees in Philadelphia correlates with greater nitrogen sources, elevated temperature, and metal stress. Similarly, foliar polyamines (e.g., putrescene, spermidine) and amino acids (e.g., arginine, proline, glutamic acid) and sub-canopy soil metals (e.g., Pb, Al, Na, Cu, Zn) were significantly greater (p<0.05) in Philadelphia forests. Increased foliar polyamines and amino acids are synonymous with increase nitrogen uptake and metabolism in response to cellular oxidative stress, particularly heavy/toxic metals. Putrescene, proline and carotenoids syntheses are upregulated to protect the chloroplasts from elevated reactive oxygen species stress caused by heavy metals and drought. Sub-canopy R. multiflora did not influence foliar physiology and biochemistry (p>0.05). In these forests, urbanization has a greater influence on red maple tree physiology and biochemistry than sub-canopy plant invasion impact on edaphic conditions. Our results suggest urban trees are acclimating physiologically to environmental abiotic drivers. In order to isolate above- and belowground drivers of tree foliar physiology, we will conduct a manipulative field experiment in 2018.