Human behavior has immensely impacted the urban nitrogen (N) cycle. A major contributor of this change is the increase in N inputs in cities via atmospheric deposition and fertilization. More research is needed to understand how these different sources influence urban vegetation. Thus, this project compared how traffic density (a proxy for vehicle emissions, which is a source of atmospheric deposition) and fertilization alters foliar N and carbon (C) content and stable isotope composition. To address this, we collected leaves from 3 species of unfertilized street trees (Acer platanoides, Koelreuteria paniculata and Gleditsia triacanthos). Roughly ten trees of each species were sampled within 10 meters of a high traffic density (> 10,000 vehicles per day) and low traffic density (non-primary or secondary road) road, resulting in a total of 59 street trees. Tree leaves of Acer platanoides, Populus tremuloides, and Pyrus calleryana were also collected in residential yards that varied in age and affluence around Salt Lake Valley. 78 residential trees were sampled. Specific leaf area (SLA, cm2/g) was calculated for both residential and street trees and all comparisons were assessed using ANOVA and Welch’s t test.
Results/Conclusions
There was a marginally significant relationship between traffic and nitrogen stable isotope composition (δ15N), with enrichment in high traffic trees (t=1.9474, p=0.057). However, δ15N did not vary among species (p=0.593). Similarly, δ13C did not vary among species (p=0.245), but was impacted by traffic (t=-2.0048, p=0.05). Traffic density (p=0.273) did not explain variations in leaf %N, although Gleditsia triacanthos had higher leaf N than the other two species (r2=0.231, p=0.001). G. triacanthos also had the lowest SLA (r2= 0.225, p= 0.001). SLA was not influenced by traffic (p= 0.638) but was negatively correlated with leaf N (r2=0.320, p=0.034) and δ13C (r2=0.307, p=0.011) in A. platanoides. SLA and δ13C were also negatively related in G. triacanthos (r2=0.236, p=0.035). For A. platanoides, δ15N and δ13C were similar between residential and street trees. However, residential trees had significantly higher leaf %N (r2=0.201, p=0.001), showing a fertilization effect. These results suggest that high traffic density trees are impacted by C and N pollution but are still N limited relative to fertilized residential trees. More research is needed to understand the implications of these patterns for tree and forest growth and N relations.