Nitrogen resorption of urban trees in high N deposition forests and fertilized lawns

Background/Question/Methods

Anthropogenic activities greatly increase reactive nitrogen (N) in urban ecosystems via indirect (i.e., regional N deposition) and direct (i.e., fertilization) N inputs.  Current knowledge about how trees respond to increased N availability in urban ecosystems is sparse.  The goal of this study is to measure the N resorption efficiency (percent reduction in element content) and proficiency (terminal content in senesced leaves) of urban trees in high N deposition urban forests relative to rural forests and in fertilized urban lawns relative to unfertilized trees.  We expect that trees in urban forests and fertilized lawns will be less efficient and proficient at N resorption than rural forests and unfertilized trees.  We studied Quercus prinus and Acer saccharum trees in urban and rural forests in Louisville, KY where regional N deposition was found to be greater in the urban than rural forests.  Additionally, we studied Gleditsia triacanthos trees in fertilized lawns and unfertilized areas on the University of Utah campus in Salt Lake City, UT.  Pre-senescent (peak growing season) and senesced leaves were collected from five trees in each site type (i.e., urban, rural, fertilized, unfertilized) and were dried at 55°C to constant mass, weighed, and ground prior to total N analysis.

Results/Conclusions

Preliminary results for Quercus prinus (QUPR) and Acer saccharum (ACSA) trees in urban and rural forests did not support our expectation that urban trees in high N deposition environments resorb less N.  We found that urban QUPR and ACSA trees were more proficient (QUPR urban = 51.1 vs. rural = 68.2 µg cm^-2; ACSA urban = 29.7 vs. rural = 37.7 µg cm^-2) and efficient (QUPR urban = 73.7% vs. rural = 64.1%; ACSA urban = 71.9% vs. rural = 61.6%) at N resorption than rural trees.  Greater N resorption efficiency and proficiency of urban trees indicates they are acting to minimize N loss.  While this is contrary to our expectation, previous research on soil N mineralization in these forests revealed lower N production rates in urban forests relative to rural forests.  Thus, internal soil N processes may be more important than external N deposition in controlling N cycling within urban trees.  Furthermore, lower soil N production rates and greater N resorption in urban trees suggest these urban forests are not leaky with respect to N.  Future analysis of fertilized versus unfertilized trees will provide further insight as to how urban trees respond to altered N inputs across urban ecosystems.