Biological, social, and urban design factors affecting young street tree mortality

By Jacqueline W.T. Lu, Erika S. Svendsen, Lindsay K. Campbell, Jennifer Greenfeld, Jessie Braden, Kristen King, and Nancy Falxa-Raymond
New York, NY (February 1, 2010)- In dense metropolitan areas, there are many factors including traffic congestion, building development and social organizations that may impact the health of street trees. The focus of this study is to better understand how social, biological and urban design factors affect the mortality rates of newly planted street trees.

Prior analyses of street trees planted by the New York City Department of Parks & Recreation between 1999 and 2003 (n=45,094) found 91.3% of those trees were alive after two years and 8.7% were either standing dead or missing completely. Using a site assessment tool, a randomly selected sample of 13,405 of these trees was surveyed throughout the City of New York during the summers of 2006 and 2007. Overall, 74.3% of the sample trees were alive when surveyed and the remainder were either standing dead or missing.
Results of our initial analyses reveal that highest mortality rates occur within the first few years after planting, and that land use has a significant effect on street tree mortality. In terms of a tree’s urban design and neighborhood context, this study confirms the observations of many urban foresters that curbside trees planted in lawn strips and in low-vehicular traffic areas are more likely to survive. This study also quantifies the disproportionately high mortality rates of trees that are planted in street medians compared to trees located on the curb. Based on this result, NYC Parks has already changed their planting policies for median trees, and is planting trees in only the widest street medians, where adverse factors like collisions, salt exposure, and minimal soil volume are less likely.
Similarly, our observation of the effectiveness of tree guards in protecting young street trees is corroborated by the experiences of NYC’s practicing urban foresters. Such demonstrated effectiveness may justify the expense of securing street tree guards at the time of planting. Our results suggest that civic stewardship and neighborhood sociability is a critical complement to municipal management and investment in new street tree plantings.
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Biological, social, and urban design factors affecting young street tree mortality
Young Street Tree Mortality