The future urban forest: a survey of tree planting programs in the Northeastern United States

Cities around the world are pursuing tree planting as a way to increase tree cover. Despite the growing interest in planting trees as a way to offset climate change, counter the negative impacts of urbanization, and provide benefits to city dwellers, there has not been a recent effort to quantify the number of trees being planted nor the species composition of these plantings. Because ecosystem services and ecosystem threats can transcend municipal boundaries, understanding trends in tree planting at multiple spatial scales is critical. To overcome this knowledge gap, we used a survey to collate recent tree planting data from 52 cities with populations greater than 50,000 people in the Northeastern USA. The four largest cities in our study (New York, Boston, Philadelphia, Washington D.C.) planted over 87% of all the trees that were planted in the region. Smaller cities, which are numerous in region, planted proportionally fewer trees and, in over 40% of the small cities surveyed, planting palettes included invasive tree species, highlighting both a resource and knowledge gap in smaller cities as compared with larger ones. Regardless of city size, records also illuminated an overreliance on certain genera for specific ecosystem services; nearly 20% of all shade trees were Quercus species and over 40% of ornamental trees were either Syringa or Prunus species. As cities continue to rely on tree-planting as a form of green infrastructure, our results demonstrate that more consideration to establishing diverse planting palettes will be an important way to ensure that ecological resilience is maintained. Achieving this will depend on increased opportunities to collaborate across municipal boundaries and promoting cross-learning from the experiences of more innovative urbanized regions to urban regions with less infrastructure and expertise.

Methods
In the winter of 2017, we sent a survey via e-mail to municipalities and non-profit organizations in cities across the Northeastern United States. The structured survey asked respondents to report on how many and which species of trees were planted in their city for each year from 2012-2017. We supplemented data from these survey responses with additional datasets to characterize the taxonomy, nativity, and height of tree species.