An Assessment of Street Tree Diversity: Findings and Implications in the United States

Integrating tree species diversity into urban forest management can help create resilient tree populations. In this study, the abundance of trees within families, genera, and species levels was determined through a system developed to evaluate the diversity of urban street trees at different scales. Municipal foresters were asked to report the six street tree species most commonly used in urban forests as an assessment of tree diversity. Through the use of this question in a series of urban forest management surveys, we were able to describe the diversity of urban street tree species in the 48 continental United States across time (1974 to 2014) and space (national, regional, and local levels). Throughout the United States the top six street tree species were distributed in 115 species, 71 genera, and 32 families. At the national scale, no one tree species or family dominated, but the Acer genus was 21.2% (0.8 standard error, or SE). At the regional level, 16.5% (3.3 SE) of all reported street trees were Acer platanoides in the Northeast region, and 8.1% (1.1 SE) of the South region were Quercus virginiana. At the local level, however, a lack of tree diversity becomes apparent. In any community, the top six reported tree species account for 61.5% (0.4 SE) of the total street tree population and the most common tree accounts for 23.7% (1.0 SE) of the total. When assessed over a 40-year period, changes in tree species and genera in each region were relatively minor. Among them, the Mideast and Northeast regions continued to be dominated by Acer and the South region maintained its preference for Quercus. This study provides a method to assess street tree populations when using surveys to quantify municipal forestry programs at state and national levels.