Arboriculture & Urban Forestry 43(1): January 2017 Wiener is sometimes preferred because it gives more weight to Simpson to less abundant species and is more sensitive to sample size (Bar- bour et al. 1987; Colwell 2009). Despite these distinctions, Simpson and Shannon-Wiener have been applied somewhat interchangeably in assessing street tree diversity. For example, Maco et al. (2005) used Simpson’s Diversity Index to assess street tree diversity in Berkeley, California, Dobbs et al. (2013) used the Shannon-Wiener Diversity Index to compare tree species composi- tion between streets, parks, and private property in Melbourne, Australia, and Jim and Chen (2009) and Kara (2012) used both the Simpson and Shannon-Wiener indices to assess street tree diversity in Taipei, Taiwan, and Aydin, Tur- key, respectively. Additionally, Sun (1992) and Sreetheran et al. (2011) used the inverse of Simp- son’s Diversity Index (1/SDI) to equate an inverse SDI value of 10 with Santamour’s 10% rule for species and an Inverse SDI value of 20 with a 5% benchmark for species; Subburayalu and Sydnor (2012) used a Simpson Diversity Index weighted by environmental benefits, pest vulnerability, and taxon adaptability to identify areas requiring increased street tree diversity in four Ohio, U.S., communities. As with Santamour’s 10-20-30 rule, results reported for the Simpson and Shannon- Wiener indices have not been equated scien- tifically with effective thresholds for street tree diversity. However, the Simpson and Shannon- Wiener indices and relative abundance metrics have been employed not only to make quan- titative comparisons for street tree diversity between neighborhoods, municipalities, regions, and other geographic levels, but also to explore explanatory factors. Thus, McPherson and Rown- tree (1989) and Pauleit (2002) found greater street tree diversity to be associated with warmer climate, Jim and Chen (2009) found greater street tree diversity in older neighborhoods, and Kara (2012) found differences in street tree diversity based on land use and street type. This paper assesses street tree diversity for three states in the northeastern United States— New Jersey, New York, and Pennsylvania—based on street tree inventory data obtained from municipalities in these states. It quantifies dif- ferences in diversity, between municipalities 3 and between states, and considers explanatory factors. Finally, this paper makes recommenda- tions for increasing diversity so as to enhance the resilience of street tree populations to pests, diseases, and climate change, and ensure the continued provision of ecosystem services and social benefits associated with street trees. METHODS Street tree inventory data were obtained for 57 municipalities in New Jersey, 164 municipalities in New York, and 54 municipalities in Pennsyl- vania (Figure 1). All municipalities from which data were obtained are Census Places. The United States Census Bureau defines a Place as a legally bounded and incorporated concentration of pop- ulation, such as a city, town, village, or borough, or an unincorporated concentration of popula- tion identifiable by name whose boundaries may change from one decennial census to the next (U.S. Census Bureau 2010). In 2014, there were 545 Census Places in New Jersey, 1,196 in New York, and 1,762 in Pennsylvania (U.S. Census Bureau 2015). Therefore, street tree inventory data were obtained from 10.5% of all Census Places in New Jersey, 13.7% of all Census Places in New York, and 3.1% of all Census Places in Pennsylvania. These data were used to calculate the relative abundance percentages of street tree species and genera comprising each inventory. Additionally, sta- Figure 1. Street tree inventories obtained in New Jersey, New York, and Pennsylvania, U.S. ©2017 International Society of Arboriculture
January 2017
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