6 Cowett and Bassett: Street Tree Diversity in Three Northeastern U.S. States Table 1. Summed selected street length contained within the 2012 USDA Plant Hardiness Zones in New Jersey, New York, and Pennsylvania, U.S. 2012 Plant Hardiness Zone New Jersey Street Length in Meters Percent of Statewide Total New York Street Length in Meters Percent of Statewide Total Pennsylvania Street Length in Meters Percent of Statewide Total Zone 3 0 0.000% 51,687 0.059% 0 0.000% Zone 4 0 0.000% 3,043,301 3.462% 0 0.000% only in New York State. Zone 3 is sparsely popu- lated and not associated with any municipal street tree data, containing only 0.059% of all selected New York State street length. Therefore, for weight- ing purposes, selected New York State street length contained in Zone 3 was aggregated with selected street length contained in Zone 4. Finally, a regional weighted mean was created from the statewide weighted means based on the percentage of each state’s selected street length relative to the sum of selected street length found in all three states. RESULTS Species and Genus Composition Weighted statewide relative abundance percentages were calculated for street tree species and genera from collated street tree inventory data (Table 2; Table 3). Acer platanoides (Norway maple) was found to be the most prevalent street tree species in all three states, with a regional weighted mean of 16.34% (14.63% in New Jersey, 19.80% in New York, and 15.08% in Pennsylvania). Acer spp. (ma- ple) was found to be the most prevalent street tree genus in all three states, with a regional weighted mean of 38.94% (36.72% in New Jersey, 40.91% in New York, and 38.96% in Pennsylvania). On both regional and statewide levels, these results exceed Santamour’s 10% rule for species, and his 20% rule for genus, and reflect abundance percentages found on the municipal level. Municipal species composition revealed that, for those municipalities from which street tree inventory data at the spe- cies level were obtained, 47 of 50 municipalities in New Jersey (94.0%), 152 of 153 municipalities in New York (99.3%), and 38 of 43 municipalities in Pennsylvania (88.4%) exceeded the 10% rule ©2017 International Society of Arboriculture Zone 5 3,394 0.006% 24,698,158 28.095% 7,830,965 8.160% Zone 6 16,915,805 30.165% 25,043,999 28.488% 70,396,700 73.357% Zone 7 39,159,020 69.829% 35,072,884 39.896% 17,736,544 18.482% proposed by Santamour. In most but not all cases, this was due to the percentage of street trees that were Acer platanoides (Norway maple), although in many municipalities the percentages of Acer rubrum (red maple), Acer saccharum (sugar maple), Gleditsia triacanthos (honeylocust), Platanus × acerifolia (London planetree), Pyrus calleryana (Callery pear), Quercus palustris (pin oak), and/ or Quercus rubra (northern red oak) surpassed 10%. Municipal genus composition revealed that, for those municipalities from which street tree in- ventory data at the genus level were obtained, 56 of 57 municipalities in New Jersey (98.2%), 162 of 164 municipalities in New York (98.8%), and 53 of 54 municipalities in Pennsylvania (98.1%) exceeded the 20% rule proposed by Santamour. In most but not all cases, this was due to the per- centage of street trees belonging to the Acer genus, although in many municipalities the percentages of Gleditsia spp., Malus spp. (crabapple), Platanus spp. (planetree), Pyrus spp. (pear), Quercus spp. (oak), and/or Tilia spp. (linden) surpassed 20%. Minimum average winter temperature as repre- sented by the 2012 USDA Plant Hardiness Zones appeared to impact species and genus composition, as substantial differences were found between the zones for many but not all species and genera. Differences were more pronounced in New York and Pennsylva- nia than in New Jersey. For example, in New Jersey, differences were found for some, but not all, street tree genera (Figure 4); in New York, differences were found for most street tree species (Figure 5); and in Pennsylvania, differences were found for most street tree genera (Figure 6). In addition, the mean num- ber of street tree species and genera per zone for the three states was found to increase as minimum average winter temperature increased (Figure 7).
January 2017
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