200 Cowett and Bassuk: Is Street Tree Diversity Increasing in New York State, USA? Lockwood and Berland (2019) had sufficient concern with possible tree species misidentification that it con- tributed to a decision to limit results to the genus level. In addition to variability in observer expertise and experience, data obtained for this paper are not stan- dardized, not only between those inventories con- ducted by the tree inventory firms and those conducted by others, but also between the inventories conducted by tree inventory firms. In particular, there are differ- ences among the inventories in tree classification at the species level. For example, some inventories clas- sify all species in a genus, such as Amelanchier, as Amelanchier spp.; some inventories classify all mem- bers of a genus subset, such as ornamental cherry trees, as the species of that genus (e.g., Prunus spp.); and some inventories classify all trees by species. Variability may exist as well in the geographic extent and completeness of the inventory. For example, state highways may or may not be inventoried, mainte- nance responsibilities for some streets may change between governmental agencies, and some areas may not be surveyed due to budget or time constraints. Data sets frequently specify latitude and longitude GPS coordinates for each tree, which enables the extent for most, but not all, inventories to be known. The great majority of the inventories obtained for this paper were complete inventories with GPS coordi- nates. Finally, some inventories include not only street trees, but park trees. Previous studies have found the dynamics and population structures of street trees and park trees to be significantly different, including their species composition and diversity (Welch 1994; Nielsen et al. 2007; North et al. 2018). This suggests that street tree diversity is best assessed independently of park tree diversity. Accordingly, for municipalities where street trees and park trees were both inventoried, and where it was practicable to do so, street trees were differentiated from park trees and analyzed separately. Park trees have not been analyzed. Ideally, all inventories obtained for this study would have been conducted by the same personnel utilizing the same inventory collection methods and protocols, especially for those municipalities for which data were collected at multiple points in time. Doing so would have maximized data accuracy, made data sets fully comparable, and improved the validity of find- ings. That this did not happen is unfortunate, but not surprising, since it is common in urban tree invento- ries that different approaches are used at different ©2021 International Society of Arboriculture times by different personnel (Morgenroth and Östberg 2017). In other words, everybody does things differ- ently. However, where we had sufficient concern about data accuracy and variability potentially impacting the validity of findings, especially for municipalities with multiple inventories, those data were excluded from assessment. Data Analysis Whereas the municipalities with newer and older inventories are directly comparable, the 75 inventories in the statewide diversity assessment comprise a non- random data sample with the potential for selection bias that could reduce the accuracy of any findings, especially in comparison to previous statewide assess- ments. The 2014 and 2017 assessments (Cowett and Bassuk 2014; Cowett and Bassuk 2017) both employed post-stratification of data and weighting with auxiliary information to correct for selection bias (Bethlehem 2010). Data were stratified by USDA Plant Hardiness Zones and weighted by measures of street length con- tained within those zones. The 2014 assessment strat- ified data with the 1990 version of the zones and created weighted measures from New York State ALIS (Accident Location Information System) street cen- terline files. The 2017 assessment stratified data with the 2012 version of the zones and created weighted measures from United States Census Bureau TIGER- Line All Roads files. This statewide assessment used the same post-stratification and weighting steps as the 2017 assessment and the same weighted measures (Table 1). Inventories were assigned to a 2012 Plant Hardiness Zone based on location of the municipality’s inner centroid (i.e., a geometrically calculated center point within municipal boundaries). Zone 3 is sparsely populated and not associated with any municipal tree data, and tree species and genera hardy in Zone 3 are not meaningfully different from those hardy in Zone 4 Table 1. Summed selected street length contained within the 2012 USDA Plant Hardiness Zones in New York State. 2012 Plant Hardiness Zone Zone 3 Zone 4 Zone 5 Zone 6 Zone 7 Street length (meters) 51,687 3,043,301 24,698,158 25,043,999 35,072,884 Percent statewide total 0.059% 3.462% 28.095% 28.488% 39.896%
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