208 Cowett and Bassuk: Is Street Tree Diversity Increasing in New York State, USA? the dominant street tree species, and maple continues to be the dominant street tree genus (Tables 3 and 4), and nearly all the 75 inventories comprising the state- wide assessment and the 32 sets of newer and older inventories fail to meet Santamour’s 10% and 20% benchmarks for species and genera diversity. Therefore, more work appears to be required to further increase diversity. Should those state and municipal officials responsible for street tree man- agement keep doing what they are doing, or should they be doing some things differently? The annual rate of decrease for Norway maple found in the municipalities with multiple inventories suggests that, if that rate remains constant, the percentage of Norway maple for these municipalities could decline to 10% in 18 years. Similarly, the annual rate of decrease for maple found in the municipalities with multiple inventories suggests that, if that rate remains con- stant, the percentage of maple for these municipali- ties will decline to 20% in 16 years. However, these estimates may be optimistic, since they are based on a subset of municipalities in the state comprising a nonrandom data sample with the potential for selec- tion bias that could reduce their accuracy. For exam- ple, since the prevalence of maple and Norway maple was found previously to vary geographically within the state (Cowett and Bassuk 2014), these estimates may or may not accurately reflect this geographic variability. Furthermore, Santamour’s benchmarks for species and genus diversity may be widely used, but there is no scientific basis confirming their effi- cacy (Kendal et al. 2014), and, since polyphagous pests such as the ALB and SLF attack multiple tree species and genera rather than a single species or genus, they may not be stringent enough (Laćan and McBride 2008). Accordingly, Bassuk et al. (2009) have recommended limiting any one tree species to between 5% and 10% of an urban tree population; Ball and Tyo (2016) have proposed a 5% rule for tree genera and placed greater emphasis on genus diver- sity than on species diversity, since many pests func- tion at the genus level; and Simons (2018) has suggested not only lower limits on genera percent- ages, but also tailoring these percentages to the USDA Plant Hardiness Zones, with colder zones having greater limits (e.g., 10% for genera in Zone 3) than warmer zones (e.g., 3% to 4% for genera in Zone 7). In addition to the issues described above, there are practical difficulties to be faced in continuing to ©2021 International Society of Arboriculture diversify New York State’s street tree population. As stated previously, not all tree species are good candi- dates to be street trees, since some tree species are less resilient to streetscape conditions and environ- mental stressors (Roloff et al. 2009). Some less com- monly used tree species might be resilient to streetscape conditions and environmental stressors, and would therefore be good street tree candidates, but sourcing these trees can be a problem due to limited nursery availability (Sydnor et al. 2010; Conway and Vecht 2015). The choice of street tree species will be nar- rowed further if emphasis is placed on planting native trees to the exclusion of non-native trees (Sjöman et al. 2016). Notwithstanding the findings made by Bar- ron et al. (2016) cited earlier, the importance of tree diversity is not universally recognized by municipal and nursery personnel (Polakowski et al. 2011; Lohr 2013), factors other than diversity may be prioritized when making planting decisions (Conway and Vecht 2015), and preferences for commonly planted tree species are persistent (Simons and Hauer 2014). Finally, smaller-sized municipalities have a knowledge and resource gap relative to larger-sized municipalities, and this gap can contribute to the continuation of established planting practices (Doroski et al. 2020). Given the complexities involved, continuing the diversification of New York State street trees, if not increasing it to levels more ambitious than those sug- gested by Santamour, will require not just intensified effort, but coordinated action due to the multiple actors (e.g., property owners, arborists, tree boards, public utilities, nurseries, state and local officials) and geographic scales (e.g., parcel, block, neighborhood, municipality, region, state) involved in street tree management (Clark et al. 1997; Mincey et al. 2013). For example, Hilbert et al. (2020) have called for state and Extension officials to work with individual municipalities to identify underutilized tree species (i.e., less than 1% of the total tree population by stem count) that are readily available and appropriate choices based on a matrix of tree and site characteris- tics; Avolio et al. (2018) have encouraged ecologists who are working with planners, nurseries, and the public to build support for tree diversity by coupling resident preferences for key plant attributes, such as shade provision, with scientific information when making planting recommendations; Hirons et al. (2020) would like arboreta and botanic gardens to collabo- rate with nurseries and the plant-user community to
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