Arboriculture & Urban Forestry 47(1): January 2021 Connecticut (Parker 1907) and Newark, New Jersey (Forest Park Reservation Commission of New Jersey 1915), inventoried municipal trees at that time, although, to the best of our knowledge, the records of many of these inventories have not survived. Ithaca’s 1902 inventory, in which sugar maple and American elm comprised 66.36% of the street tree population, reflected prevailing planting practices which included “entire blocks…[to be] planted with one tree [spe- cies] alone” (Landreth 1895). Hope et al. (2006) have defined a “legacy effect” as an inherited present day situation due to past events. Because most street trees are intentionally planted and the urban landscape is not created instan- taneously, but develops over time, urban tree popula- tions, in the absence of major ecological disturbances, are typically the product of past landscape prefer- ences and practices, which can persist for long peri- ods (Boone et al. 2010; Greene and Millward 2016; Larson et al. 2017). Although the City Beautiful movement waned after World War I, its legacy endured in Ithaca. In the 1928–1947 inventory, mono- cultural stands of trees can be found on some Ithaca streets (Figure 2). In fact, the diversity of the city’s street tree population declined between the 1902 and 1928–1947 inventories, and genera richness (i.e., the number of genera) also decreased (Table 5). Maple and elm, which together comprised 74.16% of all street tree genera in the 1902 inventory, comprised 81.73% of all street tree genera in the 1928–1947 inventory (Table 3). This lack of diversity in the city’s street tree population and its dominance by a few tree genera rendered it exceptionally vulnerable to a pest or disease such as Dutch elm disease. Ithaca’s 1987 inventory appears to represent an inflection point in the structure and management of its municipal trees. First, it revealed the decimation of the city’s elms by Dutch elm disease. Elms, which totaled 1,650 trees and comprised 29.35% of all trees in the 1902 inventory and totaled 1,604 trees and comprised 30.88% of all trees in the 1928–1947 inventory, totaled just 5 trees and comprised 0.09% of all trees in the 1987 inventory (Table 3). The Ameri- can elm’s loss proved to be the Norway maple’s gain. Recognized today as an invasive species which depresses species richness in forests (Webb et al. 2000), Norway maple was one of the few street tree species available in large enough quantities to meet the replanting needs created by Dutch elm disease in the eastern and north-central United States (Nowak 17 and Rowntree 1990). By 1987, it had become Ithaca’s most prevalent street tree species, comprising 34.62% of the street tree population, and maple, which had comprised 50.86% of the street tree population in the 1928–1947 inventory, was found even more domi- nant in the 1987 inventory, comprising 67.40% of all street tree genera (Table 3). Second, the 1987 inven- tory was accompanied by an unpublished document which included management recommendations pred- icated on an analysis of the inventory data and, in par- ticular, the impact of Dutch elm disease. After stating that “planting a large percentage of any one species… [increases] the risk of disease or insect infestation which could seriously alter the green character of the city,” the document recommended a moratorium on planting Norway maple and that no single street tree species should comprise more than 5% of the street tree population. Finally, in a dramatic departure from the methodology employed in the 1928–1947 inven- tory, in which data were recorded by hand and stored on 180 maps of city streets, the 1987 inventory incor- porated computer technology in the inventory pro- cess and especially in data storage and analysis. Data collected in the field on paper forms were entered into a computer database which could then be queried to generate charts and graphs and also be updated with future changes, such as new plantings and removals. The inventories from 1996 through 2019 further suggest that the 1987 inventory was an inflection point. After 1987 and continuing through 2019, the percentages of Norway maple and maple relative to all street tree species and genera steadily declined (Table 3). At the same time, the number of street tree species and genera, the diversity of the street tree population, and the evenness of the species and genus distributions increased (Tables 5 and 6). The DBH size class distributions for street trees in the 1987 and 1996 inventories reveal that a large number of new plantings occurred prior to those inventories and, for the 1996 inventory at least, these plantings consisted in large part of species other than Norway maple and genera other than maple (Figure 3). While the DBH size class distributions in the 2006, 2013, and 2019 inventories indicate that the pace of new plantings may have slowed after 1996, the cumulative effect of both new plantings and species selection since 1987 has been to increase street tree diversity and to approach, although not achieve, Santamour’s 10% threshold for species and his 20% threshold for genera. ©2021 International Society of Arboriculture
January 2021
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