©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(5): September 2023 239 vulnerability scores; this was true for all 30 of the most planted tree species with the exception of white ash (Table 2). When applying the High scenario to the Milwaukee metropolitan area inventory, the number of trees that fell under the low to low-moderate vul- nerability score dropped to 34.72%. An additional 2 species, black alder and northern catalpa, entered the high vulnerability class, and a total of 16.06% of spe- cies fell under the moderate-high to high vulnerabil- ity rankings. Notable species classified as having moderate-high vulnerability under the High projec- tion include sugar maple, American basswood, Call- ery pear, silver maple (Acer saccharinum), Colorado blue spruce (Picea pungens), northern white-cedar (Thuja occidentalis), English oak (Quercus robur), London planetree (Platanus × acerifolia), and white oak (Quercus alba). Perhaps most alarming, 55.52% of the trees inventoried in the Milwaukee metropoli- tan area (244,273 trees) are considered moderately, moderately-high, or highly vulnerable to the High cli- mate change projection (Figure 8). Additional vulnerability scores were applied for 2 distinct diameter classes, trees that were under 25.4 cm DBH and those under 12.7 cm DBH, and a com- parison was made between the 3 diameter classes as they relate to vulnerability within the canopy under a Low scenario (Figure 9A). Likewise, a comparison was made between the 3 diameter classes as they large portion of both the overall Milwaukee metro- politan area inventory as well as the smaller DBH classes. However, because there are so many com- mercially available cultivars and hybrids, no single elm cultivar or hybrid made it into the top 20 species in either inventory above. Most of these elm species have similarly high adaptive capacity and low vulner- ability to climate change. Overall, it appears that spe- cies selection and diversification is increasing as urban forest management practices improve over time. Vulnerability Projections Using the initial Low climate change projection (cold hardiness zone 6 and heat zone 7), only one species, red pine (Pinus resinosa), was classified as having high vulnerability by the end of the century, which only accounted for 0.08% of the inventory. No spe- cies fell under moderate-high vulnerability with the Low projection. Some notable species classified as having moderate vulnerability under the Low climate projection included white ash, northern catalpa (Catalpa speciosa), tulip poplar (Liriodendron tulip- ifera), black cherry (Prunus serotina), and black alder (Alnus glutinosa). The moderately vulnerable species combined made up 3.08% of the overall inventory, or 13,551 trees. Overall, under a Low climate projection scenario, 90.15% of trees in the Milwaukee metro- politan area fell under the low to low-moderate Figure 7. The Shannon Diversity Index (H = −Σπ × ln[π]) of each diameter class. The larger the value, the greater the diversity. Notice the increase in values for lower diameter classes amongst the Milwaukee metropolitan tree inventory data. The Hutcheson t-test was also performed on each DBH class and showed statistically significant (P < 0.001) differences between each diameter class’s diver- sity (H). The error bars represent confidence intervals (95%) from the Hutcheson t-test. .... .... .... .... .... ............... ........ ........ ........ ........ ........................
September 2023
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