176 McPherson and Peper: Urban Tree Growth Modeling km north of Westminster. Six species were measured in Chey- enne that also grew in Fort Collins. With the exception of the native Colorado blue spruce (Picea pungens), every species in Cheyenne was significantly smaller when compared to same- aged trees in Fort Collins. Beginning 30 years after planting, green ash (Fraxinus pennsylvanica) growing in Cheyenne consis- tently remain about 7 m shorter than the same species growing in Fort Collins (Figure 2). Crowns are pruned up about 1.5 m higher in Fort Collins than in Cheyenne, in relation to larger growth and proximity to roadways. Leaf area for the Cheyenne trees is estimated to be about 55% of Fort Collins ash tree leaf area. Because Cheyenne’s municipal trees were generally Figure 1. The comparison of predicted DBH size of Fort Collins trees with actual measurements of Westminster trees 16 years after planting indicates that mean tree growth over 16 years is generally greater in Fort Collins. Figure 2. Tree height, crown base, and leaf area for same-aged green ash in Fort Collins and Cheyenne. Upper lines represent height. Lower lines represent height to first branch. Cheyenne ash have 55% of the Fort Collins’ ash leaf area. tion in management, growing space, location, climate, and soils. Fort Collins has slightly lower average monthly sum- mer temperatures (up to 2°C lower) and up to 2.5 times the average late-summer precipitation than Westminster, which may boost growth. The similarities between the Fort Col- lins predictive models and the Westminster actual mea- surements suggest that model predictions are reasonable. However, comparisons between Fort Collins and predictive models developed from data collected in Cheyenne, Wyoming, illustrate how differently the same tree species may grow within a single region. Cheyenne is 75 km north of Fort Collins and 156 healthy and well-managed (94% in fair or better condition), the climatic and soil differences between the two cities were examined. At the time of the study, Cheyenne was listed as the fourth windiest city in the country with the highest in- cidence of hailstorms. At an elevation of 1860 m, the city receives an average 366 mm of precipitation annually. The harsh climate and elevation combine with alkaline soils that have low water-holding capacity. These challenging grow- ing conditions were a primary reason why the U.S. Congress established the USDA High Plains Horticultural Research Station in Cheyenne in 1928 (Torpey 2007). If trees, shrubs, and other plants could survive the conditions in Cheyenne, it was likely they would grow in other semiarid or dry land regions of the U.S. Fort Collins is lower in elevation (1525 m) and receives only slightly more precipitation (383 mm), but generally has better soils than Cheyenne. Located adja- cent to the Rocky Mountains rather than the open prairie, Fort Collins also receives more protection from the elements. Differences in growth rates translate into significant differ- ences in tree benefits (Table 1). At USD $66,000 and $201,000, respectively, the value of services produced by Cheyenne trees is about one-third of those produced by Fort Collins trees. Growth models within and across different climate re- gions also present opportunities to examine tree management effects on growth and provisioning of services as demon- strated by a comparison of Siberian elms (Ulmus pumila) in Cheyenne and San Francisco, California. Trees were exten- sively pruned in San Francisco to limit conflicts with build- ings, roads, and overhead wires (Figure 3). Although growing in a much milder climate than Cheyenne, the elms averaged only 9.8 m in height 40 years after planting, equal to the ma- ture size of a small tree. Using the same pricing index for both cities, the value of ecosystem services provisioned by 100 elms over 40 years in San Francisco is $97,000, or about 70% of those from the same species in Cheyenne (Figure 4). Table 1. The total service values for 100 green ash trees (Fraxinus pennsylvanica) grown over 40 years in Fort Collins, Colorado, and Cheyenne, Wyoming. Benefit comparisons were calcu- lated using the same pricing index for both cities. Currency is expressed in U.S. dollars. City Ft. Collins, CO Energy CO2 $85,845 Cheyenne, WY $29,887 $17,603 $5,623 Air quality $14,476 $2,586 Stormwater Total $82,816 $27,660 services $200,740 $65,756 ©2012 International Society of Arboriculture
September 2012
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