292 (Table 1) by using regression relationships among emission values, pollutant concentrations, and population numbers (Wang and Santini 1995). This regression provides estimates of the costs of dam- ages to human health resulting from air pollution. Rainfall Interception Intercepted rainfall can evaporate from the tree crown, thereby reducing stormwater runoff. A numerical interception model accounted for the amount of annual rainfall intercepted by trees, as well as throughfall and stem flow (Xiao et al. 2000). The volume of water stored in tree crowns was cal- culated from tree crown leaf and stem surface areas and water depth on these surfaces. Hourly meteo- rological and rainfall data for 1996 from California Irrigation Management Information System sta- tions in Santa Monica (Coastal) and Claremont (Inland) were used because total rainfall in that year was close to the average annual amount. The rainfall interception benefit was priced by estimating costs of treating sanitary waste and con- trolling stormwater runoff in Los Angeles. During small rainfall events, excess capacity in sanitary treatment plants can be used to treat stormwater. In the Los Angeles region, it costs approximately $0.48 per m3 to treat sanitary waste (Condon and Mori- arty 1999). Although storm drains are designed to manage 25-year events, localized flooding is a problem during smaller events. Approximately $50 million is spent annually controlling floods in Los Angeles and $1.42 per m3 is spent annually for controlling flooding caused by a typical 25-year winter event (170 mm) (Jones and Stokes Associ- ates 1998). Water quality and flood control ben- efits were summed to calculate the total hydrology benefit of $1.90 per m3 . This price was multiplied by the amount of rainfall intercepted annually. RESULTS Tree Planting and Mature Size Information from the MTLA databases indicated that 91,786 trees were planted from 2006 to 2010 and 87.5% were planted in the Inland climate zone (Table 2). This amount is substantially less than the 407,000 re- ported as planted through 2013. The majority of trees (56,453) were planted in street locations (61.5%), with 73% of these along residential streets and the remain- ©2014 International Society of Arboriculture McPherson: Monitoring Million Trees LA der along commercial streets. Approximately 22,861 trees (24.9%) were planted in private yards and 12,472 (13.6%) in parks. The number of trees planted each year ranged from 13,557 (14.8%) in 2006 to 24,608 (26.8%) in 2009. The MTLA planting palette contained a di- verse mix of species, with 149 taxa planted along streets alone. However, 57 taxa had fewer than 20 individuals planted. The most abundant known species planted were Prunus cerasifera (6.3%), Lagerstroemia indica (4.6%), Quercus agrifolia (3.7%), Platanus spp. (2.5%) Jacaranda mimosifolia (2.2%), Ginkgo biloba (2.2%), Pistacia chinensis (2.2%), Magnolia grandiflora (2.1%), Pyrus kawakamii (2.0%), and Cedrus spp. (2.0%). Because the magnitude of ecosystem ser- vices trees produce in the long-term are directly related to their mature size, the percentages of small- (<10 m tall), medium- (10–20 m tall), and large- (>20 m tall) stature trees were calculated for street, park, and yard locations. Street tree spe- cies were quite evenly distributed among the three mature size classes. However, 65.5% and 14.0% of the park trees were large and medium stature, respectively. The opposite was found for yard trees, where 45.7% were small and only 14.2% were large. Growth and Survival The typical dimensions of a #15 tree at time of planting are 1.9 cm DBH and 0.6 m crown diam- eter. MTLA street (n = 67) and yard (n = 54) trees that were surveyed four to five years aſter planting had a mean DBH of 6.4 cm (S.E. = 0.43 cm) and 5.9 cm (S.E. = 0.41 cm), respectively. Mean crown diameter was 2.0 m (S.E. = 0.12 m) and 1.8 m (S.E. = 0.13 m) for street and yard trees. The average an- nual DBH and crown diameter growth rates for street and yard trees were 1.1 cm and 0.99 cm DBH per year and 0.36 and 0.30 m per year, respectively. There was a threefold range in mean DBH and crown diameter among species sampled (Table 3). The largest species were Platanus acerifolia, Pis- tacia chinensis, Pyrus calleryana, and Olea euro- paea. The smallest trees were Lagerstroemia indica, Lophostemon confertus, and Magnolia grandiflora. The street tree survey found 67 trees alive, 1 dead, 16 missing, and 14 could not be located by the address provided (Table 4). The 14 trees with inac- cessible addresses were removed from the sample, resulting in a 79.8% survivorship and 4.4% annual mortality rate for the first five years of street tree
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