296 By removing pollutants from the air through deposition to tree surfaces, the tree planting was projected to improve air quality in Los Angeles and enhance human health. The value of this co-benefit was estimated to be $4.78 million over the 40-year planning horizon (Table 6). Interception of PM10 and uptake of O3 lion) and 147 t ($1.6 million), respectively. Uptake of NO2 sition and PM10 interception totaled 268 t ($2.2 mil- , an ozone precursor, was estimated at 103 were especially valuable. O3 t over the 40-year period ($858,799). The average annual deposition rate for all air pollutants per tree planted over 40 years was 0.14 kg, valued at $1.30. Air pollutant emissions from power plants were million), respectively. Smaller amounts of SO2 and VOC emissions were reduced. Altogether, and PM10 avoided emissions totaled 390 t ($3.25 million). The average annual avoided emissions per tree planted over 40 years were 0.11 kg, valued at $0.89. DISCUSSION Tree Planting and Mature Size The number of trees planted (91,786) from 2006 to 2010 is substantially less than the targeted one million or the 407,000 reported as planted through 2013. Pincetl (2010) described some reasons for this, including lack of public fund- ing during this era of fiscal constraint. An he- donic pricing analysis found that additional trees on housing parcels in Los Angeles would not appreciably increase property values, suggesting that there is little financial incentive for residents to pay for more trees (Saphores and Li 2012). Using MTLA database information on tree species planted and the mature size of each, this study found percentages of small, medium, and large trees were 36.7%, 33.6%, and 29.7%, respec- tively. The preliminary study assumed percent- ages of 52.3%, 38.0%, and 9.7% for small, medium, and large trees, based on the size distribution of vacant planting sites. The MTLA program appears to be filling relatively more of the available vacant sites for large-stature trees and relatively few of the sites for small-stature trees. This finding is supported by the size distribution for the 152 ©2014 International Society of Arboriculture estimated to be reduced because of energy sav- ings. The MTLA trees were projected to reduce NO2 by 195 t and 107 t ($1.7 and $1.2 depo- McPherson: Monitoring Million Trees LA sampled street and yard trees, which consisted of 27.6%, 37.5%, and 34.9% small, medium, and large trees, respectively. It appears that the MTLA program is preferentially planting larger-stature trees, probably to reap increased future benefits. Survivorship and Growth The MTLA average annual mortality rates for street (4.4%), park (3.1%), and yard (4.6%) trees were less than the 6.6% rate for Sacramento shade trees during the first five years (70.9% survivorship), as well as the 5.6% rate for small trees (<7.6 cm DBH) in West Oakland, California, U.S. (Roman 2013). Other studies have reported even higher average annual mortality rates for small trees, such as 9% in Baltimore, Maryland, U.S. (Nowak et al. 2004) and 12% for trees (7.7 to 15.2 cm) in Houston, Texas, U.S. (Staudhammer et al. 2011). MTLA survivorship rates, 79.8%, 90.7%, and 77.1% for street, park, and yard, are comparable to the 78.2% reported for trees planted for three to six years in New York City, New York, U.S. (Lu et al. 2010). Miller and Miller (1991) reported street tree survival rates that ranged from 58.8% to 76.5%, four to nine years aſter planting in Wisconsin, U.S., communities. Somewhat higher survival rates were found for trees planted four to five years in San Francisco, California, U.S. (86.4%) (Sullivan 2004) and small Iowa, U.S. communities (87%) (Thompson et al. 2004). In summary, MTLA tree survival rates are rela- tively high for a large city in an arid environment, where transplants face extended periods of summer drought. Results from analysis of a program in Sac- ramento similar to MTLA indicate that stewardship and maintenance are the most critical factors influ- encing young tree survival (Roman 2013). Activities that may increase survival include more frequent site visits, follow-up tree care tips, systematic monitoring, and planting of species with high sur- vival rates, such as those that are drought tolerant. MTLA tree growth rates compare favorably with values reported in the literature. The average annual DBH growth across all species was 1.06 cm per year (n = 121) for four- and five-year-old trees. This mean DBH growth rate is greater than 0.56 and 0.82 cm reported for trees with DBH less than 7 cm in commercial and residential land uses in Gainesville, Florida, U.S. (Lawrence et al. 2012). Also, it exceeds
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