290 comm., Ryan Allen, Tree Care Manager, Tree- People, Los Angeles, CA; July 7, 2011). Annual mortality is defined by mannual where No and the number alive at the end of the five-year interval t (Roman and Scatena 2011). The equation used to calculate the total number of live trees at the beginning of year t (TLTt and Nt ) across species s is: [1] [1] TLTt = β =ππ π π =1 TLTs, t-1 x (1-Ns, t-1) AΕΏter the five-year establishment period, annual mortality rates were assumed to be constant over time. Species Matching, Biomass, and Carbon Dioxide Calculations Each planted species was matched to one of the 20 or 22 species that were intensively studied in the reference cities following methods reported previ- ously (McPherson et al. 2013). Correctly matching species planted to their corresponding reference city species ensured that the appropriate allome- tric and growth equations were applied to cal- culate biomass and annual tree planted, climate zone, species name, and DBH were used with 26 species-specific equa- tions for trees growing in open, urban conditions. Urban-based biomass equations were developed from street and park trees measured in California (Pillsbury et al. 1998) and Colorado, U.S., cities (Lefsky and McHale 2008). The rationale for nearly exclusive use of To calculate biomass and CO2 these equations is that trees in open-grown conditions partition car- bon differently than closely spaced trees in forest stands because they do not compete as directly with other trees. Also, urban tree growth can be enhanced by periodic irrigation and care, as well as elevated levels of carbon and nitrogen deposition (Jo and McPherson 1995; Nowak and Crane 2002). Wood volume estimates were converted to green and dry-weight estimates (Markwardt 1930) and divided by 78% to incorporate root biomass (Nowak 1994a). Dry-weight was converted to carbon (50%) (Leith 1975), and these values were converted to CO2 biomass . The lated as the difference between the amount stored in year x+1 and the amount stored in year x. amount of CO2 sequestered in year x was calcu- Β©2014 International Society of Arboriculture sequestration rates. stored in each are the number of trees sampled = 1 β (Nt/No)1/t , McPherson: Monitoring Million Trees LA Calculation of Energy Effects and Avoided Emissions Calculations of energy effects of street and yard trees on buildings were based on computer simulations that incorporated tree loca- tion and building vintage information from the monitoring survey. Climate and shading effects were modeled following methods out- lined by McPherson and Simpson (1999). Park trees were omitted from the analysis because trees shaded very few air-conditioned buildings in parks. Changes in unit energy consumption due to the effects of trees were calculated on a per-tree basis by comparing results before and after adding trees. Weather data (e.g., hourly air temperature, wind speed, irradiance) for a typical meteorological year (TMY2) from Ontario International Airport (Inland) and Los Angeles International Airport (Coastal) were used (Marion and Urban 1995). Shad- ing effects for each of the 20 to 22 tree species were simulated at three tree-to-building dis- tances, eight orientations, and nine tree sizes. The shading coefficients of trees in-leaf (i.e., gaps in the crown as a percentage of total crown silhouette) were estimated using a pho- tographic method (Wilkinson 1991). Values for tree species that were not sampled, and leaf- off values for calculating winter shade, were based on published values (McPherson 1984). Foliation periods for deciduous trees were obtained from the literature and local arborists. Tree distribution by location [e.g., frequency of occurrence at each location determined from distance between trees and buildings (four set- back classes) and tree orientation with respect to buildings (eight azimuth classes)] was used to calculate average energy savings per tree as a function of distance and direction. Tree loca- tion distributions were from the random sample of 98 street and 96 yard trees. Simulated savings per tree at each location were multiplied by the percentages of sampled trees at each location to determine location-weighted savings per tree for each species and DBH class that was inde- pendent of location. Location-weighted savings per tree were multiplied by number of trees in each species and DBH class. These values were summed to calculate total savings for each year.
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