Million trees Los Angeles canopy cover and benefit assessment
Research highlights
▶ Los Angeles's existing tree canopy cover was 21%, and ranged from 7 to 37% by council district. ▶ There was potential to add 2.5 million additional trees to the existing population of approximately 10.8 million. ▶ Benefits for the 1-million-tree planting for the 35-year period were $1.33 billion and $1.95 billion for the high- and low-mortality scenarios, respectively. ▶ Average annual benefits were $38 and $56 per tree planted. ▶ Eighty-one percent of total benefits were aesthetic/other, 8% were stormwater runoff reduction, 6% energy savings, 4% air quality improvement, and less than 1% atmospheric carbon reduction.
Introduction
Urbanization creates significant changes in land use and land cover, affecting the structure, pattern, and function of ecosystems. Ecologists, planners, designers, and the public are increasingly concerned about how these changes influence daily life and affect the sustainability of “quality of life” for future generations. In Los Angeles, California, a rapidly growing region of nearly 4 million people, improving air and water quality, alleviating water shortages, cooling urban heat islands, and reducing local flooding are mounting challenges. For example, between 627,800 and 1.48 million gastrointestinal illnesses are caused annually by swimming in contaminated beaches in southern California (Given et al., 2006). This public health impact corresponds to an economic loss of $21 to $51 million related to health care costs. Rainfall interception by Santa Monica's municipal forest (29,299 trees) reduced stormwater runoff by 193,168 m3 (1.6% of total precipitation) with an estimated annual value of $110,890 (Xiao and McPherson, 2002).
Long-term effects of exposure to high air pollution levels in southern California have been associated with decreased respiratory health (Gauderman et al., 2004). Exposure to freeway-related pollutants has been found to impair children's lungs and is associated with increased asthma (Gauderman et al., 2005). Although pollen produced by certain tree species can exacerbate the incidence of asthma, some studies report reduced respiratory disease associated with increased tree cover (Lovasi et al., 2008). Tree planting was found to reduce ozone concentrations in Los Angeles, provided species were low-emitters of biogenic volatile organic compounds (Taha, 1996). Increasing tree canopy cover is one of the most cost-effective ways to reduce urban heat islands and conserve energy for heating and cooling buildings (McPherson and Simpson, 2003, Rosenfeld et al., 1998, Rosenzweig et al., 2006).
The presence of trees and green spaces in cities is associated with increases in property values, perceived consumer friendliness, and sense of well-being (Payton et al., 2008, Wolf, 2005). Views of trees and nature from homes and offices provide restorative experiences that ease mental fatigue and help people concentrate (Kaplan and Kaplan, 1989). A series of studies on human stress caused by general urban conditions and city driving show that views of nature reduce the stress response of both body and mind (Parsons et al., 1998). Hospitalized patients with views of nature and time spent outdoors needed less medication, slept better, had a better outlook, and recovered more quickly than patients without connections to nature (Ulrich, 1985). A number of studies have found an association between access to green space and human health (Gidlof-Gunnarsson and Ohrstom, 2007, Maas et al., 2006). For example, the presence of tree lined streets was associated with children walking to school (Larsen et al., 2009).
On September 29, 2006 Antonio Villaraigosa was elected mayor of the city of Los Angeles. The following day he planted a tree, kicking off his plan to plant 1 million trees in the next several years and said, “Los Angeles, the dirtiest big city in America, has the opportunity to be the greenest” (Hymon and Merl, 2006). The ambitious tree initiative was dubbed Million Trees LA (MTLA) and is integral to the city's climate action plan, which aims to reduce greenhouse gas emissions 35% below 1990 levels by 2030 (City of Los Angeles, 2007). This research addresses questions posed by the MTLA initiative—How many trees already exist in Los Angeles? Is there room for a million more trees? What environmental and other benefits will 1 million new trees provide?
Tree canopy cover (TCC) is the percentage of a site covered by the canopies of trees. American Forests and others advocate that communities identify current TCC, and then set targets for TCC increase (Grove et al., 2006, Kollin, 2006). TCC is an increasingly popular metric because it is relatively easy to measure with remote sensing technology and less costly than field sampling (Poracsky and Lackner, 2004). It is comparable across a city and among cities because the size of the area measured does not matter. Success meeting TCC targets can be measured across time as well as space. Finally, TCC is an easy-to-understand concept that is useful in communicating to the public.
However, TCC is two dimensional, only indicating the spread of canopy across land surfaces. It does not provide information on the vertical extent of tree canopy, species composition, age diversity, or health. Many functional benefits have been linked to the leaf surface area of trees, which is difficult to estimate with accuracy using only TCC. Moreover, predicting future trends in urban forest structure, function, and management needs requires a richer data set than TCC alone provides.
Many studies have used remote sensing data and GIS to map TCC. American Forests has used satellite imagery and CITYgreen GIS software to map historical TCC change, as well as the value of annual benefits from urban forests for cities such as Atlanta, Georgia, Washington, D.C., and Roanoke, North Carolina (American Forests, 2002a, American Forests, 2002b). Irani and Galvin (2003) used IKONOS data (10-m spatial resolution) to map TCC in Baltimore, Maryland. Goetz et al. (2003) found the accuracy of tree cover estimates mapped with IKONOS imagery in the mid-Atlantic region to be comparable to manual aerial photo interpretation. Poracsky and Lackner (2004) compared Portland Oregon's tree canopy in 1972, 1991, and 2002 by using Thematic Mapper and multispectral scanner data (30-m plus resolution). High-resolution infrared photography and light detection and ranging (LIDAR) data were used to map TCC in Vancouver, Washington (Kaler and Ray, 2005). Urban cover was mapped with 82% accuracy for Syracuse, New York, using high-resolution digital color-infrared imagery (Myeong et al., 2001), and similar data were used to assess New York City's TCC (Grove et al., 2006). AVIRIS (airborne visible infrared imaging spectrometer) data were used to map urban tree species in Modesto, California, but developing spectral signatures for each species was time consuming (Xiao et al., 2004).
Potential TCC (PTCC) is the percentage of area on the ground without TCC that could be covered by additional tree canopy. Traditionally, PTCC is the amount of residual pervious surface, including all grass and bare soil. It does not include tree cover that could be achieved by adding trees to impervious surfaces like paved parking lots and plazas.
We differentiate between two other terms related to TCC, technical potential and market potential (McPherson, 1993). Technical potential is the total amount of planting space – existing TCC plus potential TCC (TCC + PTCC) – whereas market potential is the amount of technical potential that is plantable given physical or preferential barriers that preclude planting. Physical barriers include conflicts between trees and other higher priority existing or future uses, such as sports fields, vegetable gardens, and development. Another type of market barrier is personal preference to keep certain locations free of TCC. Whereas technical potential is easily measured, market potential is a complex socio-cultural phenomenon that has not been well studied. The only study we are aware of is a survey of nonparticipants of the Sacramento Shade program (M. Sarkovich, personal communication, October 11, 2006). The two most common reasons customers chose not to accept a free shade tree were lack of space (34%), a physical constraint, and “Do Not Want Any More Trees” (25%), a personal preference. This finding applies primarily to low-density residential land uses and suggests that a substantial amount of technical potential is likely to remain tree-free because of market forces.
The i-Tree software suite contains two programs, Eco and Streets (formerly UFORE and STRATUM), that use numerical models to calculate annual benefits per tree in common engineering units called Resource Units (RUs) (Maco and McPherson, 2003, McPherson et al., 2005). Individual tree benefits are monetized using control or damage costs and then aggregated for the tree population. Both models rely on ground survey data as input, and use growth rate information to “grow” the tree for one year. The modeling approach directly connects benefits with tree size variables such as diameter at breast height (dbh), crown diameter, and leaf area to directly calculate the annual flow of benefits as trees mature and die (McPherson, 1992).
Projecting future benefits from a proposed tree planting project requires tree growth data because as trees grow larger the benefits they produce increases. Tree size and growth data have been developed in 16 US cities based on extensive measurements of about 900 trees randomly sampled — 40 trees of each of the 22 most common species (Peper et al., 2001a, Peper et al., 2001b). For each species, five to ten trees from each dbh size class were measured for dbh, tree height, crown diameter, crown shape, and tree condition. Planting dates were determined from city records and other local sources. Crown volume and leaf area were estimated from computer processing of tree-crown images taken with a digital camera (Peper and McPherson, 2003). Curve-fitting models were tested for best fit to predict dbh as a function of age for each species. Leaf area, crown diameter, and tree height were then modeled as a function of dbh.
Tree size and growth data were used with numerical benefit models to calculate annual benefits at 5 year intervals for a 40-year period after planting. To account for differences in the mature size and growth rates of different tree species, results were reported for a typical small-, medium-, and large-stature tree species, where mature tree height is used to characterize each species. To make benefit calculations realistic, mortality rates were included based on surveys of regional municipal foresters and commercial arborists. Tree benefit projections were published in a series of Community Tree Guides, one for each of the 16 US regions (http://www.fs.fed.us/psw/programs/cufr/tree_guides.php).
This study is unique in that it combines tree benefit projections with TCC assessment to determine: (1) existing TCC, (2) PTCC, and (3) the value of future benefits from planting 1 million trees in Los Angeles.
Section snippets
Study site
Los Angeles (latitude: 34°06′36″N, longitude: 118°24′40″W) is one of the largest metropolitan areas in the United States (Fig. 1). It has a land area of 1225 km2 and a population of 3,694,820 (U.S. Census Bureau, 2000). There are 15 council districts and 86 neighborhood councils. Topographic gradients are small in the coastal areas and inland valleys; however, within the city limits there are mountain ranges with steep slopes. Elevation changes from sea level to 1543 m at Mount Lukens in the
Existing tree canopy cover
The TCC in the city of Los Angeles was 21% (21,243 ha) (Table 2). Irrigated grass and dry grass/bare soil accounted for 12% (12,628 ha) and 6% (5581 ha) of the cover, respectively. Impervious (e.g., paving, roofs) and other surfaces (i.e., water) made up the remaining 61% (62,684 ha) of the city's land cover (excluding mountainous areas). Hence, one-third of Los Angeles's land cover was TCC and grass/bare soil with potential to become TCC.
TCC was strongly related to land use. As expected,
Discussion
In Los Angeles the existing TCC (20.8%) was close to values reported for Baltimore (20%) and New York City (23%) (Galvin et al., 2006, Grove et al., 2006) (Table 5). This is surprising given Los Angeles's Mediterranean climate, which makes irrigation essential for establishment and growth of many tree species. However, the technical potential TCC was much less in Los Angeles (33%) than 66% and 73% reported for New York City and Baltimore. In Los Angeles, the PTCC represented only a 12% increase
Acknowledgments
This research was supported by funds provided by the city of Los Angeles, California, and we thank Paula Daniels, George Gonzalez, Lisa Sarno, and Lillian Kawasaki for their support. We wish to acknowledge Patrice Gin, Randy Price, and Kirk Bishop (Public Works/Bureau of Engineering/Mapping Division, city of Los Angeles) for sharing their GIS data and aerial imagery with us. Rebecca Drayse, Edith Ben-Horin, and David O’Donnell of TreePeople led the survey of field plots. Thanks to Dan Knapp,
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