Appendix A: CITYgreen Arcgis and i-Tree Methodolgy 16
Public, green space has been an integral part of the early design of many U.S. cities. Taking their cue from European examples, planners lined streets and boulevards with trees and shrubs to enhance the aesthetic appeal of the urban landscape. The park movement of the 19th century sought to introduce a piece of the natural world into an urban setting. The result was the creation of renowned spaces, such as New York City’s Central Park, a scenic refuge from the chaos of city life. Such trends continued into the mid-1900s, when tree planting was used in many cities to revitalize and “beautify” declining, downtown neighborhoods and commercial districts (Lawrence 1995). However, for the greater part of the 20th century, urban parks and planted trees were generally viewed (with some exception) as places distinct from the city and as decorative elements. Their benefits were seen mainly in the context of their social contributions to city life.
Perspectives changed dramatically in the early seventies, when organizations, such as the International Union of Forestry Research Organizations (IUFRO), and academic institutions initiated scientific investigations into the role of urban vegetation (Bradley 1995). Since then, a large body of literature has grown up around the topic of urban trees, lining streets, in gardens and parks, downtown, and in city fringes, which are collectively termed the urban forest. These studies have found that the urban forest is not only a provider of social benefits, aesthetic, emotional, and recreational, but also many environmental ones. The urban forest is now recognized as being inextricably linked to the ecology of the city— the interactions between urban wildlife, water, energy, climate, and humans (Rowntree 1995). It has become an important player in the movement to create more livable, sustainable cities.
THEORETICAL AND APPLIED BENEFITS
Urban Heat Island
The urban heat island (UHI) is the phenomenon whereby urbanized areas exhibit significantly higher temperatures than their outlying, rural areas (Figure 1). These temperature differentials vary among urbanized regions, and are related to city characteristics, such as population size and topography. However, differences have been found to range, on average, from 1C to even 10C (Bridgman et al. 1995).
Figure 1 Schematic of air temperature along gradient of urbanization
The causes of UHI phenomenon are diverse. The shape, orientation, and density of urban structures, which tend to trap solar radiation, is one major culprit. The physical properties of urban surfaces, which tend to have low reflectivity and the capacity to store considerable amounts of heat, are another. Cities also have less vegetation and moisture than rural areas, leading to less transpiration (the release water vapor from vegetation) and evaporation of water—processes that reduce temperatures.
The UHI effect has the potential to pervade many aspects of urban life. Although it has been suggested that cities with severe winter conditions and cold climates might actually benefit from the UHI during certain months, it is generally accepted that, overall, the harmful aspects of the UHI tend to outweigh its benefits. In many cities, particularly ones in hot climates, the UHI negatively impacts thermal comfort, especially in the summer. But at even higher temperatures, issues of comfort may be overshadowed by increases in illness and mortality, which has been linked to extreme heat conditions. Increased temperatures are also known to enhance the pathways by which ground level ozone (to be distinguished from the stratospheric ozone) is formed (Cardelino and Chameides 1990, Taha et al 1997). Member of the DOE’s Lawrence Berkeley Laboratory Heat Island Group found that the ozone levels in Los Angeles related linearly to the daily maximum temperatures, and at temperatures above 32C, ozone levels were very likely to exceed national standards (120 ppb) (Heat Island Group 2004). Ground level ozone is known to cause a number of human health problems ranging from eye and lung irritation to asthma aggravation and lung damage.
The UHI also increases demand for energy via air-conditioning use. According to Stone and Rodgers (2001) a 1996 study by Rosenfield et al. estimated that the U.S. spends in excess of $10 billion per year on energy to deal with UHI related heat costs. However, tempering the effects of the UHI with air conditioning is not only fiscally costly, but also environmentally. Air-conditioning, by expelling exhaust into the city, adds heat to the urban environment, potentially exacerbating the heat island. In fact, air conditioning is one of the most significant contributors to anthropogenic heat emissions in cities (Landsberg, 1987).
Augmenting greenspace in cities is currently recognized as a key urban cooling strategy. Trees lower temperatures by directly shading surfaces and by absorbing radiation for transpiration, thus reducing the amount of radiation that can become sensible heat. The so-called ‘oasis’ effect produced by vegetated urban sites is well documented. A review by Taha (1997) found that vegetated spaces could be 2-8°C cooler than their surroundings. The same review found that studies in Montreal, Tokyo, and Davis, CA reported that vegetated regions and parks were 1.6°C, 2.5°C, and 2°C respectively cooler than neighboring, non-green regions.
The cooling effect that vegetation has on site microclimate is largely undisputed. However, there is much interest in the larger scale impacts of these green sites. A number of studies and models have found that greenspaces can also lower the temperatures of tangential, downwind areas, and, when the percent cover of such spaces across a city is large enough, mesoscale cooling is possible. A study in Tel Aviv found that small wooded green areas had a cooling effect up to 100 meters from the site’s boundary (Shashua-Bar and Hoffman 2000). For optimal urban cooling, the same authors recommended “small, wooded gardens”, approximately .25 acres, situated 200 meters away from one another. Another study that used a numerical model suggested a similar urban layout (Honjo and Takakura 1991). Using a two dimensional model of downwind cooling, the authors found that small green spaces spaced several hundred meters apart were preferable for cooling surrounding areas. Taha et al (1997) modeled the impact of various mitigation strategies in California’s South Coast Air Basin and calculated that increasing tree canopy cover by 10 million trees would have a regional influence, cooling the central valley and surrounding regions by 2C and 1C respectively.