D. J. Nowak and E. G. McPherson
David J. Nowak and E. Gregory McPherson work as researchers at the Forest Service of the United States Department of Agriculture, at the Northeast Forest Experiment Station, Chicago.
This article reports on the methodologies and the first results of a research project on urban forestry implemented in Chicago (Illinois). It examines the interrelated functions of the urban forest ecosystem currently under study – climate changes, energy savings, air quality and carbon dioxide retention – and analyzes the cost-benefits of urban vegetation.
The Chicago Urban Forest Climate Project (CUFCP), a Chicago project concerning the effects of urban wooded areas on the climate, was set up to better understand how vegetation in urban areas affects the local climate, energy consumption and air quality. The CUFCP, currently at the mid-term stage, after intensive data collection during the summer of 1992, is a multi-annual research project that should be completed at the end of 1993. In 1990, when Richard M. Daley, Mayor of Chicago, advocated research in urban forestry in support of his general plan to create green spaces in Chicago and called “Green-Streets”, this gave a strong impetus to the CUFCP. A research plan was developed, and Congress allocated $900,000 to the Forest Service of the United States Department of Agriculture in favor of the project. Two researchers and five technicians are currently working at the CUFCP in Chicago and eleven other technicians participated in the data collection in 1992.
The objectives of the CUFCP are as follows: to quantify the ecological benefits and costs related to the urban forest ecosystem of Chicago and assign monetary values to them; to develop development variants to increase the environmental benefits of urban forestry; and to develop methods and models that can be applied in other cities. The results and recommendations resulting from the project will be published in various scientific journals, in non-specialized publications and in the reports presented to the Municipality of Chicago and the surrounding communities. It is expected that the information will then be used by local urban foresters, building owners, non-profit groups, utility companies and planners, in order to make more informed decisions about the future configuration of urban vegetation.
The more extensive the vegetation cover, the greater the relative influence of trees on the environment of a given city. One of the most cost-effective analyses of the urban forest structure is that of the canopy (the proportion of the area occupied by the treetops seen from above). The analysis of the cover targeted by the CUFCP study (the city of Chicago, the rest of the surrounding Cook County and the adjacent Page County), carried out using a grid of sampling points randomly distributed on aerial photographs, revealed a vegetation cover of 19 percent. In the city of Chicago, the coverage is only 11 percent (see table, page 42), but it varies a lot from place to place, ranging from a minimum of 1 percent to a maximum of 37 percent (Figure 1). If we consider the different types of land use in Chicago, tree cover reaches its maximum in protected forest areas (54 percent of the cover), parks (26 percent) and vacant lots (20 percent); it falls to a minimum for transport networks (2 percent) and commercial/industrial land (3 percent) (McPherson et al., 1992).
Among the factors that have an impact on the urban vegetation cover in general, we can mention the ecoregion (that is, the natural environment in which the city develops), the age and the size of the city. For example, preliminary analyses show that cities that have developed in wooded areas in the eastern United States have an average urban vegetation cover of 30 percent; in the western wooded areas, of 26 percent; in the central areas occupied by grasslands/forests, of 22 percent; and in the western regions where the scrub, meadows and desert originally dominated, by 17 percent. In the United States, the urban vegetation cover is estimated at an average of 27 percent.
In addition to the tree cover, other characteristics of urban wooded areas are important to quantify the urban forest structure, namely the composition of the species, the diameter of the trees and their height, the biomass and the leaf area. However, very little is known about the overall urban forest structure. Most of the work on urban wooded areas has focused on tree populations along avenues, which often represent only a small percentage of the total urban woody vegetation. Research has quantified the composition of species and other structural characteristics for various parts of urban forests around the world (for example, Tokyo and Sendai, Japan – Iizumi, 1983; Singapore – Wee and Corlett, 1986; Reykjavik, Iceland Svanbergsson, Sigtryggsson and Andresen, 1988; Athens, Greece – Profous, Rowntree and Loeb, 1988; Oakland, California, USA – Nowak, 1991; Beijing, China – Profous, 1992; Hong Kong – Jim, 1992).
To estimate the vegetation and other physical parameters of the entire area of Chicago, 615 plots were chosen at random and inventoried with regard to all types of land use. This information was used to quantify the physical characteristics of vegetation, roads, buildings and other structures. In addition to the data relating to the plots, more than 120,000 leaves of urban trees were randomly collected from the foliage using a forklift truck, taking for the analysis of the leaf area and biomass (dry weight) a sampling frame of 0.4 m 3. These structural features will be used in functional models (for example, hydrocarbon emissions). To anticipate the future growth of urban trees, hundreds of logs, cut to human height and mainly coming from dead or almost dead trees, were provided by the Chicago Forestry Office and peripheral communities. Probing cores of healthy trees are also analyzed to verify the applicability of the growth data obtained with diseased trees.
The structure of urban forests directly influences the functions of these (for example, transpiration and reduction of the air temperature related to it, energy savings, mitigation of air pollution). The quantity, type, location and condition of urban vegetation have direct effects on the number of benefits that vegetation gives and on the associated costs.
Impact on the climate
The rapid expansion of cities in the United States over the past 50 years has been associated with the constant rise in temperatures in the center of cities (varying between 0.1 and 1°C per decade). Since the demand for electricity in cities in the United States increases by 3 to 4 percent when the temperature rises by one degree (°C), about 3 to 8 percent of the electricity needed for cooling is used to compensate for this urban heat island effect (Akbari et al., 1992). The warming of cities compared to the surrounding rural areas has other consequences, in particular the increase in carbon dioxide emissions from power plants using fossil fuel; the increase in urban water demand; the unhealthy concentration of ozone; the discomfort and diseases of city dwellers. Due to the acceleration of the trend towards urbanization around the world, and in particular in tropical regions, it is urgent to mitigate the effects of urban heat islands by economic means. To maximize the energy savings of buildings and mitigate heat islands, the planting of trees and correct cultural care can be cheaper than other methods (for example, painted surfaces in light colors and modification of urban geometry) (McPherson, 1991).
Buildings, paved surfaces and vegetation act as thermal interfaces between the atmosphere and the floors of cities. The structure of urban wooded areas influences to a certain extent the temperature behavior of different locations inside a city. The maximum temperatures in the green spaces of the various built-up areas can be 3°C lower than outside the green spaces (Saito, Ishihara and Katayama, 1991).
Urban forests improve the climate thanks to several factors: the shade they give and which reduces the radiated energy absorbed, retained and reflected by the surface of buildings; evapotranspiration, which transforms radiated energy into latent energy, thus reducing the sensitive heat that warms the atmosphere; the modification of the air flow which influences the transport and diffusion of energy, water vapor and pollutants.
The relative importance of these effects depends on the area, the surface roughness and the configuration of the vegetation as well as other elements of the sites (Wilmers, 1991). In general, the climatic effects of large green spaces are felt at greater distances (from 100 to 500 m) than those related to smaller areas (Honjo and Takakura, 1991). Tall trees influence surface roughness, and deciduous trees contribute to seasonal variations in turbulence (Oke, 1989). The spacing between the trees, the extent of the crowns and the vertical distribution of the leaf area, as well as the height, act on the displacement of cold air and pollutants in the horizontal direction, along the streets and by turbulent mixing in the vertical direction (Oke, 1989; Barlag and Kuttler, 1991). In living quarters, a very extensive canopy has been associated with inversion phenomena that retain cold air and pollutants under the foliage (Grant, 1991).
Urban foresters in Chicago and other cities are seeking to gather information to guide decision-making regarding the size, distribution and design of green spaces along streets, in parks and private properties. There is reason to think that green spaces can have beneficial effects (for example, improvement of thermal comfort and retention of rainwater) and undesirable (for example, reduction of solar radiation, spread of pollution) on the urban hydroclimate (Oke, 1988; Westerberg and Glaumann, 1991), but there is still a need to know more about the fundamental processes.
It is important to better understand how different morphologies of urban buildings and vegetation configurations affect local relative humidity, air temperature and wind speed. Many studies have analyzed urban heat islands in a city at certain times of the day, but very little continuous data has been collected. To better understand the effect that trees in urban areas have on the local microclimate throughout the year, empirical models that link the climate under the foliage to the various elements of the local site (for example, vegetation and structure of buildings near the sensors) are developed. The dependent variables are the differences in the values of climatic variables between a reference station at the airport and five portable weather stations. The creation of these models must face certain conceptual challenges, such as the difficulty of elaborating morphological parameters that will be used as independent variables in the model and of dealing with the collinearity between these variables. The need to use portable equipment has led to a certain loss of sensitivity and has also required an intensive campaign for local residents to authorize the installation and monitoring of equipment in their property.
Another study on the effect of urban vegetation on the hydroclimate analyzed the flows of water and energy in a large neighborhood. Data on the surface of the soil as well as on the consumption of water, electricity and gas were collected to verify the interception evaporation model of Grimmond and Oke (1991). After validation, the model will be applied to the effects of the increase and decrease of the cover on the local energy and water balance.
Land use, available space for vegetation and cover stands in the Chicago area (as a percentage)