D.J. Nowak and E.G. McPhersonDavid J. Nowak and E. Gregory McPherson are Research Foresters with the United States Department of Agriculture Forest Service, Northeastern Forest Experiment Station, Chicago.
This article reports on the methodologies and initial results of an urban forestry research project, based in Chicago, Illinois. It discusses the interrelated urban forest ecosystem functions that are currently being studied - climate modification, energy conservation, air quality and carbon dioxide sequestration - and considers the cost-benefit implications of urban vegetation.
The Chicago Urban Forest Climate Project (CUFCP) was established to increase the understanding of how vegetation within urban areas influences local climate, energy use and air quality. Currently in mid-operation, having completed intensive data collection during the summer of 1992, the CUFCP is a multiyear research project scheduled for completion in late 1993. The impetus for the CUFCP came in 1990 when Chicago Mayor Richard M. Daley expressed a desire for urban forest research in support of his comprehensive plan for greening Chicago, called GreenStreets. A research plan was developed and Congress appropriated US$900000 to the USDA Forest Service for the project. Two scientists and five technicians are currently working on the CUFCP in Chicago and another 11 technicians were involved in the 1992 data collection exercise.
The goals of the CUFCP are to: quantify and assign monetary values to many of the environmental benefits and costs associated with Chicago's urban forest ecosystem; generate management alternatives to enhance urban forest environmental benefits; and develop transferable methods and models for other cities. Results and recommendations derived from the project will be reported in various scientific journals, non-technical publications and reports to the city of Chicago and surrounding communities. It is anticipated that the information will then be used by local urban foresters, home-owners, nonprofit groups, utility companies and planners in order to make more informed decisions about future urban vegetation configurations.
Figure 1 - Percentage tree cover by community area in Chicago
The greater the tree cover, the greater the relative importance of trees in influencing the environment of a given city. One of the most cost-effective analyses of urban forest structure is that of tree cover (the proportion of area occupied by tree canopies when viewed from above). Tree cover analysis of the CUCFP study area (the city of Chicago, the rest of the surrounding Cook County and adjacent Du Page County), conducted using a dot grid sampling of random points on aerial photographs, revealed forest cover to be at a level of 19 percent. Tree cover in the city of Chicago itself is only 11 percent overall (see Table) but varies widely from place to place and ranges from a low of 1 percent to a high of 37 percent (Fig. 1). Land-use types with the highest tree cover in Chicago were forest reserves (54 percent tree cover), parks (26 percent) and vacant land (20 percent); the lowest tree cover land-use types were transportational (2 percent) and commercial/industrial lands (3 percent) (McPherson et al., 1992).
Factors that influence overall urban tree cover include ecoregion (i.e. the natural environment in which the city is developed), city age and city size. For example, preliminary analyses indicate that cities developed in forested areas in the eastern United States average 30 percent in urban tree cover; in western forested areas, 26 percent; central areas covered by grassland and forests, 22 percent; and, in western areas originally dominated by shrub, grassland and desert, 17 percent. Average urban tree cover in the United States is estimated to be 27 percent.
Beyond tree cover, other attributes that are important for quantifying urban forest structure include species composition, tree diameter and height distribution, biomass and leaf surface area. However, very little is known about the comprehensive urban forest structure. Most urban forest work has been conducted on street tree populations which often comprise only a small percentage of total urban woody vegetation. Research has quantified species composition and other structural attributes for various parts of urban forests across the world (e.g. 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, United States Nowak, 1991; Beijing, China Profous, 1992; Hong Kong - Jim, 1992).
To estimate vegetative and other physical parameters of the entire Chicago area, 615 plots were randomly selected and inventoried across all land-use types. This information will be used to quantify the physical attributes of vegetation, roads, buildings and other structures. In conjunction with the plot data, more than 120000 urban tree leaves were randomly sampled from tree canopies in a high-lift truck and a 0.4 m3 sampling frame for analyses of leaf surface area and dry weight biomass. These structural attributes will be used in functional models (e.g. of hydrocarbon emissions). To project future urban tree growth, hundreds of tree disks, cut at diameter at breast height (DBH) mainly from dead or dying trees, were supplied by the Chicago Bureau of Forestry and suburban communities. Increment cores of healthy trees are also being analysed to test the applicability of growth data obtained from unhealthy trees.
Urban forest structure directly affects urban forest functions (e.g. transpiration and associated air temperature reductions, energy conservation, air pollution mitigation). The amount, type, location and condition of urban vegetation directly affects the amount of benefits derived from the vegetation and their associated costs.
Impact on climate
Rapid urbanization of United States cities during the past 50 years has been associated with a steady increase in downtown temperatures (ranging from 0.1 to 1°C per decade). Because demand for electricity in United States cities increases by 3 to 4 percent per degree (C) of temperature increase, approximately 3 to 8 percent of current electric demand for cooling is used to compensate for this urban heat island effect (Akbari et al., 1992). Warmer temperatures in cities compared with surrounding rural areas have other implications, including increases in carbon dioxide emissions from fossil fuel power plants; increases in municipal water demand; unhealthy ozone levels; and human discomfort and disease. The accelerating world trend towards urbanization, especially in tropical regions, hastens the need for cost-effective urban heat island mitigation. Proper planting and care of trees to maximize building energy savings and to mitigate heat islands can be more economical than other methods of reducing electrical demand and heat islands, e.g. light-coloured surfaces; and urban geometry modifications (McPherson, 1991).
Buildings, paving and vegetation act as thermal interfaces between the atmosphere and the urban land surface. Urban forest structure measurably affects the thermal behaviour of different sites within a city. Maximum temperatures within the green space of individual building sites may be 3°C cooler than outside the green space (Saito, Ishihara and Katayama, 1991).
Urban forests ameliorate climate through: shading, which reduces the amount of radiant energy absorbed, stored and radiated by built surfaces; evapotranspiration, which converts radiant energy into latent energy, thereby reducing sensible heat which warms the air; and air flow modification, which affects the transport and diffusion of energy, water vapour and pollutants.
The relative importance of these effects depends on the area, surface roughness and configuration of vegetation and other landscape elements (Wilmers, 1991). Generally, the climatic effects of larger green spaces are noticed at greater distances (100 to 500 m) than those associated with smaller areas (Honjo and Takakura, 1991). Tall trees influence surface roughness while deciduous trees contribute to seasonal differences in turbulence (Oke, 1989). Tree spacing, crown spread and the vertical distribution of leaf area with height influence the transport of cool air and pollutants along streets by advection and out of urban canyons by turbulent mixing from above (Oke, 1989; Barlag and Kuttler, 1991). Extensive tree cover in residential areas has been associated with inversions that trap cool air and pollutants below the canopy (Grant, 1991).
Urban foresters in Chicago and other cities seek information to guide decision making regarding the size, distribution and design of green space along streets, in parks and on private property. There are indications that green space can have both desirable impacts (e.g. improved human thermal comfort and storm water detention) and undesirable impacts (e.g. reduced solar access and pollution dispersion) on urban hydroclimate (Oke, 1988; Westerberg and Glaumann, 1991), but a greater knowledge of underlying processes is needed.
Street trees in Chicago comprise approximately one-third of total city tree cover
There is the need for a better understanding of how different urban building morphologies and vegetation configurations affect local relative humidity, air temperature and wind speed. Many studies have analysed urban heat islands across a city over selected periods of a day, but very little continuous data have been collected. To gain a better understanding of the effect of urban trees on local microclimate over the course of a year, empirical models that relate subcanopy climate to local landscape features (e.g. vegetation and building structure near the sensors) are being developed. The dependent variables are the differences in the values of climatic variables between an airport reference station and five portable meteorological stations. Some conceptual challenges to the generation of these models include the difficulty in developing the morphological parameters that will be used as the independent variables in the model and having to deal with colinearity among these variables. The need to use portable equipment meant some loss of sensitivity and also required an intensive campaign to acquire permission from local residents to place and monitor equipment on their property.
Another study analysing the effect of urban vegetation on hydroclimate was conducted by analysing water and energy fluxes over a large neighbourhood. Land surface data were collected, along with water, electricity and gas usage, to calibrate the Grimmond and Oke (1991) evaporation-interception model. After validation, the model will be applied to project effects of increased and decreased tree cover on the local energy and water balances.
Impacts on use of energy
The amount of energy required to heat and cool buildings depends on their thermophysical properties, occupant behaviour and local climate. By modifying local climate, urban forests can increase or decrease building energy use (Heisler, 1986). Measured (Meter, 1991) and simulated (Huang et al., 1987; McPherson, Herrington and Heisler, 1988) energy reductions caused by vegetation around individual buildings generally range from 5 to 15 percent for heating and 5 to 50 percent for cooling. The aggregate effects of neighbourhood trees on air temperature and wind speed are just as important as more localized shading effects (Huang et al., 1987; Heisler, 1990).
Projections from computer simulations indicate that 100 million mature trees in United States cities (three trees for every two homes) could reduce annual energy use by 30000 million kWh (25800 million kcal), saving about US$2000 million in energy costs (Huang et al., 1987). Avoided investment in new power supplies and an estimated 9 million tonnes (8165 million kg) annual reduction in carbon dioxide emissions from existing power plants could augment these savings considerably. Even when the costs of planting, watering and maintaining trees are considered, tree-planting is a more cost-effective energy and carbon dioxide conservation strategy than many other fuel-saving measures.
More than 75 percent of Chicago households use electricity for air-conditioning during the summer. Initial computer simulations indicate that three 7.6 m trees around a well-insulated new home would reduce annual heating and cooling costs by 8 percent (US$96) compared with those for the same building without trees. Annual savings created per tree would be broken down as follows: reduced cooling requirements in summer as a result of shade (37 percent); reduced cooling requirements in summer as a result of evapotranspiration-lowered air temperature (42 percent); reduced heating requirements in winter as a result of lowered wind speeds (21 percent).
CUFCP energy research is using empirical relations between subcanopy climate and landscape features to estimate effects of existing, increased and decreased tree cover on building microclimates. Modified climate data will be used with building energy analysis models to identify optimal tree locations and species for heating and cooling energy savings.
Percentage of land cover, available growing space and canopy stocking for the Chicago region
|
Region |
Tree |
Grass 1 |
Building |
Paved |
Water |
AGS 2 |
CSL 3 |
|
Chicago |
11.1 |
26.9 |
27.4 |
32.4 |
2.2 |
38.0 |
29.2 |
|
Cook County 4 |
22.5 |
44.7 |
12.6 |
18.2 |
2.0 |
67.2 |
33.5 |
|
DuPage County |
18.6 |
56.0 |
9.4 |
13.9 |
2.1 |
74.6 |
24.9 |
|
Overall region |
19.4 |
44.4 |
14.5 |
19.7 |
2.0 |
63.8 |
30.4 |
1 Percentage of area occupied by both grass and soil.
2 Available growing space (percentage of tree and grass cover).
3 Canopy stocking level (percentage of available growing space occupied by trees).
4 Cook County exclusive of Chicago. Source: McPherson et al., 1992.
Impacts on air quality
Trees in urban areas help to improve air quality by presenting a large surface area in which: particulate pollutants can be trapped; gaseous pollutants may be bound or dissolved, particularly when wet; and gaseous pollutants may be taken up during gas exchange at leaf stomates. Information on the rate at which various pollutants are deposited to surfaces in urban areas, including trees, is very limited.
Besides directly absorbing or intercepting pollutants, trees can also influence the formation of a secondary pollutant, ozone. Modelling of a June day in Atlanta (United States), indicates that reducing tree cover by 20 percent would increase maximum ozone concentrations from 123 ppb to 140 ppb, mainly because of a 2°C temperature increase (Cardelino and Chameides, 1990). Research by the CUFCP is investigating the magnitude of vegetative emissions of volatile organic compounds (a precursor of ozone) and air temperature reductions caused by trees to determine how these factors influence ozone formation in the Chicago area.
Impacts on atmospheric carbon dioxide
Increasing levels of atmospheric carbon dioxide (CO2) and other greenhouse gases are thought by many to be leading to increased atmospheric temperatures through the trapping of certain wavelengths of heat in the atmosphere. This increase in atmospheric CO2, the predominant greenhouse gas, is largely attributable to fossil fuel combustion and, to a much lesser extent, deforestation. Trees, through their growth process, act as a sink for atmospheric carbon dioxide. Thus, increasing the amount of trees can potentially slow the accumulation of atmospheric CO2, so long as the trees are healthy and growing vigorously.
In terms of reducing levels of atmospheric CO2, trees in urban areas offer the double benefit of direct carbon absorption and reduction of the CO2 produced by fossil fuel power plants through energy conservation from properly located trees. At present, the CUFCP is quantifying the degree to which urban trees in the Chicago area sequester atmospheric CO2 and reduce emissions from power plants through energy conservation. In addition, a neighbourhood carbon budget is being researched to determine how much carbon (in fossil fuels) is being expended in the maintenance of urban vegetation and what carbon benefits are being derived from vegetation at the neighbourhood level.
To illustrate the potential impact of urban trees on atmospheric carbon dioxide, the effect of planting ten million urban trees (3.0 cm in diameter) annually in the United States, between 1991 and 2000, was modelled over a 50-year period (Fig. 2; Nowak, in press). The resulting 100 million trees were assumed to be planted in proper locations around buildings to conserve energy.
In the year 2040, these trees would have stored 85 million tonnes and prevented the production of another 315 million tonnes of carbon, a 4:1 carbon avoided to stored ratio. The total 400 million tonnes of carbon stored and avoided is a liberal estimate, as all 100 million trees were assumed to survive the entire 50 years and there is no calculation for the eventual CO2 released to the atmosphere when trees might have to be removed. Even so, this estimate is less than 1 percent of the amount of carbon emissions forecast for the United States over the same 50-year period (Nowak, in press).
Although the relative impact of urban trees on the increasing amount of atmospheric carbon dioxide is small, carbon sequestration and avoidance are only two of the many benefits derived from urban trees.
Urban green space provides many environmental and social services that contribute to the quality of life in cities. However, economic approaches used to estimate the value of green space services (e.g. willingness to pay, travel cost) are of limited use to policy-makers, planners and managers because the underlying values they estimate only indirectly reflect the flow of multiple benefits and costs. Benefits from trees are environmental externalities because these benefits are not reflected in consumer prices - we do not pay money to trees for cooling our homes but we do pay utility companies for the power to run air-conditioners to cool our homes. Two approaches used to estimate values for external environmental benefits from trees are direct estimation and implied valuation (McPherson, 1992). Computer simulation of the effects of trees on building energy use provide direct estimates of benefits. Implied valuation relies on the costs of environmental control to estimate a benefit to society of reducing externalities such as air pollution, water runoff from storms or highway noise. For instance, if a society is willing to pay a given figure (e.g. US$5/kg) for air pollution control, a tree that intercepts 1 kg of pollution should be worth $5 for that function.
A green space accounting approach that directly connects vegetation structure with the spatial-temporal flow of functional benefits and costs was applied for a 500000 tree-planting programme in Tucson, Arizona. Prices were assigned to each cost (planting, pruning, removal, irrigation, etc.) and benefits (savings in cooling energy requirements, interception of pollution particles, storm water runoff reduction) were calculated through direct estimation and implied valuation. The trees were projected to provide a $236.5 million net benefit over a 40-year planning horizon (McPherson, 1992). Trees planted in parks were projected to provide the highest benefit-cost ratio (2.7) and trees along residential streets the lowest (2.2).
Application of this benefit-cost approach is limited by the inability to value more intangible benefits of trees (e.g. mental health improvement, aesthetic improvement) as well as by the current lack of detailed information regarding urban tree growth and mortality rates, leaf area, rainfall interception, pollution absorption rates and management costs. Despite these uncertainties, this green space accounting approach will be applied in Chicago, as it offers a relatively sophisticated approach for evaluating some of the economic and environmental implications of possible urban forestry efforts. Tree numbers, locations and species to be planted in Chicago during the next five years will be determined by a survey of agencies and contractors. Prices will be assigned to each cost and benefit. Findings will depict the net present value of benefits and the annual flow of benefits and costs over a 30-year planning horizon. Management recommendations will address issues such as the cost-effectiveness of plantings in different locations with different species; optimal rotation lengths, pruning cycles and initial planting sizes; and net benefits of investments in urban forests as compared to other environmental control technologies.
Trees can significantly influence the urban environment, yet relatively little research has been conducted to quantify their effects. In addition to the CUFCP, urban climatology is being researched as part of the Tropical Urban Climate Experiment (TRUCE) (Oke, Taesler and Olsson, 1991). Methods and models from both projects will be transferred to planners in other cities.
The knowledge gained by the CUFCP over the next few years is expected to be significant, but much more research will be needed to quantify and monetize better the environmental benefits of urban forest ecosystems. A better understanding of how and to what degree urban trees influence the environment will lead to better management of urban trees, significant monetary savings for urban residents and a more pleasant and healthy urban environment.
Akbari, N., Davis, S., Dorsano, S., Huang, J. & Winnett, S., eds. 1992. Cooling our communities: a guidebook on tree planting and light-colored surfacing. Washington, DC, USEPA.
Barlag, A. & Kuttler, W. 1991. The significance of country breezes for urban planning. Energy and Buildings, 15-16: 291-297.
Cardelino, C.A. & Chameides, W.L. 1990. Natural hydrocarbons, urbanization and urban ozone. J. Geophys. Res., 95(D9): 13971 - 13979.
Grant, R. 1991. Evidence for vegetation effects on the daytime internal boundary layer over suburban areas. In American Meteorological Society, ed. Tenth Conference on Biometeorology and Aerobiology. Boston, American Meteorological Society.
Grimmond, C.S.B. & Oke, T.R. 1991. An evaporation-interception model for urban areas. Water Resour. Res., 27: 1739-1755.
Heisler, G.M. 1986. Energy savings with trees. J. Arboriculture, 12: 113-125.
Heisler, G.M. 1990. Mean windspeed below building height in residential neighborhoods with different tree densities. ASHRAE Trans., 96(1): 1389-1396.
Honjo, T. & Takakura, T. 1991. Simulation of thermal effects of urban green areas on their surrounding areas. Energy and Buildings, 15-16: 433-446.
Huang, J., Akbari, H., Taha, H. & Rosenfeld, A. 1987. The potential of vegetation in reducing summer cooling loads in residential buildings. J. Clim. Appl. Meteorol., 26.
Iizumi, S. 1983. The urban vegetation of Tokyo and Sendai, Japan. In W. Holzner, M.J.A. Werger & T. Ikusima, eds. Man's impact on vegetation. Boston, Dr W. Junk Publishers.
Jim, C.Y. 1992. Provenance of amenity-tree species in Hong Kong. Arboricultural J., 16: 11-23.
McPherson, E.G. 1991. Cooling urban heat islands with sustainable landscapes. Tucson. Arizona, Drachman Institute for Land and Regional Development Studies.
McPherson, E.G. 1992. Accounting for benefits and costs of urban greenspace. Landscape and Urban Planning, 22: 41-51.
McPherson, E.G., Herrington, L.P. & Heisler, G. 1988. Impacts of vegetation on residential heating and cooling. Energy and Buildings 12: 41-51.
McPherson, E.G., Nowak, D.J., Sacamano, P.L., Prichard, S.E. & Makra, E.M. 1992. Chicago's evolving urban forest. General Technical Report, Radnor, USDA Forest Service, Northeastern Forest Experiment Station.
Meier, A. 1991. Strategic landscaping and air-conditioning savings: a literature review. Energy and Buildings, 15-16: 479-486.
Nowak, D.J. 1991. Urban forest development and structure: analysis of Oakland, California. Ph.D. dissertation. Berkeley, University of California.
Nowak, D.J. Atmospheric carbon reduction by urban trees. J. Environ. Manage. (In press)
Oke, T.R. 1988. Street design and urban canopy layer climate. Energy and Buildings, 11: 103-113.
Oke, T.R. 1989. The micrometeorology of the urban forest. Phil. Trans. R. Soc. Lond., 324: 335-349.
Oke, T.R., Taesler, R. & Olsson, L.E. 1991. The Tropical Urban Climate Experiment (TRUCE). Energy and Buildings, 15-16: 67-73.
Profous, G.V. 1992. Trees and urban forestry in Beijing, China. J. Arboriculture, 18(3): 145-153.
Profous, G.V., Rowntree, R.A. & Loeb, R.E. 1988. The urban forest landscape of Athens, Greece: aspects of structure, planning and management. Arboricultural J., 12: 83-107.
Saito, l., Ishihara, O. & Katayama, T. 1991. Study of the effect of green areas on the thermal environment in an urban area. Energy and Buildings, 15-16:493-498.
Svanbergsson, A., Sigtryggsson, V. & Andersen, J.W. 1988. Icelandic social forestry in metropolitan Reykjavik. Arboricultural J., 12: 53-64.
Wee, Y.C. & Corlett, R. 1986. The city and the forest: plant life in urban Singapore. Singapore, Singapore University Press.
Westerberg, U. & Glaumann, M. l991. Design criteria for solar access and wind shelter in the outdoor environment. Energy and Buildings, 15-16: 425-431.
Wilmers, F. 1991. Effects of vegetation on urban climate and buildings. Energy and Buildings, 15-16: 507-514.
