ENERGY CONSERVATION POTENTIAL OF URBAN TREE PLANTING

Findings from monitoring and computer simulation studies indicate that trees can be a cost-effective energy conservation measure for some electric utilities. Our simulations suggest that a single 25-ft tall tree can reduce annual heating and cooling costs of a typical residence by 8 to 12 percent ($10-25). Assuming annual savings of $10 per household, a nationwide residential tree planting program could eventually save about $1 billion each year. A study of the potential for energy-conserving shade tree plantings within residential sections of San Diego found that over 40 percent of all houses surveyed had space available for a tree opposite their west wall. The 30-year net present value of proposed shade tree plantings for demand side management in Fresno was projected to be $22.3 million, with an overall benefit-cost ratio of 19. The largest benefits were attributed to property value enhancement, energy savings, avoided stormwater runoff, and atmospheric carbon removal, while greatest projected costs were from pruning, planting, and program administration. The importance of trees in our communities is taking on greater significance as we learn more about their potential to improve quality of life. Environmental concern about global warming, urban heat islands, and air pollution has brought attention to the potential of trees to ameliorate climate and conserve energy. This paper addresses a number of questions about the energy conservation potential of urban forests. What is their potential to improve environmental quality and conserve energy? Are trees cost-effective compared to otherenergy conservation measures? How does the structure of energy efficient landscapes change within a city and across climatic regions? Answers to these questions should assist arborists, urban foresters, policy makers, utility personnel, natural resource managers, design and planning professionals, and concerned citizens who are planting and managing trees to improve their local environments. Vegetation and Urban Climate Rapid urbanization of U.S. cities during the past 50 years has been associated with a steady increase in downtown temperatures of about 1 °F (0.8°C) per decade. Because electric demand of U.S. cities increases about 1 to 2 percent per degree F (3-4% per°C) increase in temperature, approximately 3 to 8 percent of current electric demand for cooling is used to compensate for this urban heat island effect (2). Warmer temperature in cities compared to surrounding rural areas has other implications, such as increases in carbon dioxide emissions from fossil fuel power plants, municipal water demand, unhealthy ozone levels, and human discomfort and disease. These problems could be accentuated by global climate change, which may double the rate of urban warming. The accelerating world trend towards urbanization, especially in tropical regions, hastens the need for energy efficient landscapes. Buildings, paving, and vegetation measurably affect the ambient temperatures of different sites within a city. Maximum temperatures within the greenspace of individual building sites may be 5°F (3°C) cooler than outside the greenspace (32). At the larger scale of urban climate (6 miles or 10 km square), temperature differences of more than 9°F (5°C) have been observed between city centers and more vegetated suburban areas (24). Urban forests ameliorate climate and human comfort through 1) shading, which reduces the amount of radiant energy absorbed, stored, and radiated by built surfaces, 2) evapotranspiration, which converts radiant energy into latent energy, thereby reducing sensible heat that warms the air, and 3) air flow modification, which affects transport and diffusion of energy, water vapor, and 322 McPherson: Energy Conservation With Trees pollutants. The relative importance of these effects depends on the area, surface roughness, and configuration of vegetation and other landscape elements (37). Generally, large greenspaces affect climate at farther distances (300 to 1,500 ft, 100 to 500 m distance) than do smaller greenspaces (12). Tall trees influence surface roughness and deciduoustrees contribute to seasonal differences in turbulence (27). Tree spacing, crown spread, and vertical distribution of leaf area influence the transport of cool air and pollutants along streets, and out of urban canyons by turbulent mixing from above (3,27). For individual buildings, solar angles and infiltration are often important. Because the summer sun is low in the east and west for several hours each day, shade to protect east and especially west walls helps keep buildings cool. Rates at which outside air infiltrates into a building can increase substantially with wind speed. In cold windy weather the entire volume of air in a poorly sealed home may change two to three times per hour (8). Even in newer or tightly sealed homes, the entire volume of air may change every two to three hours. Measured and Simulated Energy Savings from Landscapes About 7 percent of the total energy consumed in the United States during 1990 was used for household heating and cooling at a cost of $98.1 billion (6). The average household spent $370 for heating and $186 for air conditioning. These expenditures accounted for 32% and 16% of the typical annual energy bill ($1,172), respectively (6). Results of experimental studies and computer simulations reviewed in the following section suggest that energy savings from a 25-ft tree range from $10 to $25 yearly. A nationwide residential tree planting program could eventually save about $1 billion each year assuming a savings of $10 per household. Additional savings would accrue from effects of lower summertime temperatures on energy used by commercial buildings, many of which are air-conditioned. Electric utility customers could also benefit from reduced capital investment in peak electric generating capacity or power purchases and power plant emission controls. Measured savings. Relatively few studies have monitored effects of landscapes on building energy use. Monitoring studies are expensive and somewhat risky because factors such as occupant behavior, thermostat settings, and changing weather make it difficult to isolate the effects of landscapes on heating and air conditioning. In a review of measured cooling savings from landscapes, vegetation was reported to consistently lower wall surface temperatures by about 30°F (17°C) (23). Air-conditioning electricity savings ranged from 10 to 80 percent (Figure 1). In the Arizona studies (15,34) turf alone provided cooling savings of 10 to 25 percent, largely due to evapotranspirational (ET) cooling. Shading from shrubs and trees in Florida (29) and Pennsylvania (4) resulted in cooling savings of 30 percent and greater. Studies dating back to the 1930's have monitored heating savings from windbreaks and more recently have measured windspeed reductions in residential neighborhoods resulting from the combined effects of buildings and landscapes (10). Reported heating savings from windbreaks ranged from 3 to 40 percent (8), with 10 to 12 percent savings found for a mobile home and detached houses in Pennsylvania and New Jersey, respectively (Figure 2). Simulated savings. Effects of landscapes on building energy use are easier to simulate than to