Urban Forests and Climate Change

Chapter 1 Urban Forests and Climate Change What Is Climate Change? ases that make up the Earth’s atmosphere trap the sun’s heat, creating a natural “greenhouse Geffect” that makes our life on the earth possible. Recent human activity has led to an accumulation of greenhouse gases (GHGs) in the atmosphere. Estimated U.S. emissions of carbon dioxide (CO2) increased from 5 to 5.5 billion tonnes from 1990 to 1996 (DOE/EIA 1997). The globally averaged temperature of the air at the Earth’s surface has warmed between 0.3 and 0.6 ºC (0.5-1 ºF) since 1900 (Hamburg and others 1997). Evidence of this temperature increase includes the observed level of sea rise of 10 to 25 cm (4-10 inches), the shrinkage of mountain glaciers, and increasing sub-surface ground temperatures. The current best estimate of the expected rise of globally averaged surface temperature relative to 1990 is 1 to 3.5 ºC (2-6 ºF) by the year 2100. This rate of warming will probably be greater than any that has occurred in the past 10,000 years; however, specific temperature changes will vary from region to region. This warming is expected to further increase sea level rise by 15 to 95 cm (6-37 inches) by the year 2100. With 50 to 70 percent of the global human population living in coastal areas, sea level rise could have significant effects. The frequency and duration of extreme events such as heavy rains and drought are likely to increase as the climate changes. In winter at mid-latitudes, warming is expected to increase precipitation in the form of rain rather than snow. This is likely to increase rates of wintertime soil moisture and runoff, while reducing summertime runoff. In spring, more flooding may result from faster runoff. During summer, increased heating will increase the probability of severe drought and could promote the spread of diseases formerly limited to the tropics. The number and duration of heat waves is projected to increase, resulting in increased mortality from heat stress, especially where air conditioning is not widely available. Even ground-level air pollutants such as ozone (smog) could increase, because the chemical reactions that form ozone accelerate as temperatures rise. Without humans, the Earth’s atmosphere maintains a delicate balance of GHGs. These gases are released and removed from the atmosphere by a variety of natural sources. For example, the natural decay of organic material in forests and grasslands, such as dead trees, results in the release of about 196 billion tonnes of carbon dioxide (CO2) annually (Hamburg and others 1997). This release of CO2 is nearly balanced by physical and biological processes that remove CO2. Sea water into which CO2 dissolves and the growth of plants are natural pools or reservoirs of CO2 (fig. 1). Approximately 97 percent of total CO2 emissions would occur even if humans were not present on Earth. Figure 1—Forests in and around cities are sites where CO2 can be stored. USDA Forest Service Gen.Tech.Rep. PSW-GTR-171. 1999. 1 Chapter 1 Urban Forests and Climate Change Human Activities and Climate Change Human activities add GHGs to the atmosphere at a rate of about 3 percent of annual natural emissions. Although they are a small percentage of total emissions, human-produced GHGs are enough to exceed the balancing effects of natural sinks. Carbon dioxide (CO2) and methane (CH4) are the two most important GHGs produced by people. Carbon dioxide is emitted when we burn fossil fuels to produce energy and heat, and to power vehicles. Methane is emitted in urban areas when garbage and waste products decompose in landfills and sewage treatment plants. Our focus is on CO2 rather than methane or other GHGs because urban forests can store CO2 as trees grow. Urban areas are population and economic centers where large quantities of energy are consumed and CO2 released. The total GHG emissions from the 10 largest U.S. cities account for 10 percent of total U.S. emissions. As urban centers grow both in terms of population and geographic area, fuel consumption increases, resulting in greater GHG emissions (fig. 2). Controlling GHG emissions to protect the climate can produce multiple benefits (ICLEI 1997). Energy efficiency measures can provide financial savings. Strategies to reduce emissions from the transportation sector can improve local air quality. The development of compact communities, transit-oriented projects, and multi-use facilities can catalyze local economic development, create new jobs, and enhance community livability. Figure 2a Figure 2b Figure 2—As agricultural and forest land is converted to urban land uses, GHG emissions increase. These photos show urban development in Sacramento between approximately 1970 (a) 1980 (b). As part of the Cities for Climate Protection Campaign, 48 U.S. cities inventoried their GHG emissions and developed reduction targets (ICLEI 1997). Strategic tree planting has been adopted as an emission reduction strategy in communities such as Chula Vista, Calif.; Dade County, Fla; Austin, Texas; Portland, Ore.; and Tucson, Ariz. For example, increasing residential tree planting in Austin from the current 4,700 to 15,000 trees per year is expected to provide annual CO2 reductions of 33,000 t after 12 years. This savings accounts for about 1 percent of Austin’s targeted reduction of 4.5 million tonnes (Mt) (City of Austin 1997). How Urban Forests Can Influence Atmospheric CO2 Urban forests can reduce atmospheric CO2 in two ways. As long as trees are actively growing, their rate of uptake of CO2 through photosynthesis is greater than their release of that gas through respiration, and the net result is a reduction of CO2 in the atmosphere. Trees around buildings can reduce the demand for heating and air conditioning, thereby reducing emissions associated with production of electric power. On the other hand, CO2 is released by vehicles, chain saws, chippers, and other equipment during the process of planting and maintaining trees. 2 USDA Forest Service Gen.Tech.Rep. PSW-GTR-171. 1999. Chapter 1 Urban Forests and Climate Change Eventually, all trees die, and most of the CO2 that has accumulated in their woody biomass is released into the atmosphere through decomposition. Nonetheless, an urban forest can become an important storage site for CO2 through tree planting and stewardship that increases canopy cover, as well as through strategic planting that cools urban heat islands and saves energy used for space heating and air conditioning. Carbon Dioxide Sequestration Carbon dioxide sequestration refers to the annual rate of storage of CO2 in above- and below- ground biomass over the course of one growing season. During photosynthesis, atmospheric CO2 enters the leaf through surface pores, combines with water, and is converted into cellulose, sugars, and other materials in a chemical reaction catalyzed by sunlight. Most of these materials become fixed as wood, although some are respired back to CO2 or used to make leaves that are eventually shed by the tree (Larcher 1980). Sequestration depends on tree growth and mortality, which in turn depends on species composition, age structure, and health of the forest. Newly planted forests accumulate CO2 rapidly for several decades, and then the annual increase of sequestered CO2 declines (Harmon and others 1990). Old-growth forests can release as much CO2 from the decay of dying trees as they sequester from new growth. When trees are stressed, as during hot, dry weather, they can lose their normal ability to absorb CO2. Trees close their pores as a defensive mechanism to avoid excess water loss. Hence, healthy, vigorous, growing trees will absorb more CO2 than will trees that are diseased or otherwise stressed. Because of higher tree densities, rural forests sequester about twice as much CO2 as urban forests per unit land area, between 4 to 8 t/ha on average (Birdsey 1992). However, because urban trees tend to grow faster than rural trees, they sequester more CO2 on a per-tree basis (Jo and McPherson 1995). Data on radial trunk growth were used to calculate annual sequestration for major genera in Chicago (Jo and McPherson 1995, Nowak 1994). Sequestration can range from 16 kg/yr (35 lb/yr) for small, slow-growing trees with 8- to 15-cm dbh (3-6 inches diameter at breast height) to 360 kg/yr (800 lb) for larger trees growing at their maximum rate. Although rapidly growing trees sequester more CO2 initially than slow-growing trees, this advantage can be lost if the rapidly growing trees die at younger ages. Figure 3 illustrates the difference between CO2 sequestration by a rapid growing, short-lived tree such as hybrid poplar 200 150 100 50 0 0 10 20 30 40 50 60 Tree Age (Years) Sugar Maple Hybrid Poplar Figure 3—Growth rate and life span influence CO2 sequestration. In this example, the total amount of CO2 sequestered over 60 years by the slower growing maple (3,225 kg) is greater than the amount sequestered by the faster growing but shorter-lived poplar (2,460 kg). Growth curves and biomass equations used to derive these estimates are based on data from urban trees (Frelich 1992, Pillsbury and Thompson, 1995). USDA Forest Service Gen.Tech.Rep. PSW-GTR-171 . 1999. 3 Chapter 1 Urban Forests and Climate Change (Populus ‘Robusta’) and a slower growing, longer-lived tree such as sugar maple (Acer saccharum). The poplar is estimated to sequester about 2,460 kg (5,420 lb) over 30 years, while the maple sequesters 3,225 kg (7,100 lb) during 60 years. Survival of urban trees is another important variable influencing long-term sequestration. Loss rates for street and residential yard trees are on the order of 10 to 30 percent over the first 5 years of establishment, and 0.5 to 3 percent each year thereafter (Miller and Miller 1991; McPherson 1993).

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