City and Environment

6. Green Spaces, Green Governance, and Planning

6 Green Spaces, Green Governance, and Planning ONLY FROM the window of an airplane does it become apparent that U.S. cities are often heavily forested while surrounding areas are not. On average, more than a quarter of urban land is covered by trees (Nowak et al. 2001). Tree cover is especially apparent on the older fringes of cities, where pressure on land is not as great as in the center, and where time has given trees a chance to grow large canopies. In the spring, the green lawns of the suburbs radiate skyward, while surrounding farm fields, in hues of beige and brown, wait for the farmer’s plow. There is already plenty of “nature” in cities even if much of the ecosystem is highly managed. Given the multiple benefits that accrue from green space and urban trees, it is no wonder that many municipalities are making renewed efforts to “green” their cities. The environmental benefits of green spaces and trees are many, from cooling cities to improving water and air quality (see Chapter 4). Trees also play a role in slowing global climate change. The 3.8 billion city trees in the United States are estimated to store 700 million tons of carbon and sequester 23 million tons per year. The amount stored (not sequestered) is the amount of carbon emitted by the American population every six months. Annual sequestration of carbon by city trees, however, is the equivalent of fewer than six days’ worth of emissions nationwide. While city trees sequester only a small portion of the carbon emitted by human activity, they store large amounts of carbon that is better left in the tissue of the tree than released into the atmosphere. Maintaining trees is important for locking up carbon in plant tissue instead of releasing it into the atmospheric carbon cycle. New York City’s 5.2 million trees store 1.2 million tons of carbon and sequester thirty-eight thousand tons of carbon every year. The economic value of carbon storage and sequestration in city trees is estimated to be $14.3 billion (Dwyer et al. 2000; Nowak et al. 2001; Nowak and Crane 2002). Parks, playgrounds, trees, and recreation areas also provide health benefits to mind and body. Although some parks may be seen as dis- amenities, especially if they are not policed or maintained, most urban dwellers welcome them as amenities and are willing to pay for them. Higher housing values near parks increases property taxes, which help to maintain the parks. Evidence also shows that mature trees, like parks, can add to the value of a home or neighborhood (Crompton 2001). But green space comes with costs. Mature trees, while beautiful, can also pose a hazard if they topple or their limbs fall. Trees in public places must be maintained with public dollars, a cost that many cash-strapped municipalities are quick to slash when budgets are drawn. Poor selection of tree species can create problems. Some tree species are ill suited to the compacted soils or poor air quality of urban areas. Others create root systems that damage underground conduits, crack sidewalks, or damage building foundations. Maintaining diversity of tree species of different ages is also an important strategy for city arborists. Dutch Elm disease devastated urban forests in the twentieth century partly because of the monoculture practiced (Johnson et al. 2003). The fungus (carried by bark beetles) that infected the trees spread easily from tree to tree. A street lined with old elms can be beautiful, but many cities have recognized the need to adopt an ecosystem approach to urban forestry, planting a diversity of species that are better able to resist pests and disease while providing habitat for birds and other urban critters. Despite the costs and hazards that come with large trees (or even open space, which can invite crime), most cities are working to increase green spaces because of the environmental, health, and economic bene fits they provide. The transition from a complete reliance on “gray infrastructure” to incorporation of “green infrastructure” is part of the renewed commitment. As cities repair aging water and sewerage systems , for example, they are beginning to explore the use of riparian buffers, pervious surfaces, and even artificial wetlands as an ecologically and in some cases economically friendlier means of providing city services than entirely engineered systems (see Chapter 4). The decline of population in central cities has forced municipalities to explore ways to slow the drift...