Mit-dusp urban Sustainability Evaluation Green Infrastructure: Urban Tree Planting
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EnergyIssue[TBE] Tree Planting ImpactsThe lowered temperatures achieved by greenspace (see ) may not only positively impacts urban health and comfort, but can also lower energy use and related costs in commercial buildings and residences. Carefully planted deciduous trees, which lose their leaves in the winter and do not significantly block solar gain, can control building temperatures and lower energy demand (Meier 1990). In the winter, strategic landscaping can establish natural wind-blocks that reduce heating requirements and associated costs. A modeling effort examining residential energy use found that trees placed to shade a home’s roof, as well as its east and west side, reduced heating and cooling energy costs by 20-25% when compared with the same house in the open (Heisler 1986). Akbari et al (1997) planted 16 trees at two residential sites in Sacramento, CA and reported that summer cooling savings averaged 30%. While most studies have looked specifically at residential units, which, given their size, are easier to influence with plantings, there is evidence that larger structures can benefit from properly placed vegetation as well (Papadakis 2001). Air PollutionIssue[TBE] Tree Planting Impacts REWRITE (FROM NOWAK) Urban trees are able to remove air pollutants in two ways. First, they Trees remove gaseous air pollution primarily by uptake via leaf stomata, though some gases are removed by the plant surface. Once inside the leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner-leaf surfaces. Trees also remove pollution by intercepting airborne particles. Some particles can be absorbed into the tree, though most particles that are intercepted are retained on the plant surface. The intercepted particle often is resuspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig fall. Consequently, vegetation is only a temporary retention site for many atmospheric particles. Standardized pollution removal rates differ among cities according to the amount of air pollution, length of in-leaf season, precipitation, and other meteorological variables. Large healthy trees greater than 77 cm in diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small healthy trees less than 8 cm in diameter (0.02 kg/yr). Air quality improvement in New York City due to pollution removal by trees during daytime of the in-leaf season averaged 0.47% for particulate matter, 0.45% for ozone, 0.43% for sulfur dioxide, 0.30% for nitrogen dioxide, and 0.002% for carbon monoxide. Air quality improves with increased percent tree cover and decreased mixing-layer heights. In urban areas with 100% tree cover (i.e., contiguous forest stands), short-term improvements in air quality (one hour) from pollution removal by trees were as high as 15% for ozone, 14% for sulfur dioxide, 13% for particulate matter, 8% for nitrogen dioxide, and 0.05% for carbon monoxide (Nowak) Carbon DioxideIssue[TBE] Tree Planting ImpactsAs part of the process of photosynthesis, trees remove carbon dioxide from the air, expelling oxygen as a byproduct. The carbon is integrated within the tree as biomass— its roots, branches, leaves, trunk. (Rewrite)Trees remove carbon dioxide from the air through their leaves. Carbon storage is the total amount of carbon held in a tree’s wood (biomass). Carbon sequestration is the rate at which trees store carbon. Older trees have more carbon storage; younger trees have a higher sequestration rate. Approximately half of a tree’s dry weight is carbon. For this reason, large-scale tree planting projects are recognized as a legitimate tool in many national carbon- reduction programs. WaterIssueUrbanization transforms the landscape from one dominated by forests, wetlands, and vegetation to one covered by buildings, asphalt, pavement, and compacted soils, all impervious to water. Indeed, city centers can be over 90% impervious. Storm water pathways are thus limited and the majority of rainwater becomes runoff (Table 1). Table 1 Water flows with varying levels of impervious surface
(After Paul and Meyer 2001) Runoff moves more quickly through an urban environment than a natural one, as surfaces are smoother (lowered coefficient of roughness) and flatter (lowered storage capacity). This is also a result, an intended outcome in fact, of conventional storm water systems, which have been engineered to provide efficient and rapid removal of rainwater from urban surfaces. Landscapes have been graded, piped, and paved in an effort to quickly move water off urban surfaces to storm water sewers and into treatment plants, or, more commonly, into receiving waters (Coffman et al 1998). The overall result is a larger quantity of runoff moving across surfaces and entering waterways more rapidly than it would have pre-development. More and faster moving runoff causes river bank erosion, increases the magnitude and frequency of floods, and mobilizes the myriad contaminants that lie across the urban surface, such as salts, metals, particulates, and sediments, concentrating them in the receiving rivers and lakes. Tree Planting ImpactsThe concern over the degradation of waterways by runoff coupled with the EPA’s storm water permitting system (under the National Pollutant Discharge Elimination System) have provided impetus for planners and engineers to investigate ways to mitigate the impact of impervious surfaces. Such methods are directed at reversing the effects of ISC by: increasing the permeability of surfaces increasing depression storage allowing infiltration and ground water recharge allowing infiltration and pollutant remediation The suite of methods, which include techniques, measures, and structures, that manage and control the quantity and quality of runoff cost effectively, are termed best management practices (BMPs). They include structural and non-structural methods, such as retention basins and sand filters, as well signage and good housekeeping. In urban areas, where there is little space to implement land intensive controls, replacing impervious surface with trees and vegetation is recognized as excellent runoff control options. In addition to the increasing pervious surface area, trees intercept large quantities of water with their leaves and bark, slowing the flow of runoff and increasing opportunities for evaporation. Their roots increase the permeability of soils, aiding infiltration and groundwater recharge. A study on one large deciduous tree in Southern California found that it reduced runoff by over 4,000 gallons per year (Xiao 1998). This type of mitigation is especially beneficial because it is an accessible technology and can be applied on many scales. Furthermore, unlike methods such as detention basins, trees and vegetation control runoff at the site. While simply replacing impervious surfaces with trees provides benefits passively, integrating them into more comprehensive site design that employs structural BMPs can enhance runoff control. Structural BMPs that use vegetation are based on retention and or direct infiltration methods. Native vegetation is often recommended for use in these BMPs, as such plants are accustomed to the climate, and require less water and pesticide/herbicide use. Structural BMPs that can be incorporated on urban sites include:
Swales: Open channel devices that are designed to detain and filter water from nearby parking areas, playgrounds or sidewalks. They can be grassed or planted with native vegetation. Dry swales, where a highly permeable soil is used to ensure infiltration, are most common. Wet swales, which mimic wetland conditions, can also be used. Rain Gardens: Lushly planted with vegetation and slightly depressed to store and infiltrate several inches of water (not as much as swales). Vegetated Filter Strip: It does not employ depression storage, but is typically used to slow the flow of runoff and provide some infiltration. They can be covered with grass or trees. The vegetated strip was developed for use as a riparian buffer along agricultural land. Now, they are commonly used along sidewalks and to intercept roof runoff. These techniques also provide opportunities for contaminant remediation, thus improving water quality. Soils, vegetation, and trees intercept and filter sediments, prevent erosion, provide binding sites for ions, and take up nutrients. Collaborative BMP monitoring between the EPA and American Society of Civil Engineers’ (ASCE) Urban Water Research Council found that removal efficiencies of swales and vegetated strips could be as high as 80% for nutrient and suspended solid removal (USEPA 2002). Quantifying ImpactsSummaryAn important aspect of urban forestry research has been the development of generalizable models that can be used to quantify the environmental services that urban trees provide, as well as convert these benefits to dollar values. As one of the leading urban forestry researchers noted in the 1990s, “efforts to preserve natural areas, acquire new greenspace, initiate plantings and manage existing greenspace are frequently hampered by our inability to fully appraise the environmental services greenspace (i.e. the urban forest) provides (Mcpherson 1992). Since then, a number of such models have been developed, many of which are programmed into computer based modules that provide a tool with which to quantify the structure and function of local or regional urban forests. Primarily building on research undertaken by scientists at the USDA Forest, the models also convert these benefits into a dollar value. Directory: cron -> Backup -> project -> urban-sustainability urban-sustainability -> Discouraging Driving Download 282.87 Kb. Share with your friends:
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