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Increasing the number of urban trees, improving their growth rates, and prolonging their life spans are effective ways of increasing urban ecosystem carbon stocks (Table 4-12). Urban spaces that may be covered with trees include parks, green areas, forests, roadsides (alleys and street trees), riversides, and gardens. Consideration also should be given to the tree home gardens that are common in the urban tropics, which often consist mainly of fruit and nut trees. Because urban trees are often planted in dry soils that are disturbed and compacted by construction, and in small spaces, it is necessary to pay attention to site conditions, including addition of special soil amendments or facilities to improve soil aeration, moisture, and nutrient levels.
| Table 4-12: Rates of potential carbon gain under urban land management. | |||||
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| Practice |
Country/Region
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Rate of Carbon Gain
(t C ha-1 yr-1) |
Time1
(yr) |
Other GHGs and Impacts |
Notes2
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| Planting trees |
4.6-4.8
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Enhanced biodiversity |
a
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1 Time interval to which estimated rate applies. This interval may or may
not be time required for ecosystem to reach new equilibrium. 2 a. Sampson et al. (1992). Carbon sequestration opportunities on available urban growing space in the United States of America. |
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Urban tree and other vegetation waste constitutes a significant portion of the waste stream in many urban areas. Local programs to convert this material to energy may provide benefits such as reduced need for landfill space, reduced methane emissions from landfills, and economic benefits that help offset costs. Although the energy production benefits would be accounted for in fossil fuel accounting, reporting reduced methane emissions from improved urban waste management activities may be feasible in some situations.
Prolonging the life span of trees is an effective way to maximize changes in carbon stores and delay carbon emission. Because urban trees are surrounded by air that often contains high levels of pollution, they have short life spans compared to their rural counterparts (Kielbaso and Cotrone, 1990). Programs that improve tree maintenance and care can extend life spans significantly. In urban areas, dead or cut-down trees are seldom left to decompose naturally; they are collected and often burnt. Carbon emission can be delayed somewhat by reusing the wood to produce compost and mulch chips, which slowly decompose in soils and contribute to long-term SOC accumulation.
Important associated impacts of urban forests include contributions to biodiversity, purification of the atmosphere, improvement of scenic beauty, and support for environmental education.
The degree of urban greenness can be remotely sensed. Estimating the resultant carbon store will require on-site measurements, given the wide range of forms that an increase in vegetation can take.
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