Urban forestry is the care and management of tree populations in urban settings for the purpose of improving the urban environment. The concept of urban forestry, which advocates the role of trees as a critical part of the urban structure, was developed to address the issue of impact on forestry by urbanization. The urban forestry comprises all green elements under urban influence.
Changes in structure of society have accelerated the urbanization. Urbanization is considered as the main driver for eco system change (Konijnendijk, 2003). As of 2010, 50.6 % of the world’s population lives in cities (UNDP, 2010). In the tropics and sub tropics, the urban population is expected to grow to 4 billion by 2025, and major cities are expected to grow substantially in surface area (Avijit, 2002). Developing countries in particular are urbanizing rapidly thus emitting more greenhouse gases (GHG). Urbanization generally has adverse effects on eco system like destruction in habitat and watershed, change in forest structure, etc.
Urban forestry is the care and management of tree populations in urban settings for the purpose of improving the urban environment. The concept of urban forestry, which advocates the role of trees as a critical part of the urban structure, was developed to address the issue of impact on forestry by urbanization. The urban forestry comprises all green elements under urban influence, (FAO, 2001), such as:
- Street trees and road plantations
- Public green areas, such as parks, gardens, cemeteries,
- Semi-private space, such as green space in residential areas and in industrial or specially designated parks
- Public and private tree plantations on vacant lots, green belts, woodlands, rangelands, and forests close to urban areas
- Natural forests under urban influence, such as nature reserves, national parks and forests for eco-tourism.
- Urban agricultural land, such as orchards, allotments etc.
Other than just defining urban forestry as an art of managing trees and forest around urban area for the physiological, sociological, economic and aesthetic benefits, urban forestry can be a tool for mitigating carbon-dioxide emissions (Carter, 1995) as trees planted in urban areas can help in its sequestration of carbon dioxide (CO2) (Escobedo et al. 2010). Urban forestry can help offsetting 18.57% carbon emitted by the industries in urban areas and also store substantial amount of carbon, equivalent to 1.75 times the amount of annual carbon emitted by industries energy use in the cities (Zhao et al., 2010). In Shenyang, China, the urban forestry helped store 337,000 tC ($ 13.88 million in monetary value), at a carbon sequestration rate of 29,000 t/yr ($1.19 million) (Lui and Li, 2011).
Many cities and mega cities are major GHG emitters because of high energy use due to industrialization and urbanization. Studies assessing the GHG mitigating potential through urban forestry show that trees with diameter greater than 77 cm sequester approximately 90 times more carbon than those with diameter less than 8 cm. Large trees also store approximately 1000 times more carbon than small trees (Nowak, 1994). Besides, large and healthy trees with relatively long life spans will generally have the greatest overall positive effect. Healthy and larger trees have potential of sequestrating more carbon than the small healthy trees.
Urban forestry has been in practice since 1960’s. The emission mitigation by urban forest is by increasing the vegetation carbon density in the settlements. However, it may have other co-benefits indirect effects which need to be evaluated as well, such as reducing heating and cooling energy use in residential and commercial areas and increased albedo of paved parking lots and roads (IPCC, 2007).
A forest management activity by urban forestry is done by planting trees and maintaining them within cities, suburbs and towns. Selections of trees are important for urban forestry as trees in urban areas face more stress than those in rural areas. Few common stresses faced by urban trees are the restrictive soil volume and crown space, soil pollution, air pollution, wind and drought (Saebo et al., 2008). Generally, a tree in humid tropics can absorb 50 pounds (22 kilograms) of carbon dioxide annually. Trees in tropics grow three times faster than that in temperate zone; this indicates that tree planting in tropics will have larger cooling effect (Gibbard et al. 2005).
Development of market-based environmental services from carbon sequestration is receiving attention as a GHG mitigation tool as well as for sustainable forest management. Conversely, expansion of these markets is still a challenge as it heavily depends on government interventions (Katila and Puustjarvi, 2004). The municipal policies play an important role in urban forest development, but very little or no attention has been given to the urban forest policies. Urban forestry needs good planning and participation for its successful operation. Shenyang city in China introduced a Tree Planting Program (TPP) in 2001 to promote urban forestry, and the program helped planting around 19 million trees (Liu & Li, 2011). Cases of social conflicts regarding urban forestry makes it essential that participation from all levels must be involved during the planning and policy making (Konijnendijk, 2008).
The figure below shows urban population growth projections by region. In 2008, over half of the world’s population lived in urban areas and by 2050 this will rise to 70 per cent and more than 90 percent of total population growth in cities in developing countries (UNHABITAT, 2009). Hence, in this context urban forestry plays a key role in developing countries. But, the concept of urban forestry is yet to be developed in many developing countries compare. Urban forestry has been in practice in industrialized countries for long. However, after 1990’s, urban forestry found broader acceptance and support (Konijnendijk, 2003). Urban forestry in Asia is still underdeveloped. Only China, Singapore, and Malaysia have maintained or increased their urban forest cover (Carter, 1995). Nearly 90% of the forests are managed in industrialized countries (FAO, 2001) whereabouts 6% of the total forest areas in developing countries are covered by a forest management plan (IPCC, 2007).
Tree planting by private industry can be an easier way to promote urban forestry in developing countries. Companies land has much potential for tree cultivation as it will help them create successful corporate image and also the greening of industrial sites. Tree planting in the public land may have access to local people or may be controlled to some extent. Most of the tree planting in developing countries for urban forestry is done in public land. Means of involving local people and forging links between the private, public and academic sectors should be encouraged (Carter, 1995). Urban forestry offers various potential benefits, including providing the urban poor with forestry products, mitigating the ecological effects of urban sprawl, and improving the environment in urban areas.
The need to reduce GHG emission has introduced the cap and trade system and the carbon trade market is flourishing slowly. According to the study conducted in USA, it is estimated that urban forest can absorb 22.8 million tons of atmospheric carbon annually (Nowak and Crane, 2002).
Urban forestry not only helps in GHG mitigation but it also can help in socio-economic development and also provide environmental protection benefits. Urban forests can help improve the quality of urban life. The benefits could be both tangible and intangible. Tangible benefits are fuelwood, food, fodder and building materials. The intangible benefits are related to social and environmental issues like human health, recreation and others.
Urban forest has high positive benefits in social development such as recreation opportunities, improvement of home and work environments, cultural and historical values of green areas (1999). Urban forest not only has aesthetic benefits but also has physical and psychological benefits of well-being. Urban forest can contribute significantly to the food requirements of the urban poor. Certain species of trees can be an option for urban tree plantation which can provide food, edible leaves, shoots and flowers. China has been growing walnuts and Singapore has been growing fruits in parks to benefit people (Carter, 1995). Few other examples of urban tree plantation which can provide food are peach, pear, blueberries, almond, jujube, blackcurrant, fig, etc. (Clark, 2011).
Urban forest also helps in maintaining the quality of life of residents. Urban forest provides recreation activities for residents and can act as a social integrator (Chiari and Seeland, 2004). Plantations of medicinal plants and spices can potentially have economic contributions. In any countries believe trees are used as a source of animal fodder. Urban forest can be a good source of feedstock for domestic livestock especially in dry seasons when fodder collection can be a problem (Carter, 1995). In table 1 study done in Lisbon, Portugal shows that for every dollar invested in tree management by residents, benefit of $ 4.48 is obtained (Soares et al., 2011).
The urban forestry provides many other economic benefits including carbon sequestration, pollution control and looming income derived from non-timber forest product (NTFPs).
Urban forests have several environmental benefits including the following.
- Trees reduce air pollution: Urban forestry provides the most cost effective air pollution measure for any city. New York city in 1994 was able to remove 2,007 million tons of air pollution (USDA, 2003). Trees help in reducing air pollution and help settle out, trap and hold particulate pollutants. The particulate pollutants like ash, smoke, dust are the major sources that damages human lungs. An acre of tree produces oxygen enough for 18 people every day and absorbs CO2 equal to the amount CO2 emitted by a car driven for 26,000 miles (USDA, 1990). Street trees can absorb 9 times more pollutants than more distant trees, converting harmful gasses back into oxygen (Burden, 2006).
- Trees help mitigate greenhouse gas emissions: Urban trees have greater potential to mitigate greenhouse gas emissions as compared to the rural trees. This is due to the secondary effect of reducing the energy use. A healthy tree stores about 13 pounds of carbon annually or 2.6 tons per acre per year (USDA, 1990).
- Trees conserve water and reduce soil erosion: Trees help conserve water by reducing the water runoff. It increases the groundwater recharge which is lessened by paving. Trees can help capture water; 100 mature trees can capture about 379 m3 of rainfall per year in their crowns (USDA, 2003).
- Trees modify local climate: Deciduous trees shed leaves in winter and buildings can receive winter sun. Coniferous trees can provide windshield as it has needles year round and can help reduce energy use in winter as it works as a shield. A temperature difference of 41-59 ⁰F can be felt under tree canopied streets (Burden, 2006).
- Trees helps conserve energy: A single well shaded mature tree can reduce the annual air conditioning use in the range of 40-300 kWh. Similarly, planting trees in sunny side of house can help reducing air conditioning cost by 30% (USDA, 2003).
- Trees reduce noise pollution: Trees can act as a sound absorber in the urban areas. Trees have capacity to absorb as much as 50 % of the urban noise (USDA, 2003).
- Low urban air temperature: Heat island effect in urban inner cities increases temperature of the city. The thermal energy is stored in the concrete, steel and asphalt. Asphalt, concrete, steels and parking lots are known to increase urban temperatures 37-45⁰F (Burden, 2006). Nanjing city of China has been successful in reducing the heat island effect using urban forestry. The summer temperature of the city reduced from 32 ⁰C to 29.4 ⁰C over the period of 1949 to 1981 (Carter, 1995). The trees were planted 32 years ago on degraded hillsides, along railway lines, streeta etc. Around 23 trees per city inhabitants were planted.
The cost of urban forestry depends on location, species and maintenance required. Urban forest cost depends on variables like weather and policies. Proper planning and designing are required to gain maximum benefits from the urban forestry and also to minimize cost. The national mean expenditure for urban forestry for USA in 1986 was $ 10.62 per public tree. The general cost for tree ranges from $ 12.87 to $ 65 per tree (McPherson et al. 2005). The detailed activities that would incur cost in urban forest management are given in the table below.
With the enforcement of Kyoto protocol, afforestation and reforestation can also generate carbon credits. Urban forestry is also eligible to be a Clean Development Mechanism (CDM) project and can generate forestry carbon credits. Currently, AR-AMS0001 and AR-AMS0002 are used as the CDM methodologies for urban forestry (World Bank, 2010).
Current CDM methodologies cover interventions that result in GHG removal on land along transportation infrastructures, streets, waterways, human settlements like lawns, golf courses and forestation activities on degraded lands, croplands, and grasslands in urban and peri-urban areas.
There are total. The host parties of the 15 CDM projects under methodologies AR-AMS0001 currently are from India, Kenya, Nicaragua, Paraguay, Uganda, Chile, Vietnam and Bolivia (CDM, 2012).
Barriers and limitations of urban forestry
Few barriers and limitations faced in promoting urban forestry are listed below:
- Conflict between the land owner and municipalities can be a major hurdle (Carter, 1995).
- Failing in empowering the urban forestry related policies in action due to improper planning and convincing the citizens to implement those policies (Choi, 2011).,
- Lack of attractive carbon market for the urban forestry.
- The scale of the project is another limitation which is hindering its successful deployment. The small scale sellers are forced out of market due to high production and transaction costs (Beddoe, 2010).
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Asian Institute of Technology