Opportunities for Sustainable Green Urban Areas
Trees offer opportunities to create multi-strata natural areas and absorb 25–30 times more CO2 than mono-strata grass surface areas, and therefore a scientifically guided restoration or reconstruction of green environments such as vegetation ecology or ecotechnology is desirable (Miyawaki 1998). Ecotechnology is a technological means of ecosystem management based on deep ecological understanding, to minimize costs of management measures and their harm to the environment (Straskraba 1993). Ecotechnology uses a vegetation ecology approach to build multi-strata greenery (e.g., canopy, understory, shrub, and herb layers) to fulfill the ecological potential of the area. Forests have different ecological functions and urban greening programs should not rely on the functions of green vegetation in general, but should address the suitability of different plants in light of the complexities of urban design and management. Such complexities may include: (1) open spaces such as parks, (2) areas along roads and walkways, (3) tracts of land along riverine areas and in wetlands, (4) mixed-use areas such as those around and within residential districts and other built-up areas, and (5) office and industrial zones.
Trees are the most memorable aspect of a roadside planting design. They have an appropriate scale for a road corridor, are clearly noticed when travelling and are the best means for ameliorating the hard built elements of the road corridor. Subject to their safe use, they should be the primary element of a landscape design. Trees should, however, be used selectively in a corridor. For example they should not obscure expansive views and they should be located carefully and deliberately, outside clear zones and away from utilities (Chang and Collins 2008)
Each urban district occurs in a distinct ecological area with unique naturally occurring plant species. These are species that have long adapted in these ecosystems and established their own niches in the communities that they occur in. In selecting tree species to plant in urban areas, this should be a key consideration to maintain natural environments. In many upcoming cities in the developing world, and in Kenya in particular, there seem to be no scientifically informed criteria for urban greening programs, and most of the time exotic species are planted. This may mainly be due to their visual appeal, availability, and growth under a wide breadth of ecological conditions (e.g., Grevilia robusta). Indigenous species are usually overlooked and this could be mainly because of insufficient understanding of their local suitability, optimum combinations, and interaction with other species, as well as impact on the infrastructure, among other considerations. Nonetheless, some other countries have made significant progress in urban green environments based on theories of vegetation ecology (Miyawaki 1998; Muller and Fujiwara 1998).
Some key benefits of green urban environments include carbon assimilation, disaster prevention and reduction, beautification and nutrition.
Urban environments are major sources of carbon dioxide (CO2) and other green house gases (methane, nitrous oxide, and fluorinated gases) that contribute to global warming, a major contributor to climate change today. CO2 is the primary greenhouse gas emitted through human activities. The majority of GHGs emanate from burning fossil fuels, mainly from motor vehicles and industry (International Energy Agency 2012). These emissions have risen steeply over the last century with some cities now being covered in smog that not only reduces visibility, but also reduces air quality, generating air pollution which in turn can cause major respiratory issues and associated health complications. Smog generally can affect plant development and human health, as well as cause damage to materials such as rubber, textiles, and paint (Marcella et al. 1957; Rani et al. 2011). Three major outdoor air pollution problems are industrial smog from burning coal, photochemical smog from motor vehicle and industrial emissions, and acid deposition from coal burning and motor vehicle exhaust (Rani et al. 2011). A wide range of experts have advocated decreasing individual carbon footprints and investing billions to reduce the risks of a major change in the earth's environment (Stern 2008). Green environments can help absorb the CO2 in the air which can be converted into stored carbon through the process of photosynthesis. Different plant species vary in the amount of CO2 they can absorb and this variation is determined by various morphological and physiological characteristics of the plants as well as land use (Houghton 1989). Some characteristics that can help determine which plants would have high carbon assimilation rates include growth rate, leaf area, wood density, and seasonal vegetation changes, including whether they are deciduous or not.