The Use of Trees to Modify Urban Micro-Climate in West Africa

Article extracted from “The Use of Tress to Modify Microclimate in Hot-Humid Tropical West African Urban Centers”, 1985
After so many years of wanton and haphazard urbanization and urban development, man has finally realized the need to design with nature for harmony and ecological balance (McHarg, 1969). However, it is not until recently that research and political attention was drawn in the direction of climatic consequence of urbanization, global warming, climate change and green house gases.

Man’s urbanization process has destroyed many desirable microclimates. The process promotes extremes in daily maximum and minimum temperatures, and modifies the total microclimate of urban centers. This has resulted in climate change. Emerging studies in landscape architecture and urban design have explored the reversibility of the uncomfortable climatic condition in urban areas through natural means http://edis.ifas.ufl.edu/eh144. A fair amount of microclimatic manipulation could be practiced by planting trees around houses and in urban open spaces and along sidewalks and streets to block solar radiation hitting the earth surface. Physical functions of trees should be emphasized by urban designers, landscape architects and policy makers in their decisions.

The design of outdoor building space microclimate in hot-humid cities must be directed toward shading and temperature reduction, especially by trees for human thermal comfort and building space conditioning. Trees have been found to be very effective in improving human thermal comfort and building space conditioning by shielding us from direct solar radiation because we are exposed to less solar radiation beneath tree canopies. Trees provide shade and also have an active cooling mechanism in the form of evapotranspiration. Temperature in forested plot or under tree canopy is 5 ºC (41 ºF) lower than temperature in cleared plot. Tree canopy intercepts about 90 % of the solar radiation, 10 to 25 % of which is reflected back to space, and as little as 1% is used for photosynthesis or for making the tree’s food.
Since contact with European powers in the last hundred years, West African urban centers have undergone tremendous changes in every aspect of their construction and physical organization. The old style mud houses with thatched roofs have been replaced by concrete cement blocks, and dirt roads replaced by paved ones. The small traditional community settlement is being replaced by more complex urbanized settlements. The introduction of these buildings and construction materials has far-reaching consequences. “The urban natural ground surface has been obscured by building and pavements; the blue sky is ashed by air pollutants; and vegetation is diminishing to seemingly and artifact of culture, a mere reflection of man’s desire rather than his needs.” (Detwyler and Marcus, 1972).
In all developing regions of the world, there is a need to realize the potential benefits and consequences of growth. The rapid rate of urban growth started in the sixties has created very serious problems. Radical industrialization program of most West African countries including Ghana and Nigeria has let to rural-urban migration. In 1950 there was only four areas significantly urbanized. Nigeria was the only one in Sub-Sahara region. Today, more than 30% of Africans lives in urban centers. The cityscapes are changing under economic and social pressures. The existing urban forest and fertile lands have disappeared without replacement. Urban renewal, street widening, various demand for open space and new concerns for safety have all contributed toward the removal of trees.
The effects of urbanization on microclimate were recognized in Nigeria and other West African states back in 1960’s. Urban settlement has reduced the capacity of surface materials to reflect solar radiation over build-up areas in Nigeria more than 22%. Rural areas have higher reflective coefficient because of the forest and vegetation. Increase in combustion, metabolism replacement of vegetation with concrete, asphalt and other building material has led to the development of urban heat island, and phenomenon whereby heat is absorbed by building structures, tarred roads and other hard surfaces and slowly released at night to warm up the urban atmosphere. http://www.state.nj.us/dep/dsr/research/urbanheat.pdf
Climate is known to be the most influential variable on man’s health, energy, and comfort. Climate changes such as temperature, humidity, wind and atmospheric pressure determine human physical and mental health. Man’s physiological vigor may decrease with significant increase in temperature and humidity. Man functions better physically and mentally under certain optimum climatic conditions. His efficiency decreases outside this range. Maximum temperature that one can perform extended work without increase in body temperature has a threshold of 65.6 ºC (150 ºF) in dry air, but can be reduced to only 34.4 ºC (94 ºF) in full humidity of 85% condition in West Africa. Productivity in Ghana and other West African countries are low because it is very uncomfortable to work during hot days, and very difficult to sleep at night without air conditioning device, and as a result one wakes up in the morning exhausted from inadequate sleep. The comfort zone for humid tropical region has been defined by as temperature between 23 and 29 ºC (73.4 – 84.2 ºF) within humidity range of between 30 and 70 %. Temperature range of 24-31 ºC (75 – 88 ºF) with relative humidity 83% was recorded for Ife University Campus in Nigeria in a study by Tishler.
In the developed world, mechanical air conditioning has been developed to alleviate the discomfort associated with climate change or modification. In the developing countries this application is an affective and direct way of creating favorable indoor climate. However, it is expensive and out of reach for many millions who has no necessary resources of power or energy.
Trees have been found to be effective in alleviate discomfort associated with extreme temperature and heating effect in urbanized areas. Without shading, the average ground temperature in West Africa remains higher than the maximum air temperature of the surrounding rural areas. The temperature of an unshaded pavement can easily be 43 ºC (109 ºF) on a day when the air temperature is 34 ºC (93 ºF) (Tishler et, al. 1971). For this reason the treatment of the ground surface is very important. More shaded grass must replace concrete pavement to reduce ground surface heat. Strategic location a building beneath mature tree canopies provides the required shade at the right times of the day. Planting dense shade trees on the east side of a building reduces morning and midday inside temperature and provides a reduction of a 55 % of the cooling energy bill (McPherson, 1983) http://www.springerlink.com/content/mpxk119y7kk9gp7t/fulltext.pdf. Trees on the west reduce inside temperature by 6.5 ºC (43.7 ºF) during late afternoon and early morning hours. A monetary savings of $70 to $100 per year in air conditioning was gained by home owners who planted trees around their mobile homes on Alabama, USA where summer temperature is very hot and air conditioning is a standard requirement for homes. The result of this study is applicable to other building types anywhere in the world.
Vegetation will delay afternoon solar heating of structures, thereby reducing energy demands during peak demand periods. It has been shown that the shading of a building by tall vegetation, for example palm trees can drastically reduce and, in some cases, eliminate the need for supplementary cooling of structures during warmer seasons. The positioning of vegetation relative to the structure to be protected is of critical importance in terms of energy conservation. Shading roofs with tree canopies has also been found to be very effective. The percentage of roof shade does not need to be 100% for the best result. The most effective positions are trees on all sides overhanging the house to shade the roof, because the sun is at its highest altitude angle in the sky and overhead most of the day in West Africa. Planting tall palm trees and other evergreens to cover the roof is a good strategy, and in some cases, makes installation of air conditioners unnecessary. The south, north and west sides of the build should be covered with shade trees for optimum human thermal comfort and building space conditioning.
Urban climatology and forestry has added a new dimension to the urban planning and design profession. The layout of cities has nearly always been dictated, or almost accidentally created, by a series of mainly political, social and economic decision processes. But more recently, the principles of urban climates, green house gases, etc have been applied in an increasing number of urban growth, renewal and new city designs. In regional and town planning, a climatological input at the macro and micro-scales will determine the properties of the green field site in all major respects, such as insulation, temperature, air flow, precipitation etc. Knowledge of these influences has helped to determine the major land-use patterns, such as zoning, orientation of buildings, development densities, heights, and the size and location of open spaces. Several models can be found in many parts of developed world. They include LEED; Sustainable Development; Planned Unit Development; Walkable Communities; Commute Trip Reductions; Congestion Management; Livable Communities, Context Sensitive Solutions, Etc.
Vegetation (mango, coconut, Mahogany, Flamboyant, etc) should be included with other devices for shading and cooling in hot-humid tropical West Africa. It is cost effective to use trees as shading devices. Moreover, they have secondary benefit of cleaning the air, beautifying West African urban centers, screening and providing privacy, stabilizing the soil, attracting wildlife, etc. Techniques and criteria should be developed for management of energy-conservation and shade trees so that maximum energy conservation values and human thermal comfort of such planting are maintained over the life-span of the building.
Kwadwo (Kojo) Fordjour, AICP, Environmental and Community Development Consultancy, Ghana/USA
[email protected], (206) 234-1624