Healthier trees mean cleaner air and healthier lives: why our cities need trees

Trees provide oxygen, clean the air around us, absorbs carbon dioxide and reduces the need for air-conditioning. Cities occupy less than 3% of the global terrestrial surface, but account for 78% of carbon emissions, 60% of residential water use, and 76% of wood used for industrial purposes. By 1900, just 10% of the global population was living in urban areas which now exceeds 50% and is expected to further rise to 67% in the next 50 years (Grimm et al. 2008).

In developing countries, about 44 per cent of the population currently lives in urban areas, but in the next 20 to 30 years, developing countries in Asia and Africa are likely to cross that historic threshold, joining Latin America in having a majority of urban residents (UN-Habitat 2009, Montgomery 2008). Undoubtedly, urbanization will continue to have substantial impact on the ecology, economy and society at local, regional, and global scales. As this review will demonstrate, benefits of urban green spaces are wide-ranging including physical and psychological health, social cohesion, climate change mitigation, pollution abatement, biodiversity conservation and provision of ecosystem goods and service to urban inhabitants. The term “urban trees” includes trees growing both within the built environment as well as road-side avenues and public places in urban areas such as parks.

Currently, developed countries have tended to adopt a general standard of green space of 20 m² park area per capita (Sukopp et al. 1995; Wang 2009). International minimum standard suggested by World Health Organization (WHO) and adopted by the publications of United Nations Food and Agriculture Organization (FAO) is a minimum availability of 9 m2 green open space per city dweller (Kuchelmeister, 1998).

The information available globally suggests that the cities in developed countries, in general, have more trees compared to cities in developing countries, which often fall below the minimum standard set by WHO of 9 m2 green open space per city dweller. From an ecological perspective, some studies have suggested that a realistic target of 10% of tree cover throughout urban areas is necessary to create an ecologically sustainable city (Hashimoto et al. 2005).

Functions of urban trees
Trees in urban areas provide a variety of ecosystem services including biodiversity conservation, removal of atmospheric pollutants, purification of air, oxygen generation, noise reduction, mitigation of urban heat island effects, microclimate regulation, stabilization of soil, ground water recharge, prevention of soil erosion, increase of property values and carbon sequestration (Bolund and Hunhammar 1999). More simply and easily noticeable, trees provide shade especially in warm cities of the tropics. Urban trees in the USA store 700 million tonnes of carbon ($14,300 million value) with a gross carbon sequestration rate of 22.8 million tC/yr ($460 million/year). Pollution removal (O3, PM10, NO2, SO2, CO) varied among cities with total annual air pollution removal by US urban trees estimated at 711,000 metric tons ($3.8 billion value) (Nowak et al. 2006). Likewise, the City of Tshwane Metropolitan Municipality in South Africa has 115,200 indigenous street trees planted during the period 2002–2008. It has been estimated that the tree planting will result in 200,492 tonnes CO2 equivalent reduction and that 54,630 tonnes of carbon will be sequestrated.

The state of urban trees in Ghana
Despite all the recorded benefits of trees in urban areas, trees have been less valued in urban areas of most developing countries of which Ghana is no exception. I am unable to provide figures about trees in Ghanaian cities but what is clear is that the existing ones are not properly protected and managed. While street trees in developed countries are jealously protected, the few urban trees in Ghanaian cities are left to their fate with all the pressure that humans and animals put on them. Nails are commonly driven into trees so as to hold various posters of advertisement. These nails are a source of injury to trees and negatively affect the ability of trees to provide all the benefits named above. Both humans and animals do not only rest under trees but also urinate there. What is probably unknown is that urine is acidic in nature and hence affects tree roots. Also, people park cars under shade of trees and this causes compaction of the soil just around the tree trunk. Such compaction means that less air and water is able to penetrate down to tree roots and this tends to shorten lifespan of trees. Similarly, during construction works, no attention is given to tree roots and most trees die in the process. It is good practice to allow some space around a tree trunk during construction of pavements in cities.

How to improve?
First of all, I think more awareness has to be created among people about the importance of trees and urban green spaces. In this regard, the media should at least spend some time on such topics in order to change public perception and knowledge. Secondly, city authorities have to do more towards protection of trees and ensuring that urban trees are healthy. Currently, some urban trees pose a health hazard since they can fall at any time and crush something or someone. Thirdly, specific trees must be planted in cities. Not every kind of tree species is appropriate for cities. The function we would like a tree to provide may dictate its size, shape (form), life span, canopy density, colour, growth rate, fruit characteristics and other attributes. Particular attention therefore has to be put on which type of tree is planted under which condition. My next article will provide the qualities that a tree needs to be successful in an urban setting and provide some examples of tree species that would be better to plant.

In conclusion, a city without trees is like food without salt and we have no choice either than to seriously consider increasing and protecting trees in our cities. If temperate countries where temperatures are generally low still take tree planting in cities seriously, how much more the countries of the tropics which need trees to help regulate the usually high temperatures and especially to provide shade.
Let us all protect our city trees for healthy lives!

Reginald Guuroh (guuroh@yahoo.co.uk)
Germany