A healthy city for healthy people will necessarily associate its development with the concern of keeping or restoring an adequate tree cover, combining elements of the watershed, the landscape and the ecosystem. The integrated management of these elements will result in the better protection of the city against floods and landslides, the reduction of damages to the roads and infrastructures, the enhancement of poor neighbourhoods and the improvement of human lives. As an example, the quality and quantity of water supplied to a city may depend, in some cases, on its connectivity with its surrounding environment for filtration by the natural vegetation. The social aspect of urban trees is also a major issue. The reduction of air pollution by carbon sequestration and pollutant uptakes by trees has a major direct effect on human health. Furthermore, urban trees contribute to reduce stress and improve social ties, as well as physiological and mental health of citizen by creating recreational and meeting spaces, while contributing to the beautification of the cities.
The economical benefits obtained thanks to the main environmental functions of urban trees have been demonstrated around the world, which main elements are:
Significant savings in energy consumption for cooling (through the shading of buildings leading to the reduction of the needs in air conditioning) and heating (thanks to the windbreak effect),
Savings in need for rehabilitation and restoration works thanks to the mitigation of negative impacts from storms and other extreme climatic episodes;
Significant increase of properties value thanks to the presence of trees on a private land.
The good practices for designing, selecting and managing the tree systems need to respond to various environmental issues. For instance, urban planners must take into account:
- The use of waste water for irrigation of tree systems respecting the safety and human health norms and guidelines,
- The maintenance of greenbelts, hedgerows and windbreaks for wind and temperature control (e.g. reducing sand encroachment risks and regulating the microclimate),
- The selection of species resistant to air pollution,
- The protection of forest and tree cover surrounding the cities against degradation and land use change due to the urban pressure for wood material, fuelwood and built-up land,
- The protection of biodiversity by planting various tree species and favouring the establishment of wildlife habitat (e.g. in woodlots).
The art of developing a good network of trees creating paths through residential, industrial, private and public areas is not only related to the urban architecture landscape, but must also consider the approaches of integrated forest and agricultural landscape. The resulting mosaic of agroforestry systems, street trees, gardens, forests and recreational parks, is a key strategy to respond to the environmental and health needs of the cities, communities and people.