Mot-clé : Natural processes
Thème : Soil and vegetation
18 February 2011
Par : Saga Cité
Trees are important assets for cities and other living environments, as they:
- improve air quality;
- are efficient windbreakers;
- mitigate the heat island effect and generally lower summer local temperatures;
- contribute to rainwater retention and filtration;
- are sound absorbing;
- capture CO2;
- and of course, betters the overall living environment.
For example, a thirty-year-old tree captures on average 9.4 kg of CO2 per year, which is to say that you’d need about 130 such trees to capture the 4500 kg of CO2 an average car emits by being driven 20,000 km in a year. Moreover, planting trees and shrubs around buildings can help cut energy needs by 10 to 30%, thanks to their windbreaking (in winter) and cooling (in summer) properties.
The cooling properties of trees are due to the way they grow, which involves the absorption and evaporation of water in a process known as evapotranspiration. A mature tree can in this way release a few hundred liters of water everyday. And since evaporating water requires energy, which is drawn from the surrounding heated air, the process results in a significantly lower-temperature microclimate. A large tree, which would release approximately 450 liters of water in any given day, would for example have a cooling power equivalent to 5 medium-sized air-conditioning units running 19 hours a day – and these would merely transfer heat outside of the building without “eliminating” it. Moreover, air-conditioners also require power.
Fewer heat islands
One of the greatest benefits derived from these characteristics is the mitigation of the heat island effect. A green park and a sparsely vegetated public space located merely a couple hundred meters apart can experience a dramatic temperature difference of 15 °C or even more.
This is again due to the evapotranspiration process, but also to the better albedo (or solar radiation reflecting capacity) of the foliage and grass compared to that of asphalted surfaces.
Soils as natural rainwater processing plants
With the proliferation of impervious surfaces like asphalted roads and parking lots, rainwater (and the pollutants it captures from these) are quickly drained to the watercourse network, resulting in polluted runoff – or what is called nonpoint source pollution, a very difficult problem to tackle. In fact, vehicle infrastructure is responsible for 80% of the overall surface runoff . Conversely, permeable soils have many advantages. Firstly, they retain water for a longer time, allowing part of it to naturally evaporate (and thus lowering the ambient temperature). Secondly, they naturally filter and treat water, allowing it to continue through its natural cycle. Finally, the unhindered natural percolation process regulates the watercourses’ flow rates, which helps reduce bank erosion caused by flash floods.
Stakeholders in the greening effort
Soil sealing and the presence of trees are major issues that should concern individuals and communities alike. Indeed landowning citizens have an important role, as they are able to act directly and locally. At a higher level, municipalities have the power to switch their stormwater systems to an open-channel flow configuration (which actually costs less to maintain once it’s built), and to require minimum tree cover ratios for various areas of the city.
-  G. Lessard et E. Boulfroy, 2008. Les rôles de l’arbre en ville (PDF in French). ↩
-  McPherson, Nowak et al., et Ontario Ministry of Housing dans Micheal Hough. 1995. Cities and Natural Process. ↩
-  Michael Hough, 1995. Cities and Natural Process. ↩
-  J. Heaney, R. Pitt et R. Field, 1999. Innovative Urban Wet-Weather Flow Management Systems. ↩
Tree Canada (Web site)