Figure 7.10 illustrates an example of the principles used when planting to counteract traffic emissions, stressing the importance in design of focusing on both structural and dynamic aspects. The main purpose is to screen off the traffic from the areas behind, to reduce the psychological impact of the traffic and to reduce large-particle pollution. The plantation also has a wind-reducing effect. Similar principles can be applied when industry or allotment gardening are the main concerns. As the planted trees and shrubs face a major road, a 50 m broad planted zone has been used. Along a less heavily trafficked road, 15-20 m would be sufficient. Groups of trees and shrubs planted in overlapping patterns act as a windbreak and reduce noise pollution more effectively than a homogeneous mass plantation. A particularly dense edge at the roadside is essential for noise reduction. In Figure 7.10, zone A comprises nurse trees of birch, alder or larch (later removed) and bushes. If wind reduction is important, an open front edge with just trees is added to ‘catch’ the wind.
Zone B in Figure 7.10 is made up of wooded belts using low woodland types with standards constructing an open canopy and a dense lower vegetation with trees that are valuable in the longer term, such as oak, lime and maple. There is a scant understorey of bushes, which functions as a filter but
Planting to counteract traffic emissions
A private garden in Dalby, Sweden
does not prevent access to the interior of the plantation. To provide an efficient filter, this zone may be extended further to include spruce, which, during the summer has a filter capacity that is at least as good as that of deciduous trees. In the winter, the difference is pronounced; air only has to pass through a 10 m wide belt of spruce for the amount of particle-bound pollutants to be reduced to a few per cent. However, spruce is one of the tree species most sensitive to pollution. Having a deciduous screen in front protects the spruce sufficiently for it to also act as a filter.
The intermediate zones comprise meadows alternating with zones of shrub or ‘energy’ forest. These zones create contrast in distant views, but also improve the sustainability in zone B by giving more light to the understorey individuals.