Role of Nature / Mode of Action:
Boulevards simulate those trees growing at the edge of the woods (fringe area) and their effects on the
surrounding environment outside the tree-covered area. The trees shade adjoining land uses - in natural
forest commonly vegetated areas like fields, meadow or water surfaces. As a result the shaded surface is
cooler than surfaces without protection/tree cover). The shading effect is determined by the
characteristics of the trees (tree density, canopy density and season). Other effects are a reduced wind
velocity; transpiration/air cooling, air purification.
Technical & Design Parameters:
For boulevards in urban settings, only a limited number of tree species meet the selection criteria based
on design principles, durability and resistance against environmental stress. The area of the root space
for neighbouring trees can be connected in suitable conditions and if separated root space should be 12
m³ with a minimum depth of 1.5 m. In most urban conditions the root space need to be prepared with soil
substrates for trees. Depending on local climatic conditions, permanent or temporary irrigation facilities
need to be considered. The distance between the trees depend on road width, the maximum size of adult
trees, on the size of the tree when planted, and further design ideas. Protection measures (e.g. poles, wire
mesh against animals) may be needed as well.
Conditions for Implementation:
Local circumstances (e.g. topography, route characteristics, surrounding land use, underground
occupation with cables etc.) need to be considered when planning and establishing new boulevards.
Suitable location for the establishment of trees should offer enough space for trees to grow. Depending
on the site conditions and available space, suitable tree species have to be selected. The consideration of
the maximum height of the trees is important to avoid space problems in the future. Trees, that are not
sufficiently rooted, may cause accidents and constitute a danger for people on or beside the road. The
soil and subsurface should generally be suitable for the establishment of street trees and may, if need to
be, replaced by standard soils. Species and sub species that are suitable for urban conditions should be
Benefits & Limitations:
Benefits: Boulevards are associated with diverse benefits for urban ecosystems:
- Microclimate regulation
- Habitat provision
- Rainwater regulation (delayed runoff)
- Reduced airflow Higher pollution in street canyon
- Disservices of trees may be the allergenic potential of its pollen and BVOC emissions.