Boulevards represent a possibility to establish several trees in cities amongst others to mitigate urban heat stress. Within boulevards, trees are commonly arranged along streets, bicycle paths and sidewalks and - if circumstances allow - established on both sides of the route. The treetops of opposite trees often form a (nearly) closed canopy. As a result the street in die middle of two tree lines is protected, shaded and the air temperature is lowered.

Performance

Evapotranspiration 2 2
Shading 2 none
Reflection (Albedo) none
Water Conveyance none
Water Infiltration 1
Water Retention 1
Water Storage none
Water Reuse none
Water Filtering none
Water Bio-remediation none
Deposition 1
Bio-filtration 1
Habitat Provision 1
Connectivity 1
Beauty / Appearance 2
Usability / Functionality 1
Social Interaction 1
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 planted.
Benefits & Limitations: 
Benefits: Boulevards are associated with diverse benefits for urban ecosystems: - Microclimate regulation - Habitat provision - Aesthetics/recreation - Rainwater regulation (delayed runoff) Potential limitations/disservices: - Reduced airflow Higher pollution in street canyon - Disservices of trees may be the allergenic potential of its pollen and BVOC emissions.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 730052 Topic: SCC-2-2016-2017: Smart Cities and Communities Nature based solutions