Street trees can form an attractive and functional element of urban streets, helping to define the character of streets, from boulevards to home zones. Street trees make cities more liveable, particularly important given an increasing preference for flats and higher density housing, and as we discussed in Section 1.2, can add value to property.

Experience from cities such as Berlin, which favour continuous street tree canopies shows that streets with mature tree cover are generally the most popular amongst residents. They also provide important ecological services such as pollution control, air conditioning and noise attenuation.

Planting native species as a continuous canopy will maximise their habitat potential, particularly for birds and insects. However, some exotic species are better adapted to conditions in the urban environment. Resilience to air pollution is a major factor, as is access to an adequate water supply, suitability of root structure for planting close to buildings, and the density and maintenance requirements associated with foliage. Some key questions to answer when planning for street trees are therefore:

• Is there enough space for the tree to grow?
• How will the tree affect the building at maturity?
• How will the tree affect streets and utilities at maturity?
• Will foliage cast shadows or brush window panes?
• Will associated drains and soak aways create maintenance problems?

These factors can limit the choice of native species. Guidance is available on the selection of appropriate species from knowledgeable organisations and urban ecology texts.

In urban areas there is also pressure to plant older and larger trees to realise design objectives and property values. Smaller and younger specimens are preferable as they have a greater opportunity to grow into their surroundings, however, resilience to vandalism during initial years can be a deciding factor. In small open spaces a variety of sizes and ages of trees, planted together with elements of an understorey can provide visual interest and structural diversity, as demonstrated at Warrington New Town (Case Study 2, Section 3.4).

There may also be opportunities to retain existing mature trees. These must, however, be adequately safeguarded during development in order to avoid direct damage from equipment or indirect damage to roots or through soil compaction. Disturbance to the local water table may occur as soil moisture is depleted, and new forms of drainage may be needed.