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Urban ecosystems

Street and park trees

Trees are constantly under threat. Large areas of trees may be lost when land is developed or roads are widened, while individual trees can be harmed in many ways, e.g.

Plane fruits in car park Notice on tree
Surrounding trees by tarmac reduces their access to water Damaging bark (e.g. by nailing a poster to the trunk) allows ready access for pests and diseases

When making decisions about whether to protect or plant trees urban planners need to make sure that the benefits outweigh the costs. Different kinds of tree also vary in their characteristics and this must be taken into account in when choosing species and deciding where to plant them.

Advantages and diadvantages of urban trees

Case study: is the London plane a good tree to plant?

attractive bark of plane open canopy of plane
Attractive bark An open canopy giving pleasant dappled shade

The table below gives a format for evaluating the value of a tree species for urban planting. We have filled in data for the London plane (Platanus hispanica).

Advantageous characteristics are shown in BLUE.

Disadvantageous characteristics are shown in RED.

Characteristic The London Plane

Aesthetic value (is it attractive?)

A tall elegant tree. Summer canopy open giving pleasant dappled shade. Twisted branches with network of fine twigs and pendant fruits give pleasing winter silhouette. Flaking bark creates attractive colours on trunk

Does it make a mess?

Leaves, fruits and bark may need clearing from streets and pavements

Pollution tolerance

Very tolerant of air pollution. Hairs on young shoots and leaves help to trap particulate pollution. Mature leaves tough and glossy readily washed by rain. Flaking of pollution-blocked bark may also be important

Pests and diseases

Rarely affected by disease and pests (although some shoots killed each year by Anthracnose, a fungus)

Soil conditions

Very tolerant of poor soil conditions including compacted soil (although some stunting of growth by road salt)

Space

Although in open sunny positions a vigorously growing tree it isvery tolerant of pruning and root disturbance

Safety hazards

Open canopy offers little resistance to wind. Trees rarely blow over or shed branches. Fine hairs on young shoots, leaves and fruits may cause irritation and even allergies in some people.

Microclimate

Produces light shade, so some cooling effect and increase in humidity. Will intercept some rain especially when in leaf

Air quality

Estimates suggest its VOC (volatile organic compound) emissions are relatively high

Biodiversity

Not a native therefore relatively few associated invertebrate species. Provides valuable resting sites for birds. Sufficient light below canopy to allow significant plant growth.


Some of the disadvantageous characteristics of London plane trees.

Young plane leaf

Young leaf showing dense covering of hairs
plane leaf hairs magnified

Hairs magnified as seen under a microscope x30 magnification
fallen leaves

Mess created by fallen leaves
isoprene emissions table

Estimated isoprene emission of six common British trees (Note isoprene is the main VOC emitted by deciduous trees) Source: Stewart & Hewitt, 2002

Using this table for the London plane, it appears that advantageous characteristics outweigh disadvantageous ones The high tolerance of the London plane to air pollution and poor soil conditions, together with its aesthetic value make it a particularly valuable urban tree. It needs warm summers and so will do best in southern Britain.


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