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.
- high levels of air pollution
- excessive use of salt on roads affecting the tree's ability to absorb water and nutrients
- removal of dead leaves and the interruption to the normal woodland nutrient cycle, leading to low nutrient levels.
- over-pruning causing damage to branches and reduction in photosynthesis
- soil compaction
- trimming of roots during underground work on pipes cables etc
- attack from pests and diseases
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| 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.
Case study: is the London plane a good tree to plant?
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| 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 leaf showing dense covering of hairs |
 Hairs magnified as seen under a microscope x30 magnification |
 Mess created by fallen leaves |
 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.
