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Urban ecosystems

Woodland plants

The biodiversity of woodland plants is usually highest where there is most light. These areas provide habitats for both woodland specialists as well as species which commonly grow in the open but are able to tolerate a degree of shade. Some woodland species show adaptations which enable them to survive when light intensity is low. However, in very shady areas, only a few species will be able to survive.

Whilst light is often the most important factor determining biodiversity, other factors such as soil, water, temperature and relationships with other organisms are all important in determining which species are present. Animal species in the community are all ultimately dependent on the plants and will vary with the plant community (see feeding relationships).

What factors affect the amount of light which reaches the woodland floor?

1. Canopy density

Canopy density varies with species. Ash, birch and London plane all have relatively open canopies, whilst beech often has a very dense canopy.

Open canopy of ash
Typical open canopy of the ash
(Fraxinus excelsior)
Dense canopy of beech
Dense canopy of the beech
(Fagus sylvatica)

Density also varies with age, often increasing as the tree matures, but then opening out again as the tree ages, branches die and are shed. The closeness of the trees to each other will also affect the density of the overall woodland canopy.

Oak tree in winter

Oak (Quercus petraea),
a deciduous tree, in winter

2. Seasonal differences

Different species vary in the length of time that they remain in leaf, the most obvious difference being between deciduous and evergreen trees. Deciduous trees in Britain lose all their leaves at the onset of winter. Evergreen trees shed their leaves gradually and are constantly replacing them so they retain a full canopy of foliage through out the year.

3. Management practices

Management practices can cause significant changes in light intensity. The following for example all help to increase light intensity:

Case study of the effect of light intensity: the bluebell

This case study shows the complex relationship between available light and other environmental factors in the distribution of an individual woodland species.

bluebells bluebells

The bluebell (Hyacinthoides non-scripta) needs a lot of light to grow and flower successfully, but does not compete well with other species. It avoids competition by growing under the dense canopies of deciduous trees such as oak and beech where most species are excluded by lack of light. The bluebell is able to tolerate these conditions by using food reserves stored in its bulb. It grows rapidly in spring and is in flower by early summer before the trees come into leaf and lack of light becomes a problem.

In recently cleared or coppiced areas where light levels are high throughout the summer bluebells may initially do particularly well but eventually succumb to competition as other species invade.

The bluebell grows best on slightly acidic soils. On alkaline soils it may be replaced by species such as dog's mercury which occupy the same niche (see Opportunism).

Oak woodland

Fieldwork: measuring light intensity and interpreting results

There are many places where it is possible to compare the ground vegetation in two different types of woodland. However, making meaningful measurements of light intensity in woodland is often difficult and results must be interpreted carefully.

Because of these difficulties it is often simpler and more meaningful to obtain an indirect measure of how much light might reach the ground community (e.g. by assessing the amount of open sky which can be seen through the canopy at a number of fixed points, or by recording whether the ground at these points is in shade or sun at set time intervals during the day).

The angle at which the sun reaches the ground is also important. When the sun is low in the sky (e.g. in the evening) the light may shine underneath the canopy rather than through it. Where woodland is on a slope or in a deep valley it is particularly important to take into account the angle of the sun.

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