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Woodland Structure (2)

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The layers of vegetation in a woodland may alter dramatically with the seasons and as a result of management.

In broadleaf woodlands in Britain, high light levels reach the ground for several months in the period between the loss of the tree leaves in the autumn and the regrowth of the leaf canopy in the spring. This means that in warmer periods of the autumn and spring, species in the lower woodland layers have the opportunity to capitalize on the high light levels temporarily available. This can provide the energy essential for successful flowering and seed production.

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Species which take advantage of this include Bluebells and Wood Anemones. Bluebells flower in the spring, using reserves stored up in underground bulbs the previous year. Once flowering is completed, they will replace the reserves, utilizing the higher light levels available before the leaf canopy regrows and once more blocks out the light. By high summer, all that remains to advertise their presence are the dead leaves and spent seed heads.

This illustrates an important point - the layers within a woodland will often alter according to season. Thus, the field layer in a woodland may more or less disappear over the winter months, as all the vegetative parts of the grasses, herbs and ferns die back. New growth burgeons as warmer weather approaches, giving rise to a succession of different species with varying degrees of shade tolerance. These may range from Lesser Celandine, Dog's Mercury and Wood Anemones in early Spring, to Bluebells and Ground Ivy in May. Still later on, such plants as more shade-tolerant, delicate Enchanter's Nightshade and Yellow Pimpernel may appear.

In contrast to broadleaf woodlands, there is no period when the trees lose all their needles in a coniferous woodland. This is because the trees are mostly evergreen (with the odd exception, such as Larch). As a result, there is no seasonal period when unobstructed light reaches the woodland floor. The plant species which require a relatively high light intensity to produce flowers, will therefore be excluded from darker coniferous woodlands because of an inability to reproduce successfully. This means that the lower field and ground layers may be substantially less diverse than a comparable broadleaf woodland.

wpe7F.jpg (28267 bytes) Coniferous woodlands are also generally purpose-planted for timber production. Competing plants are therefore removed or discouraged from growing. Dense, relatively young stands of conifers will also have very low light conditions underneath. This prevents the development of field and ground layers.

Later on, the crop will be thinned. This encourages greater growth from the remaining conifers. After thinning and as the crop grows taller, enough light may reach the ground for these layers to develop. The number and variety of other plants in a coniferous woodland therefore depends to a great degree on the age and density of the dominant tree crop. A mature coniferous woodland may actually have a better development of field and ground layers than a heavily shaded broadleaf woodland such as Beech.

The underlying structure in a coniferous woodland will also depend on the type of conifer making up the woodland. For example, more light reaches the ground underneath larches, which tend to have a spreading loose canopy, which is also deciduous.

Continue to Types of Broadleaf Woodland