A woodland is a living community dominated by trees. In Summer, the crowns of the trees usually form a continuous cover of foliage, which is called the canopy. In a natural woodland, there is usually an understorey of immature canopy trees and other small trees and large shrubs which can reach maturity under the shade of the canopy trees. Lower still is the shrub layer which is composed of small shrubs and bushes, and the herb layer, composed of herbaceous flowering plants, ferns, grasses and sedges. A fifth layer, the moss layer may form a soft carpet over the woodland floor.
Depending on the amount of shade cast by the dominant tree canopy, one or more of these layers may be discernible in a particular woodland. The plant layers in a woodland will also influence the distribution of the animals within the woodland.
The number of layers which can be distinguished in woodlands is variable. In some woods, all four layers may be easily identifiable, whereas others may only have two. Some woodlands may even be composed of more than four layers. The number of layers present depends on a great many factors. These will include local climate and conditions, the species of plants present in the area (particularly the dominant trees), as well as the amount of light reaching the ground in individual areas. In woodlands where a lot of light reaches the woodland floor, all four layers may be well developed, whereas in densely shaded areas, little may be able to grow beneath the dominant tree canopy.
In reality, the plant layers are not separate, but all overlap. The distinctions between layers may be further blurred by plants such as Ivy and Honeysuckle which seek the light by climbing up through some of the layers, using other plants, such as the trees, as supports. Epiphytes (plants which grow on other plants, using them as a solid base), such as ferns, mosses and lichens, will also cover the branches of older, more mature trees. As with the climbers, this enables them to gain better access to available light.
A woodland may consist of some or all of these layers. Apart from the canopy the layers may be restricted to a particular area of a woodland. The structure of a woodland is dependent on various factors including grazing, tree felling, wind falls, regeneration and soil type
The dominant trees in the woodland have a major influence over other woodland organisms. Their leaves, flowers and fruits provide food and their canopies, branches and trunks give shelter. Their roots control the supply of water and nutrients to the soil, and their cycles of death, decay and leaf fall determine the rate at which used nutrients are returned to the soil.
Beneath the tree canopy a special environment is created. The woodland floor is a sheltered, shaded habitat, protected from wind and from extreme temperatures. Humidity is higher and more constant than in open habitats, hence the abundance of fern, moss and liverwort species that depend on high humiditiy for their lifecycles. In summer, the leaves of the trees absorb or reflect a large part of the sun’s rays and only a fraction of the light and heat filter through to the forest floor. For this reason, many species of the herb layer concentrate their growth in spring and early summer before the trees’ leaves have fully expanded