||Light is one of the most important factors in determining which plants
grow where in a woodland and how successful they are.
The amount of light available beneath a woodland tree canopy depends on two
factors: How close the trees are to each other, as well as the density of the individual
tree canopies. The closer trees are to each other, the more their individual leaf
canopies will overlap and the less light will be able to filter through to the ground. The
spacing between the trees may be determined naturally, by competition between individuals
for necessities such as space, light, water and nutrients. In plantations, original
planting spacings and subsequent thinning operations determine the distances between
All of the plants growing beneath the main tree canopy in a
woodland, are adapted to lower light conditions to varying degrees.
The level of biodiversity in a
woodland is greatly influenced by variations in light intensity.
A variety of light levels from deep shade, through to open, well-lit
clearings will encourage the development of a wide range of plant species beneath the main
tree canopy. Species with very different light requirements can then inhabit the same
general woodland area, albeit in slightly different places. If there are many different
species of plants, then it usually follows that there will also be many different species
of animals. This is because each plant species generally has its own range of animal
species associated with it. These will in turn provide food for yet other animal species.
Woodland butterflies provide a good illustration
of the importance of light for overall biodiversity in a woodland.
Woodlands support more than three-quarters of all British butterfly
species. Of these, two-thirds are currently under threat. This is partly because of the
overall loss of woodland habitats, but also as a result of the decline in traditional
woodland management techniques such as coppicing.
The caterpillars of most of the woodland butterflies actually feed
on herbaceous plants and grasses growing on the woodland floor. Of the 30-40 breeding
species which might be found in southern woods in Britain (the number declines northwards
for climatic reasons), only 4 species have caterpillars which actually feed on trees!
The decline in management practices such as coppicing in woodlands,
has meant that many woods have become dark, dense places, lacking the openings and
clearings which allow for the development of a diverse ground flora. As a result, the food
plants of many of the butterflies are absent, as are the butterflies themselves. In this
case, the trees are not as important as the spaces between them. Restoring a range of
butterfly species to a woodland may therefore involve removing trees rather than planting
Wood butterfly is on Lesser Celandine, which may carpet a woodland floor in gold in early
Spring. The Speckled Wood caterpillars feed on a variety of grasses.
The herbaceous flora in a wood is also
an important food source for the adult butterflies, as they require flowers to nectar
upon. Favourite flowers for nectaring woodland butterflies are Bramble, thistles, Ragwort
||It is not only
butterflies which benefit from having a diversity of plants in the field and ground layers
of a woodland. Many of the common tree species are wind pollinated and their flowers
therefore tend to be of little value as food for insects. In contrast to this, the
majority of the shrubs, climbers and herbaceous plants in a woodland are insect
pollinated. They therefore provide sugar-rich nectar within the flowers to attract insects
for pollination. Moths and flies, as well as butterflies will feed on this, while
protein-rich pollen may be utilized by bees and a variety of beetles and flies.
Ivy (left) is a particularly useful species as it flowers late in the year when few
other flowers remain.
The fruits and seeds of
the woodland plants are also important sources of food for invertebrates, birds and small
mammals. Different plants flower and set seed over widely differing timescales and
seasons. The greater the variety of plants present, the longer the period when flowers and
or seeds/fruits will be available to feed the woodland animals and birds. There is also
less chance that the failure of a particular species to flower, or fruit successfully one
year will devastate the food supply, because there will be other alternatives available.
This is not to say that areas where there is good tree
cover and shade should not also be present in a woodland. These areas are important for a
variety of other plant and animal species which cannot survive in more open areas. The
cover and shade provide a damp, cool, sheltered microclimate in which certain species
thrive. This includes a number of invertebrates, mosses, liverworts and ferns. It even
extends to animals living in rotting wood, in that logs left to decay in full sun usually
have a comparatively meagre biodiversity compared to those positioned in shade.