The structure of any woodland area is a very important factor in determining its biodiversity value. Broadly speaking, the greater the variety of structures provided, the more are the different kinds of plants and animals that can be supported. Native woodlands are generally richer in biodiversity than plantations, particularly coniferous plantations.
Coniferous plantations are often dense in structure and let little light onto their floor areas which significantly inhibits growth of other woodland plants and flowers. As a result, they have a lower biodiversity value although some species of bird are able to cope well with these environments.
Deciduous woodlands have a more recognisable structure, although it is not always possible to identify each layer clearly and the effect of heavy grazing can have also a significant impact as in the case of most areas of the New Forest. These layers are referred to as follows, from top to bottom:-
Canopy – This refers to the layer of leaves and branches formed by the large trees in the woodland. The canopy intercepts much of the light, so that the woodland floor can be quite shady during late spring and summer. Consequently the number of plants that can grow in the reduced light becomes limited after the early spring flush of flower. By way of example, bluebells and wood anemones grow in the spring before the canopy is fully formed.
Understory – This is often referred to as the Shrub layer and is the layer of vegetation immediately below the canopy. This layer consists of younger specimens of the canopy trees, together with smaller trees and shrubs which are adapted to grow under lower light conditions, such as hazel, holly, hawthorn and rowan. The invasive non native rhododendron which is very low value in terms of biodiversity, can dominate this layer in some woodlands and is one of the reasons why this species is gradually being eradicated from many woodland areas of the New Forest.
This layer is often insignificant or absent from a heavily grazed area of woodland as ponies, cattle and deer eat the young shoots of these shrub plants before they can develop. Instead, there is a distinctive browse line up to the extent of the animals’ reach with a clear line of vision through the wood under the canopy.
Herb Layer – Also known as the field layer, the herb layer is most developed where substantial amounts of light reach the woodland floor, for example in clearings or newly coppiced areas of woodland. The plants and flowers that are found here will depend on the openness of the canopy and the dominant trees. Generally speaking, the more open the canopy the more diverse the herb layer. Plants that are commonly found in the herb layer include bluebells, wood anemone, bramble, wood sorrel, primrose, grasses and ferns.
Ground Layer – The ground layer will largely consist of a great variety of different mosses, fungi, ivy, leaf litter and decaying wood. As mosses require constant high moisture levels, these will be less well developed in drier woods.
Overall, these vertical woodland layers also influence where different animals are likely to be found. The large branches of the canopy layer will provide nesting sites strong enough to support large birds such as Buzzards and the new spring leaf of these canopy trees, particularly Oak, provide feasts for large numbers of caterpillars. In some years, the leaf canopy may be subjected to plagues of these caterpillars when many small birds will take advantage of this seasonal bounty. A well developed understory will provide numerous nesting and food opportunities for these smaller woodland birds while the field layer of flora and grasses supplies food for a wide range of butterflies and other invertebrates. Carefully managed, a woodland can be rich in its biodiversity.