Trees of deciduous woodlands
The characteristic structure of British deciduous woodland, with its mosaic of deep cover and sunny glades, derives much from the wide variety of shrub species and their success at coexisting with the trees around them. The distinction between trees and shrubs is fairly arbitrary, but trees generally exceed 5-6m (16ft-20ft) in height and grow from a single stem, whereas shrubs are smaller and often divide at ground level. There are borderline species such as hazel which achieve the stature of trees in optimal conditions, while some trees, for example rowan and hawthorn, are often held at shrub height by a closed canopy.
The vigour of the shrub layer is strongly determined by the sorts of trees distributed among it. A tightly knit canopy, casting deep shade, inhibits the development of any shrub layer. Beech is particularly effective at blotting out light and preventing shrub development. Associated with the struggle for light, some shrub species are notable for leafing early in the year – in mild winters elder and honeysuckle start opening in December or January. In general, shrubs are competitive colonizers, quickly invading clearings made by fallen trees or created by man. In this respect the kind of patchy woodland we see today gives (if unchecked) unprecedented opportunity for luxuriant shrub growth. In prehistoric times lowland Britain was covered with a more or less continuous pelt of trees, but clearance began early with the Neolithic settlers and has accelerated ever since, so that now we have a mere scattering of trees representing about 10% of the original forest area. With rare exceptions this woodland cannot even claim to be primeval forest, for most was planted by man.
Man has not, however, been able to destroy completely the ancient pattern, and we still find many trees and shrubs distributed as nature intended – oak on rich clays, birch on lighter soils, ash on limestone and beech on southern chalk downland. Apart fromand other environmental factors, the presence of local animals may also influence the composition of woodland. Elder, for example, is rife in many woods because its astringent bark protects it from being gnawed to destruction by deer and rabbits. Young spindle, on the other hand, is relished by rabbits.