A typical British woodland is made-up of various layers: canopy (top layer), shrub, field and ground (bottom layer).
Canopy – this is created from the branches and leaves of the wood’s tallest and frequently the oldest trees. It acts as a home for a variety of tree dwelling animals such as birds and squirrels.
Shrub – the shrub, or understorey, layer houses the younger and smaller trees and shrubs. These tend to have evolved to be able to take advantage of the presence of less light reaching the woodland floor. Species of tree that are able to grow in such low light levels include hazel and holly.
Field – this layer needs a good level of sunlight to thrive. It can be mostly made-up of grasses, flowering plants (bluebells and primrose etc), ferns and nettles, for example. This layer tends to have a greater diversity in species in clearings or areas of recent coppicing.
Ground – this layer mainly consists of decomposing leaf letter, decaying wood and other rotting vegetation. A great many species of invertebrates and micro-organisms can be found in native broadleaf woodlands that rely on moisture and good quality organic matter. A substantial amount of mosses, lichens and fungi help to provide depth to this layer.
‘Scrub’ can be found where woodland begins and finishes, or in areas where it may develop, for example in an abandoned field. Scrub is usually dominated by shrubs of roughly 15 feet in height with many stems, creating a proto-canopy. Plants found within scrubland are ‘pioneer’ species – they grow fast and colonise open areas quickly. Depending on the soil type, species commonly found in scrub can be grasses, blackthorn, gorse and elder. Eventually, if a scrub’s pioneer species have been successful in developing a stronger canopy, other species may take hold further developing the vertical structure which, in time, could lead to further woodland development and the encouragement of other woodland plant and animal species.
A glade can be a very important open parts of a woodland and can be found associated with existing ‘rides’ and scrubby areas. They are usually grassy meadow type areas. When occurring naturally they may be the result of a fire or other natural processes that have cleared a given area. Human initiated processes like coppicing can mimic these processes to open up regions of the woodland to sunlight, therefore allowing a range of different plant species to flourish – along with their associated animals. Eventually, pioneer species may take over developing the scrub area and re-introducing a canopy.
The spacing and density of trees
Humans can have some of the greatest long term affects on the spacing and density of trees within a woodland through management techniques. Natural processes, however, can also have great effect. These can include topography, the placing of rocks, soil types, pests and weather including fires. Wild woodlands have a self-set nature with a greater diversity of field and shrub layer plants and animals closer to the wood’s edges. A reduction of diversity can be found deeper into the wood. Human plantations tend to be planted in a uniform or quasi-uniform structure to aid in their management for production. This affects the general diversity of both plants and animals as well as the variety and depth of the overall vertical and horizontal structures.