The woodland ecosystem

Structure

The complex structure of semi-natural broadleaved woodland offers a great diversity of habitats in which plants and animals can live. Like other aspects of the woodland environment, structural features change over time. One of the main aims of woodland management for conservation is to create, maintain or restore structural diversity where it has been lost or where, without management, it would decline.

Four layers are recognised in the woodland structure: the tree canopy, shrub layer, field layer and ground layer. The importance of the underground layer should not be forgotten.

Further layering may develop where the dominant canopy trees are overtopped by occasional emergent trees, or where they grow in the company of somewhat lower understorey trees. The field layer may develop two sub-divisions: a layer of tall herbs and undershrubs, and a layer of low herbs. Trees of the canopy and shrub layer may consist of plants of a single species, but of different ages and sizes, or they may be of different species which reach varying heights when mature. In a mixed wood, typical emergent species may include elm or beech, with oak or ash as dominants and crab apple, wild cherry, holly, rowan or field maple forming the understorey. This shrub layer may include hazel, hawthorn or blackthorn. Bracken (Pteridium aqualinum), rose-bay willow-herb (Epilogium angustifolium) and bramble (Rubus spp) may form a tall herb layer with bluebells (Endymion non-scripta), dog’s mercury (Mercuralis perennis), ramsons (Allium ursinum) and smaller ferns among the low herbs. The ground layer consists of mosses and liverworts, plus the seedlings of the plants of the taller layers. Where the woods are coppiced, the lower layers are likely to include coppiced shoots of the canopy species.

Not all woods have all these layers, and none would be likely to have them throughout. Some canopy species, such as ash, let in quite a lot of light so that full layering can develop. Others, such as beech, cast a heavy shade so that the shrub and field layers are largely absent except where there are breaks in the canopy. Plantations usually show little diversity within stands. A well-stocked oak plantation is unlikely to contain much besides sparse brambles and a ground layer of mosses. Conifer plantations are dense and dark and most have little other than fungi from the time the canopy closes over until the first heavy thinning. Wood pastures usually lack the shrub layer and higher field layer, due to grazing, while in scrublands the shrub layer forms a canopy in the absence of taller woodland trees.

Woodland plants are adapted to the structural conditions. For example, yew and holly are typical trees of the understorey, as being evergreen, they are able to compensate for the dense shade of summer by growing at other times of year. Dark leaves, such as holly and ivy, also contain a relatively large amount of chlorophyll, so they are able to use low light levels efficiently. Holly and yew are amongst the few trees which can regenerate in dense shade. Many of the species in the tree and shrub layers produce flowers before they come into leaf, as being wind-dispersed, the seeds need to ripen before the unfolding leaves shelter them from the wind. The plants of the field layer have to grow and flower early, before the canopy closes over them. Annual plants, that must germinate and complete their cycle within one growing season are at a disadvantage, and hence are poorly represented in woodlands. Most of the field layer plants are perennials, with bulbs, corms, tubers or rhizomes to give a rapid early start in spring.

Lateral structure is also important for diversity. Where woods are allowed to develop unhindered, some canopy trees eventually die and create gaps, where seedlings of canopy, shrub or field layer can spring up. Coppiced woods are often especially diverse because at any given time they are likely to contain some coupes which have recently been cut over, some where the coppice has created a dense shrub layer, and others where the coppice has matured into an understorey with standards as canopy dominants.

For some forms of wildlife, these relatively short-lived openings in the canopy are less important than longer-lasting glades, margins, lawns and rides. Such sheltered, humid but well-lit areas are much richer in species of epiphytes and invertebrates than either shaded woodland or open-ground habitats. It seems likely that they occurred frequently in the primaeval woodland, maintained under natural conditions by heavy grazing and browsing. In this respect, relatively open wood pastures may retain more similarity to the original forests than do dense plantations, or ungrazed, unmanaged woods.

However, the maintenance of shelter within a woodland is of great importance, providing a microclimate that is moist, sheltered and shaded, with smaller temperature fluctuations than those that occur outside the wood. Many woodland species, including flowering plants, mosses, ferns, liverworts and invertebrates have limited ability to disperse, and can be lost by changes in the woodland cover that are too drastic.

Topography may also influence lateral structure. For example, in those Scottish pinewoods which grow on irregular moraine topography, pines occur mainly on the moraine ridges with more open cover in the hollows between. Even small changes in topography and in soils can affect the distribution of tree species, with accompanying differences in layering.

Within a woodland, various associated features provide many additional habitats, especially for fungi and invertebrate animals which are often restricted to these situations. The most important of these from a management viewpoint include large and old trees, decaying wood, streams and ponds, and climbing plants. Important microhabitats include flowers and fruits, fungi, carrion, dung and nests.

One of the most important parts of the woodland structure is the part you can’t see. Beneath the soil surface is a complex ecosystem of roots, fungi and soil micro-organisms. Tree roots are often surprisingly shallow, with the most important feeding roots occurring in the top 30cm (1ft) or so of soil. The roots may spread out beyond the outer edge of the canopy, so damage at some distance from a tree may have adverse effects.

Amongst the most important organisms in the soil are a type of fungi called mycorrhiza, which forms a symbiotic relationship with roots of trees, shrubs and other flowering plants. Mycorrhiza occur in nearly all plant communities, and it’s estimated that 90% of the world’s plant species depend on a mycorrhizal association. Mycorrhiza fungi form highly branched, interconnected networks that invade the roots of plants in order to obtain a supply of carbohydrate. In return, the mycorrhiza converts organic nitrogen to inorganic nitrogen, supplies phosphate to the host plant, and may also confer some degree of pest, disease or drought resistance. For many plants the nutrient and water uptake is mainly by way of mycorrhiza, and not directly through the roots. It’s estimated that the fungal network extends the volume of soils that plant roots can exploit for water and nutrients by a factor of 12 to 15. Mycorrhizal threads are more efficient than plant roots at nutrient uptake, as they are finer and more active.

In woodlands, the ability of mycorrhiza to make nutrients available to the trees, shrubs and other woodland plants is vital for the self-renewing fertility of the wood. Mycorrhizal associations are partly the reason why natural regenerated tree seedlings in soils with mycorrhiza already present tend to thrive. A seedling transplanted into a cultivated soil or disturbed soil, where there is little mycorrhizal activity, will struggle.

Some species of mycorrhiza will only associate with particular plant species. Others will associate with various plant species, though possibly with little benefit to the host. ‘Easy to grow’, tolerant plant species such as ash, cherry, alder, willow and sycamore can form associations with a range of species. At the other end of the spectrum, many orchids are totally dependent on a particular mycorrhizal association, without which the seed cannot even germinate. This accounts for the rarity of orchids, their ability to thrive in particular places and not in others, and why transplanting leads to failure. With other plants, a particular mix of mycorrhiza may be needed.

Mycorrhiza are very fragile, and are easily damaged by cultivation. Being symbiotic, they need their host plants to survive, so the removal of woodland above is also the destruction of all the mycorrhizal associations below. This is why it is so difficult to create a woodland ecosystem simply by planting or seeding, and why it is so important to protect remaining areas of woodland and woodland soils.

Mycorrhiza can be artificially added to the nursery soil or at planting, or inoculated into the ground around existing trees or other plants. However, with the huge range of mycorrhizal species and the complexity of their associations, inoculation with a few of the generalist species is likely to be a pale imitation of the natural situation.

Communities

Woodlands, wood pastures and other wooded areas support a huge range of organisms. The National Vegetation Classification describes 19 major types of woodland, with their distinctive mixtures of trees and shrubs, and associated flora of flowering plants, ferns, mosses, liverworts and lichens. The biodiversity of woodlands is recognised in the Biodiversity Action Plans.

Semi-natural broadleaved woods may contain many of the 60 or so native species of trees and shrubs, as well as a wide variety of flowering plants and ferns, with even small woods containing 20 or more species, and large diverse woods supporting over 200 species. A diversity of plant species in turn supports a variety of insects, birds and other fauna. Some tree species, notably oak, willow, birch and hawthorn are outstanding in the fauna they support, with blackthorn, aspen, elm, hazel, beech and Scots pine also being important.

The bark of trees also provides a habitat for epiphytes, which are non-parasitic plants that grow on other plants. Epiphytes are normally only found on trees within woodland, where the shady, humid atmosphere allows sufficient moisture for them to survive. Mosses, liverworts, algae and lichens may all be found growing as epiphytes, with one higher plant, the fern (Polypodium vulgare) also occurring in Britain. Epiphytic growth tends to be much more lush in the moist and mild westerly areas. Lichens only survive in unpolluted air. Lichens and other ephiphytes grow slowly, with the richest communities found on the oldest trees of stable, undisturbed woodland, making an assemblage of great conservation interest.

The table below lists the total number of taxa of epiphytic lichens recorded from some trees and shrubs in Britain. ‘Taxa’ includes species, sub-species and ecologically distinct varieties.

Table 2a: EPIPHYTIC LICHEN

Succession

Woodlands, like all communities of living things, are dynamic. Sometimes they change so slowly that little seems to happen in a human lifetime. At other times, as when a felled wood is left to regenerate or a grassy area is allowed to grow up to scrub, the changes are noticeable within a few years. It is important to take account of this dynamic aspect of woodland ecology when managing woodlands, since to ignore it may make management difficult, frustrating and ultimately unsuccessful. Where woodland succession is understood, it can be accounted for, and if necessary, manipulated.

‘Natural succession’ is the process by which one community of organisms gives way to another, in a series from coloniser to climax. To give an idealised example, bare land is first colonised by annual ‘weeds’, then by grasses and mixed herbaceous meadow species, followed by shrubs and finally by trees, which grow up through the shrubs and largely suppress them. The weeds are the pioneer species, while the forest trees form a ‘climax’ community which tends to persist indefinitely.

In reality, succession seldom takes place uninterrupted by natural or man-induced agencies such as fire, grazing, felling or drainage. It is also usually much more complex that the picture given above. Certain trees and shrubs may come in immediately, depending on the proximity of parent or ‘donor’ plants, and on the feeding patterns of birds, which distribute the seeds of many of these species.

Tree species vary in their tolerance of shade and other conditions, and in their ability to regenerate within the woodland. Even without interference by man, woods may change in their species composition with time. ‘Tolerant’ species are those that can survive and regenerate under the canopy. They tend to be species that live longer, and flower later and more irregularly. They can persist in the understorey as suppressed seedlings, and then quickly take advantage of any increase in light due to canopy loss, for example when mature trees are blown down. Tolerant species are poorly adapted to long distance dispersal, so are not found early in the woodland succession, but tend to be part of the climax community. Species include hornbeam, lime, elm and beech. ‘Intolerant’ species are those which cannot tolerate shade, and tend to be succeeded by tolerant species. Intolerant species are fast-growing, quick-maturing, short-lived species which produce seed regularly and in quantity. They are pioneers in two ways. Firstly they are opportunistic, and can quickly occupy sites which come available, and secondly, they can persist and reproduce in infertile or difficult sites. Species include alder, willow, pine and birch.

Some species have characteristics of both tolerance and intolerance. Oak is intolerant of shade, but is very long- lived. Holly and yew are tolerant of shade, but are widely dispersed by birds.

The climax community may itself be more dynamic than theory may suggest. Pinewoods regenerate best at their edges, where shading is reduced, and many Scottish pine and birch woods seem naturally to shift their positions over time, unless constrained by climatic and altitudinal limits. In some of these woods, pine also seems to alternate with birch in a relatively stable long-term cycle, depending on which species is better able to regenerate in a given area at a given time.

Oak is a pioneer species and is often present very early in the succession of grassland to scrub, and is not a stable climax woodland species as is often imagined. Oak cannot regenerate under its own canopy or under the canopy of other trees, as there is not sufficient light. Acorns may germinate and seedlings appear, but unless an opening appears in the canopy due to windblow or felling, the seedlings wither away, or are defoliated by tortrix moth caterpillars which fall down from the parent tree above. Oak is dependent on being spread, mainly by jays, which select ripe and fertile acorns in autumn and bury them for food stores. The jays choose sites in long grass and under thorny bushes at the edges of woodland, in bare or waste ground, and in loose soil which is easy to penetrate. The acorns are buried singly. During late winter and through the following spring and summer the jay will return and retrieve some of the acorns, in the later part of the season by tugging at the seedling tree and removing the acorn, but normally without damaging the tree.

Oak is also adapted to withstand browsing and competition from grasses, herbaceous plants and thorns. Its large seed produces a deep taproot which sustains the seedling, and if browsed it can normally regrow.

A young oak tree can remain browsed and small for many years, and then take advantage of any break in browsing to put on a spurt of growth. Oak frequently grows up in the shelter of thorny shrubs including holly, hawthorn, blackthorn and gorse, where it is protected from grazing. There is an old English proverb, that ‘the thorn is mother to the oak’. Old oaks are often found growing in close association with holly, which is another long-lived tree.

Oak, and other species which cannot regenerate in closed woodland, may therefore be dependent on grazing animals maintaining open areas in which regeneration can take place. In natural conditions, oak would have colonised grazed areas. In managed landscapes, it can colonise cleared ground, waste ground, roadsides and railway embankments. If left unmanaged, oaks eventually get overtaken and suppressed by species such as lime, hornbeam, elm, sycamore or beech, except on poor soils or exposed sites. In western coastal areas oak is the climax vegetation of rocky dells and slopes which have little soil, where other trees cannot survive.

Woodlands with fairly closely spaced oaks are nearly always plantations. Old oaks in woodland with wide spreading branches low on the trunk show by their shape that they grew originally in open ground, and the woodland by which they are now surrounded has grown up since. Oaks were also planted and promoted as the standard trees in ‘coppice with standards’.

Other tree species take advantage of holly and other thorny shrubs for protection from browsing. Trees have to become established in holly in the first 20 years or so after the holly starts to grow, or the shade is too dense. The next opportunity does not arise until the holly degenerates, after 2-300 years.

There are many situations where natural woodland succession needs to be managed to benefit wildlife diversity. Trees encroaching on rare and valuable habitats such as heathland, downland, wet grassland, marshes or dunes may need to be kept in check. Another situation where intervention is necessary is where succession favours one species over others which are considered more valuable. Introduced species, notably rhododendron and sycamore, are the most common culprits.