Woodland structure

For recreation and visual value, the structure of the woodland tends to be more important than the particular species. For recreation use, most people like a woodland which is fairly open and light, with tall, clear stemmed or spreading, branched trees that you can see between, and with little undergrowth. This type of woodland may be highly managed, and may consist entirely of planted trees. A woodland which approaches more natural conditions, with a varied structure, an understorey, thickets of natural regeneration, fallen timber and trees of varying shapes and sizes, may not be so attractive to many people. Children, though, love bushes for dens, narrow paths for creeping through, low branches for swinging and climbing, muddy banks for sliding and plenty of interest at their level.

Open woodlands, which are attractive for recreation, can be vulnerable to storm damage. They may also be too draughty and exposed to be of much value for wildlife. Dense growth around the edges of woodland, which gives shelter to the habitat within, may not be popular with woodland users, who can be afraid of losing their way, or of being attacked.

Much of the worst damage in the storms of 1987 and 1990 was to plantations, and to planted trees.

In creating new woodlands, the challenge is to use local species in a way which fulfils the expectations and desires of users, whilst also creating a woodland which is ecologically diverse and resistant to storm damage.

Types of structure

Described below and overleaf are types of woodland structure, which relate to the way trees are managed. In planning a woodland, you need to decide on the type of structure most suitable to the purpose of the woodland. The type of structure affects the choice of species, spacing and early management.

High forest

In high forest, trees are allowed to grow to their mature height. The trees may be even aged, as in a plantation, or may be of mixed ages. Newly planted woodland will need thinning and possibly pruning to create a high forest system. Mature high forest is attractive for recreation use, as the trees are well spaced and the canopy is high.

Coppice

Coppice is produced by cutting near the base of the stem or trunk, which results in the cut stump producing many shoots. Coppicing is an ancient technique used to produce wood which could be easily cut with simple hand tools, and which had many uses. Coppiced woodland is very valuable for wildlife, especially flowers and butterflies, because of the changing conditions of light during the coppice rotation. The area of coppice, called a coupe, needs to be at least 0.1ha (c.30m x 30m) in area to develop properly, or 0.5 ha for commercial coppicing. Too small an area within a woodland will be shaded by surrounding trees and will not produce strong regrowth. Coppice is valuable for recreation in the later part of the rotation, and is ideal for involving local people in active management. All native broadleaved species can be coppiced, although the products of some species are of higher commercial value than others.

Coppice with standards

This is a two-storey forest, with scattered standard trees above an understorey of coppice. The coppice is cut on a short rotation for fuel, fencing and other purposes, and the standards are grown on a longer rotation and felled for timber as they mature. This silviculture system was widely used in the past, and was the legally required method of managing woodlands in the time of Henry VIII  (1509-47), when at least 30 standards per hectare had to be grown. With the exception of hazel, which only grows as a multi-stemmed shrub, standards can be the same species as the coppice layer, although beech is unsuitable as a standard because of the heavy shade it casts. Oak is the most common standard species. Coppice with standards requires careful long-term management, and felling can appear destructive. For amenity woodlands it is usually better to intermix areas of high forest with areas of coppice, rather than to use the coppice with standards system.

Wood pasture or parkland

Wood pasture is a system of land-use which combines trees with pasture for grazing animals. Some of the trees are pollarded, which means they are periodically cut at 2-3m above ground level. Growth sprouts in the same way as from a tree coppiced at ground level, except that the new shoots are out of reach of browsing animals. Pollards are harvested for firewood, fencing, building and other uses. Pollarding renews the tree in the same way as coppicing, and pollarded trees are amongst the oldest trees in Britain.

Wood pasture is an ancient system of management, in use from the middle ages. The landscape it produces may also have very ancient roots, as recent theories suggest that parts of post-glacial Britain may have had a savanna-type landscape of grass with scattered trees, which would have resulted from the grazing of large herds of animals. Over the centuries, an idealised landscape of trees and grass has been recreated in deer parks, landscape gardens, urban parks, golf courses and arboreta. Although not as diverse as woodland, scattered trees can have great wildlife value, and can be seen and appreciated in a way that is not always possible with trees in woodland. Many people who find dense woodland threatening will not fail to be drawn by a landscape of scattered trees and grass.

Wood pastures, wooded commons and parks grazed by stock still exist, and contain some of Britain’s most ancient trees, together with unique assemblages of lichens and other organisms. At low grazing densities, trees can regenerate without protection.At higher grazing densities, fencing will be needed to protect natural regeneration. See Rackham (1990) for details on the history of wood pastures and wooded commons, and Read (2000) for details on the management of veteran trees.

Many important landscape parks were created by tree planting, which has continued over the generations to replace and extend areas of parkland. Introduced species were often used, and were the raison d’être of arboreta, or collections of trees.

Woodland edges, spinneys and scrub

Woodland edges may be characterised by species of trees and shrubs which are different to those within the woodland. The light and space also allows more spreading, bushy growth, with lower branching, compared to trees in woodland. In some cases, edges have been created by planting species which provide shelter for the woodland within. In other cases, the characteristic edge evolves naturally from those species which are light demanding or colonising. Grazing around woodlands tends to leave an edge of hawthorn, blackthorn, elder and other unpalatable species. Some plantations had hedges planted around them, which may remain as fully grown hawthorn or other hedging species. Woodland edges are very important for wildlife, both because of their structure and variety, but also because of the particular plant species they contain. Many invertebrates, birds, small mammals and reptiles rely on woodland edge or scrub habitat.

Small areas of woodland edge habitat can be created by planting hedges and clumps of hedging species or ‘scrub’. Traditionally ‘spinneys’ are small areas of woodland, mainly comprising spiny species such as hawthorn or blackthorn. This type of planting is useful for screening, shelter, protecting vulnerable habitats and limiting access. Depending on the situation, it may develop into woodland if taller tree species seed into it. Exposed sites, very poor soils or other limiting factors may keep scrub as the climax vegetation.

Creating structure

Structure is a function of the species selected, the spacing at planting, and the way that the trees are thinned, pruned or coppiced in the years after planting. This is too complex a subject to deal with in detail in this handbook. For further information refer to Creating and Managing Woodlands around Towns (Forestry Commission Handbook 11, 1995), Creating New Native Woodlands (Forestry Commission Bulletin 112, 1994) and Silviculture of Broadleaved Woodland (Forestry Commission Bulletin 62, 1984).

The general formula for establishing trees, either in small groups or to create large areas of new woodland, is to plant trees small and close together, spaced 2 or 3m apart. This quickly produces a woodland-type environment or thicket, where shade and shelter mutually benefits the trees, and the ground quickly becomes too shady for grass to grow. The beneficial effects extend underground, where mycorrhizal activity which greatly enhances tree growth is more likely to establish than under grass. The trees must then be successively thinned to leave the ‘final crop’ trees. (This standard terminology is used here to describe those trees which are planned to be grown through to maturity, and which are the main object of the design.

In commercial woodland they will be a ‘crop’, but in amenity or conservation woodland they may never be felled for timber).

As explained below, you may thus plant anywhere between 9 and 40 young trees for every tree you want to grow to maturity. At planting this may seem very wasteful, but this system has many advantages both in the way it encourages trees to grow, and in related management. By planting densely, you are creating an environment in which the trees can flourish. The most important factor in establishing trees is keeping them weed-free, and especially grass-free. If you plant close and weed properly, within four or five years grass and other weed growth will have been suppressed, the trees will be well established and thinning can be started to select the final trees. The thicket of growth has the advantage of keeping people and mowers away, both of which can be damaging. The thicket stage may not be popular with local people, and the system needs explaining where planting is close to housing or public access areas. A thicket looks untidy, and you can’t see through it. It should be explained, for example, that what appears a jumble of growth is nurturing the four or five trees which will emerge to form the intended design. If you just plant five trees in grass, the planting is likely to be unsuccessful.

High forest

In a natural woodland, there is not a great mixture of species, but usually two or three dominant overstorey or forest trees, and two or three frequently occurring understorey species. This is the best pattern to follow when creating new woodlands, because it mimics a natural woodland and is thus likely to be the most simple and successful to establish. Soil and other conditions naturally limit the number of species, and it is not a good idea to add more just for variety. The planting of rare native species should not be generally encouraged, because part of their attraction is their rarity. Details on species are given in the next section.

The generally accepted method is to plant in groups of 9, 16 or 25 of each species. The trees in each group are then successively thinned to leave one tree which grows to maturity. These groups can be arranged in a particular pattern, or can be arranged randomly. For timber production, the final crop trees need to be evenly spaced to optimise growth. In amenity woodlands, a pattern which results in unevenly spaced final crop trees is usually preferred. The pattern must relate to the proportion of species which have been chosen.

A simplified example, which can also be used as a ‘rule of thumb’, is shown below. This uses three species in equal proportions, with the groups randomly arranged. The trees are planted 3.3m apart, or 9 trees to a 10m x 10m square, which is equivalent to 900 trees per hectare. The trees are thinned every 5-10 years over about 40 years, to leave one tree from each group of nine. The mature woodland thus has trees spaced on average 10 m apart, at a total of 100 trees per hectare. (This planting rate approximates to the Woodland Grant Scheme rate for amenity woodlands below 3 hectares, which is 1100 per hectare or a 3m spacing.).

This simplified example uses three species in equal proportions, planted on the square in a square hectare. Variation can be introduced into the planting design in many ways:

• nurse species may be included, for example fast growing or nitrogen fixing species, to aid the growth of slower growing more desirable trees. These should be removed once their purpose has been fulfilled.
• Understorey species may be included, which will be naturally suppressed as the canopy grows up.
• Choice of species must follow physical variations within the site, for example soil type.
• Groups can be varied in shape or size, while keeping to the basic plant spacing. Planting in irregular ‘drifts’ which follow land form and contour gives a more natural effect.
• Glades, rides and other open space should be allowed for, and should be left unplanted (see below).

Variation will also occur naturally  as the  woodland develops:

• Other species will move in by natural colonisation. Unplanted species, already on the site, may increase in number. Management may be needed of undesirable species.
• Some of the young trees may die, or fail to thrive. Typically 10% is allowable, for example under grant schemes, but if more than this fail, the site may fail to develop as woodland.
• Some species will grow at the expense of others. This may have been allowed for in the plan, or may be unexpected! Some species may fail due to disease or damage by animals. Squirrels for example tend to target particular species at a certain stage in their development.

‘Intimate’ mixtures, where trees are planted singly in a random pattern across the site, are not recommended. Slower growing trees may be suppressed by fast grow- ing trees, and thinning is difficult to carry out. Visually the result is rather homogenous, so the woodland lacks character. Although an intimate mixture may appear to be more natural, and have more wildlife diversity than a group mixture (as shown above), this is unlikely to be the case, because establishment of the woodland will be more difficult.

Single species woodlands can be created by planting the entire site with one species, and then thinning successively. alternatively,  the final crop species can be planted in small groups, say three or five trees at 3m spacing, with the spaces between the groups planted with nurse or understorey species, which are removed as the woodland matures. The groups of final crop species are progressively thinned to leave one tree. Single species woodlands are not natural, but they can be very attractive, and have a special character which can make them locally important features. They are suitable for small areas of woodland because they are easy to establish, and may have more visual impact than a mixed patch of woodland.

Coppice

A new coppice is fairly straightforward to plan and establish, as the plants which are to form the stools are planted at their final spacing. areas of coppice (called coupes) are normally of one species, so that they grow up at the same rate and can be cut at the same time. Mixed species coppice can also be grown.

Table 2a: Short rotation coppice

Table 2b: long rotation coppice

Coppicing can be categorised as follows:

• Short rotation coppice, worked on rotations of less than 10 years, for pea and bean sticks, baskets, hurdles, firewood and other uses, including biomass for energy generation. Typical species are willow (osier), hazel and sweet chestnut.
• Long rotation coppice, worked on rotations of between 10 and 30+ years, for turnery, fencing, tool handles, firewood, charcoal and other uses. all broadleaved species can be grown this way, but species commonly used are alder, ash, birch, hornbeam, lime, oak and sycamore.

The table on the previous page gives examples of different uses, rotations and spacing.

Coppice with standards

Standards, usually of oak, are grown at between 30-100 per hectare (16-10m spacing), depending on age and size. For a new woodland, they should be planted at the same time as the coppice species are planted, at about 100 per hectare. At each coppice rotation, some of the standards can be felled, with replacement planting or protection of natural regeneration as required, to ensure continuity of cover.

Wood pasture and parkland

Wood pasture type cover may have evolved naturally in response to grazing pressures from herds of wild animals. Regeneration would have occurred naturally in response to seed years and periods of declining grazing pressure. During the middle ages, trees in wood pastures and wooded commons were important resources for fuel, fodder and other uses, and effort must have been put into ensuring that regeneration succeeded. Numbers of trees per acre were not specified, as they were for coppice with standards for example, although pollarded trees were often identified in documents, which indicates their importance.

Many of the landscape parks of the 17th and 18th centuries were created by moving quite large trees. In modern times, most of this type of planting has been of standard trees, protected as necessary with individual tree surrounds. Establishment may be improved by planting closely-spaced groups of trees and shrubs, and then thinning, as described above. Fairly early thinning is required, to ensure that the final trees develop the wide branching habit and well developed rooting system of trees grown in open ground.

Today, management of wood pastures and wooded commons is aimed at conserving veteran trees, and managing grazing to permit natural regeneration as necessary. Management of invasive species, particularly bracken, is often needed.

A modern form of wood pasture is agroforestry, which is the growing of trees on land also used for agriculture, which may be under grass for grazing or fodder, or cultivated for crops. Agroforestry is eligible for grant aid under the Woodland Grant Scheme.

Spinneys and woodland edges

Spinneys, scrub and woodland edges should be closely planted as shown in the diagram below. Weeding will be necessary for 2-3 years, depending on the situation and soil type, but the thicket of growth should rapidly close and shade out grasses and other competing species. Brambles that occur naturally may be a useful addition, provided they are not so abundant as to smother the planted species. Self-thinning will occur, as individual plants become suppressed by others, and thinning operations are not normally necessary. To increase diversity in large areas of scrub, parts can be cut on rotation. Many of these species can spread quite rapidly, and management by mowing or grazing may be needed to maintain grassy open areas within patches of scrub.