Silvicultural Systems

Silvicultural systems

Introduction

A silvicultural system may be thought of as the process by which the crops which constitute a forest are tended, removed and replaced by new crops, resulting in the production of woods of a distinctive form (Troup, 1955). It embodies three main ideas:

1. The method of regeneration of the individual crops constituting the forest. (The concern for regeneration is the main difference between silviculture and exploitative logging).

2. The form of the crop produced.

3. The orderly arrangement of the crops over the forest as a whole.

European foresters have been treating forest crops in a systematic manner for many hundreds of years with the two-fold objective of ensuring the continuing growth of the forest and of producing certain kinds of material. The earliest systems were coppice, and coppice with standards, but in the 17th and 18th, and especially 19th centuries a number of more refined and complex systems evolved and were systematised. Most of the systemisation, which involved bringing together accumulated experience of the past, was in response to the consequences of widespread deforestation which lead to crisis of various kinds. Shortages of timber were caused by wars and the growth of populations and industries. The need for environmental protection was also recognised.

By the end of the 19th century the various systems in Europe began to be thought of not merely as silvicultural solutions to specific problems but almost as embodiments of forestry doctrine or dogma. This attitude still persists and manifests itself most strangely in the controversy about the relative merits of "natural" forests versus intensive plantation systems - in Britain the debate is concerned with the extent to which we should get involved in conservation.

Classification of systems

The classification of silvicultural systems, which are by nature often flexible and imprecisely defined, is not easy. To begin with, it is useful to consider the ways in which the systems differ and could therefore be classified.

• The method of regeneration used can be coppice, planting, direct seeding or natural regeneration. Although coppice systems are clearly distinguished most others can use any of the other three techniques. The method of regeneration does not therefore provide a clear basis for classification.

• The "even-agedness" of the stands puts selection (or polycyclic) systems at one extreme, and clear-felling and coppice systems (monocyclic systems) at the other. Some other systems have two age classes (two-storied high forest, and coppice with standards), and shelterwood systems have two age classes for part of the rotation.

• Systems also differ in the size of the "silvicultural unit". This ranges from the compartment in uniform shelterwood and clear-fell systems to progressively smaller areas in strip systems and to the groups of the group and "irregular" systems. The place of the selection system in this hierarchy is debatable, depending on whether one considers that each felling is applied to the stand as a whole, or whether each tree is treated individually.

A consideration of these three "axes" of variation suggests the following classification (see also Table 1):

1. Coppice systems. Crops originating from stool shoots of vegetative origin.

2.High forest systems. Crops predominantly of seedling origin.

a) Felling and regeneration is distributed continuously over the whole area giving rise to an uneven-aged (irregular) crop—Selection systems.

b) Felling and regeneration is concentrated on one part of the forest area only at any one time.

i) Systems of successive regeneration fellings such that the old crop is removed by several fellings over a period of years. This gives rise to an approximately two-aged stand for a period in the regeneration cycle—Shelterwood systems.

ii) Old crop is cleared by a single felling, giving rise to an even-aged stand—Clear-cutting system.

The various group systems are considered variants of the three basic high forest systems, as determined by the age structure within each group. This gives "group clear cutting", "group shelterwood" and "group selection" systems. A whole compartment of a group felling system may therefore be uneven-aged, but each individual group will be even-aged and managed on a clear-fell system.

Similarly, "strip", "wedge" and "edge" systems can be considered as variants of each of the three basic high forest systems, depending on the type of stand treatment that is carried out ahead of the advancing felling "edge". This gives "strip-felling", and "strip-shelterwood" systems; and also strip variants of the group systems, such as "strip-group shelterwood".

Figure 1. Classification of silvicultural systems
Type of forest / Systems / Variants / Regeneration by planting (P), natural regeneration (NR) or coppice (C)
Even-aged (monocyclic) / Clear felling / P, NR, C
High forest with standards / P, NR, C
Shelterwood / Uniform shelterwood / P, NR
Irregular shelterwood / P, NR
Multiple even-aged / Two storied forest / Main species in both stories / P
Different species in each storey / P
Uneven-aged (polycyclic) / Selection systems / Group selection / NR
True selection / NR

These divisions will, it is hoped, overcome some of the confusion that has arisen due to the loose use of the terms "group", "irregular" and "selection". The classification only leaves two-storied high forest as one which is difficult to accommodate.

Principal silvicultural systems

Coppice systems

These systems are too well known to need describing but are dealt with thoroughly by Matthews (1989), Rackham (1976) and Peterken (1981).

Coppice systems are regarded as most desirable for nature conservation. The alternation of light and dark phases on a short cycle encourages a rich woodland flora. The dense thicket stage is a good habitat for a wide variety of woodland birds, and coppice stools themselves are often very old and derived directly from primary woodland.

It is likely that coppicing can result in impoverishment of soil nutrients. Also, the bare ground state occurs much more often than with high forest systems, and so sites are more subject to erosion and other disadvantages associated with open ground (see next section).

High forest systems

Clear felling system

This system is universally applied and is likely to remain the predominant silvicultural system in forests managed for wood production. Its main advantages include simplicity, uniformity and, in particular, the ease of felling and extraction. The use of clear felling does not necessarily preclude the use of natural regeneration, but the system almost always operates with establishment by planting. The main advantages of planting arise from its artificiality and minimum reliance on unpredictable natural events. Enough plants can be ordered for the desired year and can then be evenly distributed across the whole area, in rows, to facilitate subsequent tending. This makes reliance on natural regeneration seem like a technique inherited from a primitive "hunter-gatherer" technology, whereby the time of arrival and dissemination of seed, the genetic quality and even the species of the regeneration are largely out of the control of the manager.

However, planting is expensive, losses may be high, especially through drought, and since stocking is usually much lower than with good natural regeneration the resulting crop may be of lower quality. Disadvantages of clear felling, rather than of planting largely arise from the lack of protection, leading to a rise in the water table, extremes of temperature including frost, leaching and soil acidification and rank weed growth. Clear felling is widely regarded as the least desirable system for both landscape and nature conservation but these disadvantages can be reduced by the use of small coupe-fellings (0.2 - 2 ha).

Uniform Shelterwood System

The essential feature of the system is that even-aged crops are established, normally by natural regeneration, under a thinned overstorey, which is removed as soon as establishment is complete. Present day techniques include:

1. Preparatory felling: essentially a late thinning to encourage development of the crowns of future seed bearers.

2. Seeding felling: once it is clear that there is going to be a good seed crop, one third to one half of the stems are removed. The understorey and any regeneration already present are also removed. Cultivation with disc harrows is normally carried out to assist seedling establishment.

3. Secondary fellings: usually 2-4 fellings, at 3-5 year intervals, with timing and intensity carefully regulated to allow seedlings to grow, but also to prevent rank weed growth.

4. Final felling: the last secondary felling which removes the remaining overstorey.

The whole series of operations normally takes 5-20 years. Infrequent mast years and frost-sensitive seedlings both necessitate long regeneration periods. The secondary fellings for a light demanding species (such as oak) must be few and rapid and the whole process may be completed in five years.

If mast years are infrequent, then it may take 20 years to obtain adequate regeneration. The crop will then be somewhat uneven-aged and patchily distributed in which case the system grades into the group shelterwood (see later).

One of the main advantages of this system is said by Matthews (1989) to be its simplicity, but in areas where most years are infrequent, obtaining a fully stocked, even-aged crop is a major managerial problem. The damage done to regeneration when felling is not usually very serious, especially if the regeneration is young and supple, dense and even-aged.

The shelterwood system is also used with planted stock in areas where natural seeding is insufficient or irregular, where a change of species is required, and where seed-bearers are insufficient in number or quality, as in the conversion of coppice with standards.

Variants of the system include both group and strip systems, which consist of shelterwood regeneration fellings, carried out in a strip ahead of the advancing "edge" of the final felling. They are considered more suitable than the uniform shelterwood systems for light-demanding species.

Stands managed under a uniform shelterwood system have many features in common with stands established by planting under a clear fell system. They can be pure, even-aged and can be uniform in structure and density over large areas. From a landscape and conservation point of view, they therefore share many of the shortcomings of the clear-felling system.

Selection system

Stands managed on a selection system are, at all times, an intimate mixture of trees of all age classes. There is no concept of a rotation length, or of a regeneration period, as both harvesting of produce and re-establishment takes place regularly and simultaneously throughout the stand. The only silvicultural interventions are "selective fellings" which are carried out every 5-10 years throughout the stand. These fellings are a combination of regeneration tendings, cleanings, thinnings, final crop felling and regeneration fellings. Making such fellings is not easy, as the needs of each of the age classes, or stories, must be taken into account and trees of all sizes are removed. An important feature of them is that they concentrate on improving the quality of the stand rather than felling to remove the largest and best stems that may result in impoverishment.

Without careful intervention there is usually a tendency for a more even-aged structure to evolve, and also for the different age classes to become spatially separated, that is, a group structure begins to evolve. In an extreme case, this would result in even-aged, single-storied groups. This occurs with light-demanding species, and such a "group selection" system is the only form of the selection system that is appropriate to them.

The length of the period between successive selection fellings varies. Short periods (less than 5 years) allow better crop management, particularly of young trees. Long periods result in larger volumes of timber being removed at each visit, making them more economical. They also improve the success of regeneration of light-demanders because the canopy is opened up more.

The best examples of application of the selection system are in the silver fir (with beech and Norway spruce) forests of central Europe. Selection systems are largely confined to mountainous regions where a continuous protection of the soil against erosion and protection against avalanches are of great importance. The system also provides protection to the soil against leaching and is very suitable for regeneration of frost sensitive species. In the last 25 years the selection system has tended to fall prey to neglect, even in Switzerland, because of excessively high labour costs and inaccurate methods of volume and yield prediction.

From a landscape point of view, selection forests are probably the ideal but contrary to popular belief they do not necessarily even approximate "natural" forests.

Variants to the systems

Group systems

Group systems are considered here as variants of the three main high forest systems.

Group clear felling involves felling all the trees in a group prior to restocking. The crop within each group will always be even-aged, but the stand as a whole will contain groups of a wide range of ages, and possibly of all ages. The individual groups maybe pure or mixed in species composition, and may be established by natural regeneration, or planting, or a combination. Group clear felling is particularly appropriate to strong light-demanders as the only protection given to the young trees is from side shelter. Group sizes commonly range from coupes of about 50 m in diameter (0.2 ha) to areas of a hectare in extent.

Group shelterwood systems involve the retention of an overstorey for a short period to provide shelter for the new crop, which is approximately even-aged. The main difference from the uniform shelterwood system, apart from the small size of the areas worked, is the fact that if advanced or existing regeneration is present, it is used as the focus of a regeneration felling. (In a strict uniform shelterwood system, existing regeneration would be removed with the understorey.) Groups are gradually enlarged by carrying out regeneration fellings (seeding, secondary and final fellings successively) around the edges until eventually they meet and merge. The regeneration period is generally longer (15-40 years) then with the uniform system, and the resulting crop is therefore somewhat more uneven-aged.

The most widely used developments of the group shelterwood system in Europe have been various "strip and group" systems with beech, silver fir and Norway spruce. Groups are established and developed in a strip or belt through a stand. When regeneration is well established the remaining overstorey is felled; meanwhile new groups are established in a strip ahead of the felled belt and the process is repeated. They have the advantage that both extraction and management is more straightforward.

Group selection systems. This term is widely used and loosely applied to any irregular or group system. It should strictly refer only to systems in which a stand is sub-divided into groups, each of which is, for a large part of its life, uneven-aged, and has more than one storey. They are referred to by Troup (1955) as "irregular shelterwood systems".

In practice, the system closely resembles the selection system, as there is usually no fixed rotation length or regeneration period. It differs in that a time eventually comes when all remaining old trees must be removed, whereas in selection working no such time ever arrives. Thus, there is still a shelterwood notion; an older crop providing protection for a younger one which is replacing it, but the period of shelter is often over 50 years. It also differs in that more emphasis is placed on obtaining and developing regeneration in groups rather than uniformly through the stand.

In all group systems, the size of the group is a critical characteristic. Large groups are easier to manage, and are essential for light-demanders. The most useful range is probably 0.1-0.5 ha, larger groups being needed in taller and more uneven-aged stands. The shape and orientation of the groups can have a considerable influence on the variation of microclimate within the group, and considerable emphasis is laid on this in central Europe. General observations are that

i) north-south orientation of an elliptical or rectangular group provides a good compromise between wind and sun, and

ii) that light-demanders should be near the north edge, and frost tender species near the south.

The layout of groups is vital in facilitating management of the stands. Wherever possible, the first groups to be regenerated are those located furthest from the road, thereby minimising the amount of timber that has to be extracted through a young crop. Fencing costs for small groups are inordinately high and this has always been considered a major disadvantage of the group system.

Group systems are almost as desirable as selection systems for landscape so long as openings are kept quite small. They come closest to imitating the structure of a natural stand, at least in temperate regions. They are therefore increasingly recommended for ancient and semi-natural woodlands in Britain.