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.
regeneration is the main difference between silviculture and exploitative logging).
(The concern for
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 clearfelling 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.
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2. High forest systems. Crops predominantly of seedling origin.
a) Felling and regeneration is distributed continuously over the whole area giving rise to an unevenaged (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
Even-aged (monocyclic)
Systems
Variants
Regeneration by
planting (P), natural
regeneration (NR) or
coppice (C)
Clear felling
P, NR, C
High forest with
standards
P, NR, C
Uniform shelterwood
P, NR
Irregular shelterwood
P, NR
Main species in both
stories
P
Different species in
each storey
P
Shelterwood
Multiple even-aged
Uneven-aged
(polycyclic)
Two storied forest
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
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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.
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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 lightdemanding 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,
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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.
Tropical systems
Silvicultural systems in tropical forests do not differ in any fundamental ways from those described above,
though variations often have to be introduced to accommodate the species-rich nature of these forests,
and the relatively small number of species with timber that is commercial by current standards. They
normally belong to one of two kinds: polycyclic and monocyclic. Whitmore (1990, p. 118) has described
polycyclic systems as being based on the repeated removal of selected trees in a continuing series of
felling cycles, whose length is less than the time it takes the trees to mature. This tends to result in
scattered small gaps in the forest canopy, and is analogous to the selection system described above for
temperate forests. Monocyclic systems remove all saleable trees at a single operation, and the length of
the cycle is the same as the rotation age. Such systems are similar to shelterwood or clear felling
systems of temperate regions, and regeneration is normally of seedling origin. The two kinds of system
tend to favour shade-bearers and light-demanders respectively, but the extent of the difference will
depend upon how many trees are felled at each cycle in a polycyclic system. Details of managing some
of the tropical forests are given by Dawkins (1958), Newman (1990), Poore (1989) and Whitmore (1990).
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Conclusion
Each silvicultural system is applicable to different situations and management objectives.
Woodland management can be thought of as grading from "intensive" through to "extensive". The
former implies careful and expensive tending of groups to produce valuable high quality timber. The
latter implies a lower input approach, accepting somewhat mixed and uneven-aged stands, and
producing, cheaply, rather lower quality timber. Intensive management is more normally associated with
clear felling and uniform shelterwood systems. The less intensive approach is more appropriate to
selection and group systems, which need careful, but not capital intensive, management to run well. The
same distinctions apply to the strategy adopted for obtaining and using natural regeneration: one could
either invest time and money in trying to get a full stocking from any one seed year (i.e. a uniform
shelterwood system with careful preparatory thinnings, cultivation and weed control); or one could
operate a group, or selection, system with minimum preparation for seed, but accepting and using the
steady trickle which establishes itself largely unaided. Both approaches have their merits and the "high
input" one is not always the most profitable. The low-input approach is particularly appropriate to small
woodland owners who do not have large sums to invest.
In Britain, the use of natural regeneration is unfortunately too often synonymous with laissez faire
management and grant and fiscal incentives are aimed at high-input management.
Stands of irregular structure and tolerant species are best suited to uneven-aged silviculture. Fragile
sites, steep slopes, high water tables and very dry sites that would be adversely affected by complete
removal of the forest cover even for short periods are better suited to uneven-aged silviculture. Evenaged most effective in stands of intolerant species and it should be used to return over mature, decadent,
diseased or insect infested stands to productivity. Most tolerant species are also amenable to even-aged
silviculture.
References
Dawkins, H.C. (1958) The management of natural tropical high forest with special reference to
Uganda. Imperial Forestry Institute paper 34, Oxford.
Matthews, J.D. (1989) Silvicultural systems. Clarendon Press, Oxford.
Newman, A. (1990) Tropical rainforest: a world survey of our most valuable and endangered habitat
with a blueprint for its survival. New York, USA; Facts on file 256 pp. ISBN 0-8160-1944-4.
Peterken, G.F. (1981). Woodland conservation and management. Chapman and Hall, London.
Poore, D. (Ed). (1989). No timber without trees - sustainability in the tropical forest. Earthscan
Publications Ltd, London. 252 pp.
Pryor, S.N. (1985). An evaluation of silvicultural options for broadleaved woodland. D.Phil. Thesis.
Department of Forestry, University of Oxford.
Rackham, O. (1976). Trees and woodlands in the British landscape. J.M. Dent and Sons, London.
Troup, R.S. (1955). Silvicultural systems. Clarendon Press Oxford. (This is the "classic" book. It
describes all the systems very thoroughly and accurately but examples of their application are now
out of date since the book has hardly been changed since the first, 1928, edition).
Whitmore, T.C. (1990) An introduction to tropical rainforest. Clarendon Press, Oxford.226 pp.ISBN 019-854276-3 (pbk.).
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