During the twentieth century large-scale high forest management normally produced crops of seedling origin, almost invariably employing variants of clear cutting, successive regeneration fellings, shelterwood or selection systems. Tree seedlings were often produced in enormous numbers in forest nurseries. In the last two decades, however, there has, in Europe at least, been a considerable movement towards continuous cover forestry where the aim is a much more natural, and often more species-rich forest, in which harvesting is gradual, natural regeneration is important, and the amount of bare ground at any one time relatively low.
With clear cutting the whole crop is cleared by a single felling and is even-aged. Stands within a forest may be felled at different times creating large open areas within the forest. Systems of successive regeneration fellings involve the crop being cleared by two or more successive fellings on the same area; in this case the crop may be somewhat uneven-aged. Uniform systems of this type involve an even opening of the canopy by removing a percentage of the trees. In group systems the canopy is opened by scattered gaps. Fellings may be confined to particular strips (strip systems) or wedges (wedge systems). Where the opening of the canopy is irregular and gradual the young crop in the resulting shelterwood system is somewhat uneven-aged. Two accessory systems are sometimes employed. These are two-storied high forest where a fresh young crop is planted beneath an existing immature one, and high forest with reserves in which a proportion of the old crop is retained after regeneration is complete. The rapidly developing practice of agroforestry is considered in Section 11.6.1.
When shelterwood systems are established the existing trees are harvested on two or more occasions, allowing seedlings which will form the future forest to develop before the canopy above recloses completely. The first stage is to remove inferior trees so that seeds falling on the forest floor are from good quality parents. If all goes well and some good seed years ensue, the resulting seedlings make rapid progress and many of the parent trees can be felled, thus providing an economic return while opening up the stand. The last of the mature trees are removed when the now transformed forest is well established. If natural regeneration is patchy, commercial seedlings may have to be employed. This is particularly the case in the extreme variation known as the seed tree system, where very few parent trees are left. These retained trees offer very little protection to the young seedlings that may be rapidly overgrown by weeds, and are themselves vulnerable to windthrow.
Selection systems come into their own where scenic beauty is important, or when the forest is on steep slopes since it helps prevent soil erosion or avalanches. They involve continuous felling and regeneration throughout the area so the crop is irregular i.e. wholly uneven-aged. The aim is to retain a balance between trees of different sizes, and often of different species. Having reached an appropriate balance the amount of timber felled each year should ideally be equal to the net annual increment. Forest structure remains broadly similar for long periods; unthrifty trees are felled along with others of various sizes. Gaps in the canopy of protection forest using this system close quickly, while the soil is never liable to severe erosion. Trees in such forests are also wind-firm, and it is best if their seedlings are shade-tolerant (as are those of hornbeam Carpinus betulinus, beech Fagus sylvatica and limes Tilia spp. in the UK) if they are to develop under such conditions. Shade-intolerant species (such as the British oaks Quercus petraea and Q. robur and Scots pine Pinus sylvestris) are best grown in shelterwood systems with cuts up to 5 ha in size. There are situations, however, when shade-intolerant species can be grown in selection systems, but then the trees must be particularly well suited to the site and the canopy more open. Useful though selection systems are, they demand a great deal of skill on behalf of both management and workers and the scattered logs are difficult to collect even with mechanized skidders which drag logs to the roadside.
The transformation of even-aged woodland to a continuous cover system, a modern concept which has much in common with a selection system, is not a simple matter; the aim is to reduce the influence of shade and root competition in some parts of the stand to a point where regeneration can commence (Helliwell, 2002). This system, defined by Mason et al. (1999) as the maintenance of the forest canopy without clear felling areas greater than 0.25 ha, involves a great deal of judgement on the part of the forester who marks the trees to be cut down. If felling is done at 5-yearly intervals, regenerating 12.5% of the total area on each occasion would in theory make it fully uneven-aged in 35 years. Some early attempts at such transformations in Sweden were based on the creation of regularly distributed gaps of fairly uniform size; these clearings attracted elk and other deer with the result that due to selective browsing most of the new trees were birch or willow rather than the pines or spruce which were needed. Even if new seedlings arise naturally or are planted, after the first 10 or 15 years the older parts of the forest gradually become fragmented so finding space for new gaps becomes increasingly difficult. It becomes virtually impossible to proceed further on an area basis, so the stand has to be treated as if it were fully uneven-aged. In practice it is usually best simply to commence reduction of the standing volume more heavily and less regularly than normally occurs during the thinning of an even-aged forest.
When this is done, as it has been with much greater success elsewhere in Sweden, the range of tree size classes is kept as large as possible and the forest becomes more open in some places without deliberately clearing gaps of predetermined size. It is sensible to keep a check on the developing situation by plotting the number of stems by diameter class although the curve obtained will be very uneven in the early stages of the transformation. Whilst it is highly desirable to maintain the forest by natural regeneration, this may not always be possible particularly as in many trees seed production is irregular (see Fig. 4.9), and it may be necessary to plant seedlings in unfavourable sites or years. The end result of a transformation to uneven-aged forest from a clear cutting or shelterwood system is effectively a form of selection forest.
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