Silviculture and the replacement of trees

10.3.1 Silvicultural systems

Silviculture treats forest trees as crops which are established, tended, harvested and then replaced by others. Essentially it is that part of forestry which involves an understanding of how trees modify, and are influenced by, the ecosystems in which they live. A silvicultural system encompasses (a) the regeneration of trees, (b) the form of the crop produced and (c) the orderly arrangement of the crops over the forest as a whole. The main groups of forest system described below have many variants; all produce woodlands of distinctive character. Choice of silvicultural system is particularly critical where soil and avalanche protection are involved. Well-established forest industries, particularly in Europe, are usually based on the concept of the normal forest, from which the same quantities of timber and other forest products are taken every year or period of years. The complete succession of age classes in such forests is so balanced that as each class matures and is harvested, it provides a yield similar to that of the classes which preceded it. Regular and sustained, rather than intermittent or spasmodic yields are highly desirable in theory, but problems can arise when the market is saturated with cheap timber from abroad or that salvaged after great storms. There are also pressures to fell more than usual if the value of timber is especially high.

Very many variants of the main silvicultural systems have developed in response to the needs of particular markets and local forestry conditions; these should be carefully considered before new forests are established or old ones radically altered. Matthews (1989) gives a detailed assessment of how they have been employed in both temperate and tropical areas, but does not deal with continuous cover forestry as such (Section 10.3.3). An unusual, but very effective, system used only in South-East Asia, is the Malaysian Uniform

System (MUS). It takes advantage of the mast fruiting of the Dipterocarpaceae (Sections 4.4.3-4), a family containing many of the most valuable Asiatic timber trees. Devised before World War II, its basis is the felling of the over-storey (harvest of mature trees) at a time when there are many dipterocarp seedlings and saplings present on the forest floor. These will have resulted from one of the irregularly produced very heavy mast years found in this family. Once released by the increased light resulting from felling, they grow away rapidly and give rise to the type of forest desired despite the presence of many other less desirable tree species.

10.3.2 Traditional management: coppicing and pollarding

Many hardwood trees and a few conifers develop from shoots which arise at the base of a cut stem; indeed coppicing and its variants are the most ancient form of silvicultural system. Using the methods shown in Fig. 10.7, workmen in much of Europe maintained stands for several centuries, often without the use of seedlings. In contrast, natural woodlands contain mainly maiden trees, which have arisen directly from seed. In the UK hazel coppice and tall oaks formed the most characteristic coppice-with-standards. Distinction was made between the wood produced by the coppice and the timber produced by the standard trees; the peasants often had rights to the wood while the timber belonged to the landowner. Storing is the process by which coppice is returned to high forest by removing all but one pole from each coppice stool. New trees of aspen Populus tremula, American beech Fagus grandifolia and most species

Figure 10.7 Production of wood from trees. Left to right: coppice stool above ground (e.g. ash); coppice stool below ground (e.g. hazel); clone of suckers (e.g. elm); stub on boundary bank; pollard; high pollard; shredded tree. The left-hand side of each has just been cut; the right-hand half is fully regenerated and is about to be cut again. Note that the area of a coppice system cut in any one year is called a cant, hagg or fall. (From Rackham, 2003. Ancient Woodland; its History, Vegetation and Uses in England. Castlepoint Press.)

Figure 10.7 Production of wood from trees. Left to right: coppice stool above ground (e.g. ash); coppice stool below ground (e.g. hazel); clone of suckers (e.g. elm); stub on boundary bank; pollard; high pollard; shredded tree. The left-hand side of each has just been cut; the right-hand half is fully regenerated and is about to be cut again. Note that the area of a coppice system cut in any one year is called a cant, hagg or fall. (From Rackham, 2003. Ancient Woodland; its History, Vegetation and Uses in England. Castlepoint Press.)

Cloning Ash Tree

Figure 10.8 Above: Stump of common oak Quercus robur showing early coppice growth at Chaddesley National Nature Reserve near Kidderminster, UK. This is one of the few reserves run by Natural England (the English government body responsible for nature conservation) on semi-commercial lines. Right: Stand of American beech Fagus grandifolia which has arisen from suckers, Massachusetts, USA. (Photograph above by John R. Packham, and right, by Peter A. Thomas.)

Figure 10.8 Above: Stump of common oak Quercus robur showing early coppice growth at Chaddesley National Nature Reserve near Kidderminster, UK. This is one of the few reserves run by Natural England (the English government body responsible for nature conservation) on semi-commercial lines. Right: Stand of American beech Fagus grandifolia which has arisen from suckers, Massachusetts, USA. (Photograph above by John R. Packham, and right, by Peter A. Thomas.)

of elm Ulmus spp. arise from genetically identical suckers produced by the roots (Fig. 10.8); the stumps normally die.

Many trees are effectively rejuvenated by coppicing. Ash, alder, maples, wych elm, oaks, sweet chestnut, bass and lime (Tilia spp.) and the tulip tree Liriodendron tulipifera are coppiced in temperate parts of the world, as are eucalypts and teak Tectona grandis in the tropics. Few conifers coppice; coastal redwood Sequoia sempervirens, canary pine Pinus canariensis and the monkey

Coppicing Alder Trees
Figure 10.8 (cont.)

puzzle tree Araucaria araucana being important exceptions. Coppicing of euca-lypts by people, although comparatively recent, is an important development. Rackham (2003) describes an area in New South Wales where eucalypts, mainly silvertop ash E. sieberiana, had recently been cut for the third time. These three rotations of felling and regrowth had converted eucalyptus forest into an ecosystem very similar to a coppice-wood with its own native coppicing flora. Bluebell (Fig. 10.9) is a characteristic member of such a flora in the coppiced oakwoods of southern England, forming sheets of vivid blue in spring.

Why Rain Forests Are Important

Figure 10.9 Bluebell Hyacinthoides non-scripta, an important member of the shade flora of coppiced woodlands. This shade-evading bulbous geophyte (main growing point below ground) has a pointed shoot that can pierce thick litter and expand rapidly on reaching the light. The black seeds germinate at the surface; as the bulbs develop they form contractile roots that draw the bulbs downwards. The rarity of bluebell on the chalk scarps of south-east England appears (Blackman and Rutter, 1954) to be due to the rapid drying out of the thin surface layer of soil and the physical barrier of the underlying chalk (which prevents the bulbs being drawn down into a deeper and moister zone), rather than to the high pH and calcium content of the soil. (From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Figure 10.9 Bluebell Hyacinthoides non-scripta, an important member of the shade flora of coppiced woodlands. This shade-evading bulbous geophyte (main growing point below ground) has a pointed shoot that can pierce thick litter and expand rapidly on reaching the light. The black seeds germinate at the surface; as the bulbs develop they form contractile roots that draw the bulbs downwards. The rarity of bluebell on the chalk scarps of south-east England appears (Blackman and Rutter, 1954) to be due to the rapid drying out of the thin surface layer of soil and the physical barrier of the underlying chalk (which prevents the bulbs being drawn down into a deeper and moister zone), rather than to the high pH and calcium content of the soil. (From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Pollards are trees whose trunks have been cut between 1.8 and 4.5 m above the ground. Old trackways in ancient woods are often marked by lines of them in the UK. They were also used where deer and cattle could browse young shoots from freshly coppiced trees, but were unable to reach shoots at the top of the remaining trunk (boiling - rhyming with rolling) of a pollard. Stubs are very short pollards 1.2-1.8 m high often found at land boundaries; margins of medieval woods were often marked by banks with stubs and can sometimes be traced long after the woods have gone. Shredding is a peculiar practice is which the side branches are repeatedly cut off leaving a tall trunk with a tuft at the top. Some trees managed in this way remain in France, Italy and Africa, though not in Britain.

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