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The Influence of competition on tree growth and form
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The influence of competition on tree growth and form

Competition and height growth
The degree to which competition affects the form of trees can be critical where is it necessary to grow straight trees or control branch size. Many plantation species, including eucalypts and softwoods commonly grown for timber, have strong apical dominance. This means that most of the growth expansion of a seedling or sapling is concentrated in the uppermost bud, allowing trees to grow tall and straight even when planted in the open. Others, like many of the rainforest species, have low apical dominance and will tend to grow broadly like an apple tree if sidelight is not controlled. In this case maintaining a sufficient level of competition to encourage reduced branching and straight growth may be essential.

Tree density also influences tree form and height. Increasing the initial stocking rate (stems/ha) of a plantation can lead to an increase in tree height, although this reaches a limit. Above this point height growth remains fairly constant, even with a trebling of the stocking rate. Research suggests that the loss of height growth at low stockings is largely the result of excessive exposure.

Dry winds in particular, can damage the growing tips and stunt growth. With adequate mutual shelter, the height of the healthy dominant trees within the plantation is surprisingly even. Even on a tree with a large open canopy the elongation of the leading shoot is driven by only the small cluster of leaves at the very top of the tree. This allows height, rather than basal area or volume, to be used as an effective indicator of site quality that is largely independent of stocking. The more productive the site, the taller the plantation will be at a particular age, irrespective of spacing. Research suggests that plantation height increases with increasing soil depth, humidity and rainfall. Farmers may be able to gauge the quality of a site for tree growth from the height of the remnant native forest.

Competition and diameter growth
Once site resources, particularly light and moisture, become limiting, any increase in competition will lead to a direct reduction in the size or efficiency of the individual tree canopy. As a result, the amount of sugars produced by the leaves and fed down the branches and trunk for cambium growth will be reduced. This results in reduced diameter growth. Increasing the basal area above 5 m2/ha in a young eucalypt plantation can cause a dramatic drop in the annual diameter increment. To maximize diameter growth sufficient trees must be initially established to allow mutual shelter to promote healthy growth. Then, when the trees grow, the forest must be thinned to reduce competition. Repeated thinning to avoid excessive competition while maintaining mutual shelter will allow the trees to maximise height and diameter growth.

The area of sapwood in a tree is related to the volume and health of its canopy and is therefore related to the level of competition. Basal area is a measure of the total cross sectional area of the forest and therefore includes both the heartwood and sapwood. Because of this, it is not correct to assume that a basal area of 10m2/ha, for instance, implies the same level of competition in plantations of different ages.

For example, to maximise diameter growth in a eucalypt plantation, the first thinning may reduce basal area to less than 5m2/ha, the second thinning to 10m2/ha and the third to 15m2/ha. The difference in basal area would be the result of the increasing area of heartwood in the larger trees. A useful guide is to thin to a third of the basal area of a fully stocked forest of the same age and species on that site. If some level of competition is required to control form or branch development then thin to half the maximum basal area.

Farmers can use unthinned eucalypt or pine plantations growing on similar soil types in their area as an indication of the basal areas of fully stocked stands. Better still, farmers can establish measurement plots in their own forests and monitor diameter growth over time. When they notice the diameter growth falling they can determine the corresponding basal area and make a judgment of the extent of thinning required.

Competition and volume production
Although individual tree diameter declines with increasing competition, the total volume of wood on the site increases. If the object is only to maximise the volume of timber, as for pulpwood or fuelwood, then the higher the stocking rate, the greater the yield. This is why pulpwood plantations are established at over 1000 trees per hectare (<3x3 metre spacing) and left un-thinned until they are harvested. The rate of volume production is greatest when the canopy of the young plantation first reaches its full potential. As trees continue to grow, competition then increases and the rate of volume production may decline.

Eventually, the plantation reaches a maximum volume for the site. At this point any further growth of individual trees can only be possible if other trees die or are removed. Currently many unmanaged native forests are at this point, as demonstrated by repeated measurements of total volume, showing no change over time.

Stability is a common problem in dense stands. Tall, thin trees are susceptible to toppling or bending in strong wind, particularly if the soils are prone to waterlogging. Even where the trees do not fall over, the swaying in the wind has been shown to increase the formation of tension wood in the butt log of eucalypts, affecting both sawn timber and pulpwood quality. Other problems common in dense plantations include increased difficulty of harvesting, lack of light to support understorey plants, and an increased susceptibility to drought, insects and disease.

Competition and branch development
Depending on the product specifications and the species involved, shading of the lower branches may be sufficient to control branch size or even encourage self-pruning. However, because competition reduces the size of the canopy it will also lead to a reduction in diameter growth. If trees do not naturally cast off branches soon after they die, the dead branches may remain in the new wood forming loose black knots and provide an access point for rot. In this case the branches would still need to be manually pruned to produce clearwood. Even where the trees are to be manually pruned, encouraging smaller branch size by allowing some competition may make pruning easier.

Epicormic shoots are new branches that form on the main stem and can be initiated by heavy thinning in forests where the competition has previously been high. Epicormic shoots come from dormant buds held in the cambium. Under normal growth these buds are held in a dormant state by hormones produced in the canopy. If there is insufficient capacity in the existing canopy to grow more leaves in response to thinning, then epicormic shoots may the initiated.



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