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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