Thinning - the management of competition
between trees
Once established, individual plants on a site will both support
and compete with each other. As the trees grow up together,
what began as welcome shelter from the elements may very soon
turn into competition for limited resources. The species composition,
spatial arrangement and the impact of natural, accidental
or managed interventions will determine how these interactions
play out over time. Being able to manage the positive and
negative interactions that occur between individual plants
within a forest is possibly the most important aspect of silviculture.
Competition can have both a positive and a negative effect
on tree growth and wood quality. In young plantations a dense
forest encourages rapid tree growth by suppressing weeds and
providing mutual shelter from strong winds. However, as these
trees continue to grow they begin to compete for light and
moisture and slow each other's growth. Although tree stocking
(stems/ha) is commonly used to describe the level of competition,
it is limited because it does not take account of the size
of the trees. A more useful measure of competition is the
basal area.
Tree
and Forest Measurement
Basal area is the cross-sectional area of all tree stems at
1.3m height per hectare and is directly related to the volume
of timber. An appreciation of how forest trees behave under
varying degrees of competition can be drawn from trials where
the same species has been planted at different stocking rates.
Inter-tree competition plays a powerful role in determining
tree diameter and stand volume growth in plantations of eucalypt,
poplar and pine. Similar relationships could be presented
for many other tall forest species. From these example and
others we are able to make some general comments about the
effect of competition of tree growth.
There is a great difference in the degree to which competition
effects different species. Tolerant trees, such as the native
and exotic pines, can form dense narrow canopies that allow
forests to reach high levels of competition before individual
tree growth is suppressed. Many of the hardwoods, including
the eucalypts and teak, are much less tolerant. The lower
shaded leaves of the eucalypts die as the competition increases,
leaving only a small amount of canopy on each tree to sustain
growth.
The basal area in a young eucalypt plantation commonly increases
rapidly until competition between the trees themselves slows
diameter increments thereby limiting basal area growth. The
basal area of eucalypt plantations in Australia appears to
reach a natural limit of around 50m2/ha even on the best sites.
On sites with medium to low rainfall, shallow soils or low
fertility, the maximum basal area for a young eucalypt plantation
may be closer to 20m2/ha. If left untended, any further growth
in diameter in the dominant trees must be offset by the death
of suppressed trees. By contrast, unthinned pine plantations
grow slower in the early years but can achieve basal areas
as high as 100m2/ha on high quality sites. Because pines are
more tolerant of competition it is not unusual for them to
yield twice the volume of timber at harvest than the native
eucalypt forest they replaced.
Another explanation for the low competition tolerance of eucalypts
is the fact that they have naked leaf buds that are susceptible
to damage during early growth. If the tree crowns rub against
each other as they sway in the wind, the buds can be lost.
This is why it is very uncommon to find the canopies of eucalypts
interlocking in the way that is common in pines or cypress
trees. Other native hardwoods, including the rainforest cabinet
timbers, and the introduced deciduous species, such as the
oaks, walnuts etc., vary in their tolerance to competition.
Although some rainforest species are able to persist in low
light environments, they may still need to be widely spaced
to achieve rapid diameter growth.
The influence
of competition on tree growth and form
Written by Rowan Reid, Marina Hurley and Peter Stephen. Melbourne
University, Australia
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