The Overstory
The Overstory #192--Thinning for wood production
25th July, 2007
The Overstory #192--Thinning for wood production
INTRODUCTION
In this article we consider thinning of tree stands as a means to
improve financial returns or achieve other management objectives of the
wood-production enterprise. Thinning for wood production generally
implies the selective removal of immature trees for specific aims -
mostly to remove the less desirable trees and concentrate the site's
potential growth on the trees of best form and vigour.
We refer specifically to even-aged stands, such as plantations and some
native forests regenerated by fire. The principles, though more complex,
apply also to stands of uneven age, such as most of our native forests.
Next to establishing the stands in the first place, thinning is the main
tool for intensive forest management.
The need for thinning can be reduced by establishing fewer trees of
higher quality in the first place, but some thinning will generally
still be necessary under intensive management because, even with the
best pedigree, not all of the trees planted succeed in becoming trees of
desirable quality. In any case, the crop trees have to be nursed' by
adjacent and eventually superfluous trees to control the development of
branches (unless they are pruned. Thinning can be avoided altogether in
crops grown for pulpwood only.
The special problem of growing trees at very wide spacing is considered
separately under agroforestry. The profitability of thinning also
depends on harvesting techniques.
AIMS OF THINNING
These can be various and have numerous components:
o Culling - removal of trees that are undesirable (because of poor
vigour or poor form, or because they are the wrong species) in order to
reduce competition with the more desirable trees and thus boost their
future growth and quality.
o Salvaging - harvesting of trees that would die (from suppression,
disease, insects, or injuries) and thus make use of wood that would
otherwise be lost to production.
o Controlling the size and quality of knots in the stem wood by
controlling the development of the branches.
o Reducing the length of the rotation by boosting the growth of the
remaining trees, and thus achieving saleable sizes in reduced time.
o Obtaining products or financial returns when required before the
harvesting of the main crop.
o Promoting resistance to damage by wind and snow.
o Promoting health, by removing trees that are susceptible to
infection and by reducing stress due to competition.
o Regulating competition, not only with the crop trees, but also with
the undergrowth, e.g. to improve grazing, natural regeneration, the
diversity of the vegetation or control of weeds.
o Increasing the yield of water from a catchment by reducing the
water consumed by the tree crop.
o Improving appearance, amenity and wildlife values.
o Reducing the fire hazard by removing cull trees and thus making
controlled burning easier.
Thinning on farms is usually done for a combination of the above aims.
VOLUME AND VALUE OF PRODUCTION
Gross volume of production increases with increasing numbers and sizes
of trees until the site is fully utilised. This occurs when the amount
of foliage per hectare is not increased by having more trees per
hectare. Additional stocking of a given species will have little or no
further effect on gross production, except in the few species (such as
white cypress pine, Callitris glauca) that sometimes 'lock-up', i.e.
virtually stop growing at extremely high stockings.
Unfortunately, full stocking is not easily recognised. It varies not
only with tree age, as above, but also with site quality. Basically,
growth depends on the amount of solar energy intercepted, and hence on
the amount of foliage per hectare. Each site is able to support a
certain maximum amount of foliage, and this corresponds to maximum gross
production.
Since thinning removes foliage, it tends to reduce gross volume of
production. The reduction is, however, minimal if the foliage removed
was largely shaded and if the remaining canopy soon returns to maximum
density. In fact, removing up to half the trees in dense pine or
eucalypt stands usually has little effect on gross wood production.
While thinning usually cannot increase gross volume of production, it
can boost net volume production. It can do so in several ways. Firstly,
thinning may serve to harvest trees that would otherwise die from
suppression. We may plant 2000 trees per hectare, but only 200 of these
may survive to maturity. Secondly, tree stems are usually saleable only
when they are larger than a certain minimum diameter at the small end,
say 100 or 200 mm, depending on the market. Thinning may therefore boost
the volume of saleable timber by stimulating the diameter growth of the
remaining trees. Thirdly, thinning boosts the growth rate of trees of
best quality and hence highest value by removing the trees of poor form.
What counts in the end is the net value of production. This is
considered below. The most profitable thinning regime depends as much on
the nature of the species and its site as it does on the nature of the
markets available for the products.
In reducing the number of trees per hectare, thinning concentrates the
site's potential growth on fewer trees and greatly affects each of the
remaining trees' future size and quality.
--> Size of crown, knots
The most obvious effect of amount of growing space on tree development
is the effect on the size of the crown. Contrast the shapes of trees
grown in the open with those grown in dense stands. Height growth of the
dominants is almost unaffected, except at extremely low and extremely
high stocking. But both the width and depth of the crown increase with
increasing growing space: the lowest branches tend to remain alive and
all branches grow longer and thicker.
Each branch means a knot of corresponding size in the wood of the stem.
Knots become increasingly undesirable as they increase in diameter,
especially above 2 or 3 cm. Knots formed while the branches are alive
are held tightly within the wood. Those formed after the branch has died
(but remains attached) are loosely held and may even include bark. Such
loose and 'bark-encased' knots are the most undesirable. Knot-free
timber (outside an inevitably knotty core) is produced if the branches
are pruned, or are shed naturally (as is common with eucalypts). The
next best thing is small tight knots, which is achieved by regulating
the growing space so that the branches remain alive but do not grow too
thick. In conifers it is desirable to maintain a long green crown of
thin branches, unless pruning is undertaken.
It is also desirable to keep the crowns symmetrical by allowing enough
growing space all round them. Asymmetrical crowns may cause the trees to
lean, to develop reaction wood and become more liable to damage by snow
and possibly also by wind. Tall trees may abrade and damage each other's
crowns when they sway in the wind. Thinning should thus also aim for
even spacing.
A major aim of thinning is culling: removing individuals with double
leaders, with excessively steep and large branches, with leaning or
injured or curved stems, and with generally poor vigour and health.
Excessive thinning, which unduly exposes the thin-barked portions of
stems to the sun, sometimes results in 'sun-scald' or epicormic shoots.
In radiata pine, large patches of bark may be killed by the 'scalding'.
Epicormic shoots may arise in pines from any bundles of green needles
still present on the exposed stem (mainly after high pruning). In
eucalypts (particularly mountain ash, F. regnans) new shoots may arise
from bud tissues within the bark. This tendency is least in trees less
than 20 years old. Such shoots will, of course, result in additional
knots, and may cause gum veins in eucalypts.
--> Diameter and taper of stems
Because growing space has little effect on height growth but much effect
on diameter growth, it greatly influences stem taper - how rapidly
diameter decreases with height along the stem. Minimising taper has been
one of the main aims in Europe, where conifers are typically grown in
very dense stands so that the green crowns are confined to the very tops
of the trees.
Stems taper mainly for two reasons: to provide conduits to their green
branches and a cantilever to support their crown (and to do so
economically). The stem tapers most strongly where its biggest live
branches are. At each set of branches the stem's cross-sectional area of
sapwood tends to change by the sum of cross-sectional areas contained at
the base of those branches. Small live branches mean less stem taper,
and dead branches or no branches mean least taper. The lowermost part of
the stem is again more strongly tapered in the 'butt-swell' region,
probably to anchor the cantilever. The portion between the butt-swell
and the base of the green crown tapers least, but enough to provide a
cantilever of uniform bending resistance, depending on the size of the
crown and its exposure to winds.
A fairly rapid stem taper is unavoidable if large stem diameters and
high resistance to wind and snow damage are desired at an early age.
Gross taper is avoided by pruning, or by spacing regimes that keep the
lower branches small.
--> Wood quality
Usually the most important features determining the value of a pine log
are its diameter, its straightness and the size and quality of its
knots, all of which are strongly influenced by thinning. Presence of
juvenile wood and reaction wood are also significant; the effect of
thinning on these is indirect and relatively unimportant. Thinning also
has some adverse effects on the properties of the wood itself. These
changes are not critical for radiata pine in Australia, though they have
been with some other conifers in New Zealand and England.
Thinning increases diameter growth, and therefore the width of the
growth rings. Generally this leads to reduced density and strength
because the proportion of earlywood is increased. In Western Australia's
mediterranean climate, however, thinning prolongs growth into the dry
season and hence increases the proportion of latewood. Thinning also
keeps the lower branches green for longer and may thus delay the natural
transition from juvenile wood to the more desirable mature wood. Cutting
the green branches off the lower stem hastens this natural transition
towards mature-wood production.
The timber of eucalypts is liable to growth stresses (and hence
splitting) in logs of small diameter. Thinning has been found to reduce
this problem by the stems being able to grow to larger diameters in a
given time.
--> Price gradients for size and quality
The value of a log tends to increase with diameter because larger logs
are usually relatively cheaper to harvest, to transport, and to convert
into sawn and other products. When sawing large logs, less wood is
wasted in the conversion, and the product obtained is usually more
valuable, e.g. wider boards. The extent and steepness of these price-
size gradients depend on the machinery available for conversion and the
prices obtained for the end product in the available markets.
Note that the best logs are many times more valuable, per cubic metre,
than the poorest logs. It is the steepness of these gradients that
largely determines the value of production, the benefits of thinning and
pruning, and the profitability of the whole silvicultural regime. Where
stumpage for pulpwood is high, compared with other classes of timber, it
may be most profitable to manage the plantation on short rotations
without any thinning.
Another major factor in profitability is the timing of income, because
time costs money in terms of interest paid or foregone. An important aim
of thinning is thus to shorten the rotation by accelerating the growth
of the crop trees so that they reach optimum sizes sooner, especially
when interest rates are high.
ECOLOGICAL EFFECTS
--> Health
Removing trees that are dying of suppression or disease can reduce the
chance of disease organisms becoming established in these and then
spreading to the stronger trees. For radiata pine this is true for the
wood wasp Sirex noctilio in much of Australia, and for the bark beetle,
Ips grandicollis in some areas. More intensive thinning can
substantially reduce competition, increase the vigour of the remaining
crop trees, and reduce the chance of dieback or death due to drought and
associated infection with the fungus Diplodea pinea. It is thought that
by increasing the separation of trees, thinning can also reduce the
spread of needle blight, caused by the fungus Dothistroma pini. Reducing
competition by thinning or weeding should always be considered when
trees of any age seem to be lacking in health or vigour. Careless
thinning operations, on the other hand, can seriously damage the butts
and even the crowns of the remaining trees and thus expose their stems
to infection and decay.
--> Stability
The resistance of trees to damage by wind and snow is very much
influenced by current and past thinning regimes. Maintaining tree
stability is an important aim of thinning practice. The immediate effect
of thinning is to make trees more vulnerable to wind damage. Therefore
particular care has to be taken when thinning overstocked stands of
slender trees, especially if these stands are located downwind of a
recent clearfelling. In the longer term, the provision of ample growing
space for each tree from an early age is the main way of developing and
maintaining stability.
--> Undergrowth
The denser the tree canopy, the more suppressed the undergrowth. This
principle is often used to help to control weeds such as blackberries,
or coppice of undesirable trees. Conversely, wider spacings may be
adopted to encourage the production of pasture for grazing. Thinning a
few years before the final harvest can also serve to encourage the
natural regeneration of crop trees.
Growing trees at wider spacings can also increase the water yield from
certain catchments and enhance the diversity of the vegetation to
improve wildlife, landscape and recreational values.
THINNING PRACTICE - TYPES OF THINNING
--> Culling and salvaging
Thinning can serve to remove the trees that are undesirable because of
various defects and to utilise those trees that would otherwise die from
suppression or disease. The trees may be 'thinned to waste', i.e.
'thinned non-commercially' and left to decay, or 'thinned commercially'
and sold.
Ideally, each thinning should pay for itself and make a profit as well.
In practice it often pays to thin non-commercially, even if the wood is
saleable. The cost of harvesting small stems may be too great,
especially where the terrain is steep or obstructed by undergrowth,
rocks or old logs and stumps. The decision to harvest the felled trees
also depends on how much damage is done during their removal to the
butts of the standing trees. More than half of the crop trees may be
scarred if the operation is done carelessly. In radiata pine modest
scars rarely lead to serious decay or attack by borers. The problem is,
however, more serious with some eucalypts, and may lead to significant
degrade of the potentially most valuable part of the tree.
In even-aged eucalypt forests, especially in naturally regenerated ones,
it is often not feasible
to do a first thinning on a commercial basis. On farm forests some
thinning may be done to obtain fence posts, but most thinning would be
noncommercial because markets for small timbers are mostly poor and the
cost of logging small areas tends to be high. First thinnings may be
commercial where there are good markets for pulpwood, mining timbers, or
posts.
--> Crown type
'Thinning from below' or 'low thinning' aims to cut out the less
vigorous trees of a stand mainly in order to benefit the better
remaining trees. A more single-minded approach to favouring the future
crop trees is to identify these crop trees and to remove only those of
the other trees that offer excessive competition, irrespective of their
size. The idea is to provide ample crown space in which the selected
trees may develop vigorously. This crown thinning' is particularly
appropriate in many-aged forests. Many of the most suppressed trees are
simply left standing because the competition they offer is not serious.
Thinning may also be 'from above', by removing some of the largest
trees. This may be done to make the thinning itself more profitable, or
to favour the intermediate trees with smaller limbs as future crop
trees. Mostly, however, this entails too much sacrifice in growth and
vigour. Once trees are suppressed or old they become less able to
respond to thinning: their crowns are slow to expand. The growth of the
remaining stand is depressed.
Usually something from each of these approaches to thinning enters into
the treatment actually employed, but the emphasis is mostly to cull, to
thin from below and to favour the crop trees by giving them ample and
reasonably evenly distributed growing space.
--> Row thinning or strip thinning
In 'row thinning' or 'mechanical thinning' all trees in a row are
removed irrespective of their merit. In a plantation, it is usual to
remove every fifth to twelfth row at the time of first thinning to allow
logging machinery access to the stand. The intervening 'bays' are then
thinned selectively. The most extreme form of mechanical thinning is to
remove every second row. This is done to make the thinning cheaper, and
to increase the immediate returns by also harvesting larger trees. A
sensible compromise, when using a harvesting machine that can reach
sideways into the adjacent row, is to remove every third row, and to
thin the other two rows selectively. Many modern harvesting machines can
reach beyond the adjacent row and selectively thin from outrows at every
fifth or even seventh row depending on row spacing.
The outcome of row thinning is most favourable if the original trees are
of uniformly high quality and are not yet seriously suppressed, so that
plenty of trees of good form and vigour can in fact be selected for
retention, even after one-third of the forest is removed
indiscriminately. If these conditions are not met, row thinning tends to
result in poorer growth and poorer tree quality than selective thinning.
Row thinning is less appropriate in small farm forests than in the
larger industrial forests, where pulpwood production is a major aim.
In naturally regenerated forest, e.g. eucalypt or cypress pine, it can
be appropriate to thin mechanically by 'strip thinning', i.e. by
removing trees in strips, using a dozer, and then perhaps thinning out
the remainder selectively.
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FURTHER READING
F.A.O. (1979). Eucalypts for Planting. F.A.O. Forestry Series No. 11,
F.A.O., Rome (677 pp.).
Hillis, W.F. and Brown, A.G. (1978). Eucalypts for Wood Production.
CSIRO, Australia (434 pp.).
Institute of Foresters of Australia (1987). Forest Management in
Australia. Proceedings of the IFA Conference in Perth, 1987.
New Zealand Journal of Forestry Science (1982). Special issue devoted to
Economics and Techniques of Thinning Plantations in Australia and New
Zealand. N.Z.J. For. Sci., Vol. 12, No 2.
Lewis, N.B., Keeves, A. and Leech, J.W. (1976). Yield Regulation in
South Australian Pinus radiata Plantations. Woods and Forests Dept,
&.A., Bulletin 23 (174 pp.).
Shepherd, K.R. (1986). Plantation Silviculture. Martinus Nijhoff Publ.,
Dordrecht (322 pp.).
Smith, D.M. (1962). The Practice of Silviculture. John Wiley and Sons,
New York (578 pp.).
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ORIGINAL SOURCE
This article was adapted with the kind permission of the author and
sponsoring organization (CSIRO) from:
Cremer, K.W., and A.G. Brown. 1990. 'Thinning for wood production' In:
Cremer, K.W. (Ed), Trees for Rural Australia, Inkata Press, Melbourne,
Australia.