22 Jul2007

Overstory #192 - Thinning for wood production

Written by K. W. Cremer and A. G. Brown.

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

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.
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.
Controlling the size and quality of knots in the stem wood by controlling the development of the branches.
Reducing the length of the rotation by boosting the growth of the remaining trees, and thus achieving saleable sizes in reduced time.
Obtaining products or financial returns when required before the harvesting of the main crop.
Promoting resistance to damage by wind and snow.
Promoting health, by removing trees that are susceptible to infection and by reducing stress due to competition.
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.
Increasing the yield of water from a catchment by reducing the water consumed by the tree crop.
Improving appearance, amenity and wildlife values.
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.

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.

About the authors

Kurt Cremer was a forest scientist. He graduated in 1957 from the Australian Forestry School in Canberra and gained an MSc degree from the University of Tasmania. He started his research in the Tasmanian forest service and retired in 1987 as Principal Research Scientist with CSIRO Division of Forest Research in Canberra. He published over 50 papers in silviculture, forest ecology, fire ecology, storm damage and, in the latter part of his carreer, on the natural spread of willows in Australia's streams. Kurt passed away in June 2005.

Alan G. Brown was Chief, CSIRO Division of Forestry, Canberra. He graduated from the Australian Forestry School in 1951 and later received an MSc from Sydney University. He lectured in silviculture at the Australian Forestry School. His research interests were in silviculture and tree breeding, and he was a member of the boards of IUFRO and CIFOR. He was a principle author of the book Growing Trees on Australian Farms and editor of Eucalypts for Wood Production, and is now Production Editor for Australian Forestry.