Principles of thinning

Objectives for Thinning

Thinning involves the removal of trees from stands for the purpose of concentrating the potential growth and wood production on a limited number of select trees left after cutting. The total yield of the stand is augmented largely by the utilisation of trees that would ultimately die of suppression, although the value and utility of the final crop may increase by virtue of the fact that the favoured trees grow more rapidly in diameter and to a larger size than they would without thinning. The fundamental objectives of thinning are:

to redistribute the growth potential of the stand to achieve the desired objective, and
to utilise all merchantable material produced by the stand during the rotation.

The objectives of thinning, therefore, are to regulate the distribution of growing space for the advantage of the existing crop and not to create space for the recruitment and establishment of a new crop. The gaps created in the canopy by tree removal in an efficient thinning regime should be such that the remaining crowns can grow into and occupy the newly created space before regeneration or increased understorey development occurs.

(From Smith, D.M., Larson, B.C., Kelty, M.J. and Ashton, P.M.S.(1997). The Practice of Silviculture: applied forest ecology (9th ed)

Principles of Thinning

The process of self-thinning during the stem exclusion stage of stand development will progressively reduce the stocking of even-aged regrowth forests as they develop through sapling and pole stages to maturity. The rate of self-thinning will be greater in the earlier than the later development stages. Where a stand is actively managed for wood production, its stocking may be reduced by thinning, preferably ahead of natural mortality. This may be done in order to achieve a number of objectives:

to remove dead, dying and suppressed trees where pathogens attracted to them may affect stand health as a whole;
to utilize commercially the potential mortality component that comprises stems that would otherwise die through the natural process of self-thinning;
to reduce competition and provide remaining trees with greater access to growing space and soil resources, thus accelerating their diameter growth and reducing the time taken to reach optimum commercial size;
to remove poorer quality trees (e.g. crooked stems, stems with sweep or heavy branching), and in so doing focus the growth potential of the site on better quality trees; and
to provide an early financial return, thus enhancing the economic viability of the investment.

Four questions are traditionally asked in prescribing an appropriate thinning regime when a decision to thin a stand is made:

at what age should the stand be first thinned?
what method should be used?
at what intensity should the thinning regime be (i.e. what proportion of the growing stock should be removed at one time, and how often should thinning be carried out)? and
how frequent should thinning be carried out (i.e. what is the appropriate time interval between thinnings)?

Age of first thinning

The timing of the first thinning will normally be influenced by the early growth characteristics of the species (crown and bole development, branch shedding, development of epicormic growth on the main stem), the effect of stand density on the condition of the trees, the level of competition between trees for space and resources, and the market for small stems. A trade-off between these factors may be necessary in reaching a decision.

Some species, particularly those which are intolerant or very intolerant to shade and competition, have a very rapid early growth phase, with the volume produced annually reaching a peak at a relatively early age. Where such a stand is thinned before the volume production has peaked, there is an opportunity to direct a high rate of volume production to the residual trees, and greatly accelerate their crown expansion and diameter growth. Where thinning is delayed well beyond the age at which peak volume is produced by the stand, there will be some response in crown and diameter growth, but it may be well below that potentially achievable at an earlier age.

The timing of the first thinning may be particularly critical in the case of an intolerant or very intolerant species. It will be desirable to thin the stand before the rise of the green crown seriously affects the green crown ratio (depth of green crown to total tree height), and hence the potential of the tree to respond vigorously to release. Where the species is very fast growing this may be within 2-4 years of crown closure. The timing of the first thinning may be less critical where the species is more tolerant, and the effects of competition on tree condition are less marked.

While too great a loss of green crown by crop trees is to be avoided, there are some species which should not be thinned too early, that is, before there has been some rise in the green crown. This is necessary to ensure a good length of branch-free bole in the case of a self-pruning species, or to restrict the size of the knotty core in others. Where the stand is thinned too early, heavy branching may persist well down the stem and reduce the ultimate value of the bole.

Despite the rationale for thinning before the current annual volume production has peaked, this may tot be possible on economic grounds. It will normally be desirable (or indeed obligatory) under market conditions to delay the first thinning of a eucalypt forest until stems of small diameter can be sold, as pulpwood, posts, case material, roofing battens or other small-dimension products. Where a commercial thinning is not possible, a non-commercial thinning (or a thinning to waste) might be considered where the condition of the final crop trees (i.e. those likely to form the stand at the final harvest) is likely to be seriously weakened by competition. However, this thinning will add substantial costs to forest management, and while economic in the long run, may be justified only where there is a very good reason for it, for example, the need to produce a commercial crop quickly. In some cases the costs of non-commercial thinning may be offset by not having to undertake other silvicultural treatments.

Methods of thinning

Thinning may be carried out in one of four ways.

Low thinning (thinning from below): the less vigorous trees of the stand are removed (e.g. the suppressed, intermediate and weaker co-dominant crown classes); essentially this mimics and speeds up the natural thinning process.
Crown thinning (thinning from above): this removes some of the dominants and co-dominants to `free up' and accelerate the diameter growth of the trees which will be retained to make the final crop. The method can require the early selection and marking of crop trees so that all subsequent operations are designed to favour them. Weaker trees may be left at the first thinning to grow on slowly, perhaps to be removed at a subsequent harvest.
Flexible or selection thinning: trees to be removed are determined by the marker to enhance stand development in a number of ways; this may involve a combination of low and crown thinning.
Mechanical thinning: a row or line thinning approach where, say, every third or fourth row is removed, partially releasing trees in the two intervening rows. This method is widely used where thinning is done by machine. Low or crown thinning may be carried out at the same time within the retained trees.

Given the intolerant nature of eucalypts to shade and competition, and the generally good expression of dominance, particularly on good quality sites, thinning is usually carried out from below. However, such a thinning should be heavy enough to free up and stimulate growth in the better co-dominants of the stand. The co-dominants may be more affected by competition than the dominants, and may respond most to additional growing space. The intensity of thinning needs to be considered, not only in relation to growth responses but also to the growth characteristics of the species.

Intensity and frequency of thinning

Whatever the method used, thinning may vary from light to heavy. A light thinning from below may remove only suppressed and intermediate crown classes, and a heavy crown thinning might retain only those trees selected as final crop trees. The decision on thinning intensity will be influenced by a number of market as well as ecological and biological factors. Where thinning must be done commercially, a minimum volume of thinnings may be prescribed to justify the costs involved and/or to meet a market demand for wood. This might be seen as setting the lower limit of thinning intensity.

The upper limit to the intensity of thinning might be established through an understanding of the effect of thinning intensity on stand volume production and the growth habits of the species concerned. It is now generally accepted that for a given species, site and age, there is likely to be a range in stand density (expressed as stocking or stand density, basal area or standing volume) within which the volume produced will be more or less constant. Where the objectives of management call for the production of as many sawlogs as possible in the shortest possible time, the stand might be maintained, through a series of thinnings, at the lower end of this optimum stand density range or normal stocking range. There would be no loss of volume production under such a regime, and that which is produced will be directed to the smallest possible number of trees, thus enhancing their diameter growth and minimizing the sawlog rotation. Where larger-boled trees attract a higher price per unit volume, or when there is some urgency in producing sawlogs, the stand might even be thinned to a level below that of the optimum stand density range. In this way some volume production may be sacrificed in order to realize the economic benefit of more rapid diameter growth and hence a shorter rotation.

The classical approach to stand management was based on frequent and light thinning. In this way the forest manager might seek to create a uniform and attractive forest in excellent silvicultural condition. This might be done by maintaining the stand within, say, the mid section of its optimum destiny range, and through a flexible and sensitive approach to thinning, ensuring that the tree crowns are as uniform as possible, and well balanced with a good depth of green branches. Much effort may be put into marking trees to achieve this. In this case the average tree efficiency will be expected to be high, bole quality excellent, and volume production close to the maximum the site can support.

The adoption of this classical approach to thinning might be appropriate where a more tolerant tree species is being grown, where a relatively long rotation is anticipated, where market conditions are appropriate, and the regime is justified on economic grounds. It might not be so relevant for fast growing intolerant eucalypts, and full site use and maximum volume production can be achieved at a relatively low stand density. In this case, market and economic objectives might be better met through a relatively short rotation based on an early heavy thinning, one additional thinning and a final harvest. Where the first thinning has been early enough and heavy enough, there is normally plenty of scope for flexibility in subsequent stand management, that is, for matching silviculture with market and economic requirements.

The First Thinning Decision

The first thinning decision of a regrowth stand of even age can be addressed at two levels:

early non-commercial thinning within overstocked regrowth-designed to reduce competition for resources and to stimulate the expression of dominance and growth of potential crop trees, and
later age thinning designed to provide commercial products (e.g. pulpwood and small logs) and maintain the diameter growth of potential sawlog trees.

Early non-commercial thinning of eucalypt regrowth can be difficult, particularly on high quality, wet sclerophyll forest sites. For example, a complex work environment may be created within the forest through the complexity and density of understorey in young natural stands, the frequency and size of timber debris on the ground, the soil conditions under high rainfall, difficult micro-terrain, and variations in the size and spatial distribution of trees. Inevitably, non-commercial thinning is costly, and may be justified only where there is a substantial increase in the production of commercial wood volume or off-set by possible future costs. For example, compare a dense stand that forces height growth and branch shedding followed by a non-commercial thinning with an open stand that develops strong branches that requires pruning.

Overstocked regrowth stands can be non-commercially thinned by treatment with herbicides or thinned mechanically using clearing saws or chainsaws. Herbicides may be applied as a foliar spray, to the bark at the base of the tree, or injected into the tree. Mechanical thinning with a clearing saw may be effective in spacing accessible young stands where the diameter of most trees removed is less than a certain maximum size.

Thinning Regimes

The effects of a single thinning are not maintained indefinitely. After a few years, the gaps in the canopy close together, and before long the same crowded condition that existed before thinning redevelops. As the crowns expand, the number of trees that can occupy the area to best advantage decrease and the surplus volume available for removal in the next thinning accumulates. The rate of growth of the final crop trees is the best single criterion for determining when thinning should be repeated. This can be expressed as either stand basal area or the average diameter of the final crop trees. However, it might also be necessary to delay thinning until the volume available for removal is such that a commercial operation can be undertaken. Consequently, the most appropriate balance between intensity, frequency, and timing of thinning in not easy to determine. The issue becomes one generally of a choice between heavy thinnings with longer intervals between, light thinnings at more frequent intervals or some combination between.

Source: Modified and supplemented by J.A Duggin from Florence, R.G. (1996) .Ecology and Silviculture of Eucalypt Forests and from Smith, D.M. et al. (1997) The Practice of Silviculture: applied forest ecology (9th ed)