Thinning: methods and grades
Forest thinning explained: crown classes, low and crown thinning, thinning grades A to E, mechanical thinning, intensity and a spacing index example.
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A young teak plantation planted at 2 × 2 m holds 2,500 trees per hectare. For a few years this is useful: the trees shade out weeds, prune their own lower branches and grow straight. Soon, however, their crowns touch and they begin to compete for light, water and nutrients. Left alone, the stand thins itself slowly through suppression and death, and growth is spread over too many thin, weak stems.
Thinning is the felling of some trees in an immature stand to give the remaining trees more growing space, to improve their form and health, and to obtain an intermediate yield before the final harvest. It does not regenerate the forest. It is a tending operation, carried out between establishment and the final regeneration fellings.
Objectives
- Increase diameter growth of selected crop trees, so they reach valuable sizes sooner.
- Improve stand quality by removing diseased, forked, crooked or suppressed trees.
- Obtain intermediate returns, such as poles, small timber and fuelwood, years before the final felling.
- Reduce risks of fire, pests, disease and windthrow. Moderately thinned stands are more stable, although heavy thinning late in life can increase windthrow.
- Salvage trees that would otherwise die.
Crown classes
Thinning methods are defined by which crown classes they remove. The usual classification follows Kraft (1884):
| Class | Description |
|---|---|
| Dominant | Crowns extend above the general canopy and receive full light from above and partly from the sides |
| Co-dominant | Crowns form the general level of the canopy, with light from above but crowded at the sides |
| Intermediate (sub-dominant) | Shorter, with crowns squeezed into the canopy and little side light |
| Suppressed (overtopped) | Entirely below the canopy, with no direct light from above |
| Dead and dying | Moribund or dead trees |
Trees may also be classed by their stem quality, for example as wolf trees (large, branchy and occupying too much space), whips (thin, flexible stems) or diseased trees.
Methods of thinning
Low thinning (ordinary thinning, thinning from below)
Low thinning removes trees from the lower crown classes first, then moves upward as the thinning gets heavier. It imitates natural mortality. Traditional Indian forestry defines it by grades:
- A grade removes only dead, dying and diseased trees. It is essentially a sanitation measure.
- B grade (light) also removes suppressed trees.
- C grade (moderate) also removes intermediates and poorly formed co-dominants, leaving a canopy of dominants and good co-dominants.
- D grade (heavy) removes more co-dominants. The canopy is broken but closes again within a few years.
- E grade (very heavy) opens the canopy strongly. It is used in some plantations to promote rapid diameter growth.
Low thinning is simple to mark and suits light-demanding species such as teak, chir pine and eucalyptus, whose suppressed trees are of little value anyway.
Crown thinning (thinning from above)
Crown thinning removes some dominant and co-dominant trees that compete with selected crop trees. Lower trees are mostly kept, because they shade the soil, suppress weeds and train the stems of crop trees. It is harder to mark, but it concentrates growth on the best trees and gives earlier valuable returns. It suits shade-tolerant species and mixed stands.
Other methods
- Selection thinning (thinning of dominants) removes the largest trees to favour smaller ones of good form. It risks degrading the stand if overused.
- Mechanical (systematic) thinning removes trees by position rather than quality, for example every third row (row thinning) or to a fixed spacing. It is often the first thinning in uniform plantations because it is quick and creates extraction lines.
- Free thinning combines methods as needed, focusing on releasing crop trees regardless of crown class.
Thinning regime
A thinning regime specifies four things:
- When to start: usually soon after canopy closure, when competition begins to slow diameter growth and lower branches have died back.
- Intensity: how much basal area or how many stems are removed each time, often 20–35% of basal area.
- Cycle: the interval between thinnings, short in young, fast-growing stands and longer later.
- Final stocking: the target number of crop trees at final felling.
Thinning intensity is often specified with a spacing index. The most widely used is the Hart–Becking index:
Lower S% means a denser stand. Many conifer and teak regimes aim for about 20–30% after thinning.
How many trees to remove?
A teak stand has 1,100 stems per hectare and a top height of 18 m. The average spacing is a = √(10,000 ⁄ 1,100) = 3.02 m, so S% = 3.02 ⁄ 18 × 100 = 16.8%, which is crowded. To reach S% = 25%, the spacing must be 0.25 × 18 = 4.5 m, which gives N = 10,000 ⁄ 4.52 ≈ 494 stems per hectare. The thinning should remove about 1,100 − 494 = 606 trees per hectare, mainly suppressed, intermediate and poorly formed stems in a low thinning.
Effects of thinning
- Diameter growth of the remaining trees increases. Height growth of dominants is hardly affected by moderate thinning.
- Over a fairly wide range of densities, total stand volume production changes little: the same site produces about the same wood. Thinning redistributes that growth onto fewer, larger and more valuable trees, and harvests what would otherwise die.
- Heavy thinning increases branchiness and taper, and can bring weeds, sun scorch and windthrow.
Key points
- Thinning is a tending operation in immature stands that gives growing space to the remaining trees and provides intermediate yields. It does not regenerate the forest.
- Kraft's crown classes are dominant, co-dominant, intermediate, suppressed, and dead or dying.
- Low thinning removes trees from below and is graded A (dead and dying only) to E (very heavy); crown thinning removes competitors of crop trees from the canopy.
- Mechanical thinning removes rows or trees by spacing; free thinning releases crop trees by any means.
- A regime defines start, intensity, cycle and final stocking. The Hart–Becking S% = spacing ⁄ top height × 100.
- Thinning increases diameter growth while total volume production stays fairly constant.
What the exams ask
- Define thinning and list its objectives.
- Describe Kraft's crown classes.
- Distinguish low, crown, selection and mechanical thinning.
- Explain the A to E thinning grades.
- Calculate stems per hectare from a spacing index.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Does thinning increase total timber production?
Usually not by much. Over a broad range of stocking, the total volume produced per hectare is similar. Thinning concentrates it on fewer, larger trees that are worth more per cubic metre, and harvests trees that would otherwise die and rot. Very heavy thinning reduces total production.
Why not plant at wide spacing and skip thinning?
Wide spacing saves planting and thinning costs, but trees grow branchy and tapered, weeds are harder to control, and there is less choice of good trees for the final crop. Close planting followed by thinning allows selection. Many fast-growing plantations, including clonal eucalyptus, now use intermediate spacing and few thinnings.
What is pre-commercial thinning?
Pre-commercial thinning, often called cleaning in young stands, removes trees too small to sell. It costs money but improves the stand early. Later, thinnings that produce saleable poles or timber are commercial thinnings.
Sources
- Khanna, L. S. Principles and Practice of Silviculture. Khanna Bandhu, Dehradun, revised edition 2015.
- Smith, D. M., Larson, B. C., Kelty, M. J. and Ashton, P. M. S. The Practice of Silviculture: Applied Forest Ecology, 9th edition. Wiley, 1997.
- Hamilton, G. J. Forest Mensuration Handbook. Forestry Commission Booklet 39, HMSO, London, 1975.
- Zeide, B. "Thinning and growth: a full turnaround." Journal of Forestry 99, 20–25 (2001).
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