Thinning and crown classification

Thinning is an important aspect of commercial forestry as it promotes the growth of the remaining trees, gives intermediate financial returns and may increase the total yield of usable timber over the life of the stand. There are three major factors in a thinning regime. Thinning intensity is the rate at which timber is removed per year; e.g. 8 m3 ha-1 y-1. Thinning yield, on the other hand, is the actual timber volume removed in any one thinning. For a thinning cycle (time between one thinning and the next) of 5 years, the thinning yield would be 40 m3 ha-1 in this case. If thinning intensity is low, stands of normal initial spacing become so overstocked that cumulative production of usable timber is reduced and suppressed trees die before harvesting. If thinning intensity is high it enables the remaining trees to increase in diameter more rapidly than if the stand is left unthinned, but they do not use the extra growing space fully in cases of excessive thinning. So either under- or over-thinning reduces timber production. It turns out that in practice cumulative volume production remains the same over a wide range of thinning intensities. In general foresters prefer to thin as much as possible because it brings in more money earlier and the resulting fewer but large trees can be more valuable per cubic metre. Marginal thinning intensity is the maximum thinning that can be used without incurring loss in this total volume. Once the stand is sufficiently well developed for thinning to commence this intensity is about 70% of maximum mean annual volume increment per year until the stand reaches maximum MAI (Rollinson, 1985). Thus for a stand with a yield class of 10 the marginal thinning intensity is 7 m3 ha-1 y-1, and this would give a thinning yield of 42 m3 ha-1 with a 6-year thinning cycle.

Trees in even-aged forests, such as that shown in Fig. 10.10, may be the same age but they differ in both size and form. This is of considerable importance

Dominiant Tree Crown Class

D C D ISCDISD

Figure 10.10 Even-aged crown classification in a stand of Douglas fir Pseudotsuga menziesii in the Wyre Forest, England. Naturally regenerating saplings are unlabelled. The tree fourth from the right is dead, while that on the extreme right, adjacent to a glade, is as close to a wolf tree (see text) as occurs in this area of Worcestershire. D, dominant; C, co-dominant; I, intermediate; S, suppressed. (Drawn by Peter R. Hobson. From Packham et al, 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 1.7. With kind permission of Springer Science and Business Media.)

D C D ISCDISD

Figure 10.10 Even-aged crown classification in a stand of Douglas fir Pseudotsuga menziesii in the Wyre Forest, England. Naturally regenerating saplings are unlabelled. The tree fourth from the right is dead, while that on the extreme right, adjacent to a glade, is as close to a wolf tree (see text) as occurs in this area of Worcestershire. D, dominant; C, co-dominant; I, intermediate; S, suppressed. (Drawn by Peter R. Hobson. From Packham et al, 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 1.7. With kind permission of Springer Science and Business Media.)

as the crop ages and has to be thinned. In some cases thinning is done on a systematic basis, such as the removal of alternate rows, but usually judgements are made about individual trees which fall into the five classes illustrated below. Crowns of dominant trees receive full light from above and, to some extent, the side. The slightly shorter co-dominant trees receive full light from above, but are obscured laterally by the dominants. These two classes of tree form the main canopy and are the most thrifty.

The lower crowns ofintermediate trees get some direct light through holes in the canopy, but are subject to severe lateral competition from their larger neighbours. Suppressed trees are in far worse state, being strongly over-topped by the previous three classes and dependent on sun flecks and light filtering through the canopy. Slow-growing and usually weak, these are often doomed to fall into the last category of the dead trees commonly found in unthinned plantations. The largest dominant trees may grow into wolf trees which are coarse, heavy-limbed individuals which lack effective lateral competition and have crowns so broad that they inhibit growth of more thrifty neighbours. Such trees should be removed from timber crops, although they may be of conservation interest. They are more common in stands that are not entirely even-aged.

Other classifications have been devised for stands of uneven age where the main criterion is degree of vigour rather than size. Vigorous trees are considered important because they are less susceptible to both disease and insect attack; it is important to identify vigorous young individuals which will respond positively to release if older and larger trees around them are felled.

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