PRUNING
Pruning is another standard silvicultural input in tree plantation which is aimed at producing trees without low heavy branches so that long clear boles are formed. The need for pruning depends entirely on the branch development on the lower bole. Well stocked, dense stands discourage early side branch formation and generally require no pruning (Evans 1982). There are two main considerations for pruning, i.e., silvicultural and technical considerations.
Silvicultural considerations
In some species, dead branches soon fall off (natural pruning); in others they persist for many years. For example, Acacia auriculiformis has persistent branches, and pruning must be done if knot-free timber is desired, while such species as Terminalia superba and Neolamarckia cadamba, and most eucalypts have good natural self-pruning ability and rarely need artificial pruning. Eucalyptus grandis and Pinus caribaea are intermediate and have branches persisting for 2-3 years after dying.
Self-pruning is, to some extent, influenced by stand density. Dense, unthinned stands, with trees close together, encourage earlier suppression and death of side branches while they are small. However, the effect on accelerating natural pruning is not much. Of greater importance is the increase in persistence of branches low down the stem in open stands. Where wide spacing and heavy thinning are practised, artificial pruning is essential for all but the most freely naturally self-pruning species if high quality timber is desired.
Technical considerations
The important technical consideration is whether clear, knot-free, and consequently high grade timber is required. For example, wood of such quality is of little importance in plantations grown for:
· Firewood or fuelwood;
· Pulpwood and particle board;
· Low grade sawntimber for uses such as shuttering, and packing cases; and
· Protection purposes such as shelterbelts, erosion control, and stabilisation – in fact, persistent lower branches are often an advantage.
In contrast, clear, knot-free timber is highly desirable or essential for:
1. Veneer production for decorative use, match making, and plywood; knot free timber is needed to ease peeling, improve appearance, and reduce blemishes;
2. High grade constructional timber where uniform strength, and good machining, finishing, and seasoning qualities are important;
3. Various types of poles, e.g. transmission, which need to be smooth for handling, and free of surface snags or holes to prevent entry of fungi or termite into the heartwood which is often not penetrated by preservatives.
THINNING
For most thinning regimes the total volume of timber removed in thinning during rotation amounts to about 40-60 per cent of total production (Evans, 1982). The effects of thinning are:
1. To stimulate diameter growth of the remaining trees;
2. To interrupt stand development by volume reduction, followed by recovery;
3. To redistribute future growth on fewer trees resulting in a stand with a few large trees as opposed to many small ones; and
4. To reduce natural mortality.
Ahmad Zuhaidi and Weinland (2002) suggested that in general, 200 to 300 potential crop trees (PCTs) should be selected for final stand. The desired characteristics of potential crop trees are:
1. Dominant or co-dominant position in the stand;
2. 8 to 9 m bole height;
3. No big side branches;
4. No visible stem damage, disease or epicormic shoots;
5. A straight and vertical trunk;
6. A distinct dominant leader; and
7. No spiral grain or heavy forking.
An even distribution of PCTs is desirable. The selection of small groups of up to 3 trees is permissible. It is not a good practice to sacrifice PCTs for reasons of even distribution when the PCTs reserve does not contain sufficient number of trees. After this stand intervention, the average distance between trees should be 6 to 7 m.
The thinning practice recommended by Ahmad Zuhaidi and Weinland (2002) comprises three main thinning interventions as follows:
First thinning
First thinning is carried out when the average top height of the main crop is about 15 m (preferably diameter at this stage should be around 10 cm). This thinning involves the following steps:
1. Selection and marking of a defined number of potential final crop trees;
2. Removal of trees hampering crown development of the PCTs which could be secondary trees;
3. Removal of wolf trees;
4. Climber control; and
5. Liberation of commercial species in the intervening areas.
Second thinning
Second thinning is carried out when the top height of the main crop averages 20 to 25 m. Three main steps are taken for this operation:
1. Reduction of main crop trees down to PCTs;
2. Climber control on PCTs; and
3. Removal of trees hampering crown development of the selected trees as well as secondary trees.
Third thinning
Third thinning takes place when the average height of the main crop is 30 to 35 m (a 20-year or more rotation is planned). This thinning has two purposes:
1. Reduction of PCTs to the final crop trees ; and
2. Regeneration and final harvest phase (for longer than 20-year rotation), e.g. in the Dryobalanops aromatica stand in FRIM if the next rotation is to be regenerated naturally. Once the regeneration has established itself in sufficient number, felling is carried out to harvest about 50% of the standing stock. The remaining standing stock can be removed when the young regeneration reaches a height of 1.5 m.
Detailed information regarding pruning and thinning.
Source: A Guide to Plantation Forestry In Sabah
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