The Farm Forest

Thinning increases the total volume yield of usable timber over the lifetime of the crop and provides an intermediate source of timber and revenue before clearfelling. Before making the decision on whether to thin the plantation, a qualified forester should be employed to assess the crop. The decision to thin is dependent on a number of factors: species and age, ground conditions, degree of access, availability of markets, suitable machinery and skilled labour.

If the farmer does not feel confident enough to carry out part or all of the thinning operation by himself, then he may consider employing a qualified forester and/or a harvesting contractor. The contractor in turn requires a certain volume of timber in order for the operation to be economically viable.

There is considerable cost attached to the extraction of the timber to the roadside and later, to the mill. This cost will decide whether the market price for pulpwood, stakewood, pallet and sawlog is sufficient to make thinning economic. It may not be possible to thin poorly drained, exposed sites because the resulting gaps in the canopy may expose the crop to an unacceptably high risk of windthrow.

In most cases, first thinning is seen as a breakeven operation at best. The farmer must decide whether there is enough timber to be harvested to make overall economic sense. If the plantation is too small to market by itself, the farmer may opt for one of the following:

• thin for his own use on the farm,
• thin to waste, an early first thinning system where the felled trees are left to rot in the plantation,
• group together with neighbouring plantations and market their timber as a co- operative/ association.

When should the plantation be thinned?
The future production is estimated using information such as species, age and Yield Class.

Normally, the higher the Yield Class, the earlier the plantation will be thinned. For example: first thinning of Sitka spruce can start at age 16 for Yield Class 24 but not until age 23 for Yield Class 14.

How will the timber be sold?
Before a farmer begins to thin or clearfell his forest crop, he must establish that there is a market available. Local supply and demand will determine the price paid and the lengths to which the timber should be cut. The farmer must also decide how he is going to sell the timber. There are three basic options available-
• Standing - the crop is sold to a contractor/ saw-mill who organises the harvesting, extraction and delivery of the timber
• Roadside - the timber is harvested, extracted to the road and then sold by category i.e. pulp, pallet and small sawlog.
• Delivery - the timber, in various lengths, is brought to the mill for sale. Where small lots of timber are involved, sawmills are only interested in timber delivered into their yard.

The delivery option would probably provide the farmer with the best price for the timber. However, very few farmers have the harvesting skills, the equipment or the time to go for this option. Instead, standing timber sales would be the most logical choice. It is important that the farmer employs a qualified/ experienced forester to make an assessment of the volume of the crop and to provide advice on how best to go about managing and marketing that crop.

• A contract should be drawn up between the farmer and contractor detailing the conditions governing the harvesting operation.

Choosing the Thinning System
Conifers
Line thinning removes all trees in selected lines, without considering the merits or quality of individual trees.
Selection thinning considers individual tree quality and is much more beneficial to the crop.
A combination of line and selection thinning whereby every seventh or eighth line of trees is removed. Using these empty lines, the timber processor can selectively remove poor or inferior trees on either side. These empty lines can then be used as extraction racks to bring the timber out to roadside.

Broadleaves
Thinning of broadleaved trees commences when the trees are between 10 and 15 metres tall (again according to species). It is always selective, focusing on quality final crop trees by promoting their crown development and diameter increment and is generally carried out during the winter months when the crowns and stems are easier to assess. Crown thinning (removal of dominant trees in the upper canopy) is required for the main broadleaved species.

Thinning cycle
The thinning cycle is the interval between thinnings. For most fast growing conifers on a short rotation, the period between thinnings varies from 4 to 6 years. For broadleaves and slower growing conifers, the period between thinnings is usually 10 years or more. Factors such as market demand, species, Yield Class, rotation length and susceptibility to windthrow will determine the frequency and number of thinnings to be carried out. It may be worth considering thinning the plantation earlier than the prescribed time if the site is vulnerable to windthrow if thinning to waste or to facilitate pruning.

Thinning yield
The thinning yield is the volume removed in any one thinning. The maximum volume to be removed is calculated as 70 per cent of the Yield Class, multiplied by the cycle. For example, the thinning yield for a Sitka spruce crop of Yield Class 20 being thinned on a 5 year cycle is 70% x 20 x 5 = 70 m3 per hectare.

Clearfelling
The purpose of commercial forestry is to maximise the value and quality of plantations. Clearfelling normally takes place at an age that maximises the financial yield. Timing of clearfelling depends largely on the species and the Yield Class. Other external factors can also have a bearing on time of felling and on the value of the crop. These include stocking rates, thinning history, previous management, plantation size, market price and distance from markets. Plantation owners can avail of economies of scale by waiting until there is sufficient timber available in the locality for harvest.