What is Thinning?
Thinning is the cutting down and removal of a proportion of trees in a forest crop. It is carried out primarily to provide more growing space for the remaining trees, which leads to an increase in volume of individual trees. Thinning also provides the farmer with an intermediate source of timber revenue before the final crop is cut down at the end of the rotation.
Thinning of conifers aims to maximize total production per hectare, in broadleaves the main purpose of thinning is to produce well balanced, even crowns on the final crop trees. Conifer thinning is carried out on a 5 year cycle normally, starting when the trees are near 20 years of age. These thinning operations are carried out until the crop is completely cut down.
Objects of Thinning
The idea behind thinning is to concentrate the volume increase in a smaller number of trees, resulting in wider individual trees and a greater proportion of sawlog, the most valuable timber size class. An important concept in thinning is the yield class (YC) of the tree. The yield class is a measure of the growth of the tree crop on an annual basis. A tree crop of yield class 20 will put on 20m3 per hectare per year over the lifetime of the crop. This volume increase holds true whether there are 2,500 trees per hectare or 460 trees per hectare.
How Thinning affects the Tree Crop
The table below outlines how thinning lowers the number of trees, while increasing the volume of the individual trees. The crop would begin with 2300 - 2500 trees per hectare (ha).
Volume production of thinned Sitka spruce, Yield Class 20
Methods of Thinning
Thinnings can be selective or systematic. A selective thinning is one in which the trees are removed or retained on their individual merits. Systematic thinning involves the removal of trees according to a predetermined system, such as line thinning.
A combination of the two systems called line + selection thinning may prove to be the most efficient management method. For example, every fifth line of trees may be cut and then the intermediate four lines of the trees may be selectively thinned.
The trees are either manually harvested, using chainsaws, or machine harvested, using processing machines. The branches are cut off, and the trees are cut into specific lengths and stacked, to await extraction by forwarder or skidder.
Skidders are tractors with cable drums attached, that extract by lifting one end of the load clear of the ground and pulling it out with the other end dragging on the ground. Forwarders are generally purpose-built tractor units that extract timber lifted entirely clear of the ground. The timber is carried on a linked trailer or integral rear bunk. All forwarders use a fitted loading crane. Having stacked the harvested timber beside a road, articulated lorries transport the timber to the mill.
Factors to Consider before Thinning
First thinnings combine small-sized timber and confined working space making them more difficult and more expensive than later thinnings and clearfells. The cost of the first thinning operation may be greater than the value of the timber produced, something the farmer must consider before he/she begins to thin or before a contractor is employed.
This situation is only acceptable if a market is definitely available for the product to offset some of the harvesting costs. Specialized harvesting and extraction machinery must be used in relatively large areas and used intensively for them to be economically efficient.
End Uses for Thinning
First thinning for conifers normally produces pulp and pallet size timber. Paper, plywood and oriented strand board are produced from these timbers. Final products include hoarding, packaging, decking and flooring. Stakewood may also be produced. As the thinning cycle continues, the proportion of small and large sawlog increases, resulting in end products such as flooring and timber frame construction timber