In forests managed for timber production, thinning is probably the most important operation carried out between canopy closure and the final harvest. By removing the smaller, weaker and poorer quality trees growth is concentrated on the better trees remaining. This results in a greater volume of good quality and larger diameter timber being harvested from a crop, which commands a higher price from the sawmills.
Thinning must be carefully carried out as the removal of too few trees will result in a greater proportion of smaller diameter trees of limited timber value. Removing too many allows light deep into the canopy resulting in heavy branches and knots, which reduce the quality of the wood and limit its range of potential uses.
Thinning is normally carried out several times during the rotation. The regular growth form and shape of conifers such as spruces, larches and pines allows the use of specially designed mechanized harvesters to select, fell, de-branch and cut trees into lengths required for the mills. Broadleaves, unless they have straight stems are usually felled by chainsaws.
Thinning of conifers is a relatively straight-forward due to their regular and predictable growth rate. The number of trees and volume to be removed in any one year is determined by reference to detailed management tables and charts. Thinning broadleaves, however, requires the forester to rely on intuition and judgment and depends on the quality of the crop as much as on management tables.
Windthrow is an important factor that the forester must take into account when planning thinning operations in Ireland, particularly on more exposed water-logged sites. Each tree within a forest is protected from the wind by neighbouring trees, and its root system develops just enough to provide adequate anchorage in these sheltered conditions. However, by removing stems, thinning suddenly introduces gaps into the canopy. In some cases, particularly on shallower, waterlogged soils, roots are unable to provide adequate anchorage against the greater degree of exposure, and windthrow occurs.
To avoid windthrow, the level of thinning is reduced to allow roots to gradually adapt to the windier conditions. In some cases, the risk might be so great that thinning is not carried out at all, thus forgoing the economic benefits this operation will have on the final crop at harvesting. Still, smaller diameter trees are better than having no trees at all!
WOOD FROM THINNINGS
Thinning operations provide timber throughout the rotation. Early thinnings from conifers are normally sold for lower value end uses due to their small dimensions. Uses include wood pulp, fence posts, pallets and increasingly for fuel – wood chips and pellets, while early thinnings from broadleaves make excellent firewood. In both cases, any revenue generated usually goes into paying for the thinning operation itself. With both species, however, material from later thinnings is larger and of better quality than that from earlier thinnings may yield a degree of profit.
Scots pine and European larch are often planted in mixture with oak, primarily as nurse species to provide shelter for the oak as it becomes established. These species are ideal for this purpose, as their growth rates are compatible with that of oak, thereby avoiding overshadowing. With careful management, these nurse species can be retained well into the rotation and felled at a later thinning This produces a ‘cash crop’ during the rotation of the main species, thereby providing some financial returns while waiting for the oak to mature.
Further information at: Thinning to improve stand quality