Harvesting

THINNING

The 70,000 hectares planted by the private sector between 1990 and 1995 are now at a stage where a decision needs to be made on whether to thin or not. Every forest is different and factors such as timber quality, current timber prices, ground conditions, extraction distances and  road access impact on the financial returns from thinning.  Thinning involves the removal of part of the crop in order to concentrate future volume growth on fewer and better quality stems. The main aim of thinning is to improve stem quality through the removal of deformed/unthrifty trees and to reduce the time taken for trees to reach valuable sawlog size. Thinning is the most powerful tool the grower possesses to manipulate the development of the plantation and the quality and log size of the final crop.


Thinning will increase overall timber revenue by increasing the volume of sawlog produced. This is due to the fact that larger trees attract significantly higher prices, as they are less expensive to harvest and yield more valuable end products.


There are many advantages to thinning including:


  1. ★It allows easier access into the crop for management inspection;

  2. ★Waste can be avoided by removing suppressed trees before they decay;

  3. ★Promotion of stand hygiene by removing dying trees;

  4. ★Provision of revenues early in the rotation;

  5. ★Improved the saleability of the plantation should the owner wish to dispose of the asset.



The two main types of thinning practised in Ireland are rack and selection and selection. In a rack and selection thinning ( normally done at time of first thinning), a line of trees, usually every seventh line, is removed and the intervening trees are selectively thinned. Selection thinning involves the removal of deformed, unthrifty and weaker trees throughout the crop and preferably should involve the marking of these trees in advance of harvesting.


In mechanised thinning, a harvester fells, delimbs and crosscuts the stem into various product assortments, e.g. pulpwood, pallet wood, stake wood and sawlog ( usually based on the top diameter and length of the log). The material is then extracted to roadside by a forwarder. This is the most common system used in Ireland and accounts for approximately 90% of thinning undertaken.


The timing of first and subsequent thinnings depends on the productivity of the site. Yield class is used to identify the potential growth rate of plantations. Yield classes vary from 4 to 30 for conifers depending on species and site productivity. A yield class of 20 means that the crop has the potential to produce 20 cubic metres per ha per annum over a full rotation.


The timing of first thinning is especially important in Ireland due to our windy climate and the wet mineral soils that so often typify plantations. If delayed, the crop trees can get drawn up and once opened up through thinning could render the crop liable to windthrow.


The economics of thinning depend on a combination of yield class, price and site stability. Thinning increases revenues but incurs a risk in terms of windthrow on unstable sites. There are silvicultural techniques which can be applied to certain crops to reduce this risk.  


Thinning will improve the quality of the final crop. Thinning will increase overall timber revenue by increasing the volume of sawlog produced and provide a source of early revenue to the owner. Thinning is more profitable than no thinning. It is better to thin early rather than late, and to gain the full advantage of thinning, crops must be capable of reaching their financial rotation. Not all crops will be suitable for thinning due to access and stability. First thinning can incur a loss, especially when access is difficult.


A new production forecast for the private sector for the period 2009-2028 has been developed. Unlike previous forecasts, net volumes are shown after making allowance for volume losses due to stocking, unproductive areas, windblow and harvesting. The overall roundwood production from privately owned forests will increase from an estimated 0.38 million m in 2009 to 2.95 million m by 2028. Volume production, compared with 2009, more than doubles within the first 10 year period and then trebles within the second 10 year period. Sitka and Norway spruce account for 78% of the total, pine accounts for 11% , other conifers total 6% and broadleaves account for 5%. The total thinning area ( first, second, third and subsequent thinnings) increases steadily from 2009 and peaks at almost 30000 ha in 2022. This scale of thinning, to be achieved within the next 13 years represents a significant challenge to the overall forestry sector. Clearfell areas become significant from 2020 onwards.

PRUNING

Knots are a major cause for timber quality downgrade. Pruning allows the early formation of clear knot free timber. Pruning in conjunction with thinning is economically more attractive. Pruning is a technique for adding values to trees growing in forest stands, it can be thought of as the growers “value added” effort. It is, however a long-term investment as the payback occurs at the time of clearfell when the final crop trees are harvested.


Pruning is undertaken in stages known as lifts. The height of the lift depends on the traditional sawlog lengths that the market demands. Under Irish conditions, pruning is undertaken in two lifts. The first to 3.5 m and the second to 6 m.


The following criteria should be used:


  1. ★Selected trees within a stand must be capable of increasing their mean diameter by a factor of 2.5;

  2. ★Stands will usually be between 13 and 22 years old or 9 and 10 m in height;

  3. ★Stands of yield class 18 or more should be selected;

  4. ★Stands must be stable with a low risk of windthrow;

  5. ★Minimum area of 0.4 ha



The objective is to produce a stand of conifers that will have 6 m of branch/knot free stem. This will increase the value of the crop significantly.


  1. ★Prune a minimum of 500 stems/ha;

  2. ★First lift involves the removal of all branches on the selected stems to 3.5m;

  3. ★Two to four years after the first lift the second lift from 3.5 to 6 m may be undertaken;

  4. ★Prune an inspection path every 100 metres.