Harvesting is a general term used to describe the felling of trees and preparing them for transport to the mills. It includes both thinning and clearfelling operations. Thinning is the periodic removal of the weaker and smaller trees from a crop to allow room for the better trees to grow and develop, while clearfelling is the removal of the entire crop. Clearfelling takes place at the end of the rotation which in the case of conifers is approximately 40 years after planting and 100 -150 years in the case of the slower growing broadleaves such as beech and oak.
The felling and extraction of timber is potentially a very dangerous operation and harvesting crews are required to be highly skilled and well trained. Strict health and safety rules are followed and are subject to regular inspections by both the harvesting managers and the Health and Safety inspectorate.
There are a number of felling and extraction systems available to harvest crops depending on the nature of the crop and the ground conditions. These include:
Today, most forest harvesting is carried out using mechanised harvesters. These machines comprise a base machine with a harvesting head mounted on a hydraulic arm that can fell a tree, remove the branches and section the stem into the desired lengths in less than a minute. The operator in the cab controls the movement of the machine and the onboard computer system can be programmed to cut the felled trees to the size and length specified by the customer (mills).
Harvesters are expensive machines to set up and operate, and require large areas of felling before they become cost-effective. While able to operate very efficiently on many forest sites in Ireland they have their limitations. They are able to harvest stands very efficiently with straight stems and consequently are used mostly on conifer crops. Harvesting broadleaved stands, however, with their greater variation in stem form and large branches is normally carried out by chainsaw. Also mechanical harvesters are not able to operate on very steep slopes or rough ground with large boulders and in these circumstances felling is best carried out using motor manual methods.
Motor manual harvesting
Today, motor manual felling or chainsaw felling is used less than in the past. However, there are situations where the chainsaw is still required such as felling large trees, trees with crooked stems and large branches such as broadleaves, trees on very steep slopes where the harvester cannot operate, or felling small stands where it would not be economical to use a mechanical harvester.
Chainsaw operators are required to be fully trained and certified on the use of the chainsaw. While the modern chainsaws have many safety features, the operator being close to the moving saw chain must be very vigilant regarding the safe use of the saw and must wear special protective clothing at all times.
Felling by chainsaw is a very skilled operation as it requires the operator to fell the tree in the right direction, delimb it and cut it into section lengths that will maximise its timber value.
Extraction is the term used to describe the process of removing timber from the point at which it is felled to the forest road. There are a number of extraction methods used depending on the site conditions:
- Forwarders – The forwarder has a similar base machine to the mechanical harvester but has a powered trailer fitted with a hydraulic grapple arm to load the felled timber. A forwarder usually follows a harvester to collect and extract the timber to the roadside.
- Skidders – Skidders are mounted on tractors which are capable of operating on steeper and rougher ground. Logs are attached by chains to a bar on the hydraulic linkage which lifts one end of the log off the ground and then they are dragged to the roadside. Often whole trees are extracted using this method and cut to the required sizes at the roadside.
- Cable systems – on very steep or rough ground cable systems are used. These consist of a pulley system, one end of which is attached to a tree up-slope and the other to a tractor mounted winch. The felled logs are attached to a moving carriage on the cable and the logs are winched down to the roadside.
- Horse extraction – In the past this was the traditional method of extracting timber from a forest but was largely abandoned in favour of machines. Today, horse extraction is making somewhat of a comeback, particularly where heavy machinery could cause serious damage to sensitive sites e.g. in biodiversity or nature conservation areas.
Harvesting, of all the forest operations, has the potential to have the greatest impact on the forest environment and the wider landscape. Strict Forest Service Guidelines have therefore been developed to ensure best practice in harvesting operations in order to protect the soil, waterways, wildlife, the landscape, and ancient sites.
Harvesters, forwarders and skidders are designed to minimise soil damage, with large soft tyres to spread their weight over a wide area. In addition, standard practice is to place the branches of the felled trees under the wheels or tracks of the harvesters to form a thick carpet of foliage which supports the machines and further protects the soil from compaction and rutting. Harvesting operations are scheduled according the nature of the soil with sites being categorised into winter and summer sites depending on ground conditions. Also, best practice is to suspend mechanised harvesting operations during and immediately after periods of particularly heavy rainfall.
Waterways are particularly vulnerable to the effects of harvesting as silt from the movement of machinery can enter streams and rivers causing blockage of gravels which affects insect and fish life. Also nutrients released from decaying branches, particularly from large clearfelled sites, can cause enrichment of the waters which in turn causes pollution. To counteract these effects careful planning is required in carrying out harvesting operations. Some of the measures taken to avoid impacts include:
- Limiting the size of the areas to be felled which reduces the amount of nutrients and silt released.
- Minimising the crossing of drains and streams, but where necessary installing temporary structures (log bridges, pipes etc) to avoid machines entering the water;
- Establishing buffer zones around waterways from which machines are excluded.
Similarly, 20 metre buffer zones are left around all known archaeological sites for their protection. Sometimes an unrecorded archaeological site or artefact is discovered and the area is excluded from the harvesting operation and left undisturbed and the relevant authorities notified.
The noise and impact of harvesting operations can have a major impact on wildlife habitat. Therefore in planning felling operations care must be taken to ensure that important wildlife habitats are retained and protected. Due regard must be given to the breeding and nesting seasons of important species and associated features such as badger setts and heronries. The timing of harvesting may be delayed until after the nesting season is completed, to minimise damage to bird life.
Deadwood is also left in situ, in the form of standing dead stems or naturally fallen trunks, or as logs deliberately left behind on the forest floor. As these decay, they provide habitat for fungi and insects which in turn supports other animal and bird life.
The effects of clearfelling, where entire forest stands are removed in one operation, can have a significant impact on the landscape unless carefully carried out. In Ireland, afforestation (establishment of new forests) has resulted in many adjacent conifer plantations being established within a 2-3 year period creating a large uniform areas of forest which require harvesting at the same time. Foresters are now re-structuring these forests to create greater diversity in the next rotation. The phased felling of small felling coupes and replanting with more diverse species will, over time, reduce the visual and environmental impacts and ensure that succeeding rotations do not inherit the same undesirable structure. Staggered felling/reforestation also benefits biodiversity and the landscape by introducing structural and age diversity.
Guidance published by the Forest Service on best practice relevant to harvesting is given in the following guidelines:
Forest Harvesting and the Environment Guidelines and with relevant measures set out in the Forestry and Water Quality Guidelines, Forestry and Archaeology Guidelines, Forest Biodiversity Guidelines and Forestry and the Landscape Guidelines.
Guidance on timber transport is provided in Road Haulage of Round Timber Code of Practice produced by the Irish Forest Industry Chain and Forest Industry Transport Group.
Timber is sold to customers (sawmills, board mills, wood fuel merchants etc) either standing or by the roadside. Timber sold standing is left in place in the forest for the customer to organise the felling and extraction of the trees. Timber sold by the roadside is felled and extracted by the forest owner’s employees or contractors and stacked on the side of the forest road for collection by the customer. As the cost of harvesting falls to the owner and not the customer, timber sold by the roadside commands a higher price than timber sold standing.
The main timber sales auction system operating in Ireland is:
- Coillte Timber Auction – an electronic auction system for selling roundwood primarily from Coillte forests. The system provides a robust and equitable method for auctioning timber nationwide to a widely dispersed customer base. This type of sales method is most suited to larger customers whose scale allow for economic transportation of large timber volumes from forests to sawmills over greater distances. Coillte hold a roundwood auction on a fortnightly basis.