Forestry Insights

The harvesting programme is the culmination of the forestry cycle and the test of the tactical plan established some years before.

Tactical planning is where foresters consider the market demand and opportunities, and produce a series of annual harvest plans for individual forests to best meet those demands. For harvesting, this will require decisions on area available to be cut (volume), mix of log grades, road and tracking requirements, logging systems, and the location and type of wood processing plants to be supplied.

Key environmental values need to be identified at this stage to ensure forest operations are carried out according to the guidelines and requirements of the Resource Management Act 1991, and the Forest Code of Practice.

Harvest Planning Processes

Harvest planning decisions are made with reference to topographic maps showing the location of compartments within a forest. A compartment is a block of crop trees within a forest, usually of similar age, and is the basic unit for record keeping of crop species, age, genetic origin and silviculture history. From this data, foresters can decide which compartments are ready for harvest and what market demand they are best suited to, the extent and difficulty of the operation, and the likely value per hectare of the harvest based on the silvicultural treatment of the crop trees and the log grades they can be cut into.

Contour maps and aerial photographs are commonly used in determining the location of roads, landings and the choice of logging system. Logging roads provide access into the forest. Considerations that affect roading:

•     Topography – steepness and tightness of curves
•     Soil type – risk of erosion
•     Geology – presence of rock
•     Anticipated usage, machinery, crews
•     Proximity to waterways – runoff may cause reduced water
      quality.

Landings or ‘skids’ are cleared to provide a flat working area to stock-pile felled trees, ready for cutting into required log grades. Prepared/graded logs are then loaded and transported by logging trucks from the landing to appropriate processing plants. Considerations when planning landing sites:

•     Topography – located on easily accessible terrain;
•     Road location – near roads for transportation;
•     Soil type – risk of erosion;
•     Proximity – to waterways;
•     Log storage – size of landing site required will depend on the
      number of log grades being cut and sorted, and the total volume
      of logs likely to build up on it over time.

Felling

After 25-30 years of nurturing and tending the decision to fell is made and with it the realisation of the success of about 30 years of work. Much of the ultimate responsibility rests with the felling crew. Thousands of dollars can be made or lost, depending on their ability to ensure each section of the stem is cut and graded to ensure the value of the whole stem is maximised.

Split butts, snapped stems and fractures are a regular event when felling, and safety is always a matter of concern. Below is a simplified list of considerations, potential impacts on the environment and methods of reducing impacts, as outlined in the Forest Code of Practice. 

Felling considerations

Safety – key factor to consider – logging crews wear protective clothing and equipment and are trained to comply with a high ‘work practice’ standard.

Tree size – larger, tall trees require greater clear areas to fell into, to reduce possible damage to adjoining trees.

Tree lean – fellers have limited directional control of trees which have a lean, which increases the risk to the feller, and damage to adjoining trees.

Topography – the lie of the land, consideration for the type of felling operation for extraction.

Skilled fellers – trained and experienced fellers are essential to achieve a successful, risk-free operation.

Proximity – reduce impact on stream life and water quality by felling trees away from waterways.

Visual impact – clearfelling leaves bare and often scarred landscapes. This can make a strong impression on the landscape and public attitudes. However, there are methods of reducing visual impacts, such as leaving treelines along roads, cutting to suit the landscape, and mixing age classes.

Harvesting Techniques

Logging Systems

The logging system used to fell trees and remove the logs will be strongly influenced by factors such as the slope of the land, softness of the soil, size of the area to be harvested and the quality of access.

In the simplest situations (e.g. flat dry ground with good road access), rubber tyred skidders are the fastest and most economical way to pull logs from the forest. At the other end of the spectrum, steep and very inaccessible country, helicopters may be the only practical way to get logs out.

The logging systems most commonly encountered in New Zealand include:

Rubber-tyred Skidders
Skidders are specialised logging machines designed to travel quite quickly over relatively level ground, having four large tyres (all powered for 4-wheel drive) and a hinged chassis to enable the vehicle to make particularly tight turns. Logs are attached to a winch or grapple system at the rear of the machine, enabling the butts of the logs to be lifted off the ground, reducing drag and soil disturbance. On stable slopes and in drier conditions skidders may be used to pull logs off quite steep land by using systems of contour tracking, etc.

Tracked Skidders/Bulldozers
Tracked machines provide access to steeper land and also are more effective where the soil is softer as the tracks spread the vehicle’s weight more effectively and provide better traction. Older style bulldozers often towed a wheeled logging arch enabling the butts of the logs to be lifted off the ground. However, newer machines have the arch built into the machine and are purpose-built for logging operations.

Cable Logging
Cable logging systems are used where steeper topography, softer soils and difficult access prevent the use of simpler systems. In its most basic form cable logging consists of a fixed winch with two cables – one to pull the line out to the trees and the other to pull the logs in once attached to the line. In this case logs are dragged across the surface of the land with no ability to avoid obstacles or disturbing the soil.

Much more sophisticated systems have been developed using winches driving four or five lines. In such cases quite complicated rigging systems may allow the whole drag of logs to be lifted clear of the ground and over streams, etc without disturbance.

Because much of New Zealand’s plantation forest is located on steeper land, cable systems are quite widely used, the most common systems being:

•     Highlead – Simple two-cable system with a tail rope to pull out
      the block (to which logs are attached), and a main rope to pull
      the logs in. In older systems the winch was separated from the
      spar (a tall pole over which the cable passed to give some lift to
      both the cable and the butt of the logs). However, modern
      winches are usually attached to a chassis which also carries
      a steel spar – the whole unit being known as a log "hauler".
 
•     Skylines – Skyline systems usually have a third cable along
      which a carriage runs, the carriage being pulled in and out by
      the tail and main ropes. Logs are attached to the carriage in
      various ways, and where the skyline is fixed at some distance
      above the ground, logs may be lifted up to the skyline’ before
      being winched in.
 
•     Skyline With Motorised Carriage – while most skyline
      systems rely on lowering the skyline itself, or mechanical
      braking systems to lower the carriage/cables to the ground
      for logs to be attached, the most complicated systems use
      a motorised carriage. Motorised carriages have their own
      power and cable and the operator (using radio controls) can
      lower the cable and pull logs up to the carriage. With these
      systems the logs are fully suspended, and depending on the
      length of cable carried by the carriage, it is also possible to
      pull in logs from quite a distance either side of the main cable.

Because cable systems pull logs for distances of a kilometre or more (although shorter distances are more usual and much more economical), for safety reasons it is necessary to pre-fell the trees under the cable. As the machinery is large and expensive to move it is also more usual to use cable systems in larger forest areas, although lighter and more mobile systems are now available.

Helicopter Logging

This is by far the most costly way to lift logs out of the forest and can usually only be justified where the value of the logs to be removed is high. Helicopter systems are also extremely useful where minimal extraction damage (to surrounding land and forest) is desired.

In many respects the helicopter acts as the carriage in cable logging systems, but can of course vary the path from the felling area to the log landing point. Best results are obtained where the distance over which the helicopter is carrying the logs is minimised – longer trips quickly make the whole operation uneconomic.

Mechanised harvesters

These are machines specially developed to enable the operator to fell the tree and cut it into logs from the relative safety of the cab of the machine. The harvester is usually similar to a bucket excavator, but with the bucket removed and replaced with a hydraulically or mechanically operated felling head and grapple to hold and lower the severed tree to the ground.

Fully mechanised systems may include all-terrain vehicles ('forwarder') with the ability to drive along behind the feller, pick up the logs and carry them out to the nearest road. Such systems are better suited to flatter terrain, although over time this capability has been improved as well.

Part of the move to more mechanised systems to fell and extract trees from the forest is due to the very considerable danger/high accident rate associated with logging operations.

Alternative Systems

Other logging systems still in use in other parts of the world include the use of animals (horses, oxen, elephants) instead of motorised skidders. In some parts of North America helium filled balloons have been used to lift logs from the forest with some success. However, such systems are very sensitive to wind and would probably not work well in New Zealand.

Harvesting Practice

The most common harvesting system for plantation grown forests around the world is clearfelling. Like many agricultural crops, plantation forests tend to be made up of stands of trees planted at the same time and are, therefore, all ready to harvest at one time. Clearfelling is the process of cutting down all the stand of same-aged trees at the one time, leaving an extensive bare area – the clearfell.

Often where trees are being cut from natural forest, where there is a mixture of species and ages, it may be desirable to remove only individual trees that have attained the right size, or which are required for a particular end use. Such operations are often referred to as selection felling, the best examples in New Zealand being the felling of individual trees in the rimu forests of South Westland.

In other cases, removal of single stems may not allow enough light to reach the forest floor to encourage growth of new seedlings to replace the felled tree, and in such cases small patches of trees may be removed to encourage regrowth. This is called coupe felling.

Classical European felling systems were designed to encourage the natural regeneration of seedlings following felling, and took into account the shade tolerance of the species, amount of soil disturbance required, and so on. Plantation forestry is much more focused on growing trees with particular size and wood properties as quickly as possible, and the use of planting stock of particular genetic qualities for forestation is more typical. Accordingly 'cutovers' (the clear site left after logging) are usually replanted as soon as possible.

Environmental Impacts