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Author(s): Editorial office waldwissen.net – LWF
Editorial office: LWF, Germany
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Long-term storage of roundwood – an overview of methods

In the wake of storm calamities, the forestry industry is often faced with having to log and then sell or store huge volumes of timber. With the right approaches to storing the roundwood, economic damage can however be kept to a minimum.

Sturmwurffläche
Fig. 1: Storm damage after Hurricane Kyrill in 2007 (Picture: G. Lobinger).
 
Holzmenge
Fig. 2: Timber volumes accumulated as a result of storm calamities may have to be stored for several years (Picture: M. Kubatta-Große).

The storms Vivian, Wiebke and Lothar in the 1990s, and Kyrill, Klaus and Xynthia between 2007 and 2010 all hit the European forest and timber industries hard, with several million cubic metres of timber being brought down in each case. Forest owners were forced to sell or store large quantities of timber. Forest owners and foresters may well have to watch helplessly as the elements rage around them, but they can protect the forest to a certain extent by implementing site-adjusted silvicultural methods that emulate nature as closely as possible, and they can reduce economic damage with the help of long-term roundwood storage concepts.

For larger private forest owners, forest associations or the state forestry administration, it is an essential part of their management responsibility to develop and implement storage concepts to cope with future calamity situations. There is also a definite need for the different types of forest ownership in a region to collaborate closely on this. Cooperation agreements can help to make sure maximum use is made of storage capacities, to reduce costs, and to take pressure off the roundwood markets. Choosing the "right" method of storage is crucial here. The main long-term methods of storing roundwood are presented below.

Content:

Storage by wrapping in film

Storage by film-wrapping is based on the principle of air exclusion. For this the logs are wrapped in an air-tight film. Usually, UV-resistant polythene film that is also commonly used in agriculture to wrap silage is used. There are currently two methods used - the Baden-Württemberg method and the Swiss method.

The Baden-Württemberg method
Baden-Württembergisches Verfahren 1
Baden-Württembergisches Verfahren 2
Baden-Württembergisches Verfahren 3
Fig. 3: Timber stored in hermetically sealed film using Baden-Württemberg method (Pictures: R. Thurn);
Top: Under the timber there is a double layer of silage film. Wedges cut at the ends of the underlying trunks prevent the trunks from rolling off.
Middle: The stack is ready to be wrapped.
Bottom: A seal-wrapped stack of timber.

With this method the timber is wrapped in hermetically sealed plastic film and stored in an atmosphere that is as free of oxygen as possible (Diagram 3). Natural processes such as breathing and fermentation reduce the oxygen level under the film to almost zero. It is essential to ensure that no further air penetrates the plastic. To ensure the conservation is successful, the oxygen content within the wrapped stack must be monitored regularly (every four weeks for coniferous wood, once per week for beech) using a gas measurement device. Smaller areas of damage to the foil can be repaired with webbing tape. In cases of more extensive damage, it is usually necessary to unwrap the stack and process the timber as quickly as possible. Most damage is caused by falling branches and mice.

Tests on spruce/fir, pine, beech, sycamore, ash and birch have shown that the timber must certainly be fresh, but that, provided the film is not disturbed, there is no or only negligible deterioration in the quality of the timber even after longer periods of storage.

Once the sealed stack has been opened, rapid processing of the wood or the artificial drying of the sawn timber go a long way towards helping to conserve the quality of the timber. Beech wood must be processed in the sawmill within one or two days. Otherwise, the oxidising processes that begin immediately lead to a grey discolouring that spreads rapidly from the ends and outer surfaces of the logs. With coniferous timber there is a little more time, as the fungi that destroy the wood grow relatively slowly.

The Swiss method

The Swiss method dispenses with the plastic film on the ground under the stack. The wrapped timber is kept permanently moist by the exclusion of air, thus preventing deterioration in the quality of the timber. There is a lack of research in this area, but current recommendations from the field are for a storage period of one year. To avoid serious losses in quality, the following points should be taken into consideration:

  • Only absolutely fresh timber is suitable for storage.
  • The timber must be sealed in the airtight film immediately after logging.
  • The logs should be as homogeneous in length as possible to avoid the creation of hollow spaces.
  • It is crucial to avoid damage to the bark and injuries to the surface of the trunk.
  • Only fresh to moist sites sheltered from the wind should be chosen as storage sites. Dry, windy sites are unsuitable.

To date, only studies for spruce wood are available. After removal from storage, if there are signs of discolouration progressing from the ends of the logs, it must be possible to trim the logs generously. For this reason, only long-length logs are suitable for conservation.

Because of the immense logistical efforts involved, both methods of storing the timber in film are only suitable for larger areas of privately-owned forest and forest associations. The relatively high costs and the regular monitoring required are disadvantages.

Wet storage

Wet storage can take the form of constant sprinkling or submerging the logs in bodies of water. Both methods can be used for years to prevent a deterioration in the value of the timber due to fungus and/or insect infestation. They thus take significant pressure off the timber market and pre-empt the use of insecticides.

Sprinkling
Beregnungsanlage
Fig. 4: Correctly set up sprinkling plants prevent the devaluation of stored timber very reliably for several years (Picture: AxelHH, wikipedia).

The constant sprinkling of stacks of trunk wood is a common process used to control production in sawmills, and it is the most important means of long-term storage of storm damage timber. It is the most common form of storage and is considered by both research scientists and forest practitioners to be the most reliable.

The water for a sprinkling system can be extracted from groundwater, from rivers and streams or standing surface water, or taken from the public water supply. In Germany the authorisation of the local water authorities is required for the establishment and running of a sprinkler site. The sprinkling plant must have the appropriate dimensions for the position and form of the storage area, as well as to cope with the type, quantity and pressure of the water. To guarantee quality assurance, 50 L/m²/day of water must be sprinkled.

If only healthy timber is stored and it is sprinkled correctly, it is possible to maintain the quality of the timber over a lengthy period. Under these conditions, spruce can be preserved for three to six years, pine for at least two years, and beech for two years.

Storage by submerging in water

The storage of roundwood in stagnant (or slow-moving) waters is a reliable and effective means of wet storage that has been used especially in Scandinavia and North America for many years. The trunks are submerged in the waters individually or in rafts or bundles.

Trunks strapped into bundles require only a small area, but a depth of at least two to three metres. A bundle can comprise some ten to twenty cubic metres of coniferous wood or six to twelve cubic metres of deciduous timbers. At least two thirds of the cross-section of the trunk must be constantly under water. The timber may need to be sprinkled in addition. After just a short time, beech trunks are no longer buoyant. The rapid sinking of the beechwood prevents fungus infestation. Beech wood stored in water remains easily peelable. With softwoods, fungus infestation cannot be avoided if they are stored for long periods. Otherwise this method is a reliable means of conservation. It does not require much technical outlay. It can however be difficult to retrieve the trunks.

Both wet storage methods protect spruce, fir, pine and beech wood very reliably from deteriorating in value. Because of the logistical outlay, however, they are only suitable for larger forest owners and forest associations.

Outlook

A severe hurricane will hit Central Europe again, and it will cause damage - including damage to the forests. It is just a question of when and where. A precautionary concept for the storage of storm damage timber can help reduce economic losses. The well-proven method of wet storage using a sprinkling system has been used for decades, and continues to promise the greatest prospects of success today. However, film-wrapping methods can in some circumstances be used to complement wet storage or as the first choice of method. In the event of particularly severe calamities or the absence of alternatives, immersion storage can be expedient. The most important thing is that forest owners decide in collaboration with their forest associations to develop and implement storage concepts.

Original article

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