Foil Storage of Storm-Felled Timber—Long-Term Monitoring of Norway Spruce Piles in Germany

Abstract

Windthrow and other calamities pose major challenges for forestry companies. In addition to difficult processing, solutions for the medium- and long-term storage of logs without the loss of quality are required in order to counteract the oversupply and falling prices of raw wood. The application of foil piles is a comparatively young and only regionally established procedure, which is of particular interest when wet storage is not possible. As part of a case study, a total of 32 piles of Norway spruce (Picea abies) wood were monitored under foil in the forest district of Dassel (Lower Saxony State Forests, Germany), and the conditions for wood-destroying fungi and the resulting wood quality were evaluated. A considerable depreciation in the stored wood was directly related to the atmosphere inside the pile and could be attributed to various types of damage to the foil. It became evident that quality losses can only be avoided by working quickly, being careful when sealing the piles and providing intensive support in the form of monitoring and rapid repairs.

1. Introduction

The storage of large quantities of calamity timber and its conservation became a decisive topic in many German forest companies with the passage of the Friederike hurricane in January 2018. Within a few hours, the storm threw up around 9.1 million bank meters (Fm), primarily in Lower Saxony, Saxony-Anhalt and North Rhine-Westphalia [1].

As a consequence, the market for spruce raw wood in Germany and its neighboring countries went down and, after two consecutive dry summers with a massive bark beetle outbreak in 2018 and 2019, almost collapsed. Entire regions are now characterized by the image of dead spruce stands (Figure 1a).

So far, an estimated 70 million cubic meters of calamity wood have accumulated across Germany, large amounts of which are still in the forest as the capacities for felling and transport are exhausted in many places. In addition, storage space for felled wood is becoming scarce. The most recent windthrow events took place in January and February 2022, when four consecutive storms, namely Nadia, Ylenia, Zeynep and Antonia, again felled several million cubic meters of Norway spruce and Scots pine (Pinus sylvestris) wood, especially in northern Germany.

Due to the partly very difficult sales situation in 2018, options for long-term storage and preservation have been sought. In particular, less durable spruce wood requires special protective measures if it needs to be stored outdoors for a period of many months to several years with as little loss in quality as possible. Traditionally, this is conducted by wet storage either in waterbodies or by sprinkling (Figure 1b). The permanent sprinkling reduces oxygen content in the wood to such an extent that wood-destroying fungi do not degrade the wood [2,3,4,5]. Due to the persistent drought in the summers of 2018 to 2020, however, the water supplies required for such irrigation were not available in many places. In addition, the establishment of wet storage areas is expensive and time-consuming to apply for; therefore, it often cannot be realized in the short term.

The massive occurrence of storm and bark beetle damage, combined with the water shortage in many places, brought the focus to foil storage [6,7,8,9,10], which had already been developed and extensively examined in the 1990s in the search for alternative storage options.

There are basically two methods of foil storage. According to the ‘Baden-Württemberg method’, the wooden piles are completely enclosed in foil and thus packed airtight; meanwhile, piles created according to the ‘Swiss method’ are open towards the ground so that soil moisture can enter the foil tent, but not be released upwards again. Numerous studies have shown that the Baden-Württemberg method has a significantly higher protective effect [11,12,13,14,15] because fungal growth and wood degradation can only be prevented if the wood is completely isolated from the ambient air. Nevertheless, both methods are still used today.

If the wood has not been freshly felled by a storm, but has been damaged by bark-breeding beetles, it can show very different degrees of vitality. Studies using miniature logs have shown that severely damaged ‘beetlewood’ is unable to reduce the oxygen levels in a foil tent sufficiently to prevent fungal activity and decay [16]. Only the storage of spruce trees that were infested with bark beetles, but still had needles, led to an atmosphere inside the foil tent that suppressed further fungal growth and significantly reduced the degradation of the wood.

Methods for storing logs in foil had already been developed after storms Vivian and Wiebke in 1990. Fresh, storm-felled wood is rich in living parenchyma cells, which use up the residual oxygen in a foil pile and reduce it so much that it is insufficient for wood-destroying fungi to respirate. To ensure that the oxygen content in the pile remains permanently low, it must be ensured that the foil tent remains tightly sealed. This requires a number of measures, some of which are complex. Storage areas must be leveled and freed from pointed, sharp-edged objects, water drainage must be guaranteed, mouse protection fabric must be applied and cut edges of logs must be rounded off. The effort involved in preparing a foil store is therefore not insignificant. In addition, even after the foil tent has been set up, the piles must be continuously monitored for leaks. This is usually conducted manually by measuring the oxygen or carbon dioxide content.

In many regions of Germany, however, there is a lack of long-term experience with wood stored in foil using the ‘Baden-Württemberg method’. The long-term change in the atmosphere in the foil piles and the associated change in wood quality have only been comprehensively documented in a few cases. The results of the long-term monitoring of spruce wood stores (Figure 2a), which were set up in the forest district of Dassel after the storm ‘Friederike’ in 2018, will be presented and evaluated with regard to the quality assurance of the roundwood that was achieved.

2. Materials and Methods

2.1. Monitoring the Interior Atmosphere of Foil Tent Stores

Between March and July 2018, a total of 32 foil piles with a volume of 286 to 343 m³ each were created in the forest district of Dassel, Lower Saxony, Germany. On average, the piles were 31 m long and 4 m high. Only storm-processed Norway spruce (Picea abies) was stored in sections of 5 m length. The time between processing and the closing of the foil tents was between two days and four weeks. Foil storage was carried out according to the Baden-Württemberg method, i.e., the piles were closed on all sides with foil.

Unevenness and other sources of potential damage to the foils, such as lying branches, sharp stones and roots, were removed from the storage area. Some of the storage areas were prepared mechanically, root stumps were milled out and the terrain was leveled.

All piles were laid out in the immediate vicinity of the windthrow areas on the edge of an A route (truck-capable route) on the shoulders or at the edge of the stand in order to create short distances between the location of the refurbishment and the piles, and thus to also keep transport costs low. Wind-exposed locations were avoided.

Below the storage areas, plastic floor grids (8 × 35 m) were placed to protect against gnawing by mice. Two foils were spread out on top of 14 placed wooden supports, which were smoothed if necessary, and the pile was built on top of them. The sections were placed on top of each other as precisely as possible to avoid protruding edges on the bolster faces. Protruding wood splinters and excess lengths were trimmed from the top of the pile. The piles had a rounded shape and were flat-ended (approx. 45°, Figure 2b). The ends of the logs were fixed with construction clips to prevent the logs from rolling off. A control hose to measure oxygen and carbon dioxide levels was used with an outward coupling. Another protective grid and two cover foils were spread out on the pile, which were welded to the two bottom foils so that two separate foil covers were created. Excess air was evacuated from the envelopes. Finally, the entire pile was covered with a bird protection grid.

With the gas-tight closure of the foil pile, an almost oxygen-free and carbon-dioxide-rich gas mixture developed inside the pile within a period of a few days to a few weeks. The tube installed during construction to monitor the atmosphere inside the pile was connected to a mobile measuring device (Gas Analyzer Geotech G110).

2.2. Assessment of Roundwood Quality

As an example, the raw wood quality was evaluated in two foil-packed piles (No. 5 and 9), which were prematurely terminated due to the increased oxygen content inside (Figure 3. Both piles were created in April 2018. Pile no. 5 was opened after 31 months, and pile no. 9 was opened after 34 months in February 2021. An increased oxygen content (max. 7%–9%) was previously measured in both piles over a period of several months. Six sections each were selected from the upper (approx. 3.0 m), middle (approx. 1.8 m) and lower (approx. 0.5 m) areas of the pile for the quality response and marked before the pile was removed (Figure 3).

The mean diameter of the sections was determined and both end-grain surfaces were photographed. The quality of the wood was evaluated using several cross-sections. For this purpose, a slice about 3 cm thick was cut off on both end-grain sides of the section. Further cuts were made after 20, 125, 250 and 375 cm. The log quality was assigned to quality classes B/C or D according to RVR (Framework agreement for the raw wood trade in Germany) [17]. In addition to the occurrence of cracks, the occurrence of rot came into play as follows:

B/C mix quality:

• Core cracks with a diameter of ≤1/2 of the log are permitted.
• Ring peeling with a diameter of ≤1/3 of the log is permitted.
• Superficial discoloration is permitted.
• Insect bore holes in the wood are not permitted.
• Hard rot (rot spots), which is located in the outer mantle of the buttresses up to 15% of the log diameter, is permitted.
• Softening due to rot is not permitted.

D quality:

• Core cracks are permitted.
• Ring peeling up to a diameter of ≤1/2 of the log is permitted.
• Insect feeding holes in the wood of <2 mm are permitted. Bore holes of ≥2 mm are not permitted.
• Hard rot (rot spots) is permitted, and softening due to rot is only conditionally permitted in the outer mantle of the buttresses.

Examples of the classification of logs in the respective quality classes are shown in Figure 4.

3. Results and Discussion

3.1. Interior Atmosphere of Foil-Packed Piles

The foil conservation in the Lower Saxonian forest district of Dassel was scientifically monitored during the period of March 2018 to February 2022. The plan was for the raw wood to be stored for around four years. However, about a third of the foil piles were broken up prematurely due to various types of damage. Other piles were broken up for the sale of wood. Damage to the foils was mainly caused by mice gnawing and wind movements of the foils. There was also damage from falling branches and birds. Mice penetrated behind the protective grid via the corners of the foil, some of which were only folded up flat, and from there entered through the foil into the pile’s interior, where they died due to a lack of oxygen. However, the resulting holes in the foil subsequently caused an increase in the oxygen content in the piles.

Bird protection nets were sometimes not sufficiently stable and were torn in strong winds. As a result, the foil underneath was more exposed to wind movements and was damaged. With increasing age, the tear strength and the tightness of the foils decreased. Damage caused by UV radiation could be prevented by additional covers, e.g., by straw mats already used in agriculture; however, this would result in additional costs.

In principle, a similar progression of the oxygen (O₂) and carbon dioxide (CO₂) content was observed in all 32 piles examined. Within a few days of closing the piles, the O₂ content fell to almost 0%. CO₂ content, on the other hand, rose slowly over several months and reached different maximum values between 27 and 56%. Afterwards, it dropped to values between 12 and 25%. Both O₂ and CO₂ content were in a similar range as those reported by Maier [10] who monitored beech (Fagus sylvatica) and Norway spruce logs packed in foil tents. During the summer months, there was often a slight temporary increase in CO₂ content. It can be assumed that these seasonal fluctuations in CO₂ content, which occurred primarily in the first year of storage, can be attributed to the temperature-dependent activity of the microorganisms in the pile. Due to the lack of oxygen inside the pile, some of the organisms contained in the pile die [12], so the effect wears off with the increase in storage time. If the foil is damaged, the oxygen inside the pile increases. With a time delay and after the foil was closed again, the CO₂ content also increased. This is due to the fact that the microorganisms in the pile were stimulated by the ingress of O₂, breaking down organic matter and releasing CO₂ in the process.

The interior atmosphere of the foil piles was differently favorable for the growth of microorganisms over the storage period and between the piles. As a result, the risk to the stored wood fluctuated greatly in some cases. In borderline cases, i.e., when there was a sharp increase in O₂ content, which could not be reduced again by simple repairs, piles were also broken up prematurely. Basically, the piles examined could be assigned to three categories, which are shown in Figure 5 as examples of three piles each:

(1)

low risk, due to consistently unfavorable conditions for fungal growth and wood degradation,

(2)

moderate risk, due to favorable conditions for fungal wood damage occurring in the short term,

(3)

high risk, due to favorable conditions for fungal wood damage after less than two years of storage.

The activity of both the cambium of the trees and the wood-destroying microorganisms is subject to seasonal fluctuations and is primarily dependent on respective temperature conditions [18,19]. Therefore, the processing period for the windthrown wood and the time at which the foil tents were closed may have an effect on the subsequent storage conditions in the piles. Figure 6 compares the interior atmosphere of three relatively “early” sealed foil piles (end of March and early May 2018) and three relatively “later” sealed piles (end of June and early July 2018). As a rule, the wood was processed about one to two weeks before the pile was closed.

The higher CO₂ levels in the piles sealed earlier in the year were noticeable, indicating that the fresher wood contained a higher number of living cells and/or microorganisms that were respirating O₂ and releasing CO₂. In addition, the maximum CO₂ content in the piles that were closed later only occurred after twice the length of time (approx. 80 days) as in the piles that were closed earlier (approx. 40 days). The time between the felling of the trees and the closing of the foil seemed to be decisive for the CO₂ content in the first months of storage. However, the O₂ content fell to almost 0% within a few days in all six foil piles under consideration. After a little more than a year of storage, the CO₂ content of the piles that were laid early and later also differed only slightly.

The connection between the temperature of the outside air and the CO₂ content, which is shown for a total of six piles in Figure 6, was striking in many foil piles. The temperature data were recorded by a weather station in Delliehausen/Uslar. After closing the foil, a strong increase in CO₂ content was observed in all piles, which was apparently not influenced by the respective outside temperature. Between June and August 2018, the CO₂ level reached a maximum, followed by a slow decrease over the following years, probably as a result of the decreasing numbers of living cells and microorganisms in the pile. In the following years, the higher summer temperatures led to a renewed increase in CO₂ content, which in turn can be explained by increased biological activity. The foil is not completely impermeable for gases, and it is more permeable for CO₂ compared to O₂ which, in combination with the still physiologically active microorganisms, explains the fluctuations in CO₂ content over the months as similarly observed in previous studies [10].

3.2. Quality Changes of Stored Roundwood

In both prematurely opened foil-packed piles no. 5 and 9, the increased O₂ level was the consequence of irreparable damage by mice. The damage was detected in foil pile 9 in January 2020 and in foil pile 5 in March 2020. After a temporary increase to up to 8 or 10%, the O₂ content in both piles fell to almost 0 or around 3% during the summer months. The reason for these fluctuations in O₂ content is the temperature-dependent activity of microorganisms in the pile. As the temperature increases, activity increases and the amount of O₂ consumed by respiration increases with. In other words, the oxygen content is reduced to almost 0% if the organisms in the pile respirate more O₂ than can flow through the holes in the foil. However, these processes are largely accompanied by the decomposition of the stored wood. After a renewed increase in O₂ content in winter 2020, storage was terminated, and the piles were finally opened.

After opening the foil, a characteristic odor could be perceived which, as previously described by Maier [20], was reminiscent of a “mixture of freshly cut wood, pomace and ensiled fodder”. Sometimes the wood smelled moldy and musty. It was also noticeable that the fungal mycelium was very different in intensity on the entire face of the pile and on the debarked lateral surfaces. A similar growth was observed by Maier [20] and attributed to the mold fungus Gliocladium solani. Stronger mycelial growth was observed on the shady side (northeast) of both piles.

In 2018, only wood of quality class B/C was stored in both piles. After about 2.5 years of storage, only about 30% of the logs from the random sample corresponded to quality B/C. In 70% of the log sections, at least one cross-sectional area was assigned to the lower-quality level D (

Table 1. Allocation of the assessed log sections from the prematurely terminated piles no. 5 and 9, separated according to their position in the pile.

Position in the Pile	Quality Class Percentage (%)
	B/C	D
top	42	58
middle	17	83
bottom	25	75
total	30	70

). However, 80% of the cases were classified as D due to the appearance of the outer end-grain surfaces, i.e., to the cross-sectional area exposed 3 cm from the end-grain. It was therefore assumed that more than 90% of the wood volume was of class B/C quality. Significant differences in quality between the sunny and shady sides of the piles were not determined.

According to the forestry office, the quality of the stored wood was rated significantly higher across all foil packed piles. For example, 88% of the wood in all the piles cleared by spring 2022 was class B/C and 11% was class D. Only 1% of the logs could no longer be sawn and were therefore industrial wood.

4. Conclusions

Over a period of about 3.5 years, the Dassel forestry office gained experience with the storage of windthrown timber using the Baden-Württemberg method of foil storage. In addition to logistical, technical and economic aspects, the efficiency of the measure was evaluated by long-term monitoring of the inside atmosphere of the foil tents and a comparative assessment of the log quality.

About a third of the foil piles had to be terminated prematurely due to various types of damage, while others were cleared for the sale of wood. Only in four out of 32 piles was the O₂ content close to 0% by the end of the measurement period or until the pile was opened, with two of these piles being terminated after less than two years.

The evaluation of wood quality based on two piles that were opened prematurely showed that only 30% of the sampled material was not devalued in the form of a downgrading to RVR quality class D. However, the reduction in quality was due to the occurrence of rot on the end-grain faces of the logs. By adding length and then ‘cutting it back to health’, most of the logs would still have been assigned to class B/C because the rot had only spread a few centimeters along the grain. Irrespective of this, however, the results confirm that once damage to the foil has occurred, there is little time left for to terminate the pile to be dissolved if rotting damage is to be avoided.

It is questionable whether the high costs for the installation and maintenance of the foil piles can be covered in the event of premature dissolution and whether there is any added value by using this measure, but this was not the subject of the present study.

With appropriate care, an early date for storage and rapid processing, bringing and closing, the immediate repair of leaks and close monitoring, as well as the investment in storage, can lead to a positive result according to the forest office. In order to increase the positive yield, further developments that protect against mechanical (better bird protection nets, locations away from trees at risk of storm damage) or biotic (mouse) damage should be. Risk locations and exposure to high wind loads should be avoided.