Enormous forest damage has been caused by climate change, extreme weather events and the mass proliferation of insect pests. In recent years, spruce trees in particular have fallen victim to bark beetles on a massive scale. The majority of these damaged or dead spruce trees cannot, however, be harvested promptly. Some of them remain in the forest for several years without being felled (dead standing trees).

Many forest owners are faced with the decision of whether to harvest infested and dead trees immediately, leave them standing, or take them to a temporary storage site. In the case of dead trees that are left standing in the forest, as well as other storage options, there is a loss of quality. This is primarily due to the damage process caused by drying, as well as additional fungi and insect infestation. As a result, the mechanical properties of the wood can change, which can influence its usability.

In the project, the partners focused exclusively on the utilization of spruce trees that had died as a result of bark beetle infestation. “The pivotal question was whether dead spruce trees that remain standing in the forest or stored horizontally — and which are therefore exposed to, for example, fungi and insects — are still suitable for processing into solid-wood products or wood-based materials. As damaged wood is often used directly for energy production, a potentially valuable raw material for the circular economy is thereby lost. We think in terms of cascade utilization, and were able to demonstrate that this damaged spruce wood can be processed into glued solid-wood products following a standing-storage period of two years. If stored for up to at least four years, it can still be processed into, for example, oriented strand boards (OSB), particle boards and medium-density fiberboards (MDF) without any major loss of quality. In collaboration with our project and industry partners and under the leadership of the Deutsche Säge- und Holzindustrie Bundesverband (German sawmill and wood industry association, DeSH), we have drawn up guidelines based on our findings on the storage and processing of damaged spruce wood. As a result, our findings are of direct help to forest owners and the timber industry. Other partners in the joint project led by the Fraunhofer WKI were the IHD Dresden, the University of Göttingen, and Wald und Holz NRW,” reported Malte Mérono, Project Leader at the Fraunhofer WKI.

At two sites in Germany — in the Harz National Park and the Sauerland — damaged and dead spruce trees that had died at different times and had remained standing for varying periods were sampled from large-scale disaster areas. On this basis, the researchers systematically recorded the degradation progress of the wood as a function of time. In addition, tests were carried out with dry-stored calamity spruce wood of different storage durations.

Furthermore, sawn-timber products, such as boards, planks and square timber, as well as glued solid-wood products, such as cross-laminated timber (CLT) and glued laminated timber (glulam), were produced and evaluated. The investigations encompassed visual and mechanical sorting as well as the testing of the bonding properties and mechanical characteristics. These included bending, tensile, block-shear and delamination tests in accordance with the standards.

“At the Fraunhofer WKI, we have produced exemplary wood-based materials from wood that has been standing-stored for a long period of time, and have evaluated them on the basis of normative requirements. In particular, we examined OSB, particle boards and MDF. The tests were oriented to the relevant product and CE standards. Among other things, we analyzed the wood moisture content, bulk density, thickness swelling, bending strength and transverse tensile strength,” explained Peter Meinlschmidt, scientist at the Fraunhofer WKI.

The project results show that the standing-stored spruce calamity wood can be utilized for the production of OSB, particle board and MDF as well as insulation materials. However, compared to fresh wood, a significantly higher proportion of fine particles and dust is to be expected during processing.

The practical, easy-to-understand guidelines developed on the basis of the project results contain product-specific sorting criteria. They therefore help the timber industry to obtain the best possible wood assortments from damaged timber for the intended end product.

“With our project, we are providing an important contribution towards making wood an efficient and sustainable resource, even under changing climatic conditions. The project, with its many facets and, in particular, the guidelines, can help other regions in Germany and Europe to respond in a better and more targeted manner to similar calamities in the future,” Malte Mérono is convinced.