Finnish Forest Research Institute  Metla

Press release 27.01.2006

Decay resistance of Siberian larch correlates with flavonoid concentration in heartwood

The heartwood of Siberian larch and Scots pine is valued for its decay-resistance. As building material for constructions subject to occasional moisture, heartwood is a choice for impregnated sapwood. This study focused on heartwood of Siberian larch (Larix sibirica Lebed.) and the correlation of certain characteristics of wood substrate with decay rate. The results indicate that both high flavonoid concentration and high total phenol concentration in heartwood correlated with slow decay rate.

Varying decay-resistance is a disadvantage for the use of Siberian larch heartwood. Variation is detected both between different trees and within each stem. Timber cannot be pre-sorted for decay resistance in a similar way as stress rating is performed.

The aims of the study were to determine wood density and water adsorbtion capacity (hygroscopicity) as well as concentration of extractives. Two groups of extractives were studied: water-soluble arabinogalactans (carbohydrates) that are known to be abundant in Siberian larch, and phenolic compounds, particularly flavonoids that are assumed to have properties that prevent fungal degradation. The objective was to determine whether such properties can be utilized in indirect measurements of decay resistance in timber.

The results suggested that decay resistance of Siberian larch could, in theory, be measured indirectly either based on flavonoid concentration or total phenols concentration. In practice, however, this is not feasible at the moment due to lack of sufficiently fast equipment. Wood density or hygroscopicity did not correlate with decay rate. As an additional result it was noted that a high arabinogalactan concentration made heartwood significantly more hygroscopic. This may associate with the Siberian larch’s strong deformation under changing humid conditions.

The material was achieved from a Siberian larch stand at Jämsänkoski. The decay tests were performed according to EN 113 Standard in the laboratory of the Swedish University of Agricultural Sciences in Uppsala. Three brown-rot fungi (Coniophera puteana, Gloeophyllum trabeum, Poria placenta), commonly found in wooden buildings, were used as the fungal agents.

Publication: Martti Venäläinen, Anni M. Harju, Nasko Terziev, Tapio Laakso and Pekka Saranpää. 2006. Decay resistance, extractive content, and water sorption capacity of Siberian larch (Larix sibirica Lebed.) heartwood timber. Holzforschung, Vol. 60, s. 99-103.

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