Finnish Forest Research Institute  Metla

Press release 21.8.2006

Aspen has potential for select grade timber products

A study by the Finnish Forest Research Institute (Metla) found the process-technological properties of hybrid aspen to be as good in reality as those of the indigenous European aspen. Properties of aspen may be modified e.g. through heat or compression treatments to correspond to the demands of furniture and interior decoration products.

The rot content of aspen logs impairs the quality of sawn timber or veneers. However, yield of sawn timber free of knots and rot may be significantly increased e.g. through the finger-jointing technique. The hybrid aspen is the only fast-growing cultivated tree variety to grow in Finland. It is not worth growing for much longer than 20 years, as its growth slows down and the risk of rot increases. So-called second generation hybrid aspen, regenerated from root suckers, grow at least as rapidly to reach final felling dimensions.

The study assessed the properties and differences of European and hybrid aspen from the point of view of wood product manufacture. The material consisted of test trees felled from stands in southern and central Finland, sawn and dried using three different methods (warm air drying, compression drying, heat treatment). Test specimens were prepared from the sawn timber for measurement of physical properties and lamination tests. At the interim stages of processing the material, quality assessments of the trunks, logs and sawn boards were made.

The results show only minor differences between the varieties of aspen in terms of trunk shape, number of knots and physical properties of the wood material. Conversely, there are clear differences between growth rates and growth rhythm across the whole life cycle. Diametral growth of the hybrid aspen is very rapid for the first 15-20 years, but after this its growth slows down to the level of European aspen and lags behind it soon after 20 years. European aspen grows evenly but rather slowly in comparison to the hybrid aspen at least until the age of 50. The wood material of hybrid aspen is on average a little lighter than that of European aspen, which is also evidenced by somewhat inferior mechanical properties. Various drying or modification methods can affect not only the appearance of aspen wood, but also its physical properties. Compression drying can be used to increase the timber density – particularly of the surface layer – which in turn shows as improved mechanical performance. In heat treatment, mechanical properties of the timber are clearly impaired, with the exception of compression strength.

With modern technology, even medium-quality aspen log can yield considerable proportion of knot-free lumber, which can be processed into high-value joinery products. If the timber procurement in a cost-effective way is successful, as may be expected by the time the hybrid aspen stands planted at the end of the 1990s and beginning of the 2000s reach harvesting age in 10-15 years, mechanical processing of aspen may grow to some tens of thousands of cubic metres, i.e. to many times the current levels. However, the cultivated area of aspen should be further increased in order to secure steady availability of raw material both for wood product and pulp industries. Increase of cultivated area is restricted by excessive elk and deer stocks and the low price of aspen pulpwood.

Publication (in Finnish): Heräjärvi, Henrik, Junkkonen, Reijo, Koivunen, Hannu, Metros, Juha, Piira, Teppo & Verkasalo, Erkki. 2006. Metsä- ja hybridihaapa sahatavaran ja jatkojalosteiden raaka-aineena. Metlan työraportteja 31. 102 s.

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