Aspen species cover ca. 1.5% (ca. 30 Mill. m3) of the total growing stock in Finland. Their principal end use is in papermaking currently, however, increasing proportion of logs is expected to be obtained from planted hybrid aspen stands in the future. The purpose of this study was to compare the technical properties of European and hybrid aspen (Populus tremula L., Populus tremula x tremuloides) stems in Southern Finland. Material consisted of five European aspen stands and seven hybrid aspen stands between 20 and 60 years of age. Routine measurements on the characteristics of the stands were done from 100 m2 circular sample plots, 6–7 of them per stand. The sample plots were located around one randomly selected, but sawlog-quality aspen that was then more comprehensively measured as a sample tree. Finally, altogether 75 of these sample trees were felled, bucked and cut into logs. The logs were graded and their technical quality characteristics were assessed in detail. The most common external quality defects included curves, sweep, and branchiness. European aspen trees showed better quality in terms of branchiness and stem form. Fast growing hybrid aspen trees had even 1.5 metre long clear distances between the whorls, which is an interesting possibility considering wood product manufacture. Both European and hybrid aspen suffered from diverse rot and surface defects. Based on the evaluation of logs, heart rot turned out to be more common than could be expected based on the evaluation of standing trees. Therefore, the overall quality of the logs was relatively poor compared to the impression obtained by pre-harvest measurements of the stands. As a conclusion, it turned out that planted hybrid aspen that is originally meant for pulpwood, has considerable potential to produce saw or veneer logs within significantly shorter rotation time than European aspen.
Keywords: European aspen, hybrid aspen, timber grade distribution, branchiness, stem form
Authors
Heräjärvi H.
Finnish Forest Research Institute, Joensuu Unit, Joensuu, Finland
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