Produktivitet vid selektiv mekaniserad bioenergiröjning av eftersatta röjningsbestånd

Abstract: In Sweden, harvesting pre-commercial thinning (PCT) stems for energy purposes is a interesting way of financing the high cost of preparing dense stands for a profitable first conventional thinning. This concept can be labelled bioenergy thinning. The aim of this study was to quantify the productivity of a Vimek 404R light harvester with an accumulating Naarva-Grip cutting head during the bioenergy thinning of a self-regenerated stand west of Sunnansjö in Dalarna. The stand was birch-dominated, contained 15 371 stems/ha, and measured on average 4,6 cm Dbh and 7 m tall. Even though the driver was well experienced with the machine, the stand in question was the first one that he ever thinned in this manner. Nevertheless, the driver strived to accumulate as many stems as possible during each crane cycle and chose to cut taller stems in half so as to make the handling and forwarding of the harvested stems easier. On average, the Vimek harvester required ca 57 effective hours per ha to harvest ca 56 ton dry matter (DM) per ha. Thus, the mean productivity measured 0,98 ton DM per effective hour, or expressed in other terms, 232 harvested stems per effective hour. The average Dbh of the harvested stems was 4,58 cm. Since there was between parcels only a small variation in stand characteristics, no statistical trend could be identified between productivity and mean height, mean Dbh and stem density. After the bioenergy thinning, there were on average 2 310 (mainly spruce) stems/ha left standing and ca 25 % of those stems was injured in some way. This high injury frequency was partly due to the form of the parcels as their form hindered an efficent work pattern, which also undoubtedly leads to a higher time requirement and lower productivity in general. In spite of the high biomass content of the unthinned stand, the net cost of harvesting (felling-bunching) the juvenille stems with the Vimek 404R (with measured productivity) and then forwarding them to roadside with a Vimek 606 was very high. The machine system was calculated to cost almost eight times more per ha than if the same stand was PCT motor manually without harvesting the stems. However, the following could help to improve the productivity and make the concept more competitive compared with motor manual PCT: geometrical driving and crane patterns; stronger hydraulics in the harvester; a combination of mechanical bioenergy thinning and motor manual PCT where the latter is used to cut the smallest stems (<2,5 cm).