Uttag av energisortiment vid gallring av contorta, ett komplement till konventionell gallring?
A recommendation for stand treatment of lodgepole pine (Pinus contorta) is, in a scientific point of view, lacking when it comes to harvesting for biomass energy. Within the next decades it is expected that 280 000 hectares of lodgepole pine will have reached thinning age on the domains of Svenska Cellulosa Aktiebolaget (SCA). Due to this the interest has grown rapidly for studying the possibilities of harvesting biofuel assortments already in the first thinning.
In this study the distribution of the biomass within lodgepole pine trees has been studied and taken into account for estimating the proportions of harvested assortments when using different methods of thinning. The methods studied were conventional biofuel-harvesting (with delimbed branches gathered separately from the round wood) and thinning with cutting of round wood and long tops (incl. branches/needles). Two stands in Medelpad, Sweden were the objects for the thinning study and data acquisition.
The study shows that lodgepole pine is a suitable species if biomass energy is to be produced. There is a higher amount of biomass allocated to branches and needles compared to Pinus sylvestris. Lodgepole pine is also a typical pioneer with rapid growth in the youth and with sustainable high production, which contributes to the possibility of using it as a source for biofuel-harvesting. If harvesting of energy is supposed to be done as a complement to the thinning of conventional timber, the long top-harvesting method is the method that should be used. Many arguments support the method, for instance the available cutting technique and the economy.
Depending of the choice of bucking limits, the fuel-harvest of long tops varied between 4.2- 11.2 ton of DW per hectare in stands containing 160-300 m3sk when the 30 percent strength of thinning was applied. The amount of fuel is equivalent to 24-57 MWh per hectare. The long tops-thinning method resulted in that a small fraction of thin pulpwood fell into the fuel assortment. However, for each cubic metre thin pulpwood that was ?sacrificed? as fuel, a total fuel quantity equivalent to three cubic metre of pulpwood was gained.
With an assumed price of 150 SEK per raw ton of wood fuel and 240 SEK per m3 (under bark) for the pulpwood, the economic calculation showed a positive outcome for the long tops-method. The net reached a peak for the bucking pattern of 7 cm (as diameter limit between round wood and long top) at the sum of 634 SEK per hectare (328 SEK per hectare for the conventional method). Corresponding figures for 9 cm bucking pattern was 114 and 119 SEK per hectare, respectively.
The stimulus from an economic point of view, to harvest biofuel assortments in thinning, might be quite moderate. However, as the demand seems to increase on the biofuel market, the price of biofuel should increase. To improve the long top method further, the vertical distribution in individual trees of stem- and branch biomass should be known. This could give a possibility to find the optimal diameter limit, depending on actual prices, between conventional timber/pulpwood and long top for every single tree. An algorithm could probably be made to be used in the computer of the harvester.