The Center for Forest Business, in cooperation with Langdale Industries recently completed a two-year study of a tree-length harvesting system modified by the addition of a small chipper to produce biomass chips from limbs, tops, and underbrush. The system was examined in two replicated studies – one a clearcut harvest in a slash pine plantation with 7-8 tons per acre of understory and the second a first thinning of a slash pine plantation with an average of 26 tons per acre of understory. At each site, two treatments were evaluated. In one treatment, limbs and tops that would normally be left on-site were chipped. In the second treatment, the feller-buncher attempted to fell as many unmerchantable pine and understory stems as possible and this material was also skidded to the landing for chipping.
The clearcut study removed around 65 tons per acre of roundwood compared to 20 tons per acre in the first thinning. If only limbs and tops were chipped, the volumes per acre of biomass chips were similar at 3.8 tons per acre in the clearcut and 3.0 tons per acre in the thinning. When understory was also chipped, the clearcut study produced 10.8 tons per acre, compared to 20.0 tons per acre with this treatment in the thinning study. In the thinning treatment, capturing this additional understory biomass came at the expense of roundwood production. Hourly production declined from 21 tons per hour to 10 tons per hour when understory stems were chipped in addition to limb and top debris. Chipping limbs and tops alone produced chips at cost of $2-3 per ton less than attempting to chip limbs, tops, and understory stems. The cost per ton varies based on the ratio of roundwood volume produced to chip volume produced and appears to be lowest with ratio values between 5 and 10.
Biomass chipping with a small chipper appears feasible at roundwood to chip ratios between 5 and 10 and with 15 tons per acre or less to recover. At higher ratios chipper utilization is insufficient while at lower ratios roundwood production is reduced. It appears, pairing a small chipper with a roundwood operation is best suited to clearcuts where roundwood production is not already challenged by the small tree size and tight operating space associated with thinnings.
For more information on this project, see the Forest Resources Association Technical Release here.
Or for more detail on the clearcut trial only, see the Center for Forest Business Research Note #21.