Silvicultural Implications in Timber Harvest Residues
When trees are harvested for sawtimber or pulpwood, the stems are either cut to length in the forest before being mechanically forwarded to the roadside or landing, or they are extracted as whole stems (with their branches attached) to the landing prior to being delimbed and/or cut to length. Often 25 to 45% of a tree’s biomass is left on-site as residues, although the amount of residue can be lower in some forests depending on harvesting equipment and condition of the stand (Hakkila and Parikka, 2002; Smith et al., 2004). Typically, logging residue harvesting removes 20 to 30 tons per acre, on a green basis (Koch, 1985), or about half this amount on a dry weight basis. About 65% of the total logging residues can easily be recovered for use as bioenergy (Perlack et al., 2005). This residue consists of treetops, branches, twigs, foliage, sometimes stumps, and small trees remaining after extracting the higher-value products.
Harvesting timber residues is most readily applicable to even-aged silviculture where stands are clearcut at the end of the rotation. If whole-tree extraction is being used, the residues are available free of charge at the roadside (Andersson et al., 2002). Where delimbing or cutting to length occurs in the forest, the residues are frequently collected in a separate, two-pass operation. In both cases, the biomass is normally allowed to dry in piles in the forest, reducing the cost of transportation to the industrial plant, maximizing delivered energy, and allowing some foliage to fall from the branches. Leaving the foliage behind reduces the ash content and increases the calorific value; it also reduces the removal of nutrients from the site. Contamination with soil, which reduces the value of biomass fuel, can be a problem. Normally, the material is chipped prior to transport to the industrial plant. Harvesting residual biomass at the time of clearcutting has both positive and negative site impacts. The higher removal of nutrients can be detrimental to long-term site productivity, particularly on lower fertility sites, if the ash is not returned or fertilizer applied. Reducing the removal of foliage and twigs is a helpful strategy to reduce the impact of nutrient removal. Residue removal may also influence biological diversity and could sometimes increase erosion on steeper slopes. Both of these aspects are discussed in the “Biological and Environmental Sustainability” section. The additional collection of biomass may also damage advanced regeneration. Care should be taken to avoid negatively impacting the long-term productivity of the site.
On the positive side, residue removal, apart from giving the owner a direct financial return, may:
- reduce the visual impact of the harvesting operation;
- allow better access to the site;
- increase natural regeneration through improving the seedbed by increasing the exposure of the mineral soil or through promoting seedlings, seedling sprouts, and stump sprouts in hardwoods;
- reduce plantation site preparation, planting, and weed control costs;
- allow for easier selection of planting spots and manual weed control;
- reduce some pest problems such as bark beetles; and
- reduce fire risk.
For those owners employing uneven-aged systems of silviculture, biomass harvesting may be less profitable or feasible. With selection systems, harvesting is more dispersed through the forest, resulting in scattered residual biomass that entails increased machine-moving and setup costs to recover. Usually logs, rather than whole trees, are extracted to reduce damage to the remaining trees. However, biomass recovery has been employed in California where whole-tree extraction was used with group selection and the residues were concentrated at a landing. An example of a timber harvest residue operation is given in “Integrating Bioenergy Harvesting with Silviculture: A California Example.” It is important to avoid damaging residual trees or to negatively impact the long-term productivity of the site
Details of these harvesting techniques are described in the “Harvesting” section and by Andersson and others (2002).