Processing

Once a tree has been felled, further operations are required to separate different products and remove material that cannot be utilized. These processing steps can be performed in the woods, at the landing, or at the wood-using facility prior to manufacture. In general, processing should be done as soon as possible in harvesting to minimize the investment in handling of waste material. Leaving as much of the logging residue in the woods keeps nutrients in the woods to retain site productivity.  In a simulation study, LeDoux (1986) found that cable logging costs could be reduced by 20 to 32 percent through bucking into optimum log lengths.

Strokeboom Delimber

Delimbing is the removal of branches from the tree. In manual felling operations, the faller usually delimbs at the stump with a chainsaw. Hardwoods with heavy limbs may require manual delimbing prior to extraction. Mechanical harvesters also perform delimbing at the stump, but their capability to remove limbs is limited to smaller diameters and to trees with a single main stem (excurrent form). Gate delimbers are used with softwoods and excurrent hardwoods. The gate is a large grid of steel pipe set vertically in the woods. Grapple skidders can back the load into the grid, where the limbs break off.  Greene and Stokes (1988) found that gate delimbing reduced the skidder’s total productivity by 32 percent, but the total harvesting cost was reduced through savings on manual delimbing. Pull-thru delimbers usually are attached to a log loader; they encircle a stem with a set of curved knives. As the loader pulls the tree through the knives, limbs are sheared off. Miller and Greene (1992) found that in some cases, pull-thru delimbing improved system output; in other situations manual delimbing or gate delimbing was cheaper. Pull-thru delimbers may meet product specifications for pulpwood, but they are not able to closely trim limbs for sawlog products.  Strokeboom delimbers are machines that pick up felled trees, grasp the butt of the tree, and slide the delimbing knives along the stem using a long straight boom.  Various bucking or topping saws on the strokeboom cut the tree into measured, processed log lengths.  Because of the size of strokeboom delimbers, they are generally employed at roadside or landing locations.  Special processing heads can also be attached to swing-type forest machines.  Using feed rollers and delimbing knives, processors measure, delimb, and buck.  Processor heads for hardwood operations may be specially designed to handle heavier limbs and may incorporate a topping saw to make it easier to buck around forks and limbs.

Topping is a processing step to remove the upper part of a tree at some defined minimum stem diameter. The minimum diameter will vary depending on product specifications. With manual felling, topping may be done at the stump while a whole-tree harvesting system may perform topping at the landing.

Bucking is the division of a tree into defined log-length segments. The lengths of the logs can strongly affect value, and determining an optimum bucking mix for a given tree can be very complex. Computer-aided bucking tools are being developed for that purpose (Frayer and others 1995). Bucking cuts must be precisely located, and they must be made perpendicular to the stem in order to avoid waste. Manual bucking can be done in the woods to facilitate the extraction of large-diameter logs or to leave cull material on the site. Harvesters use electronic sensors to closely measure tree size and cut logs to length in the woods as an integral part of the mechanical felling operation. When hardwoods are being extracted with skidders, however, the bucking function is usually done at a landing to improve the quality of the bucking decisions.

Chipping is another processing function that reduces trees or parts of trees into small, regularly-sized pieces through a slicing action.  Whole-tree chippers are available in a wide range of sizes and designs including small, towed, hand-fed models as well as large, self-propelled machines with their own loaders.  A whole-tree chipper combined with a delimber-debarker can produce clean quality chips for pulp production.  Chipping whole trees without debarking produces “dirty chips” that are generally a lower-valued energy product.  Grinding is similar to chipping except the tree is reduced through a shredding action into a coarser, irregular-sized fraction.  Tub grinders and horizontal grinders cannot produce pulp chips, but may be better suited for economical conversion of logging residues into hog fuel.