Extraction Systems

Primary extraction, also known as in-woods transport, is the process of moving felled trees or logs from the stump to the landing (Conway 1976). There are a number of ways the logs are transported to the landing. Skidding is extraction by dragging logs or trees across the ground. Pulling logs with horses or mules (animal skidding) was the main mode of extraction historically and is a method that still finds some application, particularly on small tracts and in selection cutting prescriptions. Mechanical skidding substitutes either a tracked or wheeled machine to pull the load. Another primary extraction method is called forwarding. Forwarders are similar to heavy-duty trucks; they carry the load completely off the ground rather than dragging the material. Skidding and forwarding are often referred to as “ground-based” systems because the load support and the traffic occur on the ground. Cable yarding systems leave the machinery at roadside and pull the logs out of the woods using a system of lines and blocks. Some cable logging may lift the logs completely off the ground during extraction, while in other instances the logs may be dragged. Aerial logging, mostly done by helicopter, lifts the load completely off the ground during primary extraction. Shovel logging, a technique transplanted from the Pacific Northwest, uses a tracked log loader to repeatedly lift and swing logs or trees out of the forest.

Primary extraction is a critical function of logging systems. Most soil disturbance and residual stand damage occur during primary extraction. The majority of operational costs from stump to truck are incurred in this phase. This is why it is important to match the type of primary extraction system to the terrain, soils, road network, and product specifications. The highest cost extraction systems (cable and aerial systems) require specialized crews and significant sale volumes, but create the least amount of site disturbance and can operate with lower road densities. Ground-based systems, on the other hand, are generally lower cost and work effectively on cutting units as small as several acres. Stuart and Carr (1991) examined ground-based harvesting systems in the Appalachians and found that overland skidding, within slope limitations, resulted in less potential for soil erosion and disturbance than constructing bladed skid trails.

Combinations of systems may be used to get wood from stump to roadside in a process called “swinging.” The initial extraction step brings logs or trees from the stump to an in-woods landing. At this accumulation point, the material may be sorted, processed, or simply built into larger payloads for more efficient long-distance extraction to roadside. The second part of the primary extraction may then be done with a large-capacity forwarder, a cable system, or a helicopter. The primary advantage of swing extraction systems is less road construction is needed (Stokes and others 1992).