Question 4 of 8: Full-Tree vs. Short-Wood Systems

Question 4: Full-Tree vs. Short-Wood Systems (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The core difference, as established in Question 1, is where the stem is broken down into merchantable lengths. In a full-tree system, the entire tree — stem, limbs and top — is extracted intact to a landing, and all delimbing, topping and bucking happens there, centralized around one or two processing machines. In a short-wood (cut-to-length, CTL) system, one machine at the stump performs felling, delimbing and bucking in a single pass, so what leaves the stand is already a set of sorted, merchantable-length logs; only load-carrying (not processing) remains to be done at roadside.

Full-tree system machines and limiting factors. The feller-buncher fells and groups several stems before moving on; its productivity is limited by stem size and density (larger/heavier stems slow the felling head's cycle), terrain slope and ground roughness (limiting travel speed between trees), and how efficiently it can arrange bunches for the skidder to pick up. The grapple skidder drags whole-tree bunches to the landing; its productivity is set by skid distance, ground bearing capacity and slope (wet or steep ground slows travel and can force smaller, more frequent turns), and bunch size/orientation left by the buncher. At the landing, the delimber/processor strips limbs and bucks to length; its throughput and output quality (accurate length/diameter sorting, minimal wood damage) depend on log size, branch character (heavy branching slows the delimbing pass) and how cleanly the skidder has decked the wood for the processor to reach. A key quality/output limiting factor unique to full-tree systems is landing space: all the slash (limbs, tops, bark) generated by processing accumulates at roadside and must be piled or burned, and a small or poorly laid-out landing directly caps processor throughput and can force costly slash hauling or double-handling.

CTL system machines and limiting factors. The harvester does felling, delimbing, measuring and bucking in one continuous stump-side operation, guided by an onboard computer optimizing each stem's bucking pattern against current log specifications; its productivity is limited primarily by stem diameter (a processing head has a maximum delimbing/bucking diameter capacity), reach and terrain slope (a wheeled harvester's boom reach and the machine's own slope-climbing limit determine how much ground it can process from one position), and stem form (heavy sweep or forking slows the head's grip-and-feed cycle). Output quality is largely set by the head's measuring accuracy and the operator's/computer's bucking-optimization settings, since bucking decisions made here are final (there is no later re-sort station). The forwarder then self-loads the sorted logs and carries them, fully off the ground, to roadside; its productivity depends on forwarding distance, payload capacity relative to log size, and how well the harvester has pre-sorted piles by product (a poorly sorted harvester leaves the forwarder more travel and re-sort work per trip).

Upstream-to-downstream productivity linkage. Because the two systems are series chains (Question 3), the function quality of an upstream machine directly conditions the productivity of every machine after it. In the full-tree system, if the feller-buncher piles trees poorly oriented or scattered rather than in tight, aligned bunches, the skidder's grapple cycle lengthens (more repositioning per turn), which lowers skidder productivity even though the buncher's own output figure looks unaffected; likewise a skidder that decks wood carelessly at the landing forces the processor to spend extra time re-positioning logs before it can delimb them. In the CTL system, if the harvester mis-measures or mis-sorts a stem (wrong length class, mixed piles), the forwarder either has to make an extra trip to collect the missorted piece or delivers an off-specification load to the mill, which can trigger a price discount that erases the productivity gain the harvester appeared to record. In both systems, therefore, the productivity of the system as measured at the mill gate can be materially lower than the sum of each machine's individually reported rate, unless the upstream machine's output quality (bunch/pile geometry, sort accuracy) is managed as carefully as its raw volume rate.