The forests in the Pacific Northwest are highly productive for timber and are a major factor in the economies of the region. The Pacific Northwest is the leading producer of lumber and plywood in the country. The use of harvester-forwarder cut-to-length harvest systems as a method for timber harvests in the region is increasing. Understanding which factors affect the productivity and costs of the system can help harvest managers plan harvests more effectively.
This research sought to determine if understory vegetation height affects productivity and stump-to-truck costs in harvester-forwarder cut-to-length thinning harvests. The study was a case study of two harvest units and two sets of harvester-forwarder (PONSSE Scorpion King harvester/Buffalo forwarder and PONSSE Bear harvester/Elephant King forwarder) systems. A detailed time study was completed for all equipment (harvesters, forwarders and loader) to determine which variables affected productivity. Understory vegetation was used as a variable for the harvester time study and classified into “short” (shorter than 0.91 m), “medium” (between 0.91 m and 2.44 m) and “tall” (above 2.44 m).
Regression results have “tall” understory vegetation reducing productivity of the Scorpion King harvester by 21%, and for the Bear harvester “tall” understory vegetation decreases productivity by 31% compared to “short”. The increase in understory vegetation height resulted in increased stump to truck costs by 12 to 17% . For more accurate assessment of the influence of understory vegetation on productivity and costs, more field studies need to be completed on a wide range of stand characteristics.