Length accuracy of logs from birch and aspen harvested in thinning operations

Accurate log processing by harvesters during bucking is of great importance if round timber is to be fully exploited for maximum value recovery. Logs that exceed the target length can cause difficulties in further processing (e.g., in the pulp industry), while logs which are too short may not be of further use (e.g., boards for pallets). It can be difficult to achieve accurate log lengths due to irregular tree trunk shapes or errors in the measurement system in the harvester head. The objective of the research was to investigate how tree species and log diameter affect bucking length accuracy. The research was carried out in a premature broadleaved stand of birch and aspen in the third age class in the Zaporowo Forest District, northern Poland (54 degrees 24'47 '' N, 20 degrees 8'5 '' E). The thinning operation was carried out in early spring before the start of the growing season and after the snow cover had melted. Trees were cut using a UTC 150-6LS harvester, equipped with a CTL 40HW harvester head designed for hardwood. Pulp wood was harvested from the trees with an intended length of 2.50 m and a tolerance of +/- 0.05 m. The on-board harvester computer was set to a length tolerance of between 2.46 m and 2.54 m. More than 80% of the thicker birch and aspen logs (d1/2 > 14 cm) met the target length (2.50 +/- 0.05 m), while only 70% and 50% of the thinner birch and aspen logs (d1/2 <= 14 cm) reached the target. For top logs, a different length tolerance should be applied compared to butt logs in order to achieve higher efficiency in length accuracy.