Thinning from below or above? Implications on operational efficiency and residual stand

This thesis summarises results of four studies of thinning from below and above. Two field studies concern harvesting productivity, extraction rate, logging damage, and thinning ratios. The third study, a simulation study, concerns the effect of average size of harvested tree on productivity and differences in time consumption between work elements for a harvester. The fourth study concerns the effect of thinning type on timber quality in the residual stand. The field studies were located in Degersjoe (63 dec 49 min N, 20 deg 00 min E), Gagsmark (65 deg 08 min N, 21 deg 16 min E), and Hemmesmark (64 deg 06 min N, 20 deg 20 min E). Thinning operations were carried out with two single-grip harvesters and a farm tractor mounted processor. Operations followed normal Swedish practice, i.e. selective thinning, including cutting strip road trees. It was difficult to perform thinning from above according to the theoretical definition, i.e. thinning ratio greater than 1.0, when also extracting strip road, damaged, and suppressed trees. Variation in dbh may influence the thinning ratio. Time consumption per tree for harvesting increased with dbh of harvested trees. However, increase in time consumption was proportionally lower than that of dbh. When thinning with a harvester, work elements move and boom are influenced by the number of harvested trees, and felling and processing are influenced by harvested tree size. Harvesting productivity was 20-40% higher for thinning from above than for thinning from below. Productivity increased with increased thinning ratio, since mean stem volume of harvested trees increased. No significant differences in the frequency of damaged trees were found. Damage was located higher up on the stem in thinning from below. Small differences in timber quality on the residual Scots pine trees (Pinus sylvestris L.) were found. Only two quality variables, lean of tree and stem straightness, showed significantly different mean values. Residual trees were straighter and leaned less after thinning from above. The operators had difficulties to attain a desired residual stand density when thinning from above. When thinning ratio exceeded 1.0, residual stand density deceeded desired level.