Abstract

Results of precommercial thinning (cleaning) by brush saw and by a machine-mounted device, consisting of a hydraulically-driven heavy disc with two short steel flails mounted in a journal bearing, are presented. Plots were laid out in middle and south Sweden in 1983, some being observed annually until 1989. There were significant differences between the two methods as regards the number of sprouts on European aspen (Populus tremula L.) and birch (Betula pendula Roth and Betula pubescens Ehrh.) stumps six years after cleaning. Plots cleaned by machine had a larger number of birch or aspen sprouts, than plots cleaned by brush saw. Mean height of stumps was significantly greater on plots cleaned by machine. Stumps cleaned by machine under snow conditions were 30-70 cm high. There were no significant differences in the number of sprouts or in the height of sprouts, between high and low stumps. Cleaning by machine damaged 7-13 per cent of conifers classified as main stems, as compared to no damage caused by brush saw cleaning. Practical recommendations are given: cleaning by machine under a snow cover is not recommended in most cases.