Height development of Betula and Salix species following pre-commercial thinning through breaking the tops of secondary stems: 3-year results

Reducing competition by topping secondary stems may be an attractive altern ative to traditional pre-commercial thinning in forestry, for both biologic al and financial reasons. In this study, the height development of secondar y and main stems of birch (Betula pubescens Ehrh. and Betula pendula Roth, species not separated), and secondary stems of willow (Salix spp.) followin g such topping was examined in an even-aged and naturally regenerated stand in northern Sweden. At a main stem mean height of 2.8 m, the following tre atments were applied to all secondary stems: top-breaking at 40 per cent (1 11 cm above ground) or 65 per cent (184 cm above ground) of main stem mean height. A control group (average height of secondary stems, 221 cm) was lef t untreated. Post-treatment damage to the trees was frequent for both gener a. For birch, treatment rankings for all the secondary stems, and for undam aged secondary stems, according to height development after three growing s easons, were control greater than or equal to top-breaking at 65 per cent I top-breaking at 40 per cent. For willow, the greatest height growth was ob served after top-breaking at 65 per cent. The results indicate that the 'st ump height' (break-point) could be raised to a level of at least 65 per cen t in this birch stand without risk of the main stems being overtopped. Duri ng the study period, 50 per cent of the main stems exchanged their leading shoots at least once.