Growth dynamics of hardwood stands during the precommercial thinning phase – Recovery of retained stems after competition release

Abstract

Early management of hardwood stands is important if the goal is to produce high-quality timber and it may also reduce the rotation period. Both effects are favourable for the economic outcome of hardwood forestry. Eight young hardwood stands, dominated by either birch (Betula pendula Roth and B. pubescens Ehrh.), black alder (Alnus glutinosa (L.) Gaertn.), aspen (Populus tremula L.) or lime (Tilia cordata Mill.), in need of precommercial thinning, were identified in southern Sweden. The aim was to assess the effect of delayed thinning and thinning intensity on retained trees under north European conditions. Three treatments were applied: (1) 5years delayed thinning, (2) standard thinning, and (3) strong thinning to 2/3 the stand density of treatment 2. Retained trees were followed during 11years and showed that delayed thinning resulted in smaller crowns and slower diameter development than in the thinned alternatives. There were only minor differences between standard and strong thinning. After thinning the trees of the delayed thinning alternative showed partial recovery of the crowns. There were no longer any significant differences in green crown base height although differences in green crown radius and crown ratio remained between the delayed and the other two thinning alternatives. Furthermore, diameter growth rate of the delayed thinning trees recovered to about the same level as for trees with earlier thinning. However, differences in diameter and in the ratio diameter:height persisted, indicating that diameter loss in the early rotation phase cannot be compensated for later. It is concluded that early precommercial thinning lead to better early diameter growth than delayed thinning but also that young stands with late thinning can recover in diameter growth rate resulting in shifted but parallel diameter growth curves.