Thinning increases drought tolerance of European beech: a case study on two forested slopes on opposite sides of a valley

Abstract

European beech (Fagus sylvatica L.) is one of the economically most important broadleaved tree species in Central Europe. However, beech shows high drought sensitivity and calls for profound research to test its ability to cope with limited water resources. Here, we investigated the drought tolerance of beech to the 2003 drought as influenced by Kraft class, aspect and thinning intensity. Annual basal area increment data of 126 sample trees from southwest Germany were used to assess the variability of drought tolerance indices, by comparing three social classes (predominant, dominant and co-dominant), two contrasting sites [a dry southwest (SW) aspect and a moist northeast (NE) aspect], and three treatments [control, strong thinning (stand basal area 15 m2 ha−1) and very strong thinning (stand basal area 10 m2 ha−1)] in mature beech stands. Our results show that the co-dominant and dominant trees had lower growth recovery and lower growth resilience after the drought, compared to the predominant trees. The differences between aspects pointed to a growth–drought tolerance trade-off, in which trees on the SW aspect displayed lower growth rates but higher resilience indices than trees on the moist NE aspect. Furthermore, our results suggest that the resistance to and resilience after the 2003 drought significantly increased for the thinned trees. Our results provide novel insights into the linkage between the forest stand management and drought tolerance of beech under contrasting sites. We conclude that thinning can partially alleviate effects of severe drought on European beech forests in southwest Germany and can be applied as an adaptive measure to increase the mitigation potential of beech stands.