Crop beech ( <i>Fagus sylvatica</i> L.) trees with radically released crowns as a tool for sustainable production and adaptation to climate change

Abstract European beech is one of the most important tree species in Europe, however, its sensitivity to intensifying drought caused by climate change currently results in its decline. In our experiment in the Czech Republic, we investigated the impact of crop tree selective thinning (CTST) on radial increment of beech between years 1999 (2005 in the case of the younger stands, respectively) and 2021. Our modification of CTST is one of the adaptive measures for stabilization of even-aged temperate beech forests, where 30–50 crop trees/ha grow in a stand and where crowns are continuously released. Based on the periodic current diameter at breast height increment model, the crop trees responded to CTST by significant diameter increment accelerations (compared to unreleased crop trees), also throughout a drought period that occurred during the experiment. Due to this acceleration, the crop trees gradually increased their share of the growing stock (from 10% up to 37.7%). As a result, the crop trees achieved or will have achieved the target diameter of 50 cm already at the age of 73 or 64, respectively (depending on the stand age when CTST was first implemented). This reduces the ecological and economic risks of traditional beech silviculture with a long rotation period (between 100 and 120 years). In addition, the crop trees compensated volume loss due to thinning (intensity 37–112 m3ha−1) with no negative influence on production characteristics on the level of forest stand (growing stock 212–354 m3ha−1, total current volume increment 12.34 m3ha−1 year−1 in an older stand and 16.28 in a younger stand). We also observed that under CTST, the diameter distribution was approaching a reverse J-shaped curve with higher variability. Based on our findings, a modified CTST method represents a promising adaptive measure for enhancing the vitality of individual trees, and as a tool for conversion to an uneven-aged forest.