Long-term growth decline is not reflected in crown condition of European beech after a recent extreme drought

Global warming poses a major threat to forest ecosystems around the world. In Central Europe, vitality losses and tree mortality have already been observed in various regions, especially after extreme drought episodes such as the 2018-2020 drought. European beech (Fagus sylvatica L.), which is the dominating deciduous tree species in large parts of temperate Europe, also suffered from the recent drought. In Germany, for example, losses in crown condition were observed. Within individual stands, however, the crown condition of beech differed strongly from non-/weakly damaged trees (hereafter referred to as vital trees) to severely damaged/dead trees (non-vital trees). Tree characteristics and micro-site conditions were apparently similar. Hence, we checked whether dif-ferences in growth behavior exist and (or) developed over time and assessed if observed differences between the non-/vital individuals might be genetically driven. We found that the climate sensitivity as well as the drought resistance of tree growth did not consistently differ between non-/vital trees. In both groups, a growing importance of water availability for tree growth was apparent. Further, long-term growth decline was wide-spread in both vital and non-vital trees, suggesting severe stress and future risks of tree mortality irrespective of crown condition. Only at one out of nine sites, we found significant differences in individual heterozygosity, signaling potential differences in the adaptation of trees to environmental stress. As a consequence, our study highlights that crown condition after an extreme drought is a poorer indicator of tree vitality than ring width in beech stands in Germany.

Automated summary

Global warming threatens forest ecosystems worldwide, and Central Europe has already seen vitality losses and tree mortality, particularly after extreme droughts. In German beech stands, the impact of drought is evident in decreased crown condition and long-term growth decline, highlighting the need to consider other indicators of tree vitality besides crown condition.