Response to extreme events: do morphological differences affect the ability of beech (Fagus sylvatica L.) to resist drought stress?

Adaptive silvicultural approaches intend to develop forests that can cope with changing climatic conditions. Just recently, many parts of Germany experienced 3 years of summer drought in a row (2018-2020). This study analysed the effects of this event on beech (Fagus sylvatica L.) in two regions in northern Bavaria, Germany. For this purpose, 990 beech trees were studied on 240 plots in drought-stressed forests. We examined trees of different social position and different size. Their morphology (e.g. tree height, crown volume) was recorded by laser scanning, and drought stress was quantified by tree core sample analyses. In addition to increment analyses, the delta C-13 signal was determined by year. Results show that the dominant tree collective was particularly affected by the drought. They still managed to perform well in 2018, but the radial growth decreased significantly in 2019 and 2020, partly resembling the performance values of subordinate trees. Subordinate trees, on the other hand, provide some consistency in growth during drought years. The drought was so severe that the effects of competition on tree growth began to disappear. The difference in growth of two geographically distinct study areas equalized due to drought. With continuing drought, increasing levels of the delta C-13 signal were detected. Similar patterns at different delta C-13 levels were found across the social positions of the trees. The influence of tree morphological variables on tree resistance to drought showed no clear pattern. Some trends could be found only by focusing on a data subset. We conclude that the intensity of the 2018-2020 drought event was so severe that many rules and drivers of forest ecology and forest dynamics (social position, morphology and competition) were overruled. The influence of morphological differences was shown to be very limited. The weakening of dominant trees could potentially be no longer linear and drought events like the one experienced in 2018-2020 have the potential of acting as tipping points for beech forests.