Impact of drought on the growth of European beech (Fagus sylvatica L.): A case study in the canton of Basel-Landschaft, Switzerland

The recent very dry and hot summers have induced widespread dieback of European beech (Fagus sylvatica L.) in the region around Basel. Although drought frequency, intensity and duration are expected to increase in the near future, the effects of drought events on growth of beech trees are still not completely understood. It is unclear why some beech trees died or have experienced severe vitality decline while other trees have survived the last droughts and remained vital. Here, we took advantage of the unprecedented beech mortality event that is occurring around Basel to investigate the growth response of vital and dying (unvital) beech trees to past and recent extreme droughts. To do this, different analyses were conducted. First, response and correlation functions were calculated between growth series of individual vital and unvital trees and climate variables such as temperature, precipitation and drought index. Second, a superposed epoch analysis was used to assess the impact of past extreme drought years on growth. Resilience components (resistance, recovery and resilience) were also calculated separately for vital and unvital trees and the differences between the two groups were statistically tested. Finally, linear mixed models were used to assess the influence of tree size, drought and competition on tree growth. We built three models with three time windows in order to assess the impact of the variables previously mentioned over three different time periods of the trees’ lifespan. The time windows were 1900-2019, 1970-2019, 2015-2019. The results showed a divergence in the growth of vital and unvital trees starting from the 1970s, with a strong decrease of the basal area increment for unvital trees in the last five decades. The growth of both vital and unvital trees responded negatively to summer drought. This result was also reflected in the superposed epoch analysis, which showed a strong decrease in growth in the past extreme drought years. Both vital and unvital trees were generally more resistant and had a stronger recovery during past extreme drought years. Tree resilience showed a declining trend, in particular for unvital trees. Finally, the models showed that tree vitality, drought and competition have all had a strong and significant effect on tree growth. The results highlighted a high sensitivity of beech trees to drought and climatic water stress that may impact future growth rate