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Drought & beech: effects of the 2018 summer drought on beech on the northern side of the Alps

Updated: Dec 9, 2023



WHFF project: 2019.15

Project leader: Thomas Wohlgemuth


The short video about the project on YouTube can be watched at the following link (only available in German): https://youtu.be/DWs3Z9QO-Zs



The most important facts in brief

  • The project investigates which sites in Switzerland will still be suitable for beech in a few decades or on which sites serious failures have to be expected.

  • The growth and crown damage of beech was investigated under different site conditions

  • Crown damage was greatest on the driest sites with shallower soils and on smaller and slower growing trees

  • Beech competitiveness has declined on suboptimal lowland sites and the species is expected to retreat to sites with better water retention.

  • However, complete disappearance of beech is not expected at this time.


Project description

The summer drought of 2018 was the longest and most extreme period of drought without rainfall on the northern side of the Alps since the beginning of systematic weather records in 1864. During the hottest phase of this drought in July and August, beech and other broadleaf tree species in the driest regions of Switzerland began to show premature leaf discoloration, which in some cases led to premature leaf fall.


In the following year 2019, the extent of drought damage became clearly apparent, especially in the cantons of Jura (Ajoie), Solothurn, Baselland and Schaffhausen, where locally beech stands with tree mortalities of 50% to 80% were reported.


Since, due to climate change, pronounced dry periods such as the one in 2018 are expected to occur more frequently in the future, the question increasingly arises as to which sites in Switzerland will still be suitable for beech in a few decades or on which sites serious failures must be expected.

Within the framework of Module 1, the crown condition and pathogen infestation of the trees from the previous 1000 Beech Project were surveyed again in the summers of 2020 and 2021. In Module 2, the crown condition of nearly 200 beech trees was assessed in nine beech stands in the Ajoie (canton of Jura), which were affected to varying degrees by the 2018 summer drought, in the summers of 2020 and 2021.



Conclusions

Beech trees on dry soils were most affected by the consequences of the 2018 drought in areas with climatic drought, especially in the investigated stands in the cantons of Baselland and Schaffhausen. There, both premature leaf discoloration and early leaf fall can be considered an indication of beech weakening. Of the trees with these signs, 10% died by 2021 in Schaffhausen and 7% in Baselbiet. These percentages should be considered conservative, since another 20% of the trees in the sample were felled prematurely for safety reasons, including severely damaged ones). In the Bremgarten/Affoltern region (AG/ZH), the percentage was significantly lower at 2%. Crown mortality reached a maximum of 30% on average in Schaffhausen in 2020.


The wet 2020 resulted in lush greenery and lower crown mortality in the following year 2021, due to the new formation of clovers. Trees with strong crown mortality also had the most holes drilled by bark beetles in 2021. One year after the 2018 drought, the percentage of trees with slime flow on the trunk culminated at a percentage of 22%. In the two following years, slime flow generally decreased.


The study of nine forest stands in Ajoie showed that beech stands on shallow soils and within the stands low-competitive beech trees with lower growth vigor showed greater damage than such beech stands on soils well supplied with water and within the stand dominant beech trees with high growth vigor.


Based on the results of this project, the authors conclude that beech on shallow soils with limited water storage capacity will probably cope less well with extreme droughts under climate change in the future. Regions with repeated negative precipitation balances in summer are particularly affected, as are shallow soils in climatically favorable beech locations. Small-scale differences caused by topography and edaphic differences may lead to the patterns encountered in the field: severely damaged beech trees in close proximity to nearly undamaged trees.



Click here for the full report:

2019
.pdf
Download PDF • 9.96MB

You can find more information about the project on ARAMIS.




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