As Switzerland emerges from one of the warmest summers on record, researchers from the University of Basel have found that native forest trees cope with high temperatures better than expected. The study authors caution that it is not clear whether consistently warmer and dryer temperatures still put native forests at risk. 

The three-year studyexternal link found that most native tree species are physiologically well-equipped to withstand extreme drought such as the one in 2015 by means of effective pore closure and adequate sugar storage. Initial data from the heat wave of 2018 confirm these results.

Closing the pores, however, comes at a price – a slower rate of photosynthesis. If the pores remain closed for a long time, the tree can no longer produce sugars and ultimately risks starvation. Based on this, the conventional wisdom was that trees run the risk of either desiccation or starvation during extreme dry spells. Reliable data on this phenomenon had not been available until the University of Basel study.

By closing their pores, the study found native trees prevent damage to their conductive pathways that help them absorb water from the soil and thus avoid the risk of desiccation. The researchers found no evidence that the long-lasting pore closure leads to a reduction in the trees` sugar reserves – a sign that the trees are still in good health.

The researchers indicate that people shouldn’t be alarmed by the leaves turning yellow in August – earlier than expected. “Premature leaf shedding is another safety measure to protect the trees from drying out,” explained the study leader Professor Ansgar Kahmen from the Department of Environmental Sciences at the University of Basel.

“Although brown leaves are no longer able to carry out photosynthesis, the sugar reserves are already quite full at this time of year, so the tree should be able to survive the winter as well,” says Kahmen.

While native trees are well-equipped to cope with single events such as the hot summers of 2015 or 2018, researchers caution that it remains unclear whether their safety mechanisms are sufficient to withstand a continuous increase in heat and drought events.