Heat slows tree growth as Alpine forests face rising stress

Measurements by the University of Innsbruck show that high temperatures are sharply reducing photosynthesis and growth in trees, according to ORF. The findings come from a pine forest in Mieming, where researchers are monitoring the effects of climate change on local forests.

Data collected from May to August 2026 show that trees initially continue photosynthesis when temperatures rise above 20 degrees Celsius, but gradually stop as the heat intensifies. During the most severe heatwaves, they stopped photosynthesising by midday to protect their water supply.

“It is a protective mechanism: the trees close their stomata to minimise water loss through transpiration,” ecologist Georg Wohlfahrt said.

On particularly hot days, the forest became a net source of carbon dioxide rather than a carbon sink because processes releasing the greenhouse gas were stronger than the trees’ uptake. Wohlfahrt said the behaviour resembled that of a Mediterranean ecosystem, even though the forest is located at an altitude of 1,000 metres in the Alps.

When trees absorb less carbon dioxide, they produce fewer carbohydrates, limiting their growth. Compared with the years 2023 to 2025, the trees observed in Mieming have added significantly less to their circumference this year. Wohlfahrt said the reduction in trunk growth became particularly clear after the first heatwaves in late May and early June.

Because trees usually stop growing for the year at the beginning of August, the researcher expects consequences for forestry, including narrower annual growth rings in the long term. “It is not only agricultural crops that are suffering, but also natural ecosystems, including our forests,” he said.

Wohlfahrt said forestry practices would need to be reconsidered. He also questioned whether Scots pine should continue to be regarded as a climate resilient tree, saying the species had shown clear signs of heat stress. However, he said Scots pine was still better equipped for changing conditions than Norway spruce, which could be weakened by heat and become more vulnerable to bark beetle infestations.

According to Wohlfahrt, tree species from southern regions are better adapted to high temperatures. Forestry therefore faces the challenge of planting species able to withstand extreme conditions. He said these future trees would not establish themselves naturally on their own and would have to be introduced through planting projects.

The researcher said recent extreme weather was changing established assumptions about forests. A continuation of current practices was no longer possible, he said, as forest ecosystems have a significant effect on the carbon balance and require faster, more effective climate adaptation measures.