Forests under climate stress – Why trees are growing less despite an early start

It is getting warmer. While our environment is suffering from the consequences of climate change, there has been a small glimmer of hope: our forests absorb carbon dioxide (CO₂) from the air and store it in wood. In theory, warmer conditions and an extended growing season could have helped forests capture even more CO₂. “But this assumption doesn’t hold up,” says Arun Bose, an ecologist at WSL. “It was believed that a longer warm season might offset some of the negative effects of climate change. Unfortunately, that is not the case.”

In his study, Bose and colleagues evaluated the growth of five of the most common tree species at 48 locations in Switzerland. “Over the last eleven years, the growing season has shifted forward by several days. This is the period of the year when trees can photosynthesise,” explains Bose. However, the earlier start to the season does not automatically lead to more growth – on the contrary. Between 2012 and 2022, annual radial stem growth declined in many stands. Increased heat waves and dry spells put trees under stress and inhibit their growth. The decline is particularly pronounced in silver fir, beech and spruce, while oak and pine trees annual growth rates remain stable. One thing is certain, however: none of the tree species studied benefited from the warmer climate.

Consequences for carbon storage and forestry

Trees grow when sufficient water is available. If it is too hot and too dry, more water evaporates than can be absorbed by the roots – the tree becomes stressed and growth stops. Depending on the tree species and weather conditions, this leaves only 40 to 110 days of growth per year. “In the end, a few days and hours determine how much a tree grows,” explains Bose. “If some of these are lost due to increased heat and dry periods, a large part of the annual stem growth is missing.” An earlier start is therefore of little use if heat and drought shorten the critical growth phases.

This has consequences for the services that forests provide. Trees absorb CO₂ and store it in wood. Put simply, the larger the stem diameter, the more carbon is bound in a tree. If the effective growth period of the most important species decreases, their absorption capacity also decreases.

Forestry is also affected. If the summer months become warmer and drier, forestry operations will tend to harvest less wood. In addition, management practices will have to be adapted to climate change. “How trees respond to climate change depends on both their location and species. That is why it is important to assess management strategies locally and species-specifically,” emphasises Bose.