Climate Swiss trees grow less despite earlier spring
Climate change is slowing down the growth of important tree species in Switzerland. Although trees are starting to grow earlier in the year due to rising temperatures, they are producing less wood overall than ten years ago, according to a new study.
07.01.2026, 08:00
SDA
This slower growth has consequences: A smaller increase in trunk diameter means that forests absorb less CO2 and are therefore less able to slow down global warming. The Swiss Federal Institute for Forest, Snow and Landscape Research WSL warned of this in a press release on Wednesday.
According to the WSL study, the start of stem growth has been significantly delayed by several days over the last ten years. In some years, the first stem radius increases can even be measured two to three weeks earlier than at the beginning of the 2000s. At the same time, however, periods of heat and drought are increasing - and it is precisely these that slow down growth.
In fact, between 2012 and 2022, none of the tree species studied experienced stronger growth. The growth of oaks and pines remained constant, while spruces, silver firs and beeches even grew less.
Only a few days of growth per year
The reason for this lies in the water balance of the trees. The decisive factor for how strongly a tree grows is not the length of the vegetation period, but the number of days on which stem growth actually takes place. This depends on whether there is enough water available. If it is too hot and too dry, more water evaporates than can be absorbed by the roots, the tree comes under stress and growth stops.
Depending on the species, trees can only really grow on 40 to 110 days per year.
"In the end, individual days and hours determine how much a tree grows," WSL researcher Arun Bose was quoted as saying in the research institute's press release. "If some are lost due to increased heat and dry periods, a large part of the annual trunk growth is missing."
An earlier start is therefore of little use if heat and drought shorten the critical growth phases. Forestry is also affected by this. If the summer months become warmer and drier, forestry companies tend to be able to harvest less wood.
