Spring sets course for autumn in trees, says Swiss study

Spring sets the course for autumn in trees. Keystone-SDA

A warm spring causes some trees to shed their leaves several weeks earlier in autumn, according to a Swiss experiment involving three native tree species.

The findings could help to better predict the impact of climate change on forests and the carbon cycle, the Swiss National Science Foundation (SNSF) announced on Tuesday.

A research team led by Yann Vitasse from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) studied young hornbeams, small-leaved lindens and Norway maples. To do this, it exposed half of the trees to temperatures around 5°C above the ambient temperature from January to mid-June.

Under these conditions, the hornbeams and small-leaved lindens budded two to four weeks earlier than the control plants. Leaf senescence also began three to four weeks earlier in these trees. Furthermore, both tree species grew more vigorously and had already stored more starch as an energy reserve by the start of summer.

The Norway maple reacted differently: although it also sprouted earlier, its growth and starch storage did not accelerate. Its leaves therefore did not age any earlier than those of the control plants.

Implications for the carbon cycle

For a long time, researchers assumed that falling temperatures and shorter days in autumn were the main triggers of leaf senescence. The new findings show that spring conditions also play a role, according to the SNSF.

It is still unclear which factor triggers the earlier leaf senescence. The researchers therefore intend to carry out further experiments and analyse field observations as well as satellite data.

The findings are also relevant for climate models. Trees absorb carbon dioxide (CO₂) from the atmosphere via their leaves. If the growing season ends earlier, the period during which they sequester carbon could be shortened. The extent to which this affects forests depends, among other things, on the tree species and the respective environmental conditions.

The Swiss National Science Foundation emphasised that the 5°C warming simulated in the experiment is not a direct prediction of future climate trends. However, unusually warm conditions in spring could occur more frequently in the future.

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Translated from German, reviewed by an English Department journalist.