Swiss study links warmer springs to earlier autumn leaf fall

Warmer spring conditions shifted the autumn timetable for two native tree species. In a Swiss experiment, young hornbeams and small-leaved lindens began shedding leaves three to four weeks earlier after researchers exposed them to temperatures around 5°C above the surrounding conditions. The results, announced by the Swiss National Science Foundation on October 27, point to a link between spring warmth and the end of the growing season.

That link matters beyond the appearance of Swiss forests. Trees absorb carbon dioxide through their leaves, so the timing of leaf fall affects how long they can take up carbon during the year. The study’s findings could help researchers improve predictions of how climate change will affect forests and the carbon cycle. The response was not uniform across all the trees tested: Norway maples also sprouted earlier, but their leaves did not age sooner. That difference places tree species at the centre of the findings.

Researchers raised temperatures by around 5°C for half of the young trees. The team, led by Yann Vitasse of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), applied the warmer conditions from January to mid-June. The experiment involved hornbeams, small-leaved lindens and Norway maples, allowing the researchers to compare how different native species responded to the same treatment.

Hornbeams and small-leaved lindens budded two to four weeks earlier than the control plants. They also grew more vigorously and had stored more starch by the start of summer. Their leaves began to age three to four weeks earlier. Norway maple followed a different pattern: it sprouted earlier, but neither its growth nor its starch storage accelerated, and its leaves aged at the same time as those of the control trees. The contrasting outcomes show why projections for forests need to account for the traits of individual species, rather than treating trees as a single group.

The experiment adds spring conditions to the factors researchers consider when explaining autumn leaf ageing. For a long time, falling temperatures and shorter days were seen as the main triggers of leaf senescence. The Swiss findings indicate that conditions earlier in the year can also shape when some trees begin to lose their leaves.

The researchers have not established which factor causes the earlier senescence. They plan further experiments and will analyse field observations alongside satellite data. Those next steps can help test how the controlled results relate to trees growing under outdoor conditions. The Norway maple’s response also underscores the importance of separating one species’ pattern from another: it budded earlier in the warmed treatment, yet its leaf ageing did not advance. For foresters and climate researchers, the relationship between spring warmth, summer growth and autumn leaf timing is a further piece of evidence to incorporate into assessments of changing forest seasons.