Warm Spring Trees Lose Leaves Weeks Earlier in Study

For trees in a Swiss experiment, spring set the timetable for autumn. Warm spring trees that leafed out ahead of schedule also dropped their leaves weeks ahead of schedule, according to the Swiss National Science Foundation, which announced the findings on Tuesday. The result ties the warmth of one season to the timing of the next in a way that matters for anyone trying to read what a forest will do in a warming climate.

Until now, the usual explanation for autumn leaf fall pointed to the other end of the year. Shortening days and falling temperatures were treated as the main cues telling a tree to shut down for winter. The new work does not throw that explanation out, but it shows spring conditions pulling the schedule forward as well, with the shift carrying through the whole growing season rather than fading after budburst.

How the warming experiment worked

Researchers led by Yann Vitasse at the Swiss Federal Institute for Forest, Snow and Landscape Research ran the test on young trees of three native species: hornbeam, small-leaved linden and Norway maple. Half of the trees were kept about five degrees Celsius warmer than the surrounding air from January until mid-June, while the rest grew under normal conditions for comparison. The design let the team follow warm spring trees through budding, growth and leaf fall in the same season, rather than comparing different trees at different moments.

The warmed hornbeams and small-leaved lindens budded two to four weeks earlier than the untreated trees, and their leaves began ageing three to four weeks earlier as well. Those trees also grew more vigorously and had already stored more starch as an energy reserve by the start of summer, a sign that the early start was not just cosmetic. The season ran fast from beginning to end for them, with the autumn close arriving in step with the early spring opening.

The Norway maple broke the pattern. It also sprouted earlier under the warmer conditions, but its growth did not speed up and its starch storage did not increase, and its leaves aged on roughly the same schedule as the untreated trees. That split is part of what makes the study useful: warm spring trees in the same trial did not move together, even though species sitting in the same forest and feeling the same warmth do not necessarily answer it in the same way.

Why the timing shift matters for forests

For forest managers and climate researchers, the value of the work on warm spring trees lies in prediction. If spring warmth can move autumn forward by weeks for some species, models that only watch autumn weather will miss part of the story. The Swiss National Science Foundation said the findings could help to better predict the impact of climate change on forests and the carbon cycle, the system through which forests take in and release carbon over the year.

There are limits to what one experiment can settle. The trees in the trial were young, and the warming was applied through the first half of the year, so the results describe how these species responded under those conditions rather than a fixed rule for every forest. Still, the experiment gives researchers a concrete link, and warm spring trees are now a clearer test case for it, between a warm spring and an early autumn, measured across budding, growth, starch storage and leaf ageing in the same trees.

Leaf ageing in this study is the annual wind-down that ends with leaves changing colour and falling, and starch is the energy a tree banks for growth and for getting through winter. When warm spring trees banked more starch by early summer, they had done a full season of work ahead of schedule, which is why the researchers read the early leaf fall as part of a faster season rather than a sign of damage.

Reporting by Keystone-SDA on the announcement is available in a Keystone-SDA report on the study, with further detail from the Swiss National Science Foundation on the experiment. The announcement frames warm spring trees as a useful signal for forest forecasting.