Across a Continent, Trees Sync Their Fruiting to the Sun

European beech trees more than 1,500 kilometers apart all drop their fruit at the same time in a grand synchronization event now linked to the summer solstice.

Each summer, like clockwork, millions of beech trees throughout Europe sync up, tuning their reproductive physiology to one another. Within a matter of days, the trees produce all the seeds they’ll make for the year, then release their fruit onto the forest floor to create a new generation and feed the surrounding ecosystem.

It’s a reproductive spectacle known as masting that’s common to many tree species, but European beeches are unique in theirability to synchronize this behavior on a continental scale. From England to Sweden to Italy — across multiple seas, time zones and climates — somehow these trees “know” when to reproduce. But how?

The recent discovery about European beech trees and the summer solstice was made by a team of researchers at Adam Mickiewicz University that included (from left to right) Jakub Szymkowiak, Michał Bogdziewicz and Valentin Journé, among others.

A group of ecologists has now identified the distinctive cue — what they call the “celestial starting gun” — that, along with balmy weather, triggers the phenomenon. Their analysis of over 60 years’ worth of seeding data suggests that European beech trees time their masting to the summer solstice and peak daylight.

It’s the first time scientists have linked masting to day length, though they still don’t know how the trees do it. “It is striking to find such a sharp change one day after the solstice. It doesn’t look random,” said Giorgio Vacchiano, a forest ecologist at the University of Milan who was not involved in the research.

If further research can show exactly how trees sense daylight at the molecular level, “that would be truly spectacular,” said Walt Koenig, a visiting fellow at Cornell University and a retired research zoologist from the University of California, Berkeley, who wasn’t involved with the study. The discovery of the genetic mechanism that governs this solstice-monitoring behavior could bring researchers closer to understanding many other mysteries of tree physiology.

Ecologists have floated various theories to explain the mysteries of masting. One idea is that, for wind-pollinated plants like beech trees, synchronized flower production improves pollination efficiency — the high, spreading plumes of pollen during masting produce more offspring. It may also be beneficial because masting trees go through periods of boom and bust, with high-masting, fruitful summers followed by low-masting, barren ones. (Researchers mostly agree that trees use low-masting years to store up resources for high-masting years.) Because of that variation, synchronized masting is likely to have value as a defense mechanism: Lean seed production in low-masting years can starve predators, and prolific production in high-masting years can overwhelm them.

So it’s easy to see why masting trees synchronize their seed production. Understanding how they do it, however, is more complicated. Plants usually synchronize their reproduction by timing it to the same weather signals. And warming temperatures and heavy rainfall correlate well with coordinated masting, suggesting that the trees synchronize to weather cues.

But three years ago, the ecologist Michał Bogdziewicz and his team at Adam Mickiewicz University in Poznań, Poland, found that European beeches coordinate their reproduction across 1,500 kilometers — the largest synchronization response of any tree species in Europe. By their calculations, the synchronization area is about twice that of Norway spruce, which mast over only about 1,000 kilometers and are much less tightly correlated in time…