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 their ability 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?

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.

The strength of the synchronization among the beeches seemed to challenge the standard explanation: If weather alone prompted masting, a stint of rainy days in England and a stretch of extreme heat in Italy should knock the masting out of sync. Yet European beeches reliably mast together despite huge differences in regional weather.

“It was kind of surprising and spectacular,” Bogdziewicz said. “But at the time, we just finished the paper saying … this is amazing, but we don’t know how [it works].”

Then the team stumbled across a clue by accident. One summer evening, Bogdziewicz was sitting on his balcony reading a study which found that the timing of leaf senescence — the natural aging process leaves go through each autumn — depends on when the local weather warms relative to the summer solstice. Inspired by this finding, he sent the paper to his research group and called a brainstorming session.

Valentin Journé, an ecologist and postdoc in Bogdziewicz’s laboratory, went home later that day to dig into the data. The idea that masting could be linked to the summer solstice was “so stimulating” that Journé had high hopes that it could explain the remarkable synchrony. Within hours, Journé had organized the massive beech data set, analyzing daily seed production dating back to 1952. He correlated the data with temperature and found a precise uptick in masting just after the June solstice and lasting through mid-July.

Journé’s analysis suggested that European beech trees do mast in response to summer temperatures. But the twist is that they do not drop their seeds until they have sensed the longest day of the year. That combination of signals organizes the masting of the wide-flung beech trees into a compact period.

It’s the first time that researchers have identified day length as a cue for masting. While Koenig cautioned that the result is only correlational, he added that “there’s very little out there speculating on how the trees are doing what they’re doing.”

Bogdziewicz’s team took a novel approach by analyzing daily data: It’s rare for ecologists to track behavior at such a granular level, Vacchiano said. By recording incremental changes in response to daylight, the team showed that trees react to subtle external cues within an unexpectedly narrow window.

It’s not surprising that trees synchronize their innate biological clocks with changes in light; most organisms do in some way. Species have evolved sensitivity to how much light is available in a 24-hour window, and that cue — the photoperiod — has been shown to influence a range of behaviors, from plant growth to hibernation, to migration, and to reproduction.

The European beech is also not the first organism known to keep track of day length and the solstices. For example, long-distance migratory songbirds set their internal clocks to the photoperiod and use the summer solstice to time their nesting and migration, said Saeedeh Bani Assadi, a biologist at the University of Manitoba. Corals use day length to initiate spawning, but they prefer to reproduce under cover of darkness when the days are shortest, on the winter solstice.

Bogdziewicz’s team is currently collaborating with molecular biologists to find the mechanisms that enable trees to sense the summer solstice. In particular, they’re looking at the gene CONSTANS, found in all flowering plant genomes, which activates in response to seasonal changes and helps regulate the circadian clock. Some plants use peak CONSTANS expression, combined with the expression of other genes, to time their flowering to lengthening days. CONSTANS may be involved in sensing the photoperiod around the solstice — but to be sure, researchers need to sequence beech genomes to see if the maximum gene expression occurs just after the longest day of the year.

If the solstice is shown to activate a genetic mechanism, it would be a major breakthrough for the field. Currently, there’s little data to explain how trees behave as they do. No one even knows whether trees naturally grow old and die, Vacchiano said. Ecologists struggle just to study trees: From branches to root systems, the parts of a tree say very little about the physiology of the tree as a whole. What experts do know is that discovering how trees sense their environment will help them answer the questions that have been stumping them for decades.