Across a Continent, Trees Sync Their Fruiting to the Sun

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.

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