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People have been celebrating with elaborate rituals since prehistoric times. But humans aren't the only species to take mark June 21 as a special time. Studies are showing the summer solstice is an important cue for plants too.

Recent studies, , have proposed that trees may use the longest day of the year as a key marker for their growth and reproductive cycles. The solstice seems to act like a calendar reminder for trees.

For example, at the solstice, trees growing in cold places slow down the creation of new wood cells and focus their energy on finishing already formed . This ensures trees have time to complete cell construction before winter hits. Incomplete cells are damaged by freezing winter temperatures, rendering them useless for water transport the following year.

Along similar lines, trees use the solstice to fine-tune the "winding down," or senescence, of their leaves . Senescence allows the tree to reabsorb critical nutrients from the leaves before they fall. This process is timed to balance missing out on sunlight from "winding down" too early, against leaving it too late and losing nutrients if still-green leaves are killed by autumn frosts.

Satellite observations of forests, and controlled experiments in greenhouses, reveal that immediately prior to the solstice cause the onset of leaf browning to start earlier that autumn. In contrast, warmer temperatures just after the solstice slow down the senescence process.

This means a longer transition period from green to fully brown leaves. This fine-tuning enables trees to extend the period of photosynthesis in years when temperatures stay warmer for longer, so they don't miss out on these favorable conditions.

But not all scientists are convinced. From an evolutionary perspective, the solstice may not be the for timing these transitions. For example, in forests in the far north, leaves do not appear until early June, only days before the solstice, and the growing season can extend late into October. In these forests, using the solstice to initiate the winding down process makes little sense for trees that have only just started growing for the year.

Nevertheless, there is more consensus about plants using the solstice to synchronize reproduction.

In many plants, especially trees from the temperate mid-latitudes, the number of seeds they produce varies dramatically year on year, . A can produce hundreds of thousands of seeds in a bumper year (a "mast event") and forgo reproduction altogether in other years.

Beech trees , often on a continental scale. They do this to of their reproduction.

A small moth, Cydia fagiglandana, lays its eggs in beech flowers. When the grubs hatch, they eat and destroy the developing seeds. Cycles of famine and bumper years help protect their seeds from these moths.

UK beech trees typically lose less than 5% of their seeds to Cydia because the cycles starve the moths into low numbers ready for masting years. But when trees are out of sync, seed loss can increase to .

For decades we have known that beech mast events happen a warm summer. These warmer temperatures trigger an increase in the formation of flower buds. More flower buds usually lead to a greater crop of seeds that autumn.

Scientists have long puzzled over how beech trees across Europe seem to use the same seasonal window to control mast events. Their seed production is determined by temperatures , irrespective of where they grow in Europe. But how can a beech tree know the date?

In my team's 2024 study, we showed that they . As soon as the days start to shorten after the solstice, beech trees across Europe seem to simultaneously sense the temperature.

Anywhere temperatures are above average in the weeks following the solstice can expect to have a mast event the next year. Weather conditions in the weeks before the solstice, by contrast, seem to be irrelevant. As seen on weather maps, warm and cool spells tend to occur simultaneously over large areas.

This to maximize the synchrony of their reproduction, whether that is investing in a mast year (warm temperatures), or forgoing reproduction for a year (low temperatures). Using a fixed marker like the solstice is the key to achieving this synchrony, and the benefits that come from it.

for this phenomenon has come from observations across dozens of forests across Europe. However, my research group is collaborating with about a dozen other groups in Europe to test this effect by manipulating the temperature of beech branches before and after the solstice at different sites. Ongoing research I am involved with seems to show flowering genes are activated at the summer solstice.

Also, studies into show they have mechanisms in their molecules that allow them to detect and respond to tiny changes in day length. This is the basis for that extraordinary scale of synchronized reproduction.

If the weather is warm over the next month or so, then there is a good chance that beech trees in your local area will have heavy seed crops next autumn. What's more, trees across the UK and into northern and central Europe will probably be doing the same.