For millennia, the summer solstice has marked a pivotal moment in the human calendar – a turning point steeped in mythology, when the oak king is said to yield to the holly king, and the days begin to shorten.

Now, science is increasingly revealing that trees really do respond to this celestial shift, with changes in their growth and reproductive strategies occuring immediately after the calendar’s longest day. A study gives fresh insights into why this happens, with implications for how forests might adapt to changing climates.

Although it has long been known that plants use daylight to cue seasonal activities such as leaf growth, botanists have recently begun to question whether the summer solstice itself, which occurs between 20 and 22 June in the northern hemisphere, could act as a celestial “starting gun” for certain events.

Last year, researchers led by Prof Michał Bogdziewicz at Adam Mickiewicz University in Poland discovered that, regardless of where in Europe beech trees live, they abruptly open a temperature-sensing window on the solstice, using it to decide how many seeds to manufacture the following year. If it is warm in the days after the solstice, they will produce more flower buds the following spring, leading to a bumper crop of beech nuts in the autumn. But if it’s cool they might produce none – a phenomenon known as masting.

“The window of temperature sensitivity opens at the solstice and remains open for about 30 to 40 days. There is no other phenomenon that could so tightly anchor beech trees all across Europe at exactly the same time,” Bogdziewicz said.

Swiss researchers also recently discovered that trees in temperate horthern hemisphere forests seemingly switch their growth strategy at around the solstice: warm temperatures before this date accelerate the ageing of their leaves, whereas warm temperatures after it slow down this process. This means it takes longer for their leaves to turn brown in the autumn, maximising their ability to photosynthesise and grow when conditions are favourable.

The latest study, published in Proceedings of the National Academy of Sciences, provides a fresh insight into the relationship between trees and the summer solstice.

Dr Victor Van der Meersch and Dr Elizabeth Wolkovich at the University of British Columbia in Vancouver, Canada, analysed 1,000 years of temperature records across Europe and North America, combining them with future climate projections. Their findings suggest that trees reach their thermal optimum – the temperature range where their physiological processes hum most efficiently – around the summer solstice. Remarkably, this peak has remained stable across centuries.

”In Spain the optimal period is slightly earlier, and in Scandinavia it is slightly later – on average across Europe and North America, the summer solstice seems to be an optimal time for plants to make decisions,” Van der Meersch said. “This was quite surprising because the warmest days are usually in July or August – they are not around the summer solstice.”

If the solstice consistently coincides with peak growing conditions, it could make evolutionary sense for plants to use it as a cue – especially since this thermal sweet spot appears to have held steady across time.

“Plants are always trying to find a balance between risk and opportunities,” Van der Meersch explained. “The further they go into the growing season, the easier it is for them to know if it’s a good growing year or a bad one – but there’s also less time left to use this information.”

The solstice seems to represent a critical juncture in this decision-making process. “It makes sense for them to switch between one growth strategy or another at this time.”

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However, the question of whether they are sensing the change in day length, or a coincidental sweet spot in temperature, remains open.

While researchers like Bogdziewicz are exploring how trees detect changes in day length at around the solstice, Van der Meersch thinks temperature may be playing a larger role than previously appreciated. “Rather than sensing the solstice, perhaps what really matters are the temperature accumulation patterns at around this time,” he said.

However, because there is a strong correlation between temperature and day length, “we need a lot more research to disentangle these complex signals”,” he added. “It is also possible that they rely on a combination of both summer solstice and thermal cues to optimise growth and reproductive timing.”

Understanding the mechanism is important, because it could have implications for how forests adapt to the climate crisis. “If plants are using warmth signals rather than day length this might be a good thing, because they could have a better ability to adapt to local environmental conditions,” Van der Meersch said. “If they are relying on day length, the solstice will always be fixed to around 21 June, so there is less flexibility.”