Why the summer solstice is a ‘celestial starting gun' for trees
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.'
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.'
Hashtags
Try Our AI Features
Explore what Daily8 AI can do for you:
Comments
No comments yet...
Related Articles
Yahoo
2 hours ago
- Yahoo
Antarctic seal numbers falling drastically due to melting sea ice, research shows
Antarctic seal populations are drastically declining as the sea ice melts around them, new research has shown. Researchers from the British Antarctic Survey (BAS) have been monitoring the seal population in the sub-Antarctic since the 1970s, looking in particular at three different seal species in the sub-Antarctic on Signy Island: Weddell seals, Antarctic fur seals and southern elephant seals. Using satellite records from as far back as 1982, the research team compared annual changes in sea ice concentration with seal population counts conducted since 1977. With five decades worth of data spanning a period of long-term warming and temporary cooling between 1998 and 2014, they were able to draw a comprehensive picture of how seal populations react to changing ice conditions, something shorter-term studies would not be able to achieve. The study, published in the scientific journal Global Change Biology, found Weddell seals (leptonychotes weddellii), which rely on stable sea ice to rest, breed and feed, had declined by 54% since 1977. Similarly, Antarctic fur seals (arctocephalus gazella), which breed on land but are affected by food chain shifts, have declined by 47%. The findings challenge previous assumptions that the population in the South Orkneys had stabilised. Southern elephant seals (mirounga leonina), despite sharing similar population trends, presented 'no significant overall long-term decline'. The study serves to emphasise the 'vital importance of long-term ecological monitoring' as well as the interconnectedness between the three species of seal and sea ice conditions. Michael Dunn, lead author of the study, said: 'For once, we're not just predicting how wildlife might respond to shrinking sea ice and environmental shifts, we've had the rare opportunity to confirm it, using solid, long-term data. The emerging picture is deeply concerning.' In light of the BAS's findings, researchers have also raised concerns about how climate breakdown is affecting the Antarctic food web, relied on by all three species.
CNN
3 hours ago
- CNN
An explosion of sea urchins threatens to push coral reefs in Hawaii ‘past the point of recovery'
Animal stories Climate changeFacebookTweetLink Follow The turquoise water of Hōnaunau Bay in Hawaii, an area popular with snorkelers and divers, is teeming with spiny creatures that threaten to push the coral reef 'past the point of recovery,' new research has found. Sea urchin numbers here are exploding as the fish species that typically keep their populations in check decline due to overfishing, according to the study, published last month in the journal PLOS ONE. It's yet another blow to a reef already suffering damage from pollution as well as climate change-driven ocean heat waves and sea level rise. Kelly J. van Woesik, a researcher at the North Carolina State University Center for Geospatial Analytics and a study author, first noticed unusually high numbers of sea urchins on snorkeling trips. 'I knew there was a story to be told,' she said. She and her fellow researchers used data from scuba surveys and images taken from the air to track the health of the reef. 'We found on average 51 urchins per square meter, which is among the highest recorded densities on coral reefs anywhere in the world,' van Woesik said. Sea urchins are small marine invertebrates, characterized by their spiny bodies and found in oceans around the world. They play a useful role in preventing algae overgrowth, which can choke off oxygen to coral. However, they also eat the reef and too many of them can cause damaging erosion. In Hōnaunau Bay, the coral is already struggling to reproduce and grow due to ocean heat and water pollution, leaving it even more vulnerable to the erosion inflicted by sea urchins. Its rate of growth has plummeted according to the study. Reef growth is typically measured by the amount of calcium carbonate — the substance which forms coral skeletons — it produces per square meter each year. The reef in Hōnaunau Bay is growing 30 times more slowly than it did four decades ago, according to the study. Production levels were around 15 kilograms (33 pounds) per square meter in parts of Hawaii, signaling a healthy reef, according to research in the 1980s. Today, the reef in Hōnaunau Bay produces just 0.5 kg (1.1 pounds) per square meter. To offset erosion from urchins, at least 26% of the reef surface must be covered by living corals – and even more coral cover is necessary for it to grow. Gregory Asner, an ecologist at Arizona State University and study author, said what was happening in this part of Hawaii was emblematic of the mounting pressures facing reefs throughout the region. 'For 27 years I have worked in Hōnaunau Bay and other bays like it across Hawaii, but Hōnaunau stood out early on as an iconic example of a reef threatened by a combination of pressures,' he said, citing warming ocean temperatures, pollution from tourism and heavy fishing. The implications of coral decline are far-reaching. Coral reefs are sometimes dubbed the 'rainforests of the sea' because they support so much ocean life. They also play a vital role protecting coastlines from storm surges and erosion. 'If the reef can't keep up with sea-level rise, it loses its ability to limit incoming wave energy,' said van Woesik. 'That increases erosion and flooding risk of coastal communities.' Kiho Kim, an environmental science professor at American University, who was not involved in the study, said the findings highlight the fragility of reef ecosystems under stress. 'Dramatic increases in any species indicate an unusual condition that has allowed them to proliferate,' Kim said. That imbalance can undermine diversity and reduce the reef's ability to provide essential ecosystem services including food security and carbon storage, he told CNN. Despite the challenges, researchers emphasize that the reef's future is not sealed. Local groups in Hōnaunau are working to reduce fishing pressure, improve water quality and support coral restoration. 'These reefs are essential to protecting the islands they surround,' van Woesik said. 'Without action taken now, we risk allowing these reefs to erode past the point of no return.'
Forbes
3 hours ago
- Forbes
ESA's Solar Orbiter Should Solve Mystery Of Sun's Outermost Atmosphere
ESA's Solar Orbiter mission will face the Sun from within the orbit of Mercury at its closest ... More approach. The European Space Agency's Solar Orbiter mission recently stunned the world with the first-ever full images of our Sun's South pole, proving that this was going to be a mission like no other. Using an orbital gravity assist from the planet Venus, the Solar Orbiter mission spacecraft was able to maneuver into an orbit that has taken it to an angle 17 degrees below the Sun's equator. Over the coming years, the spacecraft will tilt its orbit even further, so the best views are yet to come, says ESA. The 1.2-billion-euro Solar Orbiter mission, with NASA participation, should finally help us understand the origin of the Sun's solar winds as well as our understanding of the Sun's poles. And arguably most importantly, it should solve the puzzle of why our star's outermost atmosphere, or corona, is heated to millions of degrees Kelvin and is thus so much hotter than the Sun's own surface. By contrast, our Sun's visible photosphere, or surface, averages only 5,500 degrees K. With Solar Orbiter, we are clearly seeing energy releases on the nano-flare scale, Daniel Mueller, a solar physicist and ESA project scientist for both ESA's SOHO and Solar Orbiter missions to the Sun, tells me in his office in The Netherlands. But the question is, would these nano-flares continue like that infinitely, or is there a certain lower limit to the production of these nano-flares, Mueller wonders. The puzzle is whether these nano-flares are enough to heat up the Sun's corona to the temperatures with which it is routinely measured. A Unique View Launched in 2020, from its highly elliptical orbit just inside Mercury's perihelion, the closest point in our innermost planet's solar orbit, the ESA spacecraft offers the best views yet of our own yellow dwarf star. We can see on scales down to about 200 kilometers on the Sun, which shows us a lot of dynamics of our star, says Mueller. And thanks to its newly tilted orbit around the Sun, the European Space Agency-led Solar Orbiter spacecraft is the first to image the Sun's poles from outside the ecliptic plane (the imaginary geometric plane in which our Earth orbits the Sun), says ESA. We observed the Sun's North pole at the end of this past April, says Mueller. But we passed the Southern pole first and then the Northern pole six weeks later, he says. At the moment, as seen from Earth, the Solar Orbiter is almost behind the Sun, so the data downlink has slowed to a trickle. But by early October, Mueller expects to have downloaded all the data from Solar Orbiter's Spring polar observations of the Sun. And within a matter of two to three months after the data is on the ground, the first scientific results will have been written up and submitted to journals for publication, says Mueller. These observations are also key to understanding the Sun's magnetic field and why it flips roughly every 11 years, coinciding with a peak in solar activity, says ESA. The spacecraft's instruments show that the Sun's South pole is a bit of a magnetic mess now, with both North and South polarity magnetic fields present, ESA notes. Ready To Flip Right now, there is not a clear dominant magnetic polarity, but a mix of the two, says Mueller. And that is exactly what you would expect to find during the maximum of the Sun's activity cycle, when the magnetic field is about to flip, he says. The real applications are for space weather predictions. Case in point, better space weather forecasting may have saved many of Elon Musk's 523 Starlink satellites that reentered Earth's atmosphere between 2020 and 2024. This period coincides with the rising phase of solar cycle 25, which has shown itself to be more intense than the previous solar cycle, the authors of a 2025 paper appearing in the journal Frontiers in Astronomy and Space Sciences write. Our results indisputably show that satellites reenter faster with higher geomagnetic activity, the authors note. There was a big solar storm that caused the earth's upper Earth atmosphere to expand, so, the satellites experienced more drag, and therefore didn't make it to orbit, says Mueller. One option may have been simply to hold off on launches until this increased period of solar activity enabled a less risky geomagnetic environment in Earth's upper atmosphere. The hope is that the Solar Orbiter mission and other missions like it will lead to better and more reliable space weather predictions that could potentially save hundreds of millions of dollars in the commercial satellite industry. Solar Orbiter should do its share in solving both pure solar physics conundrums as well as in more practical applications like space weather. The good news is that the spacecraft still has plenty of fuel left. Our current funding goes until the end of 2026, but because we had a picture-perfect launch provided by United Launch Alliance and NASA, we saved a lot of fuel, says Mueller. So, the onboard fuel reserves are so large that we can keep going for a long time, he says.
