Summer solstice guides plant growth - but climate change is interfering

Each year, the summer solstice marks the longest day in the Northern Hemisphere. For most people, it brings warmth, festivals, and late sunsets. But for plants, it may mean something far more important.

New research led by scientists at the University of British Columbia (UBC) suggests that this annual milestone could be a vital signal for growth and reproduction.

Yet, as the planet warms, this natural cue may be losing its reliability. The research, published in Proceedings of the National Academy of Sciences, explores how plants use the solstice as a possible trigger.

However, with rising temperatures and increasing climate unpredictability, plants may now need to look elsewhere for guidance.

“Plants are known to use temperature to time things like leaf growth and flowering,” said Dr. Victor Van der Meersch, postdoctoral researcher at UBC’s faculty of forestry. “This study shows that the solstice is an optimal growth period and could also be an important trigger for reproduction.”

Solstice helps plants time their growth

Historically, researchers believed daylength in general helped plants schedule growth. This study goes further. It suggests the summer solstice – around June 21 in the Northern Hemisphere – marks a key thermal optimum.

Across both Europe and North America, it offers a balance of accumulated warmth and enough time left in the season to finish development.

According to the authors, this timing represents a trade-off. Plants need to accumulate enough warmth to anticipate future climate conditions, but they also need time to grow organs, set flowers, and prepare for reproduction. The solstice may offer the best bet to strike that balance.

Solstice aligns with optimal plant growth

Using historical and projected climate data, the researchers studied temperature patterns over many years. They measured this through “growing degree days” (GDD) – a way to track heat accumulation important for plant metabolism.

The goal was to find the day when plants could most confidently predict how warm the rest of the season would be and still have time to grow.

This trade-off, visible in the models and graphs of the paper, shows the solstice often sits near the point of best combined predictability and growth potential.

Local climates tell a different story

Despite this average, local conditions tell a more complex story. In warmer parts of southern Europe, plants peak earlier. In cooler northern regions, the best time for growth arrives well after the solstice.

The paper’s map on page 2 highlights this clearly. Sites in red, purple, and green mark where optimal growth timing occurs before, near, or after the solstice.

That means the solstice is not a reliable cue everywhere. “Daylength doesn’t change from year to year. But with temperatures fluctuating more, plants may be having trouble adjusting to both signals,” said Dr. Elizabeth Wolkovich, co-author and plant ecologist at UBC.

Fixed signals limit how plants respond

The study notes that relying on a fixed signal like the solstice could limit how plants adapt to changing environments. Instead, the authors argue that temperature – because it reflects real-time conditions – may now be more useful than photoperiod cues.

Given the variation across climates, it may be better for plants to track temperature directly. “That’s because it’s directly tied to the actual climate conditions plants are experiencing,” said Van der Meersch.

Tracking solstice involves not just sensing light but calculating how fast the light is changing – a process that may be more complex and costly for plants than sensing heat.

Mistiming growth harms plants

Mistimed growth carries big risks. Crops might flower too early, risking frost. Pollinators may miss blooms. Forests may grow less, weakening carbon capture.

“These timing problems can affect food security and biodiversity,” said Dr. Wolkovich. “We need to understand the signals plants use for key events like flowering and fruit ripening.”

The authors acknowledge another possibility: maybe the solstice isn’t a true biological signal for plant growth. It may just coincide with a thermal peak due to climate patterns.

To answer this, future research must test whether plants actively respond to solstice or if the alignment is a coincidence. The experts suggest decoupling temperature and light in experiments to test how plants really make decisions.

Why this matters for the future

As drought and warming reshape global climates, understanding when and how plants grow becomes critical. This affects everything from crop planning to forest management.

Models that ignore the complexity of climate cues could lead to poor predictions and missed interventions.

The study, published in Proceedings of the National Academy of Sciences, argues that the solstice may no longer serve as the dependable signal it once was.

The work opens a new window into how plants might adapt – or fail to – in a world where stability is fading.

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