People have celebrated the summer solstice—the longest day of the year—for thousands of years. But it’s not just humans who take notice of this special day. New research shows that trees may also “notice” the solstice and use it to guide important parts of their life cycle.

Trees don’t have brains or eyes, of course, but they are very sensitive to changes in light and temperature. Around June 21, when the days stop getting longer and start getting shorter, trees seem to use this moment as a signal. It helps them decide when to slow down growth, get ready for fall, or even plan for next year’s seed production.

In places with cold winters, trees usually grow new wood cells in spring and early summer. But around the time of the summer solstice, they stop making new cells and instead focus on finishing the ones they’ve already started. This is smart timing—if cells aren’t finished before winter, freezing temperatures can damage them, making it harder for the tree to move water the next year.

The solstice also seems to help trees decide when to start the process of shedding their leaves, called “senescence.” During this process, the tree pulls nutrients out of the leaves before letting them fall. Timing this right is important: if the tree starts too early, it misses out on sunlight and energy. If it waits too long, frost might kill the leaves before the tree gets its nutrients back.

Scientists have found that trees respond differently depending on when warm weather arrives. If it’s warm right before the solstice, it doesn’t have much effect. But if it’s warm after the solstice, trees delay leaf fall and keep their leaves green longer, which lets them keep making food from sunlight.

The solstice also seems to play a key role in how trees decide when to make seeds. Many trees, like beech trees, have years where they produce a huge number of seeds—called a “mast year”—and other years where they produce almost none. All the trees in an area usually do this at the same time, which helps protect the seeds. If all the seeds come at once, animals like insects can’t eat them all, so more seeds survive.

For a long time, scientists didn’t know how trees across huge areas could coordinate this. But recent research shows that trees use the summer solstice as a starting point. They “watch” the temperatures in the weeks just after the solstice. If it’s warm during that time, the tree will likely produce a lot of flower buds, which leads to more seeds the following year. If it’s cool, the tree skips seed-making that year.

This system works well because weather patterns often stretch across big areas. So trees in many different places get the same temperature signals after the solstice and respond the same way. That means more trees have mast years at the same time, which helps their seeds survive.

Scientists are still studying how this works. Experiments show that certain genes linked to flowering get switched on around the solstice. Also, trees have natural clocks inside them that can measure small changes in daylight. These clocks help them stay in sync with the seasons.

In short, even though trees can’t “know” things the way animals do, they are amazing at sensing the world around them. By using the summer solstice as a calendar marker, trees can time their growth, leaf changes, and seed production to get the most benefit from the changing seasons.