A life of luxury probably isn’t the best preparation for hardship. Soft, manicured hands are likely to blister after a day chopping wood.

The same is apparently true for some trees. A life of water-filled ease can leave them ill-prepared for a drought, a mismatch that can last for years, according to new research in the American Journal of Botany.

The findings underscore the difficulties facing forests where many of the trees came of age in a time of relatively abundant water. But it also offers the possibility that young trees germinated in a world where droughts are longer and more intense could fare better than their elders.

“This research makes us optimistic that young trees of today … may be better equipped to survive in a chronically dry world,” several of the researchers wrote recently in The Conversation.

The discovery emerges from an unusual experiment in a Swiss research forest. At the Pfynwald forest in the picturesque Rhône Valley, scientists in 2003 started giving some of the century-old Scots pine trees growing there double the amount of water they would naturally get from rainfall—an extra 60 centimeters worth of rain each year. Then, a decade later, half of these irrigated trees were cut off, forced to return to surviving on the water nature provided.

In 2022, scientists harvested new leaves and twigs from the tops of 30 different trees and rushed the fresh vegetation to what amounts to a super-powered X-ray microscope. There, they scrutinized the plant’s internal structures to see how they differed, depending on their watering history.

It turns out that the leaves of the once-irrigated trees maintained a “memory” of their past, water-rich years. Though the extra water had been cut off for seven years, the physiology of their leaves different from both the trees that were still being irrigated and the trees that had always subsisted on rain.

Unlike irrigated trees, their leaves showed they had switched into survival mode, growing small leaves with cells designed to resist insects and store water reserves, rather than prioritizing photosynthesis. But their physiological response to this researcher-induced drought was even more extreme than the adaptations of the rain-fed trees.

“Compared with leaves on trees that had never known irrigation, these looked as though they were in the middle of the drought of the century,” the researchers wrote.

While this response might help in a massive drought, it can be overkill if a plant is going to continue to grow and thrive in a drier world. That means trees accustomed to plentiful water might not be able to adjust to chronically dry condition as well as ones that have always experienced a relative shortage.

What’s not so clear is how long this leaf “memory” will persist. The scientists found that unlike leaves, woody twigs of the previously irrigated trees showed signs of changing back to resembling twigs from trees that had never gotten extra water. That suggests at least some parts of the tree could adapt more easily to new conditions.

If these century-old trees have grown “locked in” to being hyper-sensitive to a loss of water, that could be bad news for their long-term survival. “Some of today’s devastating global tree mortality events may be, in part, due to the legacy effects of past water abundance,” the researchers warned.

It could also mean that if young trees can survive all the other assaults that come in a hotter world—for instance, bigger wildfires—they might eventually grow into mature trees better equipped for the future. That, the scientists write in their paper, “provides a beacon of hope for the strength of future forests.”

This idea that a period of suffering might strengthen plants for the future isn’t confined to trees. Scientists have found evidence that crops such as maize and soybeans “remember” a past drought, and that exposure leaves them better prepared when it happens again.

But it’s not clear how the upbeat prediction from the tree scientists squares with other research that found a data-bank of tree rings from more than 20,000 trees showed that bigger, older trees were better at enduring droughts than younger trees.

We might have to wait a while before it the forest of the future becomes clear—if they are still standing.

Chin, et. al. “The memory of past water abundance shapes trees 7 years later.” American Journal of Botany. Dec. 23, 2024.

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