What is the future of forests in a warmer, drier world? Over the past 25 years, trees have been dying due to the effects of global climate change. In Africa, Asia, North and South America and Europe, drought stress exacerbated by heat is killing trees that have survived for centuries.

The old trees may have been growing for a full millennium and are wetter than they have been in the past 20 years. We are scientists studying forest dynamics, plant ecology and plant physiology. In a recent study, we found that trees can remember periods of abundant water, and that this memory continues to influence their growth many years after the wet phase ends.

This research provides optimism that today's saplings, which never experienced the rainfall of the 20th century, have not yet developed their own structures around abundant water, may be better equipped to survive in a world with prolonged drought.

What if we watered the forest?

The study builds on nearly 20 years of forest research in response to early warning signs of forest loss in the Swiss Alps' dry Rhône Valley in the 1990s. At that time, scientists observed that Scots pine trees, which were about 100 years old, were declining and dying. They wonder whether drought or other climate factors are causing the losses.

To address this problem, researchers from the Swiss Federal Institute for Forest, Snow and Landscape Research designed an ecological experiment. To understand the effects of drought, they will irrigate a mature forest to double the amount of natural summer rainfall, and then compare the performance of these water-rich trees to trees that receive only natural rainfall.

The Pfynwald experiment, launched in 2003, showed that trees in irrigated plots had better survival rates. After 17 years of irrigation, the team found that irrigation not only helped the trees survive drought periods, but also increased their growth rates.

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The legacy is the memory of the forest

Trees that experience drought change their leaves, wood, and roots to prepare for continued drought conditions. Wood under drought may have smaller cells that are less susceptible to future damage, and root relative to leaf area may increase. These structural changes persist after the drought has passed and continue to affect the tree's growth and ability to withstand stress for years.

These long-lasting post-drought effects are known as “legacy effects” and represent ecological memory of past climate conditions at the tree and forest levels. The researchers knew that trees have a lasting memory of past dry periods, so they wondered whether they might also show structural changes from past wet periods.

Eleven years after Pfenwald began summer irrigation, scientists stopped irrigating half of each field in 2013 to address the problem. These formerly irrigated trees, now about 120 years old, had experienced a sustained period of irrigation, but now those harvest days were over.

Will the trees remember? Ten years later, we found out.

Trees, trains and particle accelerators

Two of us (Alana Chin and Marcus Schaub) met in Pfynwald early one morning in March 2023 to collect very fresh leaf and branch samples so that we and our colleagues could look deep inside for signs of the lasting effects of past water abundance.

At the site, we climbed canopy access towers to collect newly grown top leaves and branches from control trees that had never been irrigated; trees that had been irrigated every summer since 2003; and trees that had not received irrigation water since 2013 of previously irrigated trees.

We took the samples to the Swiss Light Source, a powerful synchrotron—a particle accelerator that produces the most intense beams in the world. This facility is home to TOMCAT, an extremely high-resolution X-ray that allows us to look inside leaves and branches without destroying their structure.

Scanning our samples took all night, but as we stumbled out of the building, our images captured every cell in exquisite detail.

memory of water

We found that new leaves on trees that had been irrigated were different from trees that were continuously watered and control trees that were never watered. Leaves carry out photosynthesis, which provides the power for the tree's survival and growth. Inside them, we can see the legacy of abundant water from the past, recorded in the size, shape, and arrangement of the cells.

Reading this cellular signature, we observed that, at the expense of productivity-enhancing structures, previously irrigated trees showed various signs of chronic water stress—even more so than trees that had never been irrigated. By dissecting them, we learned why these trees, which had thrived during 11 wet years, were now growing slowly.

Every cell in a leaf has trade-offs. Trees must balance investments in rapid photosynthesis with other investments that promote leaf survival. Instead of building cells to collect sunlight and transport sugar to other parts of the tree, leaves on irrigated trees show all the signs of drought stress that we can measure.

After gaining and then losing extra water for up to 11 years, the trees begin to grow new, tiny leaves that they use primarily for their own survival. The leaves are structured to protect themselves from insects and drought, and to store water. Compared to leaves on trees that have never received irrigation, these leaves look like they were during a drought in the mid-century.

While this memory of water may seem negative, it may have once helped the tree "learn" from past conditions in order to survive in a changing environment. The trees that were previously irrigated had no idea humans were playing a trick on them. Like trees experiencing climate change, they have no way of knowing the water won't come back.

When trees experience a drought event, recovery may mean reaching a "new normal" state in which they are prepared to survive the next drought, with smaller, less fragile cells and increased energy reserves for future droughts. Period "storage". They may have deeper roots or smaller leaves for support, helping them prepare for unstable conditions.

We wondered whether the same was true for trees that experienced abundant water. Are they waiting in agony for the water to come back?

Tough times may make for tough trees

In some temperate forests, such as the one we studied in Switzerland, old trees that once knew the abundance of water have now disappeared due to climate change. Past abundance may have locked in structural and epigenetic changes in trees that are mismatched in today's dry world. If this is true, then some of today's devastating global tree die-offs may be caused in part by the legacy of past water abundance.

However, in most of the world's temperate forests, the current population of young forest trees (those that have sprouted within the past 15 to 20 years) has successfully established itself under conditions that were once considered chronically dry. Those small trees that survive endless periods of drought will form the forests of the future.

Taken together, our observations in Pfenwald give us some hope that the young trees that currently dominate many forests around the world may be better prepared to cope with a world shaped by humans. Climate change in recent decades has plunged them into hard times without lingering memories of water.