The study by Cal Poly Humboldt Botany Professor Alana Chin and colleagues and published in the American Journal of Botany found that trees have a long-lasting memory of the climates they grew in, shaping how they store and use water throughout their lives. Many older trees grew in a time when water was more abundant, but as climate change accelerates, they struggle to adapt to drier conditions.
Chin’s findings offer new insights into how forests adapt—or fail to adapt—to a changing climate. By understanding the lasting effects of past water conditions, researchers and land managers can develop strategies to sustain forests in an increasingly drying world.
The study was part of a research study that began in the 1990s when scientists in the Rhône River Valley of the Swiss Alps observed a decline and death of Scots pine trees that had thrived for 100 years in the valley. Researchers at the Swiss Federal Institute for Forest, Snow, and Landscape Research designed an experiment to investigate whether climate change and drought could be contributing factors.
Beginning in 2003, they irrigated a mature forest to double the natural summertime rainfall and compared the growth and health of these water-rich trees with those that received only natural precipitation. They found that irrigation not only improved tree survival but also increased growth rates over 17 years.
A key finding in the study was the concept of "legacy effects," referring to ecological memory—the idea that past environmental conditions continue to shape a tree’s growth and productivity. Trees facing drought develop smaller wood cells and adjust root-to-leaf ratios to conserve water in dryer climates.
Researchers aimed to determine if trees have a similar memory for wet conditions. In 2013, they stopped irrigating half of the plots after years of increased watering. A decade later, they investigated whether the trees that had been irrigated still retained benefits from their wetter past. This study provided further insights into how forests respond over the long term to changing climate conditions.
In March 2023, Chin and ecophysiologist Marcus Schuab collected fresh leaf and twig samples from Scots pine trees to study the long-term effects of past irrigation practices. They gathered samples from three different types of trees: those that had never been irrigated, those that were continuously irrigated, and those that had irrigation stopped in 2013.
Using the Swiss Light Source, a powerful synchrotron, they scanned the samples with an ultra-high-resolution X-ray to examine their internal structures without damaging them. After an all-night scanning session, they obtained detailed images of every cell, providing insights into how past water availability influenced tree physiology.
Chin found that trees that were once irrigated for over a decade but later left without extra water showed more signs of drought stress than trees that had never been watered. Their leaves were smaller and focused more on survival—storing water and resisting drought—rather than efficiently capturing sunlight for growth.
“Our findings showed that the trees retained long-term effects from irrigation years later. It wasn’t just in the old leaves; new leaves produced long after irrigation stopped were still affected. The previously irrigated trees showed multiple signs of stress, were less productive, and weren’t growing as fast. Our hypothesis is that they had "primed" themselves to expect water availability, and when it was taken away, they struggled,” Chin says.
This “memory of water” shaped the tree’s structure, making it less productive after irrigation stopped. Unlike trees that naturally adapt to drought by developing deeper roots and smaller, more resilient cells, these formerly irrigated trees seemed to struggle as if they were still waiting for the water to return.
This "memory" can put older trees at a disadvantage, making them more vulnerable to drought compared to younger trees that were raised in drier conditions.
These trees may still be adapted to past conditions, making it harder for them to survive in today’s harsher environment. This could help explain why so many trees are dying globally.
However, younger trees that have grown in consistently dry conditions over the past 15–20 years may be naturally better suited for the future. Unlike older trees that "remember" wetter times, these young trees have survived with less water, giving hope that future forests will be more resilient to climate change.
The study also records the longest-lasting legacy effects of water enrichment, especially concerning leaf structure.
“Most research focuses on how trees respond to drought, but this is one of the first studies to examine how trees retain the effects of being well-watered,” Chin says.
Chin’s research is helping Alameda County Cooperative Extension understand irrigation in newly planted oak trees, the effects of heavy irrigation on trees during establishment, and whether they should scale back irrigation for newly planted trees. Curious about the effects, the department is running an experiment this year with oak trees.
“The general theory for planting trees has been that you water heavily in the first year to encourage deep-root growth, but maybe that’s not the best approach. The parks department in the Bay Area is testing reduced irrigation levels to see if that improves long-term tree resilience in a drier climate. It’s exciting to see our research being applied to real-world park management decisions,” Chin says.