Quaking aspen that live through a drought do not simply bounce back when the rain returns. A University of Utah study found that drought-stressed aspen changed the chemistry of their leaves in the years that followed, building up compounds that deter leaf-eating insects while cutting back on others that help fend off microbes.

The research, published in New Phytologist, describes what the team calls a functional memory of drought. The findings were highlighted in a University of Utah release on Oct. 13, following the paper's online publication in September.

The stakes reach across the American West. Aspen is Utah's state tree, one of the most widespread trees in North America, and drought-linked aspen die-offs have worried land managers in Colorado and neighboring states for nearly two decades.

Three Years of Controlled Drought in Salt Lake City

The experiment took place at the university's Biology Research Experimental Garden in the Wasatch foothills east of the Salt Lake City campus. Researchers grew aspen from rootstock collected at mountain sites about 9,000 feet above sea level in five national forests in Utah and Colorado.

From 2021 through 2023, the team controlled how much water different trees received, mimicking a range of drought levels. Each year, they harvested leaves in late August and analyzed their chemistry.

The study was co-led by biology professors Talia Karasov and William Anderegg, with graduate student Aubrey Hawks, working with a team of student researchers and a chemist from the university's chemistry department. "We wanted to understand drought not simply as an acute stress," Karasov said, explaining that the team set out to track what happens to trees long after a dry spell ends.

More Insect Repellent, Fewer Microbe Fighters

In the years after drought, the stressed trees carried higher levels of salicinoid phenolic glycosides, chemicals that discourage insects and other animals from eating aspen leaves. Higher levels of those compounds were linked to less damage to the tree canopy.

Salicinoids belong to the same broad chemical family as salicylic acid, a close relative of the active ingredient in aspirin, and they are common in aspen, willows, and poplars. To a caterpillar or beetle, they make leaves bitter and harder to digest — a built-in pesticide that the tree can dial up or down.

The effect matters because these chemicals are a major part of an aspen leaf. Karasov said they can make up as much as 20% of a dried leaf's mass, and earlier research found that even a small increase can cut insect feeding by up to 25%. "It can basically be the difference between a plant being totally defoliated or not," she said.

At the same time, drought-exposed trees made fewer condensed tannins, compounds that help protect leaves against microbes. The changes in leaf chemistry were also tied to shifts in the fungi living on the leaves. The researchers found no evidence that investing more in insect defenses slowed the trees' growth.

Delayed Die-Offs Across the West

Foresters have long noticed that drought damage often shows up late. Trees may look fine during a dry year, then decline or die one or more years afterward. Karasov said conditions during the drought year alone do not explain all of the delayed mortality seen in forests.

The U.S. Forest Service has documented that pattern in aspen. Researchers described what they named sudden aspen decline in southwest Colorado in 2008, hypothesizing that acute, warm drought triggered rapid dieback, with canker diseases and boring insects finishing off weakened trees. Stands on low-elevation, south-facing slopes with dry soils were among the most vulnerable.

The new findings suggest one possible piece of that puzzle. If drought shifts a tree's defenses toward insects and away from microbes, it could change which threats the tree can resist in later years. "Defenses against pests and pathogens likely matter a lot for aspen to be able to survive droughts," Anderegg said.

A Garden Study, Not Yet a Forest Forecast

The study has important limits. It was carried out in a single experimental garden over three years, not in wild forests, and it does not establish whether the chemical shifts help or harm aspen recovery overall. The release does not report specific chemical concentrations or the exact pattern of drought years, and the researchers describe the year-after changes as a possible adaptation to dry conditions rather than a proven one.

Even so, the work changes how forest managers may think about drought. A dry year may leave a chemical fingerprint in a tree's leaves that lasts well after the soil recovers, shaping how it fares against insects and disease. That matters for anyone who manages aspen stands, from national forest staff to ranchers whose grazing lands include aspen groves.

For homeowners in Salt Lake City, Denver, and other Western cities who grow aspen in their yards, the study is a reminder that drought stress has lingering effects. Following local extension service guidance on watering during dry spells can reduce stress — and watching for leaf damage or cankers in the seasons after a drought can catch problems early.

The study was funded by the National Institutes of Health, the National Science Foundation, the USDA National Institute of Food and Agriculture, and several foundations, and the authors declared no competing interests. The bottom line is that aspen appear to carry a chemical memory of drought into future seasons, and the next research step is learning whether that memory helps Western forests survive a drier future or leaves them exposed in new ways.

What Readers Want to Know

What did the aspen study find?

Aspen exposed to drought produced more insect-deterring leaf chemicals and fewer microbe-fighting tannins in the years after the drought, a change the researchers call a functional memory of drought.

Where was the research done?

At the University of Utah's Biology Research Experimental Garden in the foothills east of Salt Lake City, using aspen from five national forests in Utah and Colorado.

Why does this matter for forests?

Drought damage often appears years later. Shifts in a tree's chemical defenses could affect which insects and diseases it can resist after a dry spell.

Did the extra defenses slow the trees' growth?

The researchers found no evidence that higher defense investment reduced growth.

Does this mean aspen forests are safe from drought?

No. The study was done in a single garden over three years and does not show whether the chemical changes help or harm long-term survival.

What can homeowners with aspen do?

Follow local extension guidance on watering during dry spells and watch for leaf damage or cankers in the seasons after a drought.