Aspen trees remember drought for years, and it changes how much of them gets eaten

Aspen trees that lived through drought kept making more of the chemicals that deter leaf-eating insects for years afterward, and a small increase in those chemicals can cut herbivory by as much as 25 percent. University of Utah biologists reported the result in New Phytologist after a three-year garden experiment run from 2021 to 2023. Talia Karasov, an assistant professor of biology and a plant-microbe specialist, calls the carryover a functional memory of drought.

A garden built to simulate drought

The study, titled “Functional memory of drought affects leaf chemical defenses and microbial interactions in aspen,” used a common garden at the university’s Biology Research Experimental Garden near the mouth of Red Butte Canyon. Quaking aspen rootstock came from mountain sites in five national forests in Utah and Colorado, at about 9,000 feet on south-facing slopes, and was planted in 2021, the Newswise account of the research explains. The team manipulated water to replicate different drought levels, harvested leaves each late August and analyzed their chemistry.

A common garden removes the variation of the wild. Trees from different origins grow side by side in identical soil and weather, so differences that persist can be attributed to the trees’ history, such as the drought they were given, rather than to where they happened to be growing. The USGS describes the approach as a way to test how plants respond to climate.

Defenses that outlast the drought

After drought exposure, the aspens carried higher levels of salicinoid phenolic glycosides in the following years. Those compounds deter herbivores, and higher levels were associated with less damage to the canopy. In dried leaves they can make up as much as 20 percent of leaf mass, which Karasov found remarkable. “It’s incredible. So they’re putting so much of their resources into defending themselves,” she said.

The stakes of a small shift are large. Karasov said a modest rise in these compounds “can basically be the difference between a plant being totally defoliated or not,” and prior research suggests it can reduce herbivory by up to 25 percent. William Anderegg, a biology professor and forest ecologist who co-led the work, said defenses against pests and pathogens likely matter a lot for aspen.

The word memory carries a specific meaning in this work. The researchers are not claiming that trees recall a dry summer the way people do; they mean that an earlier exposure leaves a measurable, lasting change in physiology that alters how the plant responds later. Karasov’s framing, quoted in the Utah account, is that drought should be understood as more than an acute stress with a clear start and end. In the garden, the leaves were the evidence: trees given less water in earlier years produced a different chemical profile in later years, even after conditions changed.

Trade-offs in the leaf and on it

The memory was not uniformly protective. Condensed tannins, which help defend against microbes, decreased in drought-exposed trees, and the shifts in leaf chemistry were associated with changes in the fungal communities living on the leaves. That suggests a trade: investing in defenses against insects may come at the expense of defenses against microbes. The team found no evidence that the extra defense investment reduced tree growth, but the study does not establish whether the changes ultimately help or harm aspen recovery.

Karasov said the team wanted to understand drought not simply as an acute stress, and the data suggest a legacy that lasts beyond the dry spell itself. Aubrey Hawks, a graduate student, co-led the study with Anderegg, and co-authors include Jaycie Fickle, Kelly Kerr, Dale Forrister, Allison Perkins, Efthymia Symeonidi, Madelyn Allen, Cannon Fehrenbach and Hsiao-Nung Chen of the Department of Chemistry. It is a long list for a single-garden study, and it reflects how much of the work was done by students at the University of Utah.

Why aspen needs the help

Quaking aspen is Utah’s state tree, and it is declining under pressure from insects, pathogens, fire suppression and grazing, which have let conifers displace it. The U.S. Fish and Wildlife Service offers a short profile in its tree-identification series on aspen. If drought leaves behind stronger insect defenses, some stands may be better placed against defoliators in the years after a dry spell, though weaker microbial defenses could cut the other way. The study design cannot settle that question, and the authors do not claim it does; what it shows is that a tree’s past leaves chemical fingerprints that are visible in its leaves, its canopy damage and the fungi growing on it.

The paper was published September 13, 2026, and is available at its New Phytologist page. All of the named researchers, including Karasov, Anderegg and Hawks, are based at the University of Utah, and the garden itself sits on the same campus. Experiments of this kind take years to run, which is why a three-year window from 2021 to 2023 is itself a notable feature of the work. The same findings are carried in a EurekAlert release.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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