Aspen Leaves Carry Chemical Memories of Past Droughts, Study Finds

Drought leaves its mark on trees in ways that forest scientists have long been able to read in the narrow bands of annual growth rings. But new research from the University of Utah suggests that quaking aspen may go considerably further than simply recording dry years in wood. According to a three-year experimental study published in the journal New Phytologist, aspen trees appear to create what the researchers describe as functional memories of previous drought, a legacy written not in rings but in the chemistry of their newly produced leaves. In the years after exposure to experimentally induced dry conditions, the trees shifted the concentrations of defensive compounds in ways that could alter how they interact with the insects and microbes that besiege them.

The study, led by assistant professor of biology Talia Karasov, forest ecologist and professor of biology William Anderegg, and graduate student Aubrey Hawks, focused on quaking aspen, Populus tremuloides, a signature species of the mountain West and the official state tree of Utah. The researchers documented that in the growing seasons following drought exposure, aspen leaves exhibited elevated levels of salicinoid phenolic glycosides, a class of chemical compounds known to deter herbivores, the insects and animals that feed on plants. At the same time, levels of condensed tannins, another chemical defense that helps trees ward off microbes, declined in the drought-exposed trees. The team also observed that these shifts in leaf chemistry were associated with changes in the communities of fungi living on the leaves.

The finding matters because drought is rarely a clean, self-contained event for a tree. As Karasov explained, the researchers wanted to understand drought not simply as an acute stress, but as an event that may reshape how trees interact with their biotic environment long after soils have rewetted. Because drought-year conditions alone do not explain all of the delayed mortality observed in forests, the team asked whether prior water limitation leaves a persistent signature in aspen leaf chemistry and, in turn, alters interactions with herbivores and the fungal communities that inhabit leaves. The results suggest that the effects of drought can indeed persist through subsequent growing seasons, potentially influencing how aspens recover and how vulnerable they remain to pests and pathogens.

The experimental design rested on a classic tool of plant biology known as the common garden approach. Common garden experiments bring together plants from geographically divergent populations and grow them in a single location under shared conditions, allowing researchers to separate the effects of a plant’s origin from the effects of its environment. In 2021, Anderegg’s laboratory established an experimental aspen plot at the University of Utah’s Biology Research Experimental Garden, situated along the Wasatch foothills east of the Salt Lake City campus. The team planted rootstock gathered from mountain sites in five national forests in Utah and Colorado, with the original collection sites sitting at roughly 9,000 feet above sea level on south-facing aspects.

From 2021 through 2023, the researchers manipulated how much water the trees received, replicating varying levels of drought across the plot. Each year, leaves were harvested in late August and subjected to chemical analysis to determine the levels of specific compounds that concentrate in aspen leaves as defenses against plant-eating insects and wildlife, as well as against disease-causing microbes. Biology students tended the trees throughout the experiment, an effort captured in photographs of the garden showing rows of young aspens under careful cultivation. The garden has also supported two earlier studies of aspen drought response by Anderegg and his graduate students, with more work underway.

The scale of aspen’s chemical investment is striking. According to Karasov, the percentage of a dried leaf’s mass composed of these defensive compounds can reach up to 20 percent, an extraordinary allocation of resources to self-protection. Prior research has shown that even a modest increase in phenolic glycoside levels can reduce herbivory by up to 25 percent. In the new study, the elevated levels of phenolic glycosides documented in drought-exposed trees were associated with reduced canopy damage, suggesting the chemical shift had real consequences for how much leaf tissue insects consumed. Karasov noted that these compounds can essentially make the difference between a plant being totally defoliated or not, because insects do not like them.

Perhaps the most surprising result was what the researchers did not find. Despite the apparent cost of manufacturing large quantities of defensive chemistry, the team found no evidence that the trees’ increased investment in phenolic glycoside production came at the expense of growth. The researchers hypothesized that the unexpected changes in leaf chemistry observed in the year after drought exposure could reflect an adaptation to dry conditions, a possibility that raises intriguing questions about how trees balance defense, growth, and survival under a shifting climate.

The study also revealed that drought and leaf chemistry can act as filters on microbial communities, shaping which fungi inhabit aspen leaves. However, the researchers are careful to note that the work does not yet establish whether those changes ultimately help or harm aspen recovery. The direction of the effect, beneficial or detrimental, remains an open question, and one that the team considers central to understanding how Western forests will weather the increasing frequency and severity of drought expected under climate change.

The broader context gives the research urgency. Quaking aspen, the mountain West’s only dominant deciduous tree species, is in decline as a result of insects and pathogens, as well as historic fire suppression and grazing by livestock and wildlife, pressures that have enabled conifers to displace aspen across large areas. Aspens grow in clonal stands where all the individuals are genetically identical, a reproductive strategy that makes entire groves vulnerable to the same threats. The species’ range extends well beyond the Rockies, covering parts of Canada, the Upper Midwest, and New England, but its ecological heart lies in the western mountains. Reversing aspen decline is considered important to the ecological health of Western forests, yet significant gaps persist in scientists’ understanding of the causes of the decline and the solutions to it.

Anderegg, whose research focuses on how trees respond to climate change, emphasized that defenses against pests and pathogens likely matter a great deal for aspen survival under both drought and normal conditions. In his view, the study fills a key gap in understanding how drought and defense production change both during and after the initial droughts themselves. That framing captures the study’s central contribution: drought is not merely a stress that trees endure and then forget, but an experience that can reorganize their chemical relationship with the living world around them.

The paper, titled Functional memory of drought affects leaf chemical defenses and microbial interactions in aspen, was published on September 13 in New Phytologist. Co-authors include Utah biology students Jaycie Fickle, Kelly Kerr, Dale Forrister, Allison Perkins, Efthymia Symeonidi, Madelyn Allen, and Cannon Fehrenbach, along with Hsiao-Nung Chen of the Department of Chemistry. Funding came from the National Institutes of Health, the National Science Foundation, and the U.S. Department of Agriculture’s National Institute of Food and Agriculture, with additional support from the Wilkes Center for Climate Science and Policy, a Phyllis Coley and Thomas A. Kursar Field Research Award, the David and Lucille Packard Foundation, and an Alan T. Waterman Award. As droughts intensify across the American West, the idea that a tree’s leaves can carry forward the chemical echo of a dry year offers both a warning and a tool: a warning that the damage of drought may linger in ways ecologists are only beginning to measure, and a tool for predicting which forests will be most resilient when the rains return.

Subject of Research: Drought-induced functional memory in aspen leaf chemical defenses and microbial interactions

Article Title: Aspens 'remember' past drought in their leaves

Article References: Aspens 'remember' past drought in their leaves. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: aspen, drought, leaf chemistry, phenolic glycosides, condensed tannins, plant defenses, herbivory, leaf fungi, common garden experiment, forest ecology, climate change, New Phytologist