Aspens ‘remember’ past drought in their leaves, 3-year experiment finds

Drought is written into trees, appearing as narrow bands in their annual growth rings. But aspen may go farther, creating “functional memories” of previous dry years reflected in the chemistry of their leaves, according to new research by University of Utah biologists.

In a three-year “common garden” experiment with quaking aspen (Populus tremuloides), researchers documented how trees’ drought history influenced the chemical defenses in their newly produced leaves. It was also associated with changes in the microbial fungi living on those leaves.

In subsequent years after exposure to experimentally induced drought conditions, aspen leaves exhibited higher levels of chemicals known as salicinoid phenolic glycosides, which deter herbivores, the insects and animals that eat plants, according to Talia Karasov, an assistant professor of biology and co-author.

A crucial tree species in trouble

A signature tree species in the West’s mountains and Utah’s state tree, quaking aspen are in decline as a result of insects and pathogens, as well as historic fire suppression and grazing by livestock and wildlife, which have enabled conifers to displace aspen. The Mountain West’s only dominant deciduous tree species, aspens grow in clonal stands where all the individuals are genetically identical. Their range also covers parts of Canada, the Upper Midwest and New England.

Reversing aspen decline is important to the ecological health of western forests, but gaps persist in scientists’ understanding of the causes and solutions to the problem.

“We 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,” said Karasov, who specializes in the study of plant-microbe interactions.

“Because drought-year conditions alone do not explain all of the delayed mortality observed in forests, we 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.”

Chemical defenses against microbes and insects

To further this work, the team sought to understand how drought influences the ways aspens interact with herbivores and potentially pathogenic microbes, which are exerting a great deal of pressure on these trees.

Karasov co-led the new study with biology professor William Anderegg, a forest ecologist interested in how trees respond to climate change, and graduate student Aubrey Hawks. Their results appear in the journal New Phytologist.

The team also found that levels of another chemical defense, condensed tannins, which help trees ward off microbes, went down in the drought-exposed trees. The researchers hypothesized that the surprising changes in leaf chemistry the year after drought exposure could reflect an adaptation to dry conditions.

The results also suggest that the effects of drought can persist through subsequent growing seasons, potentially influencing how aspens recover and how vulnerable they are to pests and pathogens, Karasov said. The study shows that drought and leaf chemistry can filter microbial communities, but it does not yet establish whether those changes ultimately help or harm aspen recovery.

“Defenses against pests and pathogens likely matter a lot for aspen to be able to survive droughts and also normal conditions,” Anderegg said. “And this study fills a key gap in understanding how drought and defense production change both during and after the initial droughts themselves.”

The ‘common garden’ research model

The common garden model used by the researchers brings together plants from geographically divergent populations to grow in a single location under shared conditions. The aspen research was conducted at the U’s Biology Research Experimental Garden, near the mouth of Red Butte Canyon.

In 2021, Anderegg’s lab established the experimental aspen plot at the site, where they planted rootstock they gathered from mountain sites in five national forests in Utah and Colorado. The original sites were about 9,000 feet (2,740 meters) above sea level on south-facing aspects.

Two other studies about aspen drought response have been published by Anderegg and his graduate students based on data produced at the research garden, with more on the way.

For the latest study, the team manipulated how much water the trees received to replicate varying levels of drought from 2021 through 2023. Leaves were harvested in late August and subjected to chemical analysis to determine levels of certain compounds that concentrate in aspen leaves as defense mechanisms against plant-eating insects and wildlife, as well as disease-causing microbes.

The increased levels of phenolic glycosides the team documented were associated with reduced canopy damage, while other changes observed in leaf chemistry were associated with shifts in the fungi inhabiting the leaves. Prior research has shown that increasing phenolic glycoside levels by just a little can reduce herbivory by up to 25%.

“It can basically be the difference between a plant being totally defoliated or not because the insects don’t like these phenol glycosides,” Karasov said. “In a dried leaf, the percentage of its mass that’s composed of these compounds that they make for fending off herbivores or microbes can be up to 20%. It’s incredible. So they’re putting so much of their resources into defending themselves.”

Yet the team found no evidence that the trees’ increased investment in production of phenolic glycosides came at the expense of tree growth.

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Gaby Clark

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