Quaking aspen leaves show lasting chemical changes after drought in three-year experiment

Quaking aspens grown in a University of Utah experiment showed changes in newly produced leaves after earlier drought, researchers reported in an October 9 account of a three-year study. The finding offers a way to examine how drought may affect trees beyond the dry year itself, but it does not show whether the changes help or harm aspens in forests.

The researchers call the effect a functional memory of drought. They measured differences in leaf chemicals used in defense and in the fungi living on leaves. Because aspens shed their leaves annually, the finding concerns traits detectable in leaves produced in later growing seasons. It does not mean that a leaf kept growing through multiple years or that the effect lasts indefinitely.

How the University of Utah aspen experiment worked

The team grew quaking aspens in a common garden near the mouth of Red Butte Canyon, using rootstock collected from five national forests in Utah and Colorado, according to the University of Utah account published by Phys.org. A common garden lets researchers compare trees from different origins under shared growing conditions. The study describes 360 trees arranged in 10 randomized blocks and water treatments spanning 2021 through 2023.

Researchers changed how much water the trees received to create differing drought histories. They then examined leaves for salicinoid phenolic glycosides, or SPGs, and condensed tannins. These compounds are associated with defenses against organisms that eat or infect plants. The team also studied the composition of fungal communities on the leaves, giving it a way to compare chemical changes with shifts among the fungi present.

What changed in leaves after earlier drought

The paper reports higher SPG concentrations in trees with a prior-year drought history. In 2022, the difference from trees watered the previous year was 1.46 percentage points of leaf dry weight, equivalent to an 11% increase. The reported increase in 2023 was 8%. Those figures describe chemical measurements in the experimental trees, not a measured improvement in forest survival.

The pattern was not uniform across every defense measure. In 2023, trees exposed to consecutive drought had lower condensed tannin levels than trees in an alternating drought treatment. The paper reports differences of 31% and 32% for its two-year and three-year consecutive drought groups, respectively. A higher concentration of one defensive compound therefore should not be read as an across-the-board increase in leaf defenses.

Genetic differences among the trees also mattered. The study reports that genotype explained more variation in total SPGs than the water treatment did in both 2022 and 2023: 14.5% against 7.7% in the first of those years, and 46.3% against 10.9% in the second. The comparison limits any simple claim that drought history alone determined a tree's leaf chemistry.

Across treatments, higher SPG concentrations were associated with less canopy damage. The researchers found no evidence that greater investment in those defensive chemicals reduced tree growth. Phys.org also cites earlier research suggesting that small increases in phenolic glycosides can reduce herbivory by up to 25%; that figure is context from prior work, not a 25% reduction demonstrated in this experiment.

What the fungal findings can and cannot show

The study found shifts in the fungal communities inhabiting aspen leaves alongside the drought and chemistry differences. It reports that fungi classified as capable of acting as pathogens became relatively more abundant under drought and less abundant as SPG concentrations increased. Relative abundance and pathogen potential do not establish that those fungi caused disease, damage or tree death.

That distinction matters because leaf chemistry can affect a tree's interactions with insects and microbes in different ways. The experiment connects prior water limitation with measurable changes in fresh leaves and with the fungi found on them. It does not settle whether the combined changes make an aspen better protected or more vulnerable when drought ends.

William Anderegg, a University of Utah forest ecologist and co-author, told Phys.org that the work fills a gap in understanding how drought and defense production change both during and after dry conditions. The reported experiment took place in a managed garden. Whether the same patterns affect recovery or survival among wild aspens remains an open field question, and the October 9 account did not identify a planned intervention or next study.