This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 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." 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.
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