by Audrey Langston-Wiebe

For many forest creatures, trees offer the ultimate all-you-can-eat buffet. Insects, mammals and even fungal pathogens see trees as a free meal. They’ll feast on tree leaves with no hesitation.

These trees, however, are not defenseless. To defend against predatory enemies, trees produce special chemicals called “secondary metabolites” within their leaves. If a tree produces enough distasteful metabolites, it may be able to deter hungry herbivores.

A recent study, spearheaded by researchers at the Smithsonian Environmental Research Center (SERC), found that trees growing in more diverse communities produce a wider array of chemical compounds on their leaves. This then changes how animals interact with those trees—often in ways the team didn’t expect.

“Understanding how trees interact with their natural enemies is essential to predicting and fostering forest resilience in the face of escalating losses of forest diversity,” said Catherine Fahey, lead author of the new study.

Fahey did the work as part of her postdoctoral fellowship with SERC’s Terrestrial Ecology Lab. She used data from SERC’s BiodiversiTREE forest: an experimental expanse of 70 “microforests” planted by SERC senior scientist John Parker in 2013. The plots vary in their tree diversity. Thirty-two of the plots contain just one tree species. The rest include mixtures of four or twelve species, which are considered medium and high tree diversity levels in nearby local forests.

A team of researchers with a long history of working at BiodiversiTREE gathered the bulk of the data. The team included head technician Jamie Pullen and former SERC postdoctoral fellows Karin Burghardt and Eric Griffin (now university professors). They sampled 427 young trees across 14 species. For each tree, they measured growth, collected leaves and surveyed for enemy damage from three key plant enemies: caterpillars, deer and fungal pathogens.

Leaf it to Metabolites

The team shipped their leafy evidence to chemical detective Brian Sedio for analysis.

Sedio, an assistant professor at the University of Texas at Austin and another former Smithsonian postdoc, pioneered the use of “comparative metabolomics” in forests. Traditionally, scientists have only looked at the number of chemicals in a sample. Sedio’s technique, however, looks at how different the metabolite structures are from one another, offering a more accurate idea of a tree’s chemical range.

Once Sedio had analyzed the samples, Fahey and her team were able to conclude that more diverse forests yield more chemically diverse leaves.

That, in turn, influences how different species feed on the trees. Each “enemy” species has its own dietary preferences. So, while metabolite-rich trees repel some species, others are drawn in. This is what Fahey chose to investigate next.

Very hungry caterpillars

_{Left: Purple-crested slug moth caterpillar, Adoneta spinuloides. Right: White furcula moth caterpillar, Furcula borealis. (Credit: Cathy Fahey)}

One of Fahey’s most surprising findings involves a notorious tree enemy: the caterpillar. Fahey found that leaves with a wider array of chemicals—which grow in more species-rich forests—attracted more caterpillar species. And with more caterpillars came more chewed-up leaves.

“Traditionally, it’s thought that insects would be less likely to consume more chemically diverse leaves,” Fahey said. Caterpillars “definitely have taste preferences,” she continued, but even within a single tree species, “they tended to prefer the trees that had more diverse chemicals.”

The real shock, though, is that trees with more insect damage also showed greater growth. One possible explanation is insect fertilization. As caterpillars digest leaves, they leave behind nutrient-rich frass (a.k.a. caterpillar poop). Their poop might be fertilizing the trees, promoting growth. Another theory is that the trees are experiencing “compensatory growth,” a phenomenon where plants respond to damage by undergoing a period of accelerated “catch-up” growth.

Fahey admitted that her team isn’t quite sure why insect damage promotes tree growth, but that it’s what the data shows.

“All the evidence points to it,” she said.

Deer Herbivory Hits Hard

While caterpillars might bring unexpected growth, deer browsing is a consistent knockback. Deer browsing not only removes leaves but can severely stunt young trees’ development. Some BiodiversiTREE plots struggled to grow beyond knee height due to intense deer pressure.

Species like red maples and black gum are particularly vulnerable. In monoculture plots, where all trees are the same species, the ones deer find tastiest struggle the most to thrive.

“The monoculture maples are the ones that get eaten the most here,” said Fahey.

By contrast, trees in mixed-species plots had a much lower chance of becoming a deer’s lunch. Even mouth-watering maples did better when surrounded by less tasty trees, said Fahey.

As it turns out, more diverse forests have better defenses against deer. And these maples aren’t just camouflaged—they’re more chemically equipped. Trees growing in more species-rich plots pack a richer mix of metabolites on their leaves, making them less appetizing to hungry deer.

Strength in Numbers

By studying how enemy species interact as part of the same system, Fahey and her colleagues can monitor how their patterns change.

“There are a lot of very different interactions as a forest grows,” said Fahey. “You can’t just look at them individually.”

And that complexity matters. While single-species plots remain common in commercial tree farms and restoration projects for their simplicity, this study suggests they may be more vulnerable to hungry herbivores—especially deer—and slower to grow. Diverse forests, on the other hand, create rich chemical landscapes that not only reshape enemy behavior but can help trees grow tall and strong.

The full study, “Chemically mediated plant–enemy interactions promote positive biodiversity effects on young tree growth,” is available in the Journal of Ecology at https://doi.org/10.1111/1365-2745.70062.

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