A recent investigation spearheaded by scholars at Washington University in St. Louis along with the Missouri Botanical Garden has revealed an unexpected dimension of diversity within tropical woodlands. Not only are these woodlands teeming with an astonishing variety of tree species, but each species adopts a distinct chemical strategy, expanding the spectrum of natural compounds that serve vital roles for the plants — and possibly for humans as well.
The study illuminated the ecological and evolutionary dynamics that render tropical forests significant hubs of biodiversity. Although the team was not specifically in search of compounds beneficial to humans, their results highlight the importance of tropical forests functioning as natural production centers for plant chemicals that could prove valuable in medical and various other applications, stated Jonathan Myers, a biology professor within the Arts & Sciences at WashU. “Tropical vegetation generates an immense diversity of chemicals with practical implications for human health.”
This research was funded by the National Science Foundation (NSF) and the Living Earth Collaborative, a biodiversity project involving WashU, the Missouri Botanical Garden, and the Saint Louis Zoo. The findings were published in the esteemed journal Ecology and were led by David Henderson, PhD ’23, a former graduate student in ecology and evolutionary biology. Collaborators included Missouri Botanical Garden researchers Sebastian Tello, Leslie Cayola, and Alfredo Fuentes; chemical ecologist Brian Sedio from the University of Texas at Austin; and ecologists Belen Alvestegui and Nathan Muchhala from the University of Missouri-St. Louis.
The scientists analyzed tree leaves collected as part of the Madidi Project, an extensive survey of plant life in the Madidi area of the Andes Mountains in Bolivia. The focus was particularly on chemical substances used by plants to defend against insect herbivores, pathogens, and other threats — a critical aspect for any organism thriving in the warm, humid, and pest-ridden tropics. Their aim was to gain a deeper understanding of how these substances vary among species coexisting in tree communities at differing elevations with varying climates.
Employing mass spectrometry, a technique that allows for the identification and quantification of individual molecules within a sample, researchers discovered a wealth of chemical substances. “We found over 20,000 distinct metabolites in leaf samples from 470 tree species,” Myers remarked. “This represents an incredible level of chemical diversity.”
More than a third of these substances were terpenoids, a category of natural compounds that plants utilize to combat insects and diseases. As components in pharmaceuticals, terpenoids have also shown potential in combating cancer, reducing inflammation, and eradicating harmful viruses and bacteria. Approximately a quarter of the substances were alkaloids, a category of plant chemicals that serve as the foundation for numerous drugs, including analgesics, anti-malarial treatments, and cancer therapies.
With a global compilation of chemical substances, researchers could search for unique compounds that might have significant benefits for society.
Jonathan Myers
The extraordinary chemical diversity of tropical woodlands emphasizes the necessity to investigate and conserve these biological hotspots, Myers noted. He and his team have contributed data from this study to facilitate the development of a global database of chemical compounds derived from plants. “With such a resource, researchers could seek out unique chemicals that might have real societal value,” he added.
In the current investigation, Myers and his associates assessed the variety of tree species and leaf metabolites in both wet and seasonally arid forest plots sampled at various altitudes ranging from around 2,000 to 11,000 feet above sea level. As they ascended, they encountered progressively fewer species. They identified nearly 140 distinct tree species in a single 1-hectare (2.5-acre) plot at an altitude of 4,000 feet but fewer than 20 species in a plot at nearly 11,000 feet.
As species diversity diminished, so too did the chemical variances among tree species. High in the mountains, trees from disparate species tended to employ similar types of chemicals for self-protection and managing abiotic stress. The natural chemical generators only reach their maximum capability at lower elevations, Myers explained.
In the exceptionally diverse, fiercely competitive lowland tropical forests, it is logical that a tree would utilize chemical defenses that are entirely different from its counterparts. “If a tree shares the same chemistry as a neighbor, it may become susceptible to identical herbivores and pathogens,” Myers indicated. That threat will inflict less damage overall if they require differing weaknesses for each tree, he elucidated.
The relationship between species diversity and chemical diversity is not confined to the tropics. Myers is involved in an ongoing NSF initiative studying trees at multiple locations, including the Amazon lowlands, the northern forests of Canada, and local research sites at WashU’s environmental field center, Tyson Research Center. The expanses of oak, hickory, and other tree types at Tyson may not rival the diversity of tropical forests, yet they exhibit significantly greater diversity than the coniferous forests to the north. Laboratory investigations have shown that Tyson’s trees also rank around the middle in terms of chemical diversity: They are not as heavily productive as tropical trees, but they offer a wealth of unique chemicals compared to northern woodlands.
Climate might be a crucial factor in elucidating why chemical diversity appears to correlate with species diversity. Warmer, wetter, and more stable climates can accommodate elevated species diversity if plants limit the types of herbivores and pathogens attacking them through varied chemical defenses, Myers stated. “This could be yet another explanation for the patterns of plant diversity across the globe.”
Originally published on the Ampersand website
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