Across 406 forest plots stretching from the lowland Amazon basin in Brazil to the Andes Mountains from Colombia to Bolivia, scientists have tracked the fate of tropical trees for four decades. The study’s conclusion is nuanced: While the total number of tree species has remained largely stable, tree biodiversity has subtly shifted throughout the region, with potentially important ecological consequences.

The work included important contributions from two WashU researchers: Jonathan Myers, a professor of biology and a fellow of the Living Earth Collaborative, and Toby Pennington, the David and Dorothy Kemper Professor in the Department of Biology, a joint appointment with the Missouri Botanical Garden.

The project uncovered fascinating shifts in the number of tree species across regions and habitats.

While the number of tree species declined in warmer, drier, and fragmented sections of lowland forest, cooler and wetter forests in more intact areas gained species. “Climate change and human activity are having a profound impact on biodiversity,” Myers said. “Tropical forests play a crucial role in the global environment, and this study underscores the critical need to keep forests intact to preserve local diversity.”

The international effort, supported by the National Science Foundation, the Missouri Botanical Garden, and WashU’s International Center for Advanced Renewable Energy and Sustainability, among many other funding sources, brought together more than 160 researchers representing multiple nations. “From the people on the ground to researchers studying specimens in herbaria to the study organizers, it’s a tremendous amount of work and a huge challenge,” Myers said. “We were proud to represent WashU in this important mission.”

Pennington, a renowned expert in tropical plant taxonomy, served as the co-principal investigator of a tree identification project in Colombia. In that role, he helped improve the identification of the thousands of tree species found in Latin American forest plots by formalizing a process of storing images of herbarium specimens in the ForestPlots.net database. The Missouri Botanical Garden is home to one of the world’s largest reference collections of tropical plant specimens. By consulting such collections and using DNA sequencing when necessary, botanists have greatly improved their ability to identify species and accurately track changes in forests, Pennington said.

Myers participated as part of an international group of researchers who were connected through the Living Earth Collaborative, a joint mission of WashU, the Missouri Botanical Garden, and the Saint Louis Zoo. “We started thinking about what it would take to really understand how forests were changing over time and how those changes varied from one region to another,” said Myers, who is also a long-term member of the garden’s Madidi project, which explores and documents plant diversity in Northwestern Bolivia.

The study uncovered some telling trends in a changing world. In the big picture, the total number of tree species in the entire study area has stayed roughly the same over the last 40 years. “This suggests, for now, we aren’t yet seeing massive tree species extinctions in tropical rainforests, which is certainly good news,” Pennington said.

The total number of tree species increased in the Northern Andes and the Western Amazon, home to the world’s largest intact rainforest. In contrast, the number of species dwindled in less-intact, drier, and more seasonal forests, mostly in the Central Andes, Central-Eastern Amazon, and the Guyana Shield.

Climate change clearly played a role. More than 90% of the forest plots were significantly warmer than they were 40 years ago. The study found that the losses of species were greater in forests that warmed the fastest. The same species that are vanishing in the lowlands may be migrating up to higher elevations and seeking refuge from warming, Myers said.

However, the total species counts tell only part of the story. Tree species adapted to warmer climates are becoming more common within some forest communities, while other species are decreasing in abundance, creating forests with less variety.

“This study complements others published recently that suggest some forests are shifting in composition towards higher abundances of species from warmer climates,” Myers said. “Even though the total number of species has stayed the same, tree communities may be converging towards similar species composition.” Over time, such compositional changes could lead to a loss of habitat for animals and dwindling resources for humans who continue to rely on tropical forests for food, timber, and beneficial compounds such as pharmaceuticals.

For Pennington and Myers, contributing to this long-running collaboration continues a St. Louis tradition in tropical ecology. WashU alum Alwyn Gentry, PhD ’72, a long-time senior curator at the Missouri Botanical Garden, helped build the early international networks necessary to answer some of the field’s biggest questions. Gentry died in a plane crash in 1991 while conducting fieldwork in Ecuador.

“We’re honoring his memory by continuing these vitally important collaborations,” Pennington said.

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