According to new study, oaks’ movement into rugged and complex habitats, such as Mexico’s Sierra Madre, combined with their high level of adaptability to new climates, facilitated the formation of a vast oak diversity hotspot. (Photo courtesy of The Morton Arboretum)
New Study Traces How Oaks Rapidly Diversified Across the Americas
Oak trees are a familiar part of the landscape here in the Triangle, shading us from the heat and peppering Duke’s campus with acorns of different sizes and shapes. But far beyond North Carolina, the mountains of Mexico and Central America harbor one of the richest concentrations of oak diversity anywhere on Earth.
A new international study co-authored by Professor of Biology Paul Manos provides the clearest picture yet of how that extraordinary diversity came to be.
The study, led by researchers at The Morton Arboretum and published May 4 in the “Proceedings of the National Academy of Sciences (PNAS),” reconstructed the evolutionary history of oaks across the Americas and identified the conditions that allowed the trees to rapidly diversify in mountain ecosystems.
Using data from 322 of the world’s approximately 450 oak species, the researchers found that two major oak groups — red and white oaks — independently migrated into the mountains of Mexico about 25 million years ago. Once there, both groups rapidly diversified along parallel paths, evolving into a remarkable array of new species.
Today, Mexico and Central America are home to at least 160 oak species, representing roughly 40% of the world’s oak diversity.
“The fastest expansion of oak diversity anywhere in the world has taken place in Mexico and Central America, revealing the importance of this region for these keystone species,” said lead author Kieran Althaus, a University of Chicago Ph.D. candidate affiliated with The Morton Arboretum’s Science and Conservation Department. “Mountain ecosystems prove key to generating oak diversity.”
According to the researchers, rugged mountain terrain and rapidly changing climates created opportunities for oak populations to adapt to new environments and split into new species over time.
“We’ve waited long enough to fully appreciate, in an evolutionary context, the geographic patterns of the approximately 250 oak species across the Americas,” said Manos. “This is what we learned from Darwin and Wallace, though in a mostly island context: that the distribution of species — fossil and modern — is at the heart of understanding the process of speciation.”
“For the oaks, in their mainly continental setting and with so many species,” he added, “it’s thrilling to unpuzzle the twists and turns of their complicated and clearly opportunistic biogeographic history.”
The findings help scientists better understand how biodiversity hotspots form and evolve over millions of years. They may also help researchers predict how forests could respond to future environmental change.
"For the oaks, in their mainly continental setting and with so many species, it’s thrilling to unpuzzle the twists and turns of their complicated and clearly opportunistic biogeographic history." – Paul Manos, Duke University Professor of Biology
More than 30% of the world’s oak species are considered at risk of extinction. Oaks are foundational in many ecosystems, holding deep cultural value for indigenous people in the region and supporting biodiversity, including fungi, insects, birds and mammals. Each oak species hosts a unique community of life, making their conservation vital to maintaining healthy ecosystems.
“The ability to predict how these plant communities may respond to environmental change is an important, valuable tool that should be considered in future forest management,” said co-author Socorro González-Elizondo of the Centro Interdisciplinario de Investigación para el Desarrollo Integral Regional (CIIDIR) at the Instituto Politecnico Nacional in Mexico.
The publication represents more than 15 years of fieldwork, botanical expeditions and collaboration among researchers in the United States and Mexico.
“This work reflects years of shared effort and deep, sustained collaboration across borders,” said senior author Andrew Hipp, the Arboretum’s Director of the Herbarium and Lead Scientist in Plant Systematics. “It also constitutes the first chapter of Kieran’s Ph.D. dissertation, underscoring the significance of such projects in granting exposure to the next generation of conservation leaders.”
