Why are mountain forests in Mexico and Central America hotspots for oak trees? New study led by The Morton Arboretum shows most definitive answer yet
Findings offer unique case study on species evolution
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Sierra Madre Occidental, Mexico
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LISLE, Ill. (May 4, 2026)—The mountains of Mexico and Central America harbor some of the greatest biodiversity of oak trees worldwide, and a landmark study conducted by The Morton Arboretum with U.S. and Mexican collaborators provides the most definitive answer to date as to why.
The team of researchers led by University of Chicago Ph.D. candidate Kieran Althaus, an affiliate of the Arboretum’s Science and Conservation Department, uncovered the conditions that enabled oaks to rapidly diversify in the Americas. The findings have advanced scientific understanding of the origins of one of the world’s biodiversity hotspots, forming a unique case study for evolution at a global scale.
The paper, “Timing and origins of Mexican and Central American oak diversity,” published May 4 in the Proceedings of the National Academy of Sciences (PNAS) and set to be the cover article of its May 12 digital issue, revealed that oak lineages were able to very quickly adapt—in terms of tree time—when they encountered high-elevation terrain.
“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,” Althaus said. “Mountain ecosystems prove key to generating oak diversity.”
According to the authors, oaks’ movement into rugged and complex habitats, combined with their high level of adaptability to new climates, facilitated the formation of a vast oak diversity hotspot. Using data from 322 of the world’s approximately 450 oak species, the researchers carefully reconstructed oak evolutionary history.
Two major groups of oaks—red and white oaks—independently migrated into the mountains of Mexico about 25 million years ago. The study discovered that upon their arrival, the two groups of oaks diversified along a parallel path, rapidly evolving into a remarkable array of species. Now, Mexico and Central America are home to at least 160 different oaks, representing roughly 40% of global oak diversity.
Even still, more than 30% of the world’s oak species are at risk of extinction. Oaks are foundational in many ecosystems, having 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 study co-author Socorro Gonzalez, Ph.D., researcher at Centro Interdisciplinario de Investigación para el Desarrollo Integral Regional (CIIDIR), Instituto Politecnico Nacional in Mexico.
The results of this publication represent a milestone in more than 15 years of fieldwork, botanical expeditions and data sharing between researchers in the United States and Mexico, highlighting the power of international partnerships and positive outcomes of scientific mentorship.
The study’s senior author is the Arboretum’s Director of the Herbarium and Lead Scientist in Plant Systematics Andrew Hipp, Ph.D. Co-authors include the Arboretum’s Director of the Global Tree Conservation Program Silvia Alvarez-Clare, Ph.D. and Plant Systematics Research Program Manager Marlene Hahn as well as researchers from Harvard University, Duke University and institutions in Mexico. These international collaborators include La Universidad Nacional Autónoma de México (UNAM), Morelia; UNAM, México; Herbarium and Botanic Garden, Benemérita Universidad Autónoma de Puebla (BUAP); and CIIDIR, Instituto Politecnico Nacional.
“This work reflects years of shared effort and deep, sustained collaboration across borders,” Hipp said. “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.”
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About The Morton Arboretum
The Morton Arboretum is a globally recognized leader in tree research and education. Its 1,700 acre site cares for 106,714 specimens representing 4,067 different kinds of plants. The Arboretum’s Center for Tree Science, Global Tree Conservation Program, Chicago Region Trees Initiative, and Center for Species Survival: Trees are contributing scientific knowledge and technical experience to secure the future of trees locally, nationally, and worldwide. Information about the Arboretum’s scientific work and how it contributes to a greener, healthier world where people and trees thrive together can be found at mortonarb.org.
Journal
Proceedings of the National Academy of Sciences
Article Title
Timing and origins of Mexican and Central American oak diversity
Article Publication Date
4-May-2026
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