Why Mexico’s Mountains Hold So Many Oak Species

In southern Mexico’s high mountains, even an ordinary walk can bring unexpected sights. Over a small area, oak trees repeatedly change in appearance.

Some have rough, thick bark, while others bear smooth leaves, with every tree looking a little distinct. In many other regions of the world, the same area might contain only a handful of oak types.

Why, then, are so many found in this part of the world? Scientists have now identified a clear explanation.

Mexico contains most oak species

Mexico and the adjoining parts of Central America are home to roughly 40 percent of the world’s oak species.

That is remarkable: although this is a comparatively small region, it contains almost half of all global oak diversity.

Researchers have long recognised that this area has an exceptional abundance of oak species. Surveys in the field regularly reveal much greater variety than in larger areas elsewhere.

Yet, despite this awareness, scientists had been unable to explain clearly why such a high concentration of oak species occurs in one location.

A research team examined hundreds of oak species, studying their genes and environments to discover how oaks dispersed and evolved over millions of years. Their findings indicate that the explanation is rooted far back in history.

Oak species reached Mexico in two waves

About 25 million years ago, two major oak groups arrived in Mexico’s mountains: red oaks and white oaks. They reached the region at separate times and by different routes.

After arriving in the mountains, however, both groups did something notable. They developed and diversified in broadly similar ways.

Mexico’s mountains are far from uniform. Dry zones, damp pockets, cool higher elevations and warm valleys can all be found close to one another.

This variety produces numerous small habitats. Tree populations become isolated in different settings and, over time, each population develops independently into a new species.

Oaks are also extremely adaptable. Their ability to survive under varied conditions helps them spread and evolve.

The pattern occurred twice in nature

What makes this account particularly remarkable is that the process took place twice. Both groups of oaks followed comparable evolutionary routes in the same region.

This demonstrates that the landscape itself was crucial. It was not simply a matter of chance: the mountains provided the conditions needed for diversity to grow.

“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 Kieran Althaus of the University of Chicago.

“Mountain ecosystems prove key to generating oak diversity.”

What oak diversity reveals about global biodiversity

The discovery also gives scientists a better way to understand biodiversity hotspots in other parts of the world.

Places such as the Andes and tropical islands likewise contain extraordinarily high diversity. The story of oaks provides a clear illustration of how such richness can emerge.

Where landscapes produce many small and varied environments, living organisms have more chances to adapt and evolve.

Oaks are not merely trees; they sustain whole ecosystems. Birds, insects, fungi and mammals all rely on them.

Numerous insects eat only oak leaves, and birds depend on those insects as food. Small animals shelter in oak trees, forming a connected chain of life between many species.

Many oak species are threatened with extinction

More than 30 percent of oak species are now at risk of vanishing because of climate change and human activity.

Forests are cleared for agriculture and development. Higher temperatures are also changing where trees can survive and grow. Certain oak species may be unable to adjust quickly enough.

In Mexico, oaks also form part of everyday life and local traditions. Their disappearance would affect people as well as nature.

“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.

More than 15 years of research

The research required more than 15 years to complete. Scientists from several countries collaborated, visiting forests, gathering samples and exchanging expertise.

Fieldwork frequently meant lengthy journeys into isolated mountain regions. Researchers spent days identifying tree species and documenting their characteristics. This work helped create a robust dataset.

“This work reflects years of shared effort and deep, sustained collaboration across borders,” said Andrew Hipp, senior author of the study.

“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.”

Mexico’s oak forests require protection

Mexico’s mountains demonstrate nature’s capacity to produce abundant, varied life. One hillside can reveal a long history of transformation and development.

Safeguarding these forests is a collective responsibility. Local communities, researchers and governments all have a part to play. Conservation measures may involve protecting land, planting trees and reducing damaging activities.

If these forests remain protected, they will keep sustaining life in the years ahead. Without that protection, the damage will be difficult to undo.