Celebrating and Protecting Mexico’s Amazing Oak Diversity

Posted on October 13th 2016 by Audrey Denvir

When you think of Mexico, the first thing that springs to mind probably isn’t ‘oak trees’, but expect that to change. Audrey Denvir of Morton Arboretum shows us why Mexico’s oaks are so remarkable and explains recent moves by tree conservationists to secure their future.

Mexico is the country with the greatest oak diversity in the world. Notably, of the around 425 species of oak that exist globally, Mexico is home to around 160.The diversity of oaks in Mexico is visible in the wide variety of tree heights, leaf shapes and acorn sizes that are found there. Unfortunately, many oak species in Mexico are threatened with extinction due to ecological threats like habitat loss, climate change, and increased human use. The Global Trees Campaign, along with the Morton Arboretum and others, is part of a new alliance the ‘Oaks of the Americas Conservation Network’ (OACN), which aims to protect the threatened oaks of the Americas, especially in the oak diversity hotspot of Mexico.

Eye-catching oak species in Mexico

Oaks are a group of species that are famed for their size and grandeur, and in Mexico Quercus bumelioides outstrips all the rest, growing to huge heights. On average, this tree is usually between 4 and 20 meters tall, but some individuals grow much taller than that. There is a particular Q. bumelioides tree in nearby Costa Rica’s Parque Nacional Cerro de la Muerte called the “Grandfather Oak” that has been measured at an astounding 60.4m (198 ft) tall. This species is found throughout Central America and in the State of Chiapas in southern Mexico, but it is threatened by the decline of mountainous forest habitat and as such is listed as Vulnerable on the IUCN Red List.

Towering Quercus bumelioides Credit: Flickr/Duane Hook.

On the other end of the spectrum, there are ‘shrub oak’ species that stay closer to the ground, like the diminutive scrub oak (Quercus dumosa), an Endangered species found in the Baja California Peninsula of northwestern Mexico and the Critically Endangered Hinckley’s oak (Quercus hinckleyi), found in North Chihuahua, Mexico. Scrub oak often grows to only 2-3m, and Hinckley’s oak does not exceed 1.5m. Certain varieties of both species can have small, coarsely toothed leaves, giving an appearance similar to holly.

The holly-like leaves of Quercus dumosa. Credit: Flickr/Don Laurie.

The spectacular variety of oak species in Mexico is also exemplified by the variety of acorn shapes and sizes one can find there. For example, Quercus insignis (Critically Endangered), which is found in Oaxaca and Veracruz states, has huge, rotund acorns. Quercus brandegeei, an Endangered species endemic to southern Baja California, has acorns that are similarly large, but elongated. On the other hand, species like Quercus dumosa, mentioned above, have small acorns that can be less than half the size of Q. brandegeei. Variation in acorns represents the variety of environments and evolutionary histories present throughout Mexico.

The palm-sized acorn of Quercus insignis. Credit: Guy Sternberg.

Oaks in Mexico also have a diversity of social and cultural importance. Acorns of Quercus leiophylla (Near Threatened) are collected In the Huatusco region of Veracruz, to create religious and decorative objects. The wood of the Critically Endangered Hinton’s oak (Quercus hintonii) is used in the baking of “las finas” bread in the Tejulico traditional culture. The wood’s smoke gives the bread a unique taste, giving the tree species a specific cultural significance. Quercus sapotifolia’s (Vulnerable) bark is used for tanning leather and dying textiles due to the bark’s rich tannins and coffee-colour.

The small acorns of Quercus dumosa. Credit: Flickr/Jerry Kirkhart.

The mystery of Quercus brandegeei

Despite the high level of threat to many species, there are numerous Mexican oak species whose ecology and biology remain very poorly understood. Quercus brandegeei (Endangered), a species of oak that exists in a very narrow and isolated range in southern Baja California, is one of these species.

Q. brandegeei grows in stream beds that are dry most of the year but fill with water after exceptionally heavy rains, like hurricanes. Based on its close relatives, researchers believe that this oak has a very long life span of up to up to 800 years but little more is known about the reproduction or life history of the species.

The distinctive acorns of Quercus brandegeei. Credit: Jose Luis Leon de la Luz/CIBNOR.

Recent and historic surveys of the area have revealed that no regeneration of Q. brandegeei appears to be happening in the wild. In fact, all of the individuals that experts have been able to find appear to be at least 100 years old. Now, in order to save this species, The Morton Arboretum, UNAM, and Vallarta Botanical Garden are working together to learn what is preventing seedlings from becoming established. Could the problem be droughts that are intensified by long term climate change? Or is the problem overgrazing of acorns by cattle (or other animals), which are an important part of the local economy? Knowledge about what is preventing seedling regeneration is critical to the creation of informed, effective conservation measures targeting this species.

More work like this needs to be done. Not just in Mexico but around the world, oak trees are coming under increasing pressure of climate change, habitat loss and human use. In order to save these incredible species, a concerted conservation effort is needed among gardens, non-profits, researchers and all conservation actors alike.

A Quercus dumosa leaf. Credit: Flickr/ElCajonYachtClub

The Oaks of the Americas Conservation Network

The Morton Arboretum is working on Q. brandegeei and other oaks in Mexico as part of the Oaks of the Americans Conservation Network (OACN), which was born out of the International Workshop on Oak Conservation held in March 2016 in Morelia, Mexico. The OACN is a network of researchers and conservationists at universities, botanical gardens, arboreta, NGOs and government agencies working to conserve threatened oak species around the world. An urgent need identified by the OACN is a gap analysis of the current state of oak species in Mexico and Central America. In other words, there needs to be a concerted and coordinated effort to examine the threat status of oak species in the wild and what is being done to protect them. From there, the Network can prioritise species for urgent conservation action.

Are you working on a threatened oak species somewhere in the world or interested in doing so? We have opportunities for oak conservationists working to protect the world’s most threatened species. This year, we are supporting the Conservation Leadership Programme (CLP) to provide project funding and international training for an early career tree conservationist. Please contact globaltrees@fauna-flora.org for more information.