Bark that Defies Fire

Garden Director Lew Feldman

The cork oak (Quercus suber), featured in the Crops of the World Garden, is one of the most iconic plants of the Mediterranean environment—an ecosystem where fire is a recurring feature. Its thick bark is composed of specialized, dead cells known as cork, which serve to insulate the tree’s living interior tissues from the intense heat of annual grassland fires.

A cross-section slice from a cork oak tree, showing the thick outer layer of cork

Similarly, native California oaks such as interior live oak (Quercus wislizeni), found in the Garden’s California Collection, also possess bark composed of cork. Although not as thick as that of Quercus suber, this bark has evolved to be sufficiently protective against the fast-moving, low-intensity grassland fires typical of California ecosystems. However, in areas where fire suppression has prevented regular burns, fuel has built up over time. When a fire eventually breaks out in such environments it often burns hotter and longer, potentially defeating the insulating properties of the bark. This can damage the tree’s interior, living tissues, weakening the tree and making it susceptible to disease and other stresses.

A close-up view of the light gray, thick and wavy bark of a cork oak tree, located in the Crops of the World Garden

The Role of Indigenous Burning Practices
Historically, Indigenous communities used fire as a tool for land management, intentionally setting periodic fires to maintain healthy grasslands. This practice, often referred to as cultural burning, was used to clear underbrush, create firebreaks, and promote the growth of culturally important plant species. Indigenous peoples across North America have engaged in controlled burns for millennia. A recent report (Page Buono “Quiet Fire”. Nov. 02, 2020 (Winter 2020) The Nature Conservancy) highlights how Indigenous groups in northern California, including the Yurok, Karuk, and Hoopa Tribes, used fire to encourage the growth of traditional food sources like acorns, as well as basket-weaving materials such as hazel.

Because these human-managed burns were often annual and controlled, they tended to be low in intensity and short in duration. This longstanding practice may have influenced the evolutionary development of fire-resistant traits in native plants. It’s reasonable to speculate that the thickness of cork bark—seen in both Quercus suber and native California oaks—may be a result not just of natural fire regimes, but of millennia of human-directed burning. In this way, humans have not only shaped the landscape but may also have pushed natural selection towards the evolution of a cork layer optimally adapted for fire protection.