At what thickness is a tree immune to flames?

It’s a spectacular sight: 100-metre-high, 1,500-year-old forests spreading across the primeval landscape of the northwestern United States. The giant redwoods of Yosemite National Park in California are some of the tallest and oldest trees in the world – even though they’re rooted in one of the most wildfire-prone regions on the planet. The secret of their longevity lies in their tough bark, which, at 50cm thick, offers a natural form of fire protection. This means small fires pose no risk to the giant sequoias. In fact, the opposite is true: by getting rid of pesky competitors and delivering nutrient-rich ash to the redwoods, wildfires actually have a positive impact. Certain trees have even adapted to the fire so their seed pods only burst under extreme heat. However, redwoods aren’t the only fire retardant trees: bark with a thickness of just five centimetres 
can protect against flames, as the 60-metre-high yellow pines of New Mexico have shown. These evergreen pines stretch across the southern
tail of the Rocky Mountains and have survived countless fires over the years, escaping almost all of them unscathed, although the undergrowth has frequently not been so lucky. Even some deciduous trees can stand up to wildfires: Quercus suber, an oak that grows in southwest Europe, has fireproof armour made of cork. The tree is the primary source of cork for wine bottle stoppers.

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