09.07.2022, USA, Yosemite National Park: Ein Feuerwehrmann schützt einen Mammutbaum, während das Washburn-Feuer im Mariposa-Hain der Riesenmammutbäume (Grove of Giant Sequoias) im Yosemite-Nationalpark brennt. Foto: Noah Berger/AP/dpa +++ dpa-Bildfunk +++

Forest fires have been raging in Yosemite National Park in northern California for more than a week, endangering the world-famous giant sequoias. As of Friday afternoon, the flames covered a forest area the size of almost 2,500 soccer fields. More than 1,000 firefighters have been deployed so far, and according to the US crisis platform “Inciweb”, they had about a quarter of the fires under control.

The so-called “Washburn” fire had also approached the Mariposa Grove, which attracts tourists from all over the world with its 500 giant sequoias. On average, they grow between 50 and 85 meters tall, and their trunks usually reach a diameter of up to nine meters.

So far, the centuries-old grove has been spared from the flames because those responsible for the national park have been setting controlled fires for years, regularly thinning out the surrounding forest and thus removing a lot of fuel. Above all, biologists and firefighters want to prevent the treetops of the giants from catching fire.

While the trunk of such a tree can still withstand some heat and scorching damage, high flames could reach the top of the natural giants and ignite them like a giant match. In the past few days, the fire brigade sprayed the giant sequoias with water as a precaution.

Thomas Speck, Professor of Botany at the Albert-Ludwigs-University Freiburg, has researched the giants. “Giant sequoias are one of the oldest living things on earth. Some live to be 3,000 years old and weigh 2,000 tons. Its sheer size makes people humble,” he told the Tagesspiegel.

The trees are also highly adapted to surface fire. According to the researcher, the bark, the outermost part of the bark, is up to 70 centimeters thick and protects the inner and vital cambium from the flames.

The botanist finds out how fire-tolerant different tree species are in flame tests. Speck takes bark samples from various tree species, stores them all equally dry and flames them until he penetrates to the sensitive cambium under the bark tissue. “As soon as this living cell layer gets hotter than 60 degrees, the tree dies.”

The longer it takes for the flame to advance to the cambium in the experiment, the more fire tolerant the tree species is. “With the thick bark, the giant sequoias can survive surface fires for about five to seven minutes without damaging the living tissue,” explains Speck. This makes the giants far more fireproof than trees such as beech, silver fir or the Scots pine that is often found in Brandenburg.

Whether the flames bring down the trees also depends on the moisture in the trunks and soil. “The extreme drought in California in particular can favor large forest fires and widespread destruction,” says Speck.

The most common cause of large fires are man-made, including in Europe: “Camping fires, discarded cigarettes and arson trigger almost all forest fires that get out of control,” says Speck. Smaller natural forest fires would even ensure the survival of adapted tree species. The heat of the fire helps the trees open their cones and scatter their seeds.

But the ongoing climate crisis favors longer and more frequent droughts and also fire weather in different regions of the world. In view of this development, do the giants still have a future at all?

Speck says: “It is difficult to say whether the giant sequoias will survive as a result of the climate crisis. It is plausible that more frequent forest fires will reduce the population.” Overall, the increased drought in the USA and Europe would affect many tree species, making them more susceptible to fungal diseases, pests and fires and thus reducing their chances of survival.

In any case, the bark of the giant sequoia is so resistant that Speck researched it in search of new industrial materials. “For example, it is impressive that the bark fibers are self-extinguishing up to a certain point. If you hold a detached piece of bark up to the flame of a lighter, the bark will not continue to burn.” This is due to phenolic ring compounds, which only ignite at high temperatures.

In any case, those responsible for Yosemite National Park have no hope of a quick end to the fires in the region and assume that the fires will flare up and smolder in the coming months – until it rains or snows in winter.