Nathan Stephenson, biologist and emeritus researcher at the Institute of Geological Studies in the United States, explains, in an interview with the “world”, that 13 % at 19 % of the giant sequoia of California, therefore of the world, have been destroyed by fire since 2020.
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Nathan Stephenson, biologist, emeritus researcher at the Ecological Research Center on the American West of the United States Institute of Geological Studies, has been studying giant sequoias (Sequoiadendron Giganteum) for over thirty years. These millennial trees grow only on the western slope of Sierra Nevada, California, between 1,500 and 2,100 meters above sea level. They are divided into 75 Futaies, scattered over 350 kilometers.
since 2014, you have been studying the repercussions of drought in California on giant redwoods. What did you see?
I had been alerted by the fact that the redwoods lost height leaves for the first time. It was an intelligent way to react to stress: by getting rid of certain leaves, they reduced their water needs and kept their young, more productive leaves. Besides, the following year, they returned to normal.
But, at the same time, 33 redwoods died. We determined that they had been killed by the Bark Beetles [Schénytes, Colaoptera insects]. Until then, these insects had never managed to tackle the redwoods. In itself, losing 33 trees is not a disturbing phenomenon. If winter is extremely rigorous, it can happen. But this example has shown us that, if the elevation of the temperatures continues, the bark will succeed in killing our redwoods.
and then, the lights started …
In 2015, a large fire killed at least a hundred redwoods. The previous record dated from 1987: fourteen trees, and that had seemed huge! In 2017, two other fires shot 120 trees, at least. In recent years, 2020 and 2021, have seen a massacre: in two summers, we have lost 13 % to 19 % of all the giant sequoias of Sierra Nevada, that is to say the world! Before, we had 75,000 redwoods over 1 meter in diameter. Up to 14,000 of them were destroyed. When we study the growth rings of trees, there is no clue that such severe fires have taken place in the last thousand years and probably before that.
We have also noted another phenomenon: some of the fires that decimated the redwoods were “crown fires” [Crown Fire, which spread from top to peak]. Even the leaves that are 90 meters high have been reached. The fire burned the cones. These have fallen, destroying the source of the seeds which allow new trees to push. It is to be feared that the regeneration of futaia will be threatened in certain places.
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