Fires deliberately set years earlier helped giant sequoias survive the extreme fires of 2020 and 2021 in California. A study estimates that these preventive burns prevented the death of about 1,900 trees in Sequoia and Kings Canyon national parks.
A prescribed burn involves burning an area under chosen and monitored conditions. The goal is not to burn the large trees but to remove some of the undergrowth, dead branches and small plants. Without this combustible vegetation, a future fire can remain less intense at ground level.
Giant sequoias.
Image Wikimedia
The researchers studied 26,403 sequoias spread across 19 groves affected by the Castle fire in 2020 and the KNP Complex fire in 2021. They combined satellite imagery, aerial surveys, tree maps and field observations.
Their analysis estimates that 7,974 sequoias in this group died, or about 30.2%. Trees in areas that had received a prescribed burn during the previous ten years had survival chances nearly four times higher than those in untreated areas.
This comparison does not mean that preventive fire makes sequoias invulnerable. It indicates that it strongly reduces the risk of mortality when an extreme fire occurs. The authors estimate that already treated areas prevented 1,888 deaths, with an uncertainty margin between 1,487 and 2,302 trees.
The model also suggests that about 3,888 additional sequoias could have survived if all studied areas had received comparable treatment.
Giant sequoias are adapted to periodic low- or medium-intensity fires. Their thick bark protects them, and fire opens up space on the ground and promotes germination. However, more than a century of systematic fire suppression has allowed a large amount of fuel to accumulate in some forests.
When this dry vegetation burns during a hot and arid period, the flames can become powerful enough to kill trees that are centuries old. The goal of prescribed burns is to make uncontrolled fires less violent when they occur.
The study focuses only on two exceptional fires and on groves in the Sierra Nevada. It does not allow us to assert that the same result would apply everywhere. Nevertheless, the authors consider prescribed burns to be an important tool, to be combined with monitoring, vegetation thinning and adaptive management of each forest.