Why wildfires are necessary
Did you know that several forest species need fire to survive?
In the conifer-rich forests of western North America, lodgepole pines constantly seek the sun. Their seeds prefer to grow on open, sunny ground, which pits saplings against each other as each tries to get more light by growing straighter and faster than its neighbors. Over time, generations of slender, lofty lodgepoles form an umbrella-like canopy that shades the forest floor below. But as the trees’ pine cones mature to release their twirling seeds, this signals a problem for the lodgepole’s future: very few of these seeds will germinate in the cool, sunless shade created by their towering parents.
These trees have adapted to this problem by growing two types of cones. There are the regular annual cones that release seeds spontaneously:
And another type called serotinous cones, which need an environmental trigger to free their seeds:
Serotinous cones are produced in thousands and are like waterproof time capsules sealed with resinous pitch. Many are able to stay undamaged on the tree for decades. Cones that fall to the ground can be viable for several years as well. But when temperatures get high enough, the cones pop open.
Once it’s gotten started, a coniferous forest fire typically spreads something like this: flames ravage the thick understory provided by species like Douglas Fir, a shade-tolerant tree that’s able to thrive under the canopy of lodgepole pines. The fire uses these smaller trees as a stepladder to reach the higher canopy of old lodgepole pines. That ignites a tremendous crown fire, reaching temperatures of up to 2400 degrees Fahrenheit. At those temperatures, the serotinous cones burst open, releasing millions of seeds which are carried by the hot air to form new forests. After the fire, carbon rich soils and an open, sunlit landscape help lodgepole seeds germinate quickly and sprout in abundance. From the death of the old forest comes the birth of the new.
So however counterintuitive it may seem, wildfires are important for the wider ecosystem as a whole. Without wildfires to rejuvenate trees, key forest species would disappear—and so would the many creatures that depend on them. And if a fire-dependent forest goes too long without burning, that raises the risk of a catastrophic blaze which could destroy a forest completely, not to mention people’s homes and lives. That’s why forest rangers sometimes intentionally start controlled burns—to reduce fuels in order to keep the more dangerous wildfires at bay.
From the TED-Ed Lesson Why wildfires are necessary - Jim Schulz
Animation by @provinciastudio