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  • There was a time before our ancestors smashed flint and steel together,

  • when they felt the cold lack of fire in their lives.

  • But anthropologists theorize that early hominids

  • relied on lightning to cause forest fires,

  • from which they could collect coals and burning sticks.

  • Fire gave them the ability to cook food and clear land,

  • and became central in many rituals and traditions.

  • So instead of seeing forest fires as an exclusively bad thing,

  • ancient humans may have learned to appreciate them.

  • Yet, it wasn't just humans who benefitted from these natural phenomena.

  • Even as they destroy trees, fires also help the forest themselves,

  • however counterintuitive that seems.

  • In fact, several forest species, such as select conifers,

  • need fire to survive.

  • But how can fire possibly create life in addition to destroying it?

  • The answer lies in the way that certain forests grow.

  • 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 lodgepoles' 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 waterproofed 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.

  • Let's see that in action.

  • 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 step ladder

  • to reach the higher canopy of old lodgepole pines.

  • That ignites a tremendous crown fire

  • reaching temperatures of up to 2400 degrees Fahrenheit.

  • That's well more than the 115-140 degrees

  • that signal the moment when serotinous seeds can be freed.

  • At those temperatures, the 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.

  • Fires are also 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.

  • They may be frightening and destructive forces of nature,

  • but wildfires are also vital

  • to the existence of healthy boreal forest ecosystems.

  • By coming to terms with that,

  • we can protect ourselves from their more damaging effects

  • while enabling the forests, like the legendary phoenix,

  • to rise reborn from their own ashes.

There was a time before our ancestors smashed flint and steel together,

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B1 US TED-Ed forest fire canopy shade burning

【TED-Ed】Why wildfires are necessary - Jim Schulz

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    Kristi Yang posted on 2016/04/08
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