Name: 【TED-Ed】Why wildfires are necessary - Jim Schulz
Uploaded: 2016-04-08T11:05:09.000Z
Duration: 4 min 21 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

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,

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

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,

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

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.

Once it's gotten started, a coniferous forest fire

Flames ravage the thick understory provided by species like douglas fir,

a shade-tolerant tree  that's able to thrive

The fire uses these smaller trees as a step ladder

to reach the higher canopy  of old lodgepole pines.

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,

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,

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,

to the existence of healthy  boreal forest ecosystems.

we can protect ourselves from their more damaging effects

while enabling the forests, like the legendary phoenix,