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This vast forest circling the globe contains a third of all the trees on Earth
这片环绕着地球的广大森林容纳了地球上1/3的树木
and produces so much oxygen it changes the composition of the atmosphere.
产生了大量氧气，从而改变了大气成分
As we travel south so the sun's influence grows stronger
随着我们向南行进，太阳的影响力逐渐增强
and at 50 degrees of latitude a radical transformation begins.
北纬50°的地方是一场重大变革的起点
Summers here are long enough for broadleaf trees to replace conifers.
由于有足够长的夏季，阔叶林得以取代针叶林
Broadleaves are much easier to eat and digest
阔叶树的树叶更容易食用和消化
so now animals can collect their share of the energy that has come from the sun.
因此动物们也能获得太阳馈赠给它们的那部分能量
It's summer and these forests are bustling with life.
现在是夏天，森林中一片生机勃勃的景象
But the good times will not last.
可是好景不长
Broad leaves must be shed in winter for their damage by frost.
由于严寒的原因，阔叶树得在冬天落叶
As they disappear, so the land becomes barren with little for animals to eat.
树叶消失之后，大地一片荒凉，动物们的食物日益稀少
The inhabitants must migrate, hibernate, or face months of near starvation.
居民们只得迁往他处，或者冬眠，或者忍受几个月的饥荒
