As we travel south 随着我们向南行进

111. contains a third of all the trees on Earth
     容纳了地球上1/3的树木
112. and produces so much oxygen
     产生了大量氧气
113. it changes the composition of the atmosphere.
     从而改变了大气成分
114. As we travel south
     随着我们向南行进
115. so the sun's influence grows stronger
     太阳的影响力逐渐增强
116. 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
117. 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

来自《行星地球 第一季 第一集 Planet Earth S01E01 从北极到南极》第 114 行