一个年轻的女工把地址条贴在一个棕色的纸皮运输盒外面，包装条则放在里面。

One young woman put the address label on a brown cardboard shipping box and the packing slip inside.

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结果还表明在每年的这个时间，在夏天的时候，纸皮桦给花旗松送的碳比花旗松反送给它的要多，特别是当花旗松见不到阳光的时候。

It turns out at that time of the year, in the summer, that birch was sending more carbon to fir than fir was sending back to birch, especially when the fir was shaded.

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纸皮桦和花旗松到底是怎么交流的呢？

How were paper birch and Douglas fir communicating?

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