作者对于造林的前景做出了什么样的结论？
What does the author conclude about the prospects of forestation?
所有这一切都使得重新造林的纯利益极难估算。
All of this makes the net benefit of reforestation extremely difficult to measure.
经历了多次严重自然灾害后，人们才逐渐意识到植树造林的重要性。
People didn’t come to realize the importance of planting trees until they (had) experienced a number of serious natural disasters.
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