▲其实你这个句子的正常词序应该是这样的：

He says that three times as many young trees as those that can be cut are planted.

他说，新种的幼树是那些可砍伐树木的三倍。

你的句子只不过是把 as those that can be cut 放到了句末。至于为什么会这样处理，你这里只是一个单句，没有上下文，但可能与强调或句子平衡等因素有关。

▲再举一个类似的句子：

Sixteen times as many men are colour-blind as women. 男性色盲为女性色盲的16倍。

这个句子其实相当于：Sixteen times as many men as women are colour-blind.