He says之后是that引导的宾语从句，从句是一个“倍数词+as...as”的倍数表达句式，three times as修饰形容词many（即第一个as是副词）；第二个as是连词引导一个省略形式的比较从句，从句中的that can be cut为定语从句修饰those,补全后便是：

He says that three times as many young trees are planted as those that can be cut (were planted). 他说，栽种的小树是（以前栽种的）现在可以砍伐的那些树木的三倍。

▲ 再给你提供两个词典例句（as从句中的省略部分已加上）：

Sixteen times as many men are colour-blind as women (are colour-blind).  男性色盲为女性色盲的16倍。《柯林斯高阶英汉双解学习词典》

We produced ten times as many tractors as the United States (did).  我们生产的拖拉机的数量是美国（生产）的10倍。《柯林斯高阶英汉双解学习词典》