on the tree是在树上。in the tree在树上（外来物）。on the tree例句：The leaves on the tree are yellow；in the tree例句：A little bird was singing happily in the tree。

on the tree的例句：We followed the marks he had made on the tree trunks（我们按他在树干上做的记号往前走）；in the tree的例句：The woodpecker pecked a hole in the tree（啄木鸟在树上啄了一个洞）。