Shameless Enough.

Something as absurd as the death of more than 70% of trees in the plane was never anticipated by the tree people. It is a nightmare for them.

They realized they were overreacting when the real nightmare came. The spawns of the demon God appeared amongst them. Many of them succumbed to the temptation of power. Most of the spawns appeared from within the dark elves that lived underground. It wasn't much of a surprise to the wood elves since those elves were already evil. 

It came to them as a shock that some tree elves also succumbed to the whisperings of the demon God. It couldn't be helped. The tree people are all about balance and peace but there has to be a black sheep in a flock of sheep. It is a matter of probability. After all, the dark elves used to be tree people so the tree people can be evil too.