Another thing you could do is this..
1. Make a unit and set the properties so it fits a tree.
2. Create a trigger similar to this.

Tree Respawn
You may have noticed the quotation marks before and after some words. Those simply mean that depending on what you decide to put in yourself will affect that text.
If you wish to have a wait between when it respawns add this trigger:

Wait "Desired Duration" seconds
*Also I apologize for posting in a WEHZ Solved Problem Thread. I didn't realise it was here considering the link was in the Unsolved WEHZ Problems Forum. I'm sorry.*