To respawn trees add a trigger to your map:
Respawn Trees
Events
Destructible - A destructible within (Playable map area) dies
Conditions
(Destructible-type of (Dying destructible)) Equal to Summer Tree Wall
Actions
Destructible - Resurrect (Dying destructible) with (Max life of (Dying destructible)) life and Show birth animation
Basically, that's it.
Innumerable variants are possible: for example, you could change (Playable map area) to a specific region.