Well, yeah. All the trigger does is it creates trees. The game doesn't know what you want so it only does what is in the code, hence it only creates the trees.
Is this spell supposed to be MUI (= can be cast by multiple units at same time), MPI (can be cast by 1 unit of each player at the same time) or only by 1 unit?
If it can be cast by 1 unit only, then you can create Destructible array variable, use
-
Set your_var[Ingeter A] = Last created destructible
and then outside the loop start timer, which expires after 5 seconds.
Then make another trigger that is fired by that timer.
When that timer expires, loop through your_var[1-8] and destroy that destructible.
For MUI it would use similar, but will be way more complex.