I'm rather sure he meant creating in-game? Since you cannot create a lot of trees instantly (besides copying ofc) in the editor either.
Personally, I found a lot of trouble here too... Trees cannot be revived in-game... And if you kill them, they're still unwalkable due their footprint...
After trying to play with trees desperately I didn't found a single solution, besides removing them and replacing them. But this makes the spot they were still unwalkable... I also failed in editing the footprints.
So how did I solve it?
I simply created a unit with the model of a tree and another one with the model of a "dead tree". By doing this, I don't have to use footprints and thus a "dead tree" or a "removed tree" becomes walkable. Now, the collision size of an alive tree has a value, whereas the collision size of dead trees is zero and thus they're walkable (don't forget to set push rate to 0).
Now, all you need is a trigger to replace them on revive/kill instead of kill/revive functions.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory