Here, I'll tell you a rephrased way of doing it.
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What you need: One tree character Set.
Make 6 Variables:
- Tree 1: X
- Tree 1: Y
- Tree 2: X
- Tree 2: Y
- etc.
Make a variable operation command. Use Set, and make the number determined by Random (You can choose any range as long as it's going to be within the map).
Now, make a tree event, and set it to Parallel Process. Use the command Change Event Location to, and use the tree variables as the coordinates. You should be all set.
I'm sure though that it looks like there is a bug, because the forest seems to keep randomizing since there's no end (From the looks of this tutorial). So turn the self switch A on, make a new page requiring that condition to be on and retain the Tree graphic on the new page. Repeat the events, just change the Location Event Coordinates.
Hopefully that's more understandable...^^;