You can just add them with triggers on map init.
Simply do this:
1. add an error to map script(just add "Custom Script" action that reads "sgfsg")
2. save the map -> the debug window with full map script appears
3. find the part when trees are added, copy that part.
4. delete the error
5. paste the copied part in custom map script
6. delete all trees
7. you can add more trees now, the original trees will be added with your new script