If you have resistance you can press the "Show IDs" button. Then you can zoom in the editor window until a number is displayed on every object. This number is the object ID. With the Object command you can access every object on the map by it's ID.
Look up the IDs of the trees you want to fell and use this line to chop them:
"(Object _x) SetDammage 1" ForEach [ID1, ID2, ..., IDn]