Ok make a wood sector with a good height and draw another sector around it and make the tag a 1 for example.
Make a 64x64 sector outside the map.
Click any linedef on that sector and make that tag a 1 also and above that select your action. just type 25 since its a normal FOF and select a green texture on that same linedef.
Go to your 64x64 sector and adjust your height. Example Ceiling height 1100, Floor height 800.
And there, you should have a tree.