Following are a few pics and tips on making an evergreen canopy on a flat hillside backdrop, and making an access hole with lift-out foam mountain with deciduous Fall tree canopy. The first photo shows 3/8" sponge rubber foam cut into double layer of evergreen trees. 
The sponge rubber tree strips are then sprayed green and attached to wall with spray adhesive, in descending layers. 
Access hole to end of N scale dogbone loop. Access hole covered with “lift out” hollow foam mountain with canopy of Fall colored Deciduous trees, for easy access to derailed N scale trains. 
Click on photos to enlarge them. Then, click on small photos at left, to view a few other areas of my 24’x24’ HO layout.
Great idea and reletively easy for people with imagination like your self. Thanks for sharing.