Jam a nice arching hunk of wood in the right side facing the left side. Carve out portions of the wood to plant small scale species and some mosses too.
Create some mountains out of natural stone and include caves with mosses hanging off the entrances. Start on the left side with the mountains and taper them off slightly when you reach the right side. Maybe make a taller mountain slightly off centered in the middle of the background.
On the left side use small pieces of interesting twigs to create a "dead" forest and then underplant that with varying type of plants for contrasting texture and color.
Gonna have to trim those bad boys often!
Just an idea of course!
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