In a previous thread I had commented that after creating a lichen tree and adding leaves with the arisol glue and parsley method the tree was brittle.
I found a post that reports that hairspray will remain flexible over time. Arisol glue will still work you just can't handle you diorama after it dries.

I paint on loads of white glue first. Then add the leaves. This should take care of most the problem. I have used hairspray as well but don´t have real faith in it. Remember to hang your trees upside down while drying to get the best effect and let the glue run into the leaves better!