I made a diorama of a 30cal. machine gun nest next to a hedge row with a tree in it. I used woodland scenics rail road lichen material as the basic overall tree and hedge material. I then used Elmers spray adhesive and sprinkled parsley over the top for leaves.
About six months ago this dio took a nose dive off a shelf (I told them not to throw balls in the house ).
Most of the dio was salvagable so I shelved it and am just getting back to resurecting it now.
One thing I noticed is that the leaves and lichen material are incredible brittle and very easy to crush and crumble. I compared this to a diorama I made about five years ago with just the lichen (No spray glue). The one without the glue is still plyable.

This is sort of an FYI - have you had this happen - what do you use for spary adhesive kind of a post. So feel free to take away from this to be careful with older dios that may suffer from brittle spary glue. Or maybe Elmers isn't the best one to use.

Had the same experience once... spray glue will harden and dry out the lichen which, if left alone, will attract moisture in the surrounding air and stay "soft" and moist. Best advice - leave lichen "as is" unless you are SURE the sucker won't be moved or touched!