If you use the opacity function to break down leaves, you'll probably end up with pieces of leaf flying around without stem, good chance anyway. But from a reasonable distance it won't be very visible and you can still make 'autumn degrading' trees, where for instance the tops are virtually leafless. But, mind you, I took just any tree to test this, and this happened to be one with plain, flat squares 'attached' to the final branches, and they kind of angle in any direction, so the leaves aren't attached to the stems at all. It is a low poly distance tree.
I was thinking ahead (I tested this before on grass stems) and thought that it is also possible to get tiny droplets of water on the leaves if you take a really small scale billow PF, big contrast, and give it some displacement. The way I did the ivy on the wall in Garden of Eternity. Then attach a water shader (or reflective shader), perhaps blended by a default shader where the opacity function is ruled by the same small scale fractals, to reduce the amount of 'water' to be rendered. I might try this all on a real leaf...
I don't quite understand why you'd want only the edges to be affected... but indeed working with another, smaller leaf mask will produce any affect only within that masked area, so you can leave the tiny stems alone. You can even paint exactly where you'd want anything on that leaf to happen. Don't you use an alpha channel as mask?
This is a little bigger; you'll see the green plain leaf cluster (totally flat), with a color pf into the color function + a shader attached to the specular function input.
