Thanks for letting us know about this. Several of our presets predate the introduction of that feature and so don't have the Forest Edge map applied to the materials input (required for Corona, but not for V-Ray)
Fortunately, you can easily add it automatically. Go to the Material rollout and launch the Optmize Materials tools. Make sure Apply Forest Edge is checked and click Optimise.
We'll look into adding these to the library so that you don't have to do this in future.