Schorschi,
don´t use the forest object itself for masking, use the items which are scattered by the forest object (layer "forest_templates"). Object properties will be scattered with the template objects, so it´s for example possible to mask a certain type of object by ID inside a forest scattering without dividing it up. If you wish to mask by material, you have to pick the "orignial" object material from the template, NOT the generated material on the forest object itself.
Good Luck!
P.S.: Not a Corona specific issue, same e.g. in MR