Does any one have any idea how to make a mask overlay that looks like a forest canopy shadow in gimp?

Usually when I import a map in I'll make a new layer from visible. I then darken that layer and then use the free-form mask tool with feathered edges to draw out where I want brighter ares and then delete that section so the brighter original picture is visible.

I want to do the same thing but make it look like light is shining through the forest canopy above. So, I'm thinking of some kind of template that could be applied that can be reused for any forest maps. I'm not good enough with gimp to figure out how to do something like that.