Yes, its a self shadow of the shadow planes over the tree texture. We have been working on this problem some time, but unfortunately we cannot exclude some specific geometry from the raytracing process, as we need. The only fixes are:
- 100% self-illumination
- Disable 'receive shadows' for the Forest object
(but these workarounds will disable ALL shadows on the trees, not only the self shadow)
- XShadow: Our plugin excludes the self shadow automatically, but as you know its a shadow mapper, not a raytracer... you must adjust the map and lights parameters carefully to get good results.
Personally, i would use self-illum for large scenes and XShadow for closer views.