I am using some of loopix trees and some others from au resource.
Problem I have the shadow flag set and it casts square shadows on loopix and no shadows from the au site. Same thing when cast is set too. Tried static light and dynamic light. messed with different options in the compiler too.

The leaves consist of large planes and the shadows don't look at the alpha value of the skin - that's why your getting those strange shadows.

The only way to have trees like this cast good shadows, is to either fake it with shadow sprites, or use shadow mapping.