No problem on my end...
Internal Engine rendering...
Also with OpenGL, as long as you remember to turn transparency on and turn contours off, you get a decent image as well.
What version of ArchiCAD are you using?
Because you mention in the post that you're using version 18 trees but your signature implies you're using ArchiCAD 17
I think the render engines in ArchiCAD 18 were re-configured to work with the billboard trees so that they're always facing the camera (if you refresh the view) regardless of what direction they were facing when you placed them.
I'm not sure version 17 had this feature and if not then it's likely you're trees are facing backwards (in the Internal Engine render) and giving you the black box.
(I'm not even sure the version 18 trees can work with ArchiCAD 17)
Or if you're in version 18*, you might want to refresh the view or re-load the view (close and re-open the 3D window at that view) to get the trees to face the camera again.
(if you're in version 18, why not use Cinerender? with one of the pre-loaded custom settings? It would be just as fast and much, much better quality)