Quote - I take it you used ecosystems in this scene ?  

Go to the materials tab and look under Ecosystem Materials (it's usually at the bottom of the list).  Go through those materials and make sure they're all showing correctly.  

I seem to recall having this happen a few years ago and it was one material that was causing it.

It has something to do with solid growth plants and how they cast shadows.  The shadow itself is not calculated as a shadow but as black artifacts.  Sometimes the screen goes black at the end of the render until "auto-exposure" is unchecked.  However if I use any transparent object such as Big Brader's tree-like objects, there is no problem.  Also if I add a SG tree as an object there is no problem.  There is something wrong in this scene with the ecosystems that causes shadows to equal black only.  I am using a proceedural terrain with a simple material with 8 layers, 2 of those layers are ecosystems.