Rendering multiple layers separately causes weird black artifacts (2.79b, Cycles) I have a ground mesh which I have cloned into multiple layers. It is the same mesh, except one has a grass material, and the other is sand. I wish to render these layers into separate render layers, and then combine the result with the compositor to produce a fancy effect.

When I select only the grass layer, and render that, then everything seems fine (environment &amp; background omitted):



Same goes for the sand layer:



But when I try to select both layers for rendering, and render, then the grass layer seems to have strange artifacts (just the grass layer, no compositor):



Looking carefully, it could just be Z fighting, since you can kind of see that the black parts are where the sand was displaced (as the sand uses displacement in it's material). But the grass render layer and sand render layer have separate scene layers selected, I can't see why this would happen.
In each layer, I toggled the other in the "Exclude bitfield", which is in the "Render Layers" tab.

Still slightly confused as to why this fixed the problem, but it works.