I am experiencing a similar issue. This is at horizontal joints of walls (modeled correctly).
When zoomed in Enscape the issue disappears:
Currently running the preview release 2.0.0.971-g6627c41, though it was happening in the current full release as well.
One trick we sometimes use is to render the scene with trees turned off, then render again with them on. Then you can layer the two images in Photoshop and adjust the opacity to suit. That wouldn't quite get you the variable transparency you are talking about, but I suppose you could do a series of renders, one with background trees only, one with mid-ground trees, and one with foreground trees.
