Foliage gets darker depending on placement? WHY?

This is driving me crazy. When using the foliage brush to place meshes on my landscape, the color/brightness of the meshes gets darker depending on their placement. It usually goes like this:

  1. Using the foliage brush to place meshes in a small, confined area looks great and the lighting is correct.
  2. Adding more foliage that is a somewhat large distance away causes the existing meshes to darken.

Here are some images that illustrate the problem:

Is it a problem with the foliage actor in conjunction with the skylight? Global illumination? Any help would be greatly appreciated!