Hi,
We don't recommend adding any Modifiers to a Forest object because it disables instancing but it should still render. If you are using Points-Cloud display mode you may find though that it disappears in the viewports. As an alternative there are a couple of ways you might be able to achieve this using Forest Colour or if you're a V-Ray user, using V-RayDistanceTex that maintain full instancing support.
Using Forest Colour:
* Create your wall using a vertical surface and using Forest's
UV mode to place the bricks. Make sure the surface is UVmapped.
* Create your blend material as normal but add a
Forest Colour map to the
Blend Amount map slot.
* In the Forest Colour map, turn on
Tint > Override.
* Check
Get Colour from Map.
* Add a
Gradient Ramp to
Tint map slot.
* Set the
Blending mode to
Normal.

.
Using VRayDistanceTexThis technique has the advantage of allowing you to create a gradient that follows an undulating terrain.

* Create the wall however you want. It isn't important for this technique.
* Create a
solid ground object, this will be used to drive the distance based gradient.
* Create your Blend material as normal but add a
VRayDistanceTex map to the
Blend Amount map slot.
* In the VRayDistanceTex material, add the ground object to the list of
VRayDistanceTex Objects.
* Adjust the
Distance value to control the size of the gradient.
I've attached a file to illustrate both these approaches. I hope it helps but if you have any other questions please just let me know.
Many thanks!
Paul