Sorry it's taken so long to get back to this.
I am not using Camera Density Falloff because the camera moves during the animation (and it tells me not to use this in the dialog for this reason). The Area Density Falloff is great, but as mentioned before, I have to over-create many trees (that I don't need) just to get the area and density falloff I DO need. In my case, it would be much better to be able to create an irregular Include spline and define the linear falloff direction with a 'Start/End' line that I define... something for the wishlist.
I have V-Ray but am not using it for this project... I am using Scanline. I did get the 'F' icon so I will try upping that setting.