Hi Bob,
I have taken another look at your scene now that I have a moment and I've found several other problems and unusual configuration options.
First, you do have your objects offset from the terrain in the population by specifying a non-0 number in the middle coordinate for "Area centre". This is the Y or vertical axis. This means your objects aren't sitting properly on the terrain.
Second, and perhaps most important to the shadow problem, your fill lights are at a fairly high strength. Their combined brightness is going to wash out most shadow you would see, especially with your low sun angle and the distance of your camera.
Finally you are specifying a *luminosity* texture and strength for your leaf textures in the tree object. This is going to give you a luminous tree - i.e. one that provides its own light to the scene - and this is no doubt resulting in the odd unshadowed look you're seeing in the trees themselves (regardless of shadows on the ground).
I think these issues account for the effects you're seeing and so I would say this is expected behavior given your settings. In the future if you're having problems with populations or any other particular feature I can't stress enough the value of trying it in a *default* configuration before experimenting further. That way you know that it's probably not the setting adjustments you've made that is causing any problems you see.
So in summary, to fix your scene I would recommend doing *all* of the following:
Turn on Ray Traced Shadows in the renderer's More Settings tab
Set Y axis position value for populator(s) to 0
Reduce fill light strength to 0.2-3 or less for all fill lights
Remove Luminosity texture from leaves entirely and set luminosity value to 0
- Oshyan