It's actually more noticeable on the far cylinder and sphere test props than on the tree.

But going back to the idea of making the fog thicker at top or bottom...

I suggested using the Y-component of a P node to control the DepthCue Distance parameters, but I think it would be much easier to control the DepthCue Colour. Fading the colour to black would reduce the fogging effect. I'll try this out when I get home this evening, if I'm allowed -- it's not my turn to be she who must be obeyed....

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)