Very nice!
there is just one little thing:
Although you see trees growing on rocks in nature, the log (stem?) of your fir coming directly out of the rock looks strange.
I know the fir models in that gratis x-frog set got roots.
On the other hand I wondered if one could use a local displacement shader to simulate roots crawling over your rock or into some fissure?
Maybe some little postwork would also do, considering your rendertime.
Best regards, Jan