I spent about 2 or 3 evenings adding dried roots to the first structure (just a couple of twigs glued together) but I could have spent weeks to get a beter result. The white stuff in the picture is Das airdrying modeling clay.
Here the tree is primed with a light grey primer. Next the tree was airbrushed entirely black, I forgot to take photographs, my apologies for that.
After the initial black coat, three or four coats of various greens and greys were misted on. I splatered on some yellow-green and grey spots, followed by a brown wash.