Hi there
Just a reminder to begin with, I don't know how long you've been on the forums but there are plenty of good tutorials and treemaking threads which serve as really good reference pictures.
One of them is
http://www.wayofthepixel.net/index.php?topic=10520.msg112358#msg112358I don't know how realistic you want this to be but the first thing I react on is the squareness of the foliage. The second I notice is the perspective. There are also no outlines on the foliage but there are outlines on the trunk, I would suggest go either with or without. As I mentioned there are plenty of ways to go about when drawing a tree. I usually go outside and pick a tree which I feel like would serve my purpose and style, then I alter as I go to fit perfectly. Your tree looks most like an oak, so I would go get a reference picture of an oak and go from there.
Then I get a wider brush and get a good shape going. I point out the light source and get some lumps of leaves, then you can refine everything as you go.
Here are two
unfinished trees I was working on a while ago:
Hope they can be of any help!
(Also wanted to add that most roots doesn't show that much above earth, if they do they usually grow on mountains or places where they need to search for other places to get a grip)
/coffee