It's a good start.
Balance is an issue here, at the moment i can see much more leaves on the right side. This would work, if it wasn't for the large branch coming out on the left with very minimal foliage on top (for example, it gives the impression that you didn't intend for there to be such unbalance). Big trunk? LOTS OF LEAVES. Don't be scared - for one i would increase your canvas size so you're less afraid to work on it's size more.
The trunk looks pretty fleshy atm, which is good. You'll have to add a highlight colour and see how it looks then.
So basically i would suggest adding more leaves where needed and to generally fill in the large gaps visible at the moment. More branches/leaves or altering the general shape will help this.