Hi there...
It's not too bad. but the thing with trees is (and a lot of other stuff) is not what you think you see but what you don't see is very important for building up a sense of relief and or form...
Looking at trees in reality they will allways have areas on them that you can SEE through... it's very rare that you'll find a tree that you can't see THROUGH... with that in mind you should build in holes into the leaf section...
See here>>>
Now, the other thing with trees... is that they're not Green and Brown...
? Well, they are... but they're not too... don't fall into the trap that "leaves are green and bark is brown" - they're not... LOOK at a tree... they're green, and brown, and black, and grey, and a whole heap of colors - so reflect this in your representation... unless of course you're going for that comic stylised look (which is fair enough) but even then - bear it in mind.
Regardless of what colors you choose, you should try and use the bark color and the leaf colors and try and homogenise them and use them together so that you don't get that Brown bark and leaf green standing out in relief from one another... (which is what I believe adrian is referring to) ....perhaps someone else can describe this better than me?
Finally... you need to think of the tree in layers... so you begin with the trunk and build the general branch structure, and then working from the back (with a dark tone) line in you general shape, fill it, then move up a tone and work forwards until you have you overall forms... I've supplied an rough example below showing this in action... it's by no means complete in terms of final rendered quality but it illustrates my point. At each step you can work it up to a more or less better quality - but don't go mad... all you're trying to acheive is the understanding that there's something BEHIND the stuff in front... as you get further forward in the layering that's when you spend more time on each layer until the final layer when you can go mad and complete it in detail. But, don't forget those holes, that punch through the layers revealing the darker sections underneath, or, indeed the sky (or background)
Good luck - and keep on pushin!