Just a slightly unconventional tree I was messing around with, yes the I was attempting to use the mid tone as the outline for most of the other leaves... I liked the cut out look. would love to heart peoples thoughts on it!
Though this will seem pretty painful to do, I highly recommend scrapping the work you've done with your leaves and starting over. As things stand now there are large issues with lighting, banding and pillow shading; all of which are things that you seriously want to avoid. You also choose to use an outline on the leaves (a light colored one at that, for some reason) when no such outline exists on the trunk of the tree. I know that you want a sort of "cut out" look but at the moment things look pretty disjointed.
Pixel clustering really works better for leaves and trees than drawing little individualistic details and hoping that it all comes together in the end. Pixel clustering works better because you can communicate depth, light, form and the like much more easily... all the while avoiding the uncanny valley look and feel of your piece.
All in all, you're doing OK so far for where you stand. Trees can be difficult and getting them to look natural and not artificial or too rigid can be hard. I suggest looking up tutorials for pixel clustering and examples of tree's that are made with that method as well.
I know it might not really be the "look" you're after but if the look you're after is riddled with a lot of fundamental issues then sometimes its better to take a few steps back and evaluate since going forward will usually just create bad habits and more problems.
