Wow this looks really nice! I really like this style<3
I like those leaves; it's like something I would see in a storybook. I think beyond EdJr's advice, the trunk could also use more shape to it.
Thank you for the advice! I think that is a good idea on the leaves. I was wondering about possibly outlining them a bit, but trying to spread the colours out a bit more would probably be better. I think when I go for a bigger tree, I shall definitely do more branches. It was tricky to stick ones in the leaves at this kind of size, but as I like the effect, I shall push it more next time. :)
Actually, the leaves have a very cool style! I could imagine an entire picture with foliage like this. I don't see the same depth and life in the trunk though. I don't know if my tip will be valid/useful, but I'd suggest you not to dither everything so indiscriminately, and to isolate the leafs using different tones in each one (almost like it is now). Your technique could work very nicely too if you put more branches separatedly.
Oh, I think it looks better without this background.... Or you could fill it using not so contrastant colors.
I appreciate it. :)