It's a good start! I have a few critiques that I think could help.
Where is your light source coming from? From the way to have the leaves shaded, it looks like your light source is right in front of the trees, but it's not consistent throughout the piece. Figure out where your light source is, and stick to it with every element in your art. Another tip: usually when you're outside, the light source is coming from the sky. The only time it would be eye level is during sunset or sunrise, but then the colors would be much different!
The bark on your tree, especially the smaller one, looks a bit like stone because of the way you have it separated. Consider omitting some of the darker outlines within the body of the trunk and just suggest that there are shadows there.
Talking about the tree, don't be afraid to use reference images. Using both style references and real life references can be really helpful in making sure you capture the essence of an object.
The blue green shade color in the grass is a nice hue shift, but it could be darker. If you are going to hue shift colors in your piece, do it with all of them! Use the same logic throughout your piece.
The dithering in the sky is nice (and I really like the colors). I would love to see you expand on that with different patterns of dithering as one color bleeds into the next. It can make the transitions look smoother.