Hi `Rob. It's a nice start you have, but there's plenty of things that can be improved. First of all, your tree is a blob, which doesn't look very organic. There's endless styles of trees, but you need to approach the shape of the leaves in accordance to how they would grow. Consider the left tree in this post:
link. Better yet, study some photos of real trees.
Secondly, lose the dithering on the trunk. Dithering has a few uses, but this is not one of them. Here it is noisy and confuses the form of the trunk. You have enough colours in the trunk ramp to define the form of the trunk with little to no dithering at all required. That big area of shadow at the base of the trunk is confusing as well. Really, I would think it would be one of the potentially most-lit areas on the trunk, when considering the position of the lightsource on the leaves.
Leaves look too shiny. You have your brightest highlights bleeding into the midtones as speculars. Make a more even transition from highlight to midtone.
Ground tile is flat (contrast-wise) and somewhat uninteresting. The grass looks like flat color, needs more contrast and possibly a different form. Rocks need to pop more, imo.