Simple shapes can be ok if they fit the context (are these for a game?). The smaller ones in particular could have enough detail for most uses, although the specific pixeling of that detail looks a bit random.
Agree with decroded about warmer light tones.
Shading isn't great either. For one thing, the trunk is impossibly brighter than the darkest leaves; also you used unrealistic, evenly spaced gradients which are not specific to any kind of light source.
Example:
Notice how some large areas are almost evenly lit while others change a lot in a short distance. This is something that restricted palettes are good at: use it to your advantage!
This is just a generic shading tuto for a sphere in harsh light; according to your intended light source and tree texture, the result might be entirely different but you've got to think about how a specific light works on a specific object, with help from references or IRL experiments if need be.
