I think the issue is under-developed shading practice. I'd say try shading just a log, and separately try shading a bush. This is to learn shading forms without overthinking. A log would be shaded similar to a cylinder. A bush in this case would be shaded more like a blobby sphere. It's much easier to practice shading these first and confirm that you're shading them correctly much more than it is to shade a whole tree, but once you learn how to do it, you can combine the 2 ideas together to draw a tree more easily.