Really a good game! A bit repetitive, but I’m not worried since it’s a first version. Maybe you could add something that breaks this loop during a run. I won’t influence what you should add, or even whether you should really add anything—after all, you’re the artist ^^.
I also had a strange feeling about progression. It’s not unbalanced, but at times I felt like I was stagnating while waiting for an upgrade, and at other times a single upgrade made me progress much faster. You could add an upgrade that makes trees grow faster, or something along those lines to stay within the game’s theme—maybe scout NPCs that allow you to expand the map to always have a dense area, etc.
The aesthetic is good. I’m not really a pixel art expert, but I find it simple and effective. Good project!