Very nice idea, with a nice incremental/idle game concept too, and quite relaxing too (also thanks to the music, great choice).
I have a couple of suggestions for polishing if you want to "expand" the game further:
- Zoom out (to see the tree fully)
- Make it clear that you can't add branches to the main trunk (it took me a while to understand where I could place a branch when I had no S-T or R-T trunk, and I actually couldn't place it anywhere): maybe change the color of the ball of the trunk with another color to indicate that only trunk go there, and also add some instructions.
- Add tooltips or tutorial texts appearing when you click somewhere to place something but you can't place it, with instructions explaining why you can't.
- Add an "energy per second" and "oxygen per second" indication or maybe progress bars filling up gradually over time and giving you resources based on the solar panel and branches you have.
Overall, the concept is nice for an idler game, or an incremental game, with a little bit of polish it could be a cool game to leave in the background while I do other stuff on my PC, good job :D