The concept is interesting and I loved the tree background. Body part crafting was also neat but it has place for improvements. I'll elaborate on it later.
There are several things I didn't like or could use an improvement:
1. Tree collision was unintuitive. I could fly through some areas of the trees where in other places there was a collision. I also hated the V typed tree branches where runes were placed. Was frustrating to get there and pick up the rune.
2. The movement was strange. I would expect a bit smoother & controllable movement but if that was your design choice then you can ignore this comment.
3. Only from description I understood that the "pot" is every 5 trees (Reading is hard). Even without that knowledge, it desperately needs an indication for nearest pot or cauldron. Maybe even a fast travel to it. While playing I lost it several times and had to use my memory & guessing if thats the right place where it was.
4. After crafting a body part I was expecting that something will change to my player character. Either visually or functionally (bigger; faster; more controllable) but nothing changed :( There has to be something to satisfy player. Additionally maybe some small instruction or description how/why can be added there, so that dummies like me understand how/why am I doing that.
5. From intuitive perspective it makes all the sense in the world to have a human body and body parts, but... player character isn't really a human :D Maybe it would make more sense to change it to something more fitting for the character ? It could give you lots of possibilities on how much/what kind of body parts your character has. In the long term maybe you can expand this mechanic to transform/upgrade your player character in a different form that will have different "anatomy".