Hello! I have a problem concerning rendering orders in 2D.
The problem can be seen in the following images: I have my character at a sorting layer above the tree-sprite. When the character walks behind the tree I need to swap the sorting layers somehow, anyone got any idea how to do this in unity?
Image 1. No problem with sorting here, tree is one sorting layer behind my char

You don’t need to change sorting layer. You need to swap your tree and character sprites sorting order.
Unity - Scripting API: Renderer.sortingOrder .
Sprite with bigger sorting order will be rendered in front of sprite with smaller sorting order.
you can put the tree and the character in a same layer and change the z value when evere you want to have the character behind or not.
Also if you don’t want your player to walk through the base of the tree, just add a small box collider at the base. This would mean that you can still walk behind the tree but not through the base of the tree.