My tree is clearly straight, but painting trees result in sideways ones. What can cause this?
Tree is straight when placed manually as a prefab.
Looks like its a handedness issue, in 3d graphics the choice of which axis is called what is kind of arbitrary (to a point) and there are a couple of different common standards.
Unity uses the left-handed, Y up, but other software, particularly 3d modeling software like blender, use right-handed, Y or Z up.
Freya Holmér made this really great reference image
Realistically the fix is just to transform your model by rotating it, or scaling one dimension by -1, either in unity or in some 3d modeling software, until it looks correct.