Burn a tree

Hello,
i dont know where to start, could someone push me the right direction.

Problem 1:
How would i change the Shader for Unity Trees at runtime? (Prefabs, not painted)

Problem 2:
I want the surface to become black but look realistic, at best from bottom to top, leaves should vanish

 

If not writing own shaders, then Shader Graph is good for getting some results asap.