I think there are two possibilities here:
1. He regenerates... somehow, while buried
2. He is already somewhere else, since he IS part of his vines, maybe he managed to clone himself before he died, and is sleeping within the HUGE vines they are saying are the only ones that did not crumble down or disappeared.
either way, he cant be dead cos... ya know... story got to continue hahahaha
this is NOT a spoiler, just my theories, so don't get angry at me!