Volcanic ash covered some of the trees while they were still alive, and over time, their trunks turned to stone.
Acid rain covered some of the trees, and was responsible for the death of the forrest. 
mnli hypothesis: Acid rain covered some of the trees, and was responsible for the death of the forrest.  premise: Volcanic ash covered some of the trees while they were still alive, and over time, their trunks turned to stone.