Wouldnt, also, in certain cases, the ash mix with the soil and make a more hardened clay-like mix which would harden and prevent a certain amount of erosion?
Not in California where the mountains are growing at a fast rate (for a geological process fast). The roots hold the mountain up while the trees grow, but when the trees burn those roots no longer have life and the next rainstorm lets the mountain side fall as the mountain has grown.
Read the book, it is described much better than I can summarize.
