Is there geological record on how stable these forests have been over the last few thousand years? I can't help but wonder if we're watching the west coast of n

Is there geological record on how stable these forests have been over the last few thousand years? I can't help but wonder if we're watching the west coast of north america shift to a more desert like climate through the PNW/BC/Sierra regions. Presumably if the burns are large enough, and the following wet season dry enough this process could occur over the course of only a few years.