A 3,000-year record from 52 of the world's oldest trees shows that California's western Sierra Nevada was droughty and often fiery from 800 to 1300, according to new research. The health of the giant sequoia forests seems to require those frequent, low-intensity fires, Thomas W. Swetnam said. He added that as the climate warms, carefully reintroducing low-intensity fires at frequencies similar to those of the Medieval Warm Period may be crucial for the survival of those magnificent forests.