By pulling in data on temperature variation with elevation, Johnstone and Dawson also related their fog data with a temperature inversion that each summer traps the fog between the coast and the coastal mountains. The inversion is caused by a warm, dry, high-pressure cell that sits over Northern California in late summer, bringing hot temperatures to inland areas, including the Central Valley. If the inversion is strong, its lower boundary at about 1,200 feet keeps a lid on the cool marine layer and prevents fog from penetrating over the Coast Ranges. When it is weak, the ocean air and clouds move upward and inland, resulting in a cooler interior and a warmer, drier coast.
"The data support the idea that Northern California coastal fog has decreased in connection with a decline in the coast-inland temperature gradient and weakening of the summer temperature inversion," Johnstone said.
"As fog decreases, the mature redwoods along the coast are not likely to die outright, but there may be less recruitment of new trees; they will look elsewhere for water, high humidity and cooler temperatures," Dawson said. "What does that mean for the current redwood range and that of the plants and animals with them?"
Eventually, Dawson and Johnstone hope to correlate fog frequency with redwood tree ring data in order to estimate climate trends going back hundreds of years.
"While people have used tree ring data from White Mountain bristlecone pines and stumps in Mono Lake to infer climate change in California, redwoods have always been thought problematic," Dawson said, mainly because it's hard to determine whether the width of a tree ring reflects winter rain, summer fog, temperature, nutrient supply or other factors. "Stable isotope analyses of wood cellulose allows you to pull this data out of the tree ring."
Dawson has established that the isotopes of oxygen in a t
|Contact: Robert Sanders|
University of California - Berkeley