Diminishing Coastal Fog: A Threat to Redwoods

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Monday, March 29th, 2010

Coastal FogSummer fog has decreased along the California coast over the last century, threatening redwood forests and their dependent species, reports a recent study published in the Proceedings of the National Academy of Sciences.

The paper’s authors, UC Berkeley scientists Todd Dawson and James Johnstone, say the mighty trees and the species that live in and beneath them depend on the cool, damp microclimate provided by the ubiquitous fog.

Dawson told the San Francisco Chronicle, “The redwoods along our coast are highly dependent on fog as a source of water during the summer when water in the ground is scarce. Foggy nights are needed to rehydrate the trees that can’t tolerate long droughts.” Even if established trees can tolerate the increased drought stress, the authors say, fewer young trees will be able to grow to maturity in the same areas where redwoods stand today.

Redwoods’ inability to regulate their water use as closely as other tree species is thought by scientists to be the key reason the enormous conifers are restricted to the cool, moist coastal belt that extends from the central California coast to just north of the Oregon border.

The study of summer fog decline is the latest to suggest that changes in the global and regional climate threaten the viability of familiar ecosystems. But it is among the first to point to specific climate-related concerns for the redwood forest.

Because redwoods are such long-lived organisms, and old growth redwood forests create their own microclimates, the news of their vulnerability may surprise the casual visitor. Advocates for redwood forest conservation have long argued that one of the key flaws in clearcut-based plantation forestry for redwoods is that it reduces the productivity of the overall forest by creating monocultures more vulnerable to drought and heat stress.

The Berkeley study notes several kinds of evidence of the close association between the coast redwood (Sequoia sempervirens) and summer marine fog, including biogeography, physiology, and paleoecology. It concludes that the frequency of coastal summer fog is an indicator that reflects key aspects of the coastal climate, including both the wind-driven upwelling system of the California Current and the broad ocean temperature pattern known as the Pacific Decadal Oscillation.

The study used the records of fog levels kept at local airports since 1951, including McKinleyville, then analyzed those data together with temperature records from 114 stations that go back to the beginning of the 20th century. The analysis showed that in the early part of the 20th century the California coast had about 30 percent more summer fog than has recently been the case. The foggiest year on record was 1951, with 62 percent of summer days seeing fog, while the least foggy was 1997, with only 27 percent fog through the summer.

By Scott Greacen