Seeing Green: The Redwood Climatic Distillery

Opinion by Holly Moeller
Nov. 13, 2013, 8:38 a.m.

SANTA CRUZ, CA – As a gray ceiling closed over Main Beach mid-Saturday afternoon, those of us waiting on volleyball games scrambled for our sweaters.

Anyone accustomed to spending long hours playing volleyball, surfing, hiking, or just lounging about on the Northern California coast quickly learns that packing layers is de riguer: The offshore marine layer and attendant coastal fog can sweep in at any time, turning a sunny day suddenly chilly.

The coastal fog is formed by a combination of atmospheric and oceanic phenomena. The cold Pacific waters incubate chilly, moist air just next to the coastline. When the sun heats up further inland (Think triple-degree temperatures in the Central Valley, and you’ll understand why San Francisco’s always foggy in Summer.), that hot air rises, creating a vacuum that sucks the cool air inland from the coast, condensing its moisture into fog as it goes.

Fortunately for me, I love fog and clouds for their own sake. But for those of us who prefer a sunny sky, it’s useful to remember that the uncomfortable weather also waters some of the most beautiful trees on the planet.

California’s coastal redwoods, the tallest organisms on the planet, rely on fog to meet their water needs during the long and otherwise-dry Summer season. Fog condenses on the giant trees’ branches, seeping directly into the photosynthetically active needles, and dripping down onto the forest floor below. This moisture source is so important that it accounts for up to 40% of the water in the ecosystem annually.

In addition to being giants, redwoods are also extraordinarily long-lived. Some will live for 2,000 years; many survive into their 500’s and beyond. This turns out to be both impressive, and scientifically useful.

Trees grow outwards, putting down a new layer of tissue just under their bark each year, so they form annual rings that record information about the environment. For example, we’ve known for a long time that a thick annual ring indicates a good year for growth. You can also look through tree rings to find evidence of fire scars, and the signature of nuclear testing, which produced a spike in radioactive carbon-14 in the 1950s and 60s.

Because redwood trees are so old, their tree rings read like a centuries-old book rich with information dating back long before our human scientific studies. For example, scientists at the University of Washington and UC Berkeley just published a method to tell how much fog and rain a redwood experienced in a given year.

Their study relies upon something called isotopic fractionation. Each atom – in this case, of oxygen – has a particular weight that’s determined by the number of protons and neutrons in its nucleus. Different isotopes are atoms of the same element, but with different weights (because they have different numbers of neutrons). Heavier atoms behave differently than lighter ones. For example, just like at the gym, where it’s tougher to hold up a heavier weight than a lighter one, heavier atoms tend to “fall” (e.g., be left out of various physical and biological processes) more frequently than lighter ones.

Compared to fog, rainwater is relatively “light”: that is, a rain droplet’s cluster of water molecules has more oxygen-16 compared to oxygen-18 (a heavier isotope) than a droplet condensed from fog. Both fog and rain are formed in the same way: from water that’s evaporated off the ocean. So they start out with the same isotopic signature. But while fog often forms, moves inland, and condenses in the same day, rain clouds can spend a long time hovering over the ocean. During this time, some of the water in them condenses and falls out as rain. And those heavy oxygen-18 isotopes, of course, fall out first and fastest, a phenomenon called Rayleigh distillation.

Scientists use these differing oxygen isotopic signatures to figure out where a tree’s water came from in a given year. After double-checking to make sure that their numbers line up with independently collected measurements of rainfall and fog days, they can then use this method to study the climate over a tree’s lifetime.

Unfortunately, these woody climate experts might not be available to help us track climate for much longer. The fog that sustains them has declined by 30% over the past century, and this trend is likely to continue as greenhouse gas emissions shift our climate. And while this might be great news for beach volleyball and San Francisco summer tourism, it will mean fewer awe-inspiring giants monitoring our coastline.


Holly welcomes feedback from readers (especially prospective doubles partners with perfect hand-sets) via email at hollyvm “at”

Holly is a Ph.D. student in Ecology and Evolution, with interests that range from marine microbes to trees and mushrooms to the future of human life on this swiftly tilting planet. She's been writing "Seeing Green" since 2007, and still hasn't run out of environmental issues to cover, so to stay sane she goes for long runs, communes with redwood trees and does yoga (badly).

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