by Betsy Herbert

published in the Santa Cruz Sentinel 1/28/12

Did you know that the world’s forests, including California’s coast redwoods (Sequoia sempervirens) are helping in a big way to combat climate change? As forests grow, they pull vast amounts of carbon out of the air and store it within their enormous biomass. Forests cover about 30 percent of the earth’s surface, so climate change scientists are looking at the forests with renewed interest to help solve the world’s carbon problem.

Most scientists agree that climate change is caused by increasing levels of carbon dioxide (CO2)and other greenhouse gases in the earth’s atmosphere over the last 200 years. Scientists attribute about 3/4 of the increase in CO2 to the burning of fossil fuels and about 1/4 to deforestation and land clearing.

How do forests pull carbon out of the air? Thanks to photosynthesis, all plants capture and use energy from sunlight to convert carbon dioxide and water into building blocks for growth. What makes coast redwoods so exceptional at storing carbon is their enormous size and longevity.  Redwoods can attain heights of 350 feet, with trunks exceeding 24 feet in diameter, and they can live for more than 2,000 years. So, redwoods store lots of carbon, and they store it for a very long time.

According to National Park Service Ranger Christine Walters, “One mature coast redwood tree holds more than 400,000 pounds of carbon in its trunk alone—the equivalent of about 800 tons of carbon dioxide.” That means that two mature redwoods store as much carbon as the average American produces in a lifetime, roughly 1,600 tons of carbon dioxide emissions.

Local land trust Sempervirens Fund completed a scientific inventory of the carbon stored in the 100-year old Lompico Headwaters Forest, which the group purchased in 2005.  In 2007, the 285 acre stand of redwoods on the property stored approximately 116,000 metric tons of carbon. According to Sempervirens Fund’s Laura McLendon, who oversaw the project, “Scientists project that in 100 years Lompico Headwaters Forest, if not logged, will store nearly three times as much carbon.”

Measuring the carbon stored in forests worldwide gets complicated, though. It turns out that forests not only store carbon, but they also release it. When trees are cut or die, they stop pulling in carbon. Afterwards, as they decay they begin releasing their stored carbon back into the atmosphere. Making things even trickier, different tree species decay at different rates. After an old-growth redwood dies, for example, it can take many centuries to decompose. When a young fir dies, though, it can decay in a few years.

On balance, forests anywhere between 15 and 800 years of age absorb more carbon dioxide than they release, according to Beverly Law, professor of forest science at Oregon State University. An old study in the 1960s suggested that old forests emitted as much carbon as they pulled in, and that message was taught in ecology classes until recently.  “The current data now makes it clear that carbon accumulation can continue in forests that are centuries old,” said Law.

Climate scientists continue to find more accurate ways to measure carbon stored in the world’s forests. NASA Earth Observatory recently released a map showing global carbon forest storage, the result of six years of research (http://earthobservatory.nasa.gov/IOTD/view.php?id=76697&src=eoa-iotd) The darkest green areas on the map reveal areas with the densest, tallest, and most robust forest growth. Not surprisingly, the redwood forests of the Santa Cruz Mountains are mapped in dark green.