by Betsy Herbert, Ph.D.
Redwoods are extraordinary. The more we learn about them, the more extraordinary they prove to be. We’ve known for a long time that California’s coast redwoods (Sequoia sempervirens) are the world’s tallest trees and among the longest-living. Scientists are now confirming that redwoods play an important role in the local water cycle and in achieving a healthy, stable climate.
By meticulously measuring redwoods, scientists are determining how fast they’re growing, storing carbon and capturing fog, and how they are responding to climate change. For example, a team of scientists is working on the Redwoods and Climate Change Initiative (RCCI), with support from Sempervirens Fund, to quantify how accelerating climate change is affecting California’s redwoods.
While all trees provide oxygen and help stabilize the climate, redwood trees are truly climate champions. “Ancient redwood forests store at least three times more carbon above ground than any other forests on earth,” according to RCCI findings. Two mature redwoods remove and store roughly 1,600 tons of carbon from the atmosphere, as much as the average American produces in a lifetime through his/her carbon dioxide emissions (CO2).
Forests cover roughly 30 percent of the earth’s surface and store more carbon than is contained in the entire atmosphere!
How do forests pull carbon out of the air? Through the fundamental life process of photosynthesis, all plants capture energy from sunlight to convert CO2 and water into the building blocks for growth. During photosynthesis, plants pull in CO2 and release oxygen into the atmosphere, supplying the entire animal kingdom.
Coast redwoods are exceptional at storing carbon and releasing oxygen because of their enormous size, fast — and continuing — growth, and longevity. Attaining heights up to 350 feet and trunk diameters more than 24 feet, redwoods can live more than 2,000 years.
We now know that redwoods continue to grow as long as they live, packing on the girth, growing new tops after windstorms blow off old ones and sprouting millions of new needles. Thus they continue to pull in CO2 and release oxygen as they age — long after they attain their full height.
This discovery has dispelled an old myth that old-growth redwoods stagnate and take up space in the forest. These forest elders are anything but “old and in the way!”
When trees are cut or die, they stop pulling in carbon. As they decay, they begin releasing their stored carbon back into the atmosphere. Different tree species have different life spans and decay at different rates. Once again, redwoods are extraordinary. After an old-growth redwood dies, it can take many centuries to decompose and release its stored carbon.
Redwoods in the local water cycle
Redwoods also play a critical role in local watersheds, both in terms of water quality and water supply. We’ve known for a long time that the vast root systems of redwoods carpeting our local watersheds help prevent erosion and that their deep loamy soils act as natural water filtration and storage systems. Forest soils act like giant sponges, soaking up rainwater as it falls, and slowly releasing it throughout the dry season. In addition, redwoods that grow along streams provide shade, keeping the water cool for native fish.
What’s really extraordinary about redwoods is that they’ve evolved to use fog as their primary water source during times of year when rain is most scarce in northern California.
As the fog rolls in off the coast and creeps through the redwood canopy, it condenses on millions of redwood needles. If you’ve ever walked under a big redwood when it’s foggy, you know you need a raincoat to stay dry. This fog-drip is captured by redwood roots, which spread out widely near the surface of the soil. Todd Dawson, a UC Berkeley researcher and RCCI team leader, estimates that fog-drip supplies 30 to 40 percent of the water that redwoods require to grow!
Fog-drip from redwoods also supplies summer water to fish and humans. Spencer Robert Sawaske, a Stanford researcher, measured fog-drip from individual trees in the Santa Cruz Mountains during the 2013 dry season. He found that older redwoods and Douglas firs on the Pacific Coast side near the ridgetops produced the most fog-drip: up to 38 inches recorded over 2.5 months! He also found that this fogdrip soaked into the ground and replenished stream flow.
San Vicente Redwoods (formerly known as CEMEX Redwoods) encompasses 13 square miles of forest land within a very productive area for fog-drip: between Skyline Ridge down to the Pacific Ocean. Thanks to Sempervirens Fund and our partners, the redwood trees within reserves in this area are fully protected and will continue to produce fog-drip for a healthy watershed and forest ecosystem.
Redwoods are key actors in the water cycle of the Santa Cruz Mountains. Water circulates from the soil, up their trunks, into the clouds and back to the ground. Many forces are at work to pull water up the trunks of these tallest trees in the world. There is pressure from below as roots absorb water from the soil. The cohesive properties of water keep it moving up the water column in the tree’s sapwood, much like water in a straw. Perhaps the most critical force is supplied by transpiration, as pores in the tree’s foliage release water vapor into the air, thus pulling more water up the tree.
During the current drought in California, the local redwood forest continues to tap fog as a water source, and it's deep, loamy soils slowly release the water it captured from earlier rain. No doubt the redwood forest has softened the effects of the current drought.
Can redwoods survive climate disruption?
The iconic coast redwoods have demonstrated legendary resilience to some severe onslaughts, including massive clear-cutting over the last 200 years. Today, coast redwoods are threatened by home-building, invasive species, conversion into vineyards, logging, fire suppression and habitat fragmentation. Now they face the additional threat of accelerating climate change.
Scientists are studying how rapidly changing climate conditions (such as increased temperatures, drought and altered precipitation patterns) may outpace a forest ecosystem’s ability to adapt, especially where that forest is already stressed or degraded.
Climate change is expected to bring warmer and windier conditions to northern California, along with increased wildfire frequency and severity, according to a 2004 study by U.S. Forest Service and Lawrence Berkeley National Laboratory scientists. The study was right on target for Santa Cruz County, where the 2009 Lockheed Fire burned 7,800 acres in Bonny Doon and Davenport, including 2,400 acres of redwoods. Fortunately, the California Department of Forest and Fire Protection (CalFire) post-fire report found that the Lockheed fire “will not have long term detrimental ecological effects to the redwood forest type.”
Fog may also be on the decline. Dawson’s preliminary evidence showed a 30 percent decrease in the number of fog days in the region over the past 60 years.
How can we help redwoods survive climate disruption?
Forest scientists emphasize the need to increase the adaptive ability of forests to withstand climate disruption. To help redwood forests, we can:
- minimize soil disturbance,
- protect and buffer old-growth reserves,
- reduce forest road densities,
- increase wildlife connectivity.
Sempervirens Fund is forging ahead with all of these stewardship activities in San Vicente Redwoods, which connects 27,500 acres of contiguous protected land and shelters some 90 ancient redwoods that will be protected in special reserves.
By acquiring, protecting, and caring for local redwood forests, Sempervirens Fund helps manage redwood ecosystems to increase their resilience to drought, accelerating climate change and human disturbances. We are working with our donors and partners to create the Great Park to ensure that redwoods continue their extraordinary contributions here for thousands of years to come.