Fog Sipping Coastal Redwoods

Caitlin Dempsey

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Fog is a frequent presence along the coastal areas of parts of Northern California. The coastal ecosystems of California have evolved to take advantage of this regular influx of moisture saturated air in an area that has a distinct dry season during the late spring to late fall months.

One of these ecosystems is the coastal redwood forest of Northern California. These areas are dominated by coastal redwood trees (Sequoia sempervirens), a species of evergreen trees native to the western coast of North America. Coastal redwoods stretch along a narrow strip close to the coast from the very southern area of Oregon down into the Central California region.

Coastal redwood forests are limited to the extent that coastal climate can reach inland. Therefore, most coastal redwoods are found within 50 miles of the Pacific Coast where temperate climates and persistent fog during the drier summer months provide conditions where these trees thrive.

Shaded relief map of Northern California with a dark green overlay for the range of coastal redwood trees.
The range of coastal redwood trees in Northern California based on the work of Little, 1971. Map: Caitlin Dempsey. Data: USGS.

Very limited old-growth redwood is left

Most of the redwoods growing today are second growth forests, a direct result of forest logging that began in the mid 1800s in California. According to Save our Redwoods, only about 5% (110,000 acres) of the original old-growth redwood forests remain. The vast majority of redwood forests are found on privates lands (1.2 million acres – 77% of total range) while about 382,000 acres (23%) of the coastal redwood range is found on protected land.


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A pie chart with mostly yellow to with small green wedges to show how little of the original old-growth redwood forest remains.
Pie chart created by the National Park Service showing how much of the original old-growth’s two million acre historic range remains.

A multi-layered canopy system

Coastal redwood forests have a mixture of trees and shrubs that coexist with these tall redwood trees. Common tree species in these forests can include Douglas fir, big leaf maple, California bay laurel, madrone, and tanoak. Commons shrubs found in the coastal redwood ecosystem include California hazel and wood rose. Common forest floor plants are redwood sorrel, redwood trillium, sword fern, and bracken fern.

A view of the lower forest canopy and floor in a redwood forest.
Underneath the coastal redwoods, other trees and forest plants thrive. Photo: Caitlin Dempsey, Henry Cowell Redwoods State Park.

“Cathedral forests”

Coastal redwood trees are the most dominant aspect of the coastal redwood forest. Coastal redwoods are considered the tallest tree species in the world and can reach heights as tall as 350 feet. The tallest tree in the world is a coastal redwood named Hyperion that grows in the Redwood National Park and reaches a height of a little over 380 feet (roughly 116 meters).

Coastal redwoods can grow as wide as 22-27 feet and live as long as 2,000 to 2,500 years. Fossil records show that relatives of the redwood grew 160 million years ago during the Jurassic Era.

A view straight up a grove of redwoods.
Standing in a grove of redwoods has been described as feeling like being in a cathedral. Photo: Caitlin Dempsey.

In old-growth forest, the forests have been described as like “cathedrals” due to the awe inspired by the immense height of these trees. The dry, earthy scents emitted by redwood trees and the calm rustling of the trees creates perfect conditions for forest bathing by visitors.

Albino coastal redwoods

One rarity within the coastal redwood forest is the albino coastal redwoods. These white-leaved redwoods are known by a range of nicknames, most popularly as “ghost trees” or “ghost redwoods”. These trees are very rare, one researcher in 2016 estimated there were less than 406 of them.

A small redwood sapling with white needles grows in a forest.
An albino redwood tree in Henry Cowell Redwoods States Park. Photo: Caitlin Dempsey.

The white of these albino redwood trees is a result of a genetic mutation that prevents the tree from using photosynthesis to create chlorophyll, the pigment that makes plants green. To survive, albino redwoods syphon sugars from normal host redwood trees through interconnected root systems since they are unable to make their own food from sunlight.

One researcher, Zane Moore, has hypothesized that these albino redwood trees have a symbiotic relationship with their host plants. Moore found that edwood albino trees have higher concentrations of heavy metals than their green counterparts, believed to be due to poor stomata control by the albino plants. Moore theorized that albino plants serve as a reservoir for cadmium, copper, and nickel in exchange for sugars from healthy redwood trees.

A fire adapted ecosystem

Coastal redwood trees possess remarkable adaptations that enable them to survive and even thrive in fire-prone environments. Their thick bark, rich in tannins, offers resistance against low-intensity fires, serving as a protective layer.

Redwood trees with sprouts growing from the base of the trees in a forest.
Basal sprouting, a form of vegetative regeneration enables coastal redwood trees to rapidly recover from wildfires and continue their life cycle even after experiencing significant damage. Photo: Caitlin Dempsey.

The species also has the ability to sprout new shoots from the base, known as basal sprouting, allowing them to regenerate after fire events rapidly. This fire-resilience has allowed redwoods to be a dominant species in fire-dependent ecosystems. However, their capacity to withstand fire is adapted to historic climatic and wildfire conditions — namely, low to moderate intensity fires at relatively long intervals, allowing sufficient time for recovery.

A redwood snag with fire burns.
A redwood snag with fire burns. Photo: Caitlin Dempsey.

Climate change is altering the fire regime by creating longer hotter and drier periods of drought that stress and weaken trees, leading to more frequent and intense wildfires that can overwhelm even the resilient coastal redwoods. Rising temperatures and shifting precipitation patterns are creating drier conditions, making forests more susceptible to devastating fires. This change poses a significant threat to the longevity and health of these ancient trees, thereby altering the composition and dynamics of the ecosystems they anchor.

Fog sustains the coastal redwood ecosystem

Fog is an important source of water, particularly during the drier summer months. Northern California has a Mediterranean climate which is characterized by cooler, wetter winter months and drier, hotter summer months.

A low lying cloud surrounds a tree covered mountain on an overcast day.
Coastal redwoods thrive in areas where moisture-laden fog and clouds persist during the drier summer months in Northern California. Fog and clouds roll in to the Santa Cruz Mountains during May. Photo: Caitlin Dempsey.

As temperatures increase, so does the demand for water by plants in this area. For plant communities near the Pacific Coast, this demand is met in part by the presence of fog formed by the marine influence.

Redwoods are remarkable for their ability to capture and utilize fog as a source of moisture. Coastal redwood trees capture fog through their needles, a process known as “foliar uptake,” where moisture is absorbed directly through leaf surfaces.

A view up a tall coastal redwood tree with a foggy overcast sky.
Fog brings water to the coastal redwood forest during the summer months. Photo: Caitlin Dempsey.

Coastal geography in Northern California and fog formation

In Northern California, the formation of summer fog near coastal regions is primarily influenced by the interaction between the cold California Current and the warmer inland air. The cold ocean water cools the adjacent air, creating a moist, cold marine layer. During summer, warm air from inland areas moves towards the ocean.

A satellite image of the Pacific Coast showing the marine layer along the Northern California and Oregon coast.
The marine layer is an important source of summer water for the coastal redwood forest ecosystem. A satellite image of the marine layer over the coastal areas in Northern California and southern Oregon on September 29, 2006. Image: NASA, Aqua satellite.

When this warmer, less dense air meets the cold marine layer, it causes the moist air to rise until it encounters a temperature inversion layer, usually caused by high-pressure systems. This inversion traps the moist air, forcing it to cool to its dew point, which leads to condensation and the formation of fog.

Fog supplies summer water to the forest understory in coastal redwood forests

Fog is not only important for coastal redwoods, but for the more shallow rooted understory plants that are unable to access deep soil water. Studies have shown that fog brings a significant amount of water to coastal redwood environments. Studies have also shown, like one done in 1998, found that 66% of water presence in understory plants in coastal redwood forests came from fog water dripping down from trees into the forest soil.

Fog decline and climate change

As climate change affects local conditions in Northern California, researchers are concerned about the effects on redwood trees and the underlying ecosystem that are dependent upon fog for water. Warmer air carries less moisture. As summers become hotter and even drier, this has an adverse effect on fog moisture. For example, the National Park Service has reported that the fog that redwoods need to survive at Muir Woods National Monument has decreased by a third.

The coastal redwood forest has evolved over millions of years and fog is an essential ecological component. In these forests, the presence of summertime fog affects everything from water availability, plant survivability, microclimates, and carbon sequestration. As climate change continues to acerbate favorable conditions for survival, large segments of California’s forests, including redwoods, are at risk of becoming zombie forests where newer generations of these majestic trees are unable to survive.

References

Azevedo, J., & Morgan, D. L. (1974). Fog precipitation in coastal California forests. Ecology55(5), 1135-1141.

Dawson, T. E. (1998). Fog in the California redwood forest: ecosystem inputs and use by plants. Oecologia117, 476-485. https://doi.org/10.1007/s004420050683

Johnstone, J. A., and T. E. Dawson. “Climatic Context and Ecological Implications of Summer FOG Decline in the Coast REDWOOD REGION.” Proceedings of the National Academy of Sciences, vol. 107, no. 10, 2010, pp. 4533–4538., doi:10.1073/pnas.0915062107. 

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.