Coast Redwoods Depend On Summer Fog

Coast redwoods grow in a climate with almost no summer rain. What sustains them through the dry months arrives as fog rather than as precipitation.

Fog forms offshore and moves inland

Cold water upwelling along the coast chills the air above it, and moisture condenses into a marine layer that is drawn inland when the interior heats.

That is a summer phenomenon, which places the fog exactly in the season when rainfall is absent and demand for water is highest.

The forest occupies a narrow coastal band because the fog does, and the distribution of the trees maps closely onto where the marine layer reaches.

The canopy harvests water directly

Fog droplets condense on needles and drip to the ground, adding moisture beneath the canopy that never fell as rain.

Leaves also absorb water directly from wet air, which reduces the amount that must be drawn up from the roots during the driest weeks.

That matters because moving water to the top of a very tall tree is physically demanding, and the height these trees reach is close to the limit that mechanism allows.

The canopy is a habitat of its own

Mats of soil accumulate on large limbs high above the ground, supporting ferns, huckleberry and other plants rooted in the tree rather than in the earth.

Salamanders and invertebrates live in those mats without descending, which was only documented once climbers began systematic canopy work.

Trees also grow additional trunks from damaged tops, so a single old redwood can present a structure more like a small grove than like one stem.

Fire and flood are part of the system

Thick bark and high moisture content make mature redwoods resistant to fire, and hollows burned into trunks over centuries are a common feature rather than a fatal injury.

Flooding buries the base of the trunk in silt, and the trees respond by growing new roots into the deposited layer, which is unusual among conifers.

Sprouting from the base and from root crowns allows regeneration after damage without seed, which is how cut stumps produce rings of new trunks.

Old growth is a fraction of what stood

Most of the original forest was logged, and the remaining old growth persists in scattered parks and reserves rather than as a continuous belt.

Second-growth stands are dense and structurally simple, and restoration work often involves thinning them to accelerate the development of large trees.

Because the mechanism sustaining the forest depends on the marine layer, changes in coastal fog frequency would affect the survivors regardless of how well the land itself is protected.