Ecology of the Coast Redwood

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

The Coast Redwood (Sequoia sempervirens) grows naturally along the Pacific Coast of North America from central California north to southern Oregon.  Its distribution is correlated with that of the thickest part of the "California fog belt" where, each day during the summer, cool fog moves off the ocean and onto land.   See Figure 1.

In California, relatively little rain falls in summer, so during this period redwoods are especially dependent upon fog for moisture to grow and survive.  Studies show that fog supplies 13-45% of the total water used annually by redwoods (Dawson 1998), and that redwood leaves can even absorb some water directly from the fog (Burgess and Dawson 2004).  In addition, fog blocks the evaporating rays of direct sunlight, reducing the amount of water that redwoods lose by transpiration (Byers 1953; Azevedo and Morgan 1974; Burgess and Dawson 2004).

When fog comes into contact with redwood trees, it condenses into liquid water and drips off the foliage onto the ground.  In this way, redwoods and other giant trees "strip water" from the fog and drip it onto the ground, where it is used not only by the redwoods but by other plants as well. 

During summer in California, Dawson (1998) found that two-thirds of the water used by plants growing in the understory of a redwood forest came from fog that had condensed on redwood foliage and then dripped into the soil."  When redwood trees are cut down for timber, plants growing under them receive less water and even the flow of water in nearby streams is decreased, because the redwoods are no longer present to "strip water" from the fog and put it in the ground (Ingwerson 1985).

The Coast Redwood is the tallest tree in the world, growing to heights of over 110 meters and to a very great age (Koch et al. 2004).  One Coast Redwood is confirmed to be at least 2,200 years old, but it is suspected that many other individual redwoods are much older (Sawyer et al. 2000).  Thus, some of the redwoods living today began their lives before the time of Christ.

The Redwood Canopy Community

According to Sillett and Bailey (2003), large redwood trees are among the "most structurally complex trees on earth," with individual crowns composed of "multiple, reiterated trunks rising from other trunks and branches...indistinguishable from free-standing trees except for their origins within the crown of a larger tree" (see also Sillett 1999).

For example, Sillet and Van Pelt (2000) studied a single old-growth redwood tree in Redwood National Park and found that its crown had "148 resprouted trunks arising from the main trunk, other trunks, or branches."  Five of the resprouted trunks had a basal diameter of over one meter, and the largest resprouted trunk was over 40 meters tall.  These researchers concluded that the crown of this redwood could itself be considered a forest.

Each year, redwoods shed their foliage.  While some foliage falls to the ground, other foliage accumulates on large branches of the redwood and decomposes there into an organic soil called "canopy soil"  Seeds of plants and spores of fungi colonize this soil, creating a plant community high in the canopy of redwood trees (Sawyer et al. 2000).

Plants that grow on trees rather than on the ground are called epiphytes.  Redwood trees often support sizable communities of epiphytes because their large size, great height and complex architecture make them excellent structures for soil and plants to colonize. In addition, the great age of redwoods increases the probability of colonization by epiphytes since so much time is available.

The number of epiphyte species growing in redwood canopies is quite large.  For example, Williams & Sillet (2007) sampled the canopies of nine large redwood trees and found a total of 282 plant species growing there, including "183 lichens, 50 bryophytes and 49 vascular plants...Compared to Douglas-Fir and Sitka Spruce, redwood consistently supported more epiphyte richness, except for cyanolichens."

In redwood forests, vascular plant epiphtyes grow in great abundance mainly on old-growth redwood trees located "within 10 kilometers of the ocean" (Sillett and Bailey 2003).  The most abundant vascular plant epiphyte on redwood is the Leather Fern (Polypodium scouleri) (Sillett and Bailey 2003).  It is found in large aggregations (mats) on branches and trunks high in the redwood canopy.

Of 27 redwoods sampled along the coast of Del Norte and Humboldt Counties, California, 13 had fern mats of this species which collectively weighed more than 100 kilograms, while one had fern mats weighing as much as 742 kilograms (Sillett and Bailey 2003).  Other ferns that grow as epiphytes on Redwood include the Licorice Fern (Polypodium glycyrrhiza), Sword Fern (Polystichum munitum) and Lady Fern (Athyrium filix-femina) (Sillett and Van Pelt 2000; Sillett and Bailey 2003).

Epiphytic Trees and Shrubs of the Redwood Canopy

Redwood trees are so immense that large plants (i.e. other species of trees as well as shrubs) grow on them as epiphytes.  The following species of trees and shrubs have been found as epiphytes on Coast Redwood:

Tanbark Oak (Lithocarpus densiflorus)
California Bay (Umbellularia californica)
Cascara (Rhamnus purshiana)
Sitka Spruce (Picea sitchensis)
Douglas Fir (Pseudotsuga menziesii)
Western Hemlock (Tsuga heterophylla)
Huckleberry (Vaccinium spp.)
Salal (Gaultheria shallon)
Elderberry (Sambucus callicarpa)
Gooseberry (Ribes spp.)

References: Sillett and Van Pelt (2000); Sillett and Baily (2003)

One California Bay found in the canopy of an old-growth redwood in Redwood National Park is the highest recorded epiphytic tree in the world, growing out of a knothole in the redwood located 98.3 meters above the ground (Sillett and Van Pelt 2000). 

Animals of the Redwood Canopy

The epiphytes growing in redwood canopies produce food and microhabitats for many species of animals, including beatles, crickets, earthworms, millipedes, mollusks, arthropods and amphibians (Sawyer et al. 2000; Sillett and Bailey 2003).  "Yellow-cheeked chipmunks (Tamias ochrogenys) and Band-tailed Pigeons (Columba fasciata) feed frequently on Vaccinium berries in redwood crowns (Sillett & Van Pelt 2007)."

One important animal that lives in the redwood forest canopy is the Clouded Salamander (Aneides vagrans) (Cooperrider et al. 2000), which has many adaptations for living in trees, including "a prehensile tail that it uses to assist in climbing vertical surfaces and long limbs with slender digits bearing subterminal toe pads...in the laboratory, [it] will leap from the hand to nearby objects, clinging with great tenacity, even to vertical surfaces" (Spickler et al. 2006). 

Clouded Salamanders have no lungs.  Instead they breathe through their skin, which must be keep moist for respiration to occur.  Not surprisingly, the Clouded Salamanders of redwood canopies are found most frequently in epiphytical fern mats, a microhabitat that stores water like a sponge (Spickler et al. 2006).  (Editor's Note: Genetic research suggests that the Clouded Salamander is actually two species, with the south fork of the Smith River in northern California being the dividing line between the two populations. [Jackman 1998]).

Many species of birds and mammals nest high in the canopy of redwood trees.  These include the Peregrine Falcon (Falco peregrinus), Bald Eagle (Haliaeetus leucocepahlus) Marbled Murrelet (Brachyramphus marmoratus), Northern Spotted Owl (Strix occidentalis caurina) and Fisher (Martes pennanti) (Cooperrider et al. 2000; Hunter and Bond 2001; Meyer et al. 2004; Baker et al. 2006).  

In addition, Vaux's Swift (Chaetura vauxi) and numerous species of bats nest and roost inside old, hollow redwood trees (Sterling and Paton 1996; Zielinski & Mazurek 2007).

It is possible that the California Condor (Gymnogyps californianus) also nested in the canopy of Coast Redwoods.  Two hundred years ago, this giant bird was common along the coast of northern California, but was extirpated there before modern ornithologists could conduct studies of it.  In the Sierra Nevada, where the condor survived much longer, ornithologists found it nesting in cavities of the Giant Sequoia (Sequoiadendron giganteum), a tree related to the Coast Redwood (Koford 1953; Snyder et al. 1986).  For details, see our special review:  Ecology of Condors.

At the Bolinas Lagoon Preserve, 50 kilometers north of San Francisco, over a hundred Great White Egrets (Egretta alba), Snowy Egrets (Egretta thula) and Great Blue Herons (Ardea herodias) nest high in the tops of the redwood trees of Picher Canyon.