The Amazing Coast Redwood

By: Rikke Reese Næsborg, Ph.D., the Garden's Tucker Lichenologist, and Cameron B. Williams, Ph.D., Research Associate

We’re slogging through a forest of 30-storytall trees. Logs thicker than cars force us to scramble over or explore a way around. Head-high ferns devilishly obscure the chaotic jumbles of splintered wood and craterous pits leftover from uprooted giants. If dinosaurs still roamed somewhere on Earth, this would be the place. We’re traversing through a forest of old-growth coast redwood (Sequoia sempervirens) in the northwestern corner of California. Sunshine filtered through dense fog and towering canopy barely reaches the damp leaf litter. It is so quiet. The feeling of awe and amazement is humbling.

As one of the state’s iconic trees, coast redwood is sometimes characterized as a living fossil, known as far back as 200 million years in the paleobotanical record. In addition to its lineage being ancient, individual trees can live longer than 2,000 years. Redwoods were once widespread across the Northern Hemisphere — from Spitzbergen Island near the North Pole to Texas in the south, including North America, Europe, Siberia, and Japan. Today, the natural range of redwood is restricted to a narrow coastal zone from southern Oregon to California’s Big Sur. Sadly, due to logging since the 1850s, the vast majority of remaining redwood forests are young. Only 5% of redwood forests are old-growth.

Why are trees that were once so widespread now hugging the coastline? Collections of fossil plants that included redwoods indicate that these communities thrived in environments that were mild and moist, but climatic shifts over the past 200 million years have restricted today’s redwoods to the California coast. Here, cool, humid conditions are delivered by a combination of ocean currents and wind, and the collision of this marine layer with a warmer and drier continental atmosphere generates fog that inundates the redwood zone. The humidifying and cooling effect of fog reduces a tree’s demand for water, and fog interception by coastal trees can drip to the ground to the extent that it may supply as much as 34% of redwood water uptake. And when conditions are just right — thirsty trees in a wet atmosphere — redwoods can even drink from the sky by absorbing water directly through their leaves, which bypasses the lengthy soil-root-stem pathway and more quickly quenches their towering crowns. The great height of redwoods affects the shape of their leaves. Water absorbed by the roots is transported all the way to the treetop leaves against gravity. Consequently, the leaves change with height in the tree — leaves at the bottom of a redwood are broad and long whereas those near the top are narrow and short. So stark is this gradient in leaf shape that leaf litter originating from the bottom versus top of a single redwood could be confused as different species. Is it possible that variation in leaf shape helps the redwood capture fog moisture?

This redwood branch supports at least 10 different lichen species. (Photo: Rikke Reese Næsborg)

The tiny (scale bar is 0.03 inches or 1 millimeter) redwood tripe lichen (Xylopsora canopeorum) is growing on the coarse bark of a redwood trunk. This species was described as new to science in 2018. (Photo: Einar Timdal)

The presence of summer fog and winter rain likely contributes to the height potential of redwoods. They grow exceedingly fast. Young trees can grow 2 to 3 feet (0.6 to 0.9 meters) per year under ideal conditions, and trees in northern California routinely exceed 300 feet (91 meters). The trees live so long that they inevitably endure violent storms, lightning strikes, crown fires, and impacts by neighboring treefalls. The resulting loss of branches and sometimes whole treetops awakens dormant buds that sprout into new branches and accessory trunks growing in their stead. Repeated over centuries, or even millennia, such damage-resprout events promote bizarre crown architectures frequently resembling gigantic candelabras reaching skyward. Researchers have painstakingly recorded the architecture of many redwoods by manually measuring the height, diameter, length, and trajectory of every branch and trunk. These data can then be used to produce a three-dimensional representation of the tree, such as this interactive model of the tallest redwood in Santa Barbara Botanic Garden (https://bit.ly/3yreVL1). In this model, do you see any evidence of damage to its top? This tree is only about 100 years old. Imagine what it could look like after aging 1,000 years! The primary motivation for generating such datasets is to learn more about tree structure and function, especially how trees interact with and respond to a dynamic environment.

Peeking through the head-high ferns at a swath of dimly lit trunks, one might conclude that redwoods are poor hosts to other plants. Indeed, few epiphytes (plants that grow on other plants) can tolerate such consistently dark and damp conditions found near the ground in old-growth redwood forests. The truth is that so much of the biodiversity in a redwood forest is hidden well above the ground, exposed to more sunlight, fog, and airflow, where old and coarse bark grades into young and smooth, and where a wide variety of nooks and crannies offer a diverse set of living options. The candelabra-like structures composed of accessory trunks and their supporting limbs accumulate decaying wood and leaf litter that cultivate a profusion of shrubs and ferns. These shrub and fern epiphytes are also available for colonization by other epiphytes — that is, epiphytes on epiphytes! Thus, a redwood’s towering height, old age, and resprouting ability make it a great host to biodiversity. In an exhaustive survey of 24 old redwood trees spanning the species’ geographic range, we found 373 epiphyte species, including 30 vascular plants, 40 bryophytes, and 303 lichens. Three of these lichen species were previously unknown to science. One of our study trees supported more than 100 different species of epiphytes. Humid climates in the northern part of the range favored vascular plants and bryophytes whereas lichens were more common in the drier south.

It’s a foggy morning in this redwood forest canopy. (Photo: Anthony Ambrose)

The devastating CZU Lightning Complex Fire burning through Big Basin Redwoods State Park. (Photo: Ethan Baron)

Redwoods are no strangers to fire. The naturally recurring fire interval ranges from eight years in drier sites to 600 years in very humid locations. Redwood bark, which can grow up to 2 feet (0.6 meters) thick, is fibrous and provides good protection against lowintensity, low-to-the-ground fires. Such understory fires also clear space and reduce competition for rapid growth of newborn redwoods. However, decades of fire suppression in and around redwood forests have led to an accumulation of fuel, and our rapidly changing climate has brought higher temperatures along with a reduction in summer fog. Combined with higher fuel loads, these warmer and drier conditions have sparked devastating wildfires in some of the remaining old-growth redwood forests, where the flames have reached high into the tree crowns. While an individual redwood tree is likely to survive a crown fire via resprouting, the associated organisms it supports, such as epiphytes, salamanders, tardigrades, and flying squirrels, may not. Moreover, repeated hot fires will eventually kill a redwood.

How can we best protect the amazing coast redwood against threats posed by a warming climate? One option is a style of forest management that actively reduces fuel loads in and around redwood forests, regardless of ownership — because fire doesn’t respect park boundaries. Another option is to assist the migration of redwood by planting seedlings further north than they naturally occur. Assisted migration is a somewhat controversial idea since outplanting has potential to replace or reconfigure native vegetation. Obviously, the best solution would be to do everything in our power to slow and eventually reverse climate change. O