Redwood, Sequoia sempervirens
Champion Trees
A hobby of mine since I started climbing trees as a graduate student has been finding the tallest trees. There are many record–keepers among tree–lovers, and I do not profess to be the sole authority. Today on Earth there are only 5 tree species with living individuals known over 300 feet. I have climbed and measured the total height of the tallest live–topped individual of each of these species. As of 2006, these were Sequoia sempervirens (379.1), Pseudotsuga menziesii (317.7), Picea sitchensis (316.8), Sequoiadendron giganteum (311.4), and Eucalyptus regnans (303.1 feet). There are taller dead–topped individuals of Pseudotsuga menziesii (~326) and Eucalyptus regnans (318.2), but I have not climbed them. The former tree was climbed in the 1990s by a group from Oregon. At the time, its top was dead, and it has died back further since then. The latter tree was climbed recently by Tom Greenwood and Brett Mifsud and measured with direct tape drop. Its top is dead, too, and 40-year-old sign at the base says the tree was over 99 meters tall when it had a live top.
Over 100 of the 137 known trees over 106.7 meters (350 feet) tall (all Sequoia sempervirens) occur in Humboldt Redwoods State Park. Chris Atkins, Michael Taylor, and I have been finding and measuring the tallest trees since the 1990s. Here is the 2006 leaderboard of Earth’s tallest trees. Five new trees have been discovered so far in 2007, all by Chris Atkins and Michael Taylor in Humboldt Redwoods State Park.
Here is a summary of human knowledge of maximum tree height since 1931 when the Founders Tree was measured at 364 feet. A taller tree was not measured until 1963 when the Libby Tree was found in Redwood National Park. A taller tree, the Dyerville Giant, was found in Humboldt Redwoods State Park several years later. When this tree fell over in 1991, the record returned to Redwood National Park, where two successive trees, Harry Cole and National Geographic, became the height champions. Then, in 1996 a taller tree, Mendocino, was found in Montgomery Woods State Reserve. This tree was dethroned in 2000 with the discovery of a taller tree in Humboldt Redwoods State Park. Known as the Stratosphere Giant, this tree steadily increased in height during the next several years. In July 2006, Chris Atkins and Michael Taylor discovered a tree, Helios, over 375 feet tall in a remote region of Redwood National Park. One month later, they discovered a much taller tree, Hyperion, in another remote region of the park. Whether there are any taller trees left on Earth remains to be seen. There are few places left to search.
People often confuse the concepts of "big" versus "tall" when it comes to trees. Here is a graphical summary of the biggest (5 trees on left) and tallest (5 trees on right) known Sequoia sempervirens (dead–topped Icarus, the 3rd tallest tree, is excluded). Diagrams are undistorted views of the main trunk, reiterated trunks, and limbs based on data collected during crown mapping. No branches are shown. Units are cubic feet (volume) and feet (height). Note that none of the biggest redwoods exceed 350 feet in height and none of the tallest redwoods exceed 23000 cubic feet in volume. The biggest Sequoia sempervirens (5th from left) is bigger than all but 7 other living trees on Earth (all Sequoiadendron giganteum).
Tree Structure
A redwood tree grows tall in a forest if left undamaged. The lucky few escape injury and approach their maximum heights in 600 years or so, but most trees get damaged in storms from wind and falling neighbors, and the crowns of many old trees have burned in forest fires. A high resistance to both wood decay and fire bestows great longevity on redwood, and trees can survive for more than a millennium after experiencing severe and repeated damage. Here is a brief description of what I have learned about the growth of Sequoia sempervirens from climbing in old–growth forests since 1987.
Big redwoods in old-growth forests inspire awe by virtue of their sheer size as viewed from the ground. The largest individual, shown here with my parents and wife for scale, is over 29 feet diameter at the base and has over 42 thousand cubic feet of wood and bark in its main trunk and crown.
Not much of a big redwood’s crown is visible from the ground, so climbing is necessary to fully understand the tree. A fear of heights prevents most people from accessing the crowns of tall trees, but even experienced climbers fear falling. Here is the view looking straight down from 150 feet up along the main trunk of a big redwood.
A tall, undamaged redwood has a lower crown with many large branches radiating from the main trunk. Here is the view looking up from 250 feet in such a tree.
The upper crown of an undamaged redwood is actually quite boring as most branches are simple and support little more than a few lichens.
Here is a view of the top of the world’s tallest known tree, a 379.1–foot–tall Sequoia sempervirens in Redwood National Park. The only evident damage was from a woodpecker.
Undamaged trees are referred to as "model–conforming" because their crown form is a direct result of a genetically determined pattern of development known as the architectural model. Redwood’s architectural model is similar to that of many other conifers (cone–producing trees). There is a single main trunk that grows vertically and produces numerous branches growing radially outwards from the trunk.
The tallest redwoods are increasing in total height by only a few inches per year. Here are the views of the top of the National Geographic Tree in 2005 and 2006 from the exact same vantage point. The tree grew from 369.5 to 369.9 feet.
In summer 2006, the National Geographic Tree’s top died back. The right panel in this photograph shows the tree’s top on September 19. The tree’s total height had shrunk to 369.8 feet, and the highest living leaf was 368.3 feet.
The tops of nearly all the tallest redwoods show signs of past damage followed by recovery. When the top of a redwood’s main trunk dies, new trunks often arise to re–build the treetop. The top of this tall redwood died back, leaving a spire extending to 362 feet. Two new trunks arose from main trunk around 358 feet and now both extend above 369 feet. Such trunks are known as "reiterated trunks" because their growth reiterates (repeats) the tree’s architectural model.
Reiterated trunks at the tops of tall redwoods can be quite vigorous. Here is the thrifty new leader on the Stratosphere Giant, which reached 370.5 feet in 2006. Note the cones.
Reiterated trunks continue the height growth of the tree, although it is not clear whether a reiterated trunk can achieve the same maximum height as the original trunk. In this view of the upper crown of a 375.3–foot–tall redwood, the leader is clearly a reiterated trunk.
Here is a view looking up along the highest reiterated trunk in a 375.3–foot–tall redwood, the 2nd tallest known living tree.
Reiterated trunks arise from broken main trunks, and they also arise from other damaged regions in the crown such as this small trunk arising from a broken reiterated trunk 286 feet up in a 369–foot–tall redwood.
The cumulative effects of repeated damage followed by trunk reiteration contribute to the structural individuality of trees in old-growth forests such as this graceful redwood with multiple trunks.
Many reiterated trunks in big redwoods arise near the top of the crown as this region is most vulnerable to wind damage. Here is a view looking up from 305 feet in the crown of a big redwood with multiple trunks in its upper crown.
The crowns of big redwoods often become dome–shaped after many centuries of trunk reiteration. This huge, emergent redwood crown has over 200 reiterated trunks.
If the main trunk breaks off relatively low in the crown, the reiterated trunks that re–build the crown can become quite large, as seen here in this big redwood. The trunks arising from the broken main trunk 220 feet above the ground are each over 4 feet basal diameter and 100 feet long.
The largest reiterated trunks in old-growth redwood forests can be larger than full–size trees in many other forests. Here is a view looking up from a crotch formed by two reiterated trunks arising from the main trunk 100 feet above the ground. The reiterated trunk on the left is over 5 feet basal diameter and 157 feet long.
Redwood’s incredible decay resistance allows trunks to maintain structural integrity long after sustaining severe injuries. The main trunk of this big redwood broke off 212 feet above the ground several centuries ago, but it still carries large reiterated trunks just below the break even though the heartwood has decayed somewhat and now supports a lush growth of huckleberry bushes (visible as yellow–green foliage on top of the broken trunk).
Horizontal appendages of redwood trunks are continuously produced and come in a variety of forms. The epicormic branches emerging from the right side of this big redwood trunk 300 feet above the ground are far younger than the trunk itself at this height. They arose long after the original branches in this region of the trunk died.
Some original branches persist for centuries in the crown, such as this elegant branch extending over 30 feet from the main trunk.
Over time, portions of redwood crowns rub against each other, wearing away the bark, exposing the cambium, and growing together to form elaborate fused structures. An appendage of this large original branch has fused with another portion of the branch creating a bridge.
During the life of a branch, falling debris from higher in the crown or from adjacent trees can damage or kill the branch. Branches damaged but not killed often become transformed into limbs as reiterated trunks arise from the surviving portion of the branch. This limb 262 feet above the ground, which formed after the end of this original branch was damaged, gives rise to several small reiterated trunks.
Here is another view of the limb from above that clearly shows the reiterated trunks. Note that each one bears its own set of branches.
The shape of a branch becomes transformed after it becomes a limb. A buttress of dense wood forms on the lower surface of a limb proximal to the origin of a reiterated trunk. In this reiterated trunk arising from a limb 271 feet above the ground, the buttress is clearly visible. Unlike branches, limbs are not round in cross section.
Crowns of highly reiterated redwoods have a labyrinthine quality that is evident in this limb complex 301 feet high in a 356–foot–tall redwood.
Limbs contribute greatly to the structural complexity and individuality of redwood crowns, becoming the thickest horizontal structures in old–growth forest canopies. These two redwoods standing side–by–side clearly illustrate the difference between branches and limbs. The tree on the left has a 7.6–foot–diameter limb 127 feet above the ground that carries three large reiterated trunks, including a 3.4–foot–diameter trunk with 10 of its own reiterated trunks. The tree on the right has no limbs visible in the photograph, only epicormic branches.
The broad platforms created by large limbs serve as important habitats in redwood forest canopies. This 5.9–foot–diameter limb 122 feet above the ground supports a lush growth of epiphytic ferns and shrubs. Note that the limb carries a reiterated trunk that has broken and reiterated again.
Fire caves are a distinctive habitat in old–growth redwood forests. Nearly every big redwood I have climbed shows clear evidence of fire damage within its crown. Here is a fire cave 210 feet above the ground in big redwood whose main trunk was nearly killed by the fire. Its crown is now massively reiterated and among the most structurally complex I have seen. Note the huckleberry bush (Vaccinium ovatum) growing from the cave.
An even larger fire cave occurs in the main trunk of this tree 161 feet above the ground. Here Marie and I are standing inside the cave while Cameron Williams (note foot) takes our picture.
Here is a view of the floor inside the larger fire cave. Note the soil accumulation and the huckleberry bush on the left.
In addition to quantifying redwood crown structure, my research team is studying redwood physiology. Among North American conifers, redwood has the most dramatic variation in leaf morphology. From the forest floor to the top of a tall redwood, the morphology changes from relatively broad and long leaves arrayed in one plane to much smaller scale—like leaves spirally arranged around the shoot. In this illustration of 2–year–old leafy shoots, values are in feet, and the scale bar is in centimeters.
The dramatic within-crown variation in redwood leaf morphology appears to be caused by an underlying gradient of water pressure within the tree’s vascular system. Leaf expansion in plants is driven by positive pressure (turgor) that builds inside cells as they grow. Leaves near the top of the tree, such as these 370 feet above the ground in Humboldt Redwoods State Park, can expand very little because there is so much negative pressure (tension) in the vascular system at these heights due to gravity. Note the water droplets clinging to the leaves during this foggy morning.
What is the limit to how tall a redwood can grow? At some height there must be too much tension in the vascular system to permit further leaf expansion, but the theoretical maximum height may vary from forest to forest. Here are the leaves 370 feet up in the crown of the world’s tallest known tree, a 379.1–foot–tall tree in Redwood National Park. Note that they are much more expanded than the leaves in the previous photograph. Does this mean that trees can grow taller in Redwood National Park than in Humboldt Redwoods State Park? We still have a lot to learn about redwoods.
While we are learning a great deal about redwood crowns, we have scarcely begun to unravel the biological mysteries of redwood root systems. A complete understanding of redwood will only be possible by considering the entire tree, roots, trunk, and crown. In many ways, the below–ground biology of redwood will be more challenging to study than canopy biology. Caution is warranted when exploring redwood root systems, because the trees may be damaged by excavations of roots. You never know what you will encounter when exploring uncharted territory. Here is something startling I encountered while groveling under a logjam to search for the tallest tree: redwood lignotubers.
Canopy Views
The beauty of old–growth redwood forests is easily appreciated, and I have been lucky to see these forests from vantages few others experience. Here Marie and I share some of our favorite photographs, all taken with a waterproof digital camera that fits in a vest pocket.
Streams cut their way through redwood forests creating skyscraper–like green walls on both sides. Redwood is a highly shade–tolerant species, so its foliage can extend all the way to the ground, leaving little room for other woody plants. A few tanoaks are visible near the stream’s edge in this view of Bull Creek.
Tall redwood forests are dominated by the massive trunks of trees whose crowns loom above and blot out much of the light. The first 200 feet are thus deeply shaded, and the older trees have long since lost most of their lower branches. The smaller trees that eke out a living in the lower canopy manage to do so by deploying expanded leaves acclimated to shade. Since few if any of their leaves are fully illuminated, their entire crowns grow slowly and may persist in subordinate positions within the canopy for centuries. Here is a view from 200 feet in winter.
As a climber approaches the lower crown of a tall redwood, the huge expanse of empty space can be intimidating if one looks straight down.
After the first several branches have been passed, the fear of falling dissipates as foliage obscures one’s view of the ground.
Views of the forest canopy are ever–changing, and each grove has a unique appearance. Striking differences in form and color among nearby redwood crowns hint at what must be an enormous pool of genetic variation in redwood populations. Nearly every tree visible in this photograph is a redwood.
Not all trees in old–growth redwood forests are redwoods. Here is a view from near the top of a 318–foot–tall Douglas–fir (Pseudotsuga menziesii) in a redwood forest. The two Douglas–firs in the lower center of the photograph are both over 260 feet tall.
The light environment in the redwood canopy fluctuates dramatically during the day, especially when fog is moving through the forest. Here the upper canopy on an alluvial flat is coming into full sun as the fog burns away.
Redwood forests extend from the valley bottom far up the slopes in some basins. Here the fog is lifting from the forest revealing individual crowns of redwoods and Douglas–firs on the slope.
The biggest redwoods live on alluvial flats where water supply is unlimited. The flat–topped tree in the center of this photograph is 336 feet tall one of the 10 biggest known living redwoods. Its top broke centuries ago but for some unknown reason never reiterated.
The world’s tallest forest occurs on the alluvial flats of Bull Creek in Humboldt Redwoods State Park. Over 100 of the 137 known redwoods taller than 350 feet grow here. This is a view of Bull Creek flats from near the top of a 366–foot–tall redwood. Every tree visible in the flat is over 300 feet tall.
Here is another view of the world’s tallest forest canopy just after the fog lifted in the morning.
We all owe a great debt of gratitude to the Save–the–Redwoods League for helping protect some fine examples of old–growth redwood forest. These forests were nearly destroyed, but there is still a chance to understand them and use this knowledge to restore some of our cutover landscapes.
Epiphytes
Organisms that live on the surfaces of plants without parasitizing them are called epiphytes. Redwood forests support a wide variety of epiphytes, and much of my research has addressed questions about their ecology. Here are some photographs of redwood epiphytes.
Lichens and bryophytes commonly occur as epiphytes on redwoods. In this view from 300 feet high in the canopy, lichens (mostly Sphaerophorus) can be seen on the dead branch in the foreground, pendants of the lichen Usnea hang from the ends of living branches, and mosses are visible on the larger branches in the background.
Lichens, like these pendant Usnea, are often abundant in the upper crowns of tall redwoods.
Compared to many other tree species, the bark of redwood is relatively inhospitable to epiphytes. The cause remains unclear, but is likely a combination of chemical toxicity and instability. Bark on redwood branches peels readily, sloughing away epiphytes. Here are some epiphytes in the middle crown of a redwood. The large brown–gray species is a nitrogen–fixing lichen called Pseudocyphellaria anthraspis.
A few species of nitrogen–fixing lichens are occasionally found on redwoods. These lichens are growing on branches of a 366–foot–tall redwood 250 feet above the ground. The green species is Lobaria pulmonaria, and the darker species is Pseudocyphellaria anthraspis.
Lichens are symbiotic associations between a fungal partner and one or more photosynthetic partners (green algae or cyanobacteria). The vast majority of the fungal partners are in the phylum Ascomycota, but a precious few are Basidiomycota, such as this Omphalina lichen growing on a 296–foot–tall redwood’s main trunk 220 feet above the ground.
This 5–foot–diameter limb growing 192 feet high in a 281–foot–tall redwood was heavily damaged early in its life. The limb’s cambium is now restricted to the sides and lower surface where it builds the buttress maintaining a 66–foot–tall reiterated trunk 3–feet–diameter at the base. The limb’s upper surface supports a thick and thriving epiphyte community overlying a large volume of dead wood. Moisture probes revealed a pocket of decaying wood whose volumetric water content reached 70% in the wettest winter storms and averaged 37% during 2 years of continuous monitoring.
Limbs create ideal platforms for epiphyte and soil accumulation. This limb 223 feet high in a 322–foot–tall redwood supports a mat of Selaginella oregana growing amidst the moss Antitrichia gigantea and the fern Polypodium glycyrrhiza. Only the wettest old–growth redwood forest (in Jedediah Smith Redwoods State Park) appears to have epiphytic Selaginella, as I have never seen it elsewhere on redwood.
The pendulous nature of Selaginella oregana allows it to exploit surfaces unavailable to many other epiphytes. A curtain of Selaginella, which is rooted in canopy soil, festoons the sides of this 5–foot–diameter limb 199 feet above the ground.
Limbs and crotches formed at the bases of reiterated trunks allow the evergreen fern Polypodium scouleri to form huge mats in old–growth redwood forest canopies. This fern mat 260 feet high in a 325–foot–tall redwood is among the largest I have seen. Based on non–destructive measurements and monitoring with some of the instruments visible in this photograph, the mat has over 5000 fern fronds, a dry mass of 740 pounds, and stores between 500 (summer drought) and 1500 liters (rainy seasons) of water. The yellow–green foliage is from a Vaccinium ovatum rooted in decaying wood.
Each year a redwood sheds 10 to 20% of its leaves. Some of this litterfall accumulates on limbs and in crotches, where it is processed by saprotrophic fungi and incorporated into canopy soils. Over 95% of the soil in old–growth redwood forest canopies is associated with Polypodium scouleri fern mats, which serve as litterfall traps within the tree crowns.
The bulk of canopy soils beneath fern mats consist of dead roots and rhizomes of Polypodium scouleri. The soil pit on the left was made on a 6–foot–diameter limb 155 feet above the ground, and the soil pit on the right was made in a crotch formed by three large reiterated trunks 198 feet above the ground. Both soils are over 3 feet deep; values in photographs are centimeters (cm). Note that the crotch soil is uniformly wet (31 to 35% volumetric water content late September 2001) below the litter layer. This soil extended more than 10 cm below the 70 cm mark into a saturated region created by the bowl–shaped crotch. In contrast, the limb soil is much drier with volumetric water contents ranging from 4% in the surface horizons to 21% near the bottom.
Decaying wood in redwood trunks serves as a rooting medium for epiphytic shrubs, like this Vaccinium ovatum growing 160 feet high in a big redwood. Large volumes of dead wood exist beneath broken trunks and inside cavities in the main trunk. Note the charcoal on the exposed heartwood to the right of the shrub.
Some redwoods possess huge burls, the upper surfaces of which serve as platforms for soil development. A Ribes laxiflorum grows on this burl 177 feet high in a 360–foot–tall redwood.
A number of trees occur as epiphytes on redwoods in old–growth forests. Here is a 7–foot–tall Tsuga heterophylla tree from the dead face of a huge reiterated trunk 223 feet high in a big redwood.
The trees and other woody plants that grow as epiphytes on big redwoods often support epiphytes of their own. We found a lichen (Pseudocyphellaria anthraspis) on this 8–foot–tall Picea sitchensis tree growing from the broken top of a 4–foot–diameter reiterated trunk 176 feet above the ground that we could not find anywhere on the redwood itself, despite thorough searching of the entire crown. Note the Vaccinium ovatum growing beneath the Picea tree.
Many of the epiphytic trees on redwood have wind–dispersed seeds, but some rely on animals to carry their seeds into the canopy. Here are the remains of Lithocarpus densiflorus acorns on top of a limb stub 295 feet high in a 371–foot–tall redwood. A squirrel brought them here to eat.
Occasionally an acorn or other tree seed brought into the canopy by a squirrel or bird manages to survive long enough to be lost inside a crevice with sufficient water and nutrients to become an epiphytic tree. This Lithocarpus densiflorus sapling is growing from a crack beneath a dead stub 270 feet high in a 360–foot–tall redwood.
Water availability is critically important to epiphytic trees, and there are few places suitable for them even in the biggest redwood crowns. Here is an Umbellularia californica sapling growing from a knothole 323 feet high in a 360–foot–tall redwood.
Plants are not the only epiphytes of redwood trees. Here is the fructification of a white slime mold growing saprotrophically on a dead branch 307 feet high in a 371–foot–tall redwood.
Many species exist epiphytically in the forests of the world that scientists have never seen. I observed this brown slime mold, which covers the bark of a limb 262 feet high in a 366–foot–tall redwood, for the first time in 2006 despite having climbed the tree nearly a hundred times since 1998.
Arboreal Animals
Old–growth redwood forest canopies are home to a multitude of animal species, most of which are tiny and only visible with a microscope. Some larger and very interesting animals also inhabit the canopy, but these are difficult to see without sustained effort on ropes or via continuous video monitoring. My overall impression of the redwood forest compared to other forests in which I have climbed trees extensively is that the canopy is very quiet. There are relatively few insects, birds, and mammals living in redwood crowns, perhaps because the trees are toxic to herbivores and highly resistant to pathogens.
Canopy soils in redwood forests harbor an impressive diversity of microarthropods, including at least 85 species of mites as well as spiders, insects, centipedes, millipedes, nematodes, annelids, and gastropods. One of the most surprising creatures we found in the redwood canopy is a species of harpacticoid copepod (photograph courtesy of Dr. Michael A. Camann), an aquatic crustacean that also inhabits terrestrial streams. How these copepods manage to reach the canopy (We found them up to 260 feet above the ground in 5 of 5 trees sampled.) remains a mystery.
A top predator of the redwood canopy food web is the wandering salamander (Aneides vagrans). This lungless amphibian lives and breeds in canopy soils associated with Polypodium scouleri fern mats, and it also inhabits crevices in bark and decaying wood. Here is an individual I encountered 207 feet above the ground on the main trunk of a 371–foot–tall redwood during a warm November morning following a night of heavy rain.
Wandering salamanders are excellent climbers by virtue of a prehensile tail and long, broad–tipped toes. Their skin glistens in a mottled pattern of gold–green over dark brown.
One morning in late September, I encountered a wandering salamander 305 feet above the ground climbing upwards along the main trunk of a 336–foot–tall redwood. The individual pictured here was climbing upwards along the lower trunk of a 296-foot-tall redwood in the same grove.
The marbled murrelet (Brachyramphus marmoratus) is an endangered seabird that nests on the limbs of large trees in old–growth forests from California to Alaska. These birds fledge from the canopy and fly directly to the ocean, where they begin diving and feeding on fish. They are long–lived but have a high mortality rate from egg to the sea. Adults are probably successful only a few times in their lifetimes at producing reproductively viable offspring, so their numbers are slow to increase and the species is vulnerable to habitat loss and degradation. This nest, found 318 feet above the ground in a 371–foot–tall redwood, is the highest ever recorded. Note the pink–colored ring created by defecation of the chick.
The northern flying squirrel (Glaucomys sabrinus) is one of the few mammals that regularly nests in redwood forest canopies. I startled this individual, who lived in a cozy space behind a large bark flake on the main trunk, while ascending the second tallest known redwood..
Several woodpecker species inhabit redwood forests, and I do not know which of these species caused this damage to the main trunk near the top of the tallest known redwood. Note the yellow pitch oozing from beak wounds.
