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Key takeaways
Did you know? Trees grow in two directions at once, lengthening from their roots, trunks, and branches, and thickening their trunks under the bark.
Wood is about 50 percent carbon by weight, drawn from carbon dioxide in the air during photosynthesis.
For most species, mass growth rate increases continuously with size, and a single big tree can add as much carbon in a year as an entire mid-sized tree contains!
Researchers estimate that the large coast redwoods can grow no taller than 400 to 427 feet, because gravity and friction limit how high a tree can lift water.
Hyperion, the tallest known tree in the world, grows in a closed section of California’s Redwood National Park. The coast redwood stands 380 feet tall, yet its trunk is small compared to many old-growth redwoods, and the National Park Service notes that its height cannot be observed from the ground. Researchers found it in 2006, and the park service expects the title to move on eventually, since wind and lightning can break the crown off any record holder.
In a 2004 study published in the journal Nature, George Koch, Stephen Sillett, and two colleagues climbed the tallest redwoods in Northern California and calculated that no coast redwood could grow past 122 to 130 meters, or about 400 to 427 feet, because water stress from gravity and friction increases with height. Near the top, pulling water up is "like trying to suck water through a very thin straw," Koch, an ecologist at Northern Arizona University, told the San Francisco Chronicle.
Trunks keep thickening long after a tree's top stops rising, and in most species the biggest, oldest trees are the ones adding mass the fastest.
How do trees grow?
Trees grow taller from apical meristems, clusters of dividing cells in the buds at the tips of trunks and branches, and roots lengthen the same way. A tree's terminal bud forms its trunk over time.
Width comes from the cambium, the layer just under the bark that Andrew Groover of the U.S. Forest Service describes in Annals of Botany as responsible for a stem's radial growth, producing both wood and inner bark. Cells added toward the inside become xylem, which carries water and nutrients up from the roots, and cells added toward the outside become phloem, which moves sugars and stored food through the tree. Most of a tree trunk is dead tissue supporting the crown, and only its outer layers are alive.
Each year of growth leaves a ring, with a wide, light band from spring and a narrower, darker band from summer and fall. Wide rings point to warm, wet growing conditions and narrow rings to a cold or dry year, according to the Minnesota Department of Natural Resources.
Where a tree's mass comes from
Leaves build a tree's mass largely out of the air, combining carbon dioxide and water with energy from sunlight to make sugar and release oxygen. Trees turn those sugars into wood, branches, roots, and leaves, which is why wood is about 50 percent carbon by weight. Water moves up from the roots and out through the leaves, and a large tree may move 100 gallons of it a day.
Do trees ever stop growing?
Height growth tapers off in old trees while their trunks keep adding wood. Sillett, now a professor at Cal Poly Humboldt, led a team that measured 43 coast redwood and mountain ash trees and found wood production rising with age in redwoods up to 2,000 years old.
"Barring catastrophic damage from fire or falling, our results indicate that non-suppressed trees of both species enlarge steadily and maintain high leaf area well into old age, even after height growth has nearly ceased," the researchers wrote in the study, published in Forest Ecology and Management.
The fastest growth rate known for any tree belongs to a redwood as well. In Northern California’s Del Norte County, a 704-year-old coast redwood known to researchers as tree 49 added 2,811 pounds of aboveground biomass in 2014, according to Sillett and Emily Burns of Save the Redwoods League.
Nate Stephenson, a forest ecologist with the U.S. Geological Survey, found the same pattern across 403 species on six continents in a 2014 study in the journal Nature, concluding that large, old trees actively fix large amounts of carbon compared to smaller trees.
"This finding contradicts the usual assumption that tree growth eventually declines as trees get older and bigger," Stephenson said in a statement. "It also means that big, old trees are better at absorbing carbon from the atmosphere than has been commonly assumed."
What slows tree growth?
Drought can hold back a tree's growth for years after the dry spell ends. A team led by William Anderegg analyzed tree-ring records from 1,338 forest sites and found that in the first year after a severe drought, growth ran about 9 percent below expectations, and in the second year about 5 percent below.
"What we've found is that recovery takes a while. For most forested ecosystems, growth following a dry year is slower than expected," Koch, a coauthor on that study, said in a statement.
"Knowing that water stress increases with height in these trees tells us that if the climate dries out, that should lower the height of these trees," Koch said of redwoods in an interview with the San Francisco Chronicle. "We'd expect their tops to start 'dying back.'"
The life cycle of a tree
Most trees begin as seeds that need to travel away from the parent tree, and they do it by gravity, force, animals, wind, and water. Birches release tiny winged seeds that glide on air currents, jays and squirrels bury acorns for winter and forget some of them, alder seeds float downstream to new ground, and witch hazel launches its seeds ballistically.
Pine cones produce millions of seeds, but only a few germinate and grow into seedlings. When a seed lands somewhere suitable, its first root breaks through to anchor it and take in water, and a shoot pushes up through the soil. Seedlings are most at risk from disease and grazing, and they grow slowly until they clear the surrounding brush and reach full sunlight, then grow rapidly.
A tree becomes a sapling at about 50 centimeters, roughly 20 inches, and reaches maturity once it starts to reproduce. A Douglas-fir starts producing cones after about 12 to 15 years and can continue for up to 1,000 years, while a typical English oak produces its first acorns around age 40 and peaks between 80 and 120 years.
Over decades or centuries, disease, injury, fire, overcrowding, and insects weaken a tree until it dies and decomposes. Standing dead trees, called snags, return nutrients to the forest and feed and shelter wildlife, and the Woodland Trust calls dead and decaying trees a vital part of a wood's biodiversity.
"Old trees, after all, can die and lose carbon back into the atmosphere as they decompose," Adrian Das, a U.S. Geological Survey coauthor of the Nature study, said in a statement. "But our findings do suggest that while they are alive, large old trees play a disproportionately important role within a forest's carbon dynamics. It is as if the star players on your favorite sports team were a bunch of 90-year-olds."
Jill Ettinger is a writer and editor covering the intersection of culture and sustainability. Her reporting connects our everyday choices to the environmental and social-justice stories shaping them, from the products we put in our carts to the communities a changing climate touches most. She is based in Los Angeles.
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