Offset Projects 14 Oct 2020

What Trees Sequester the Most Carbon? It's Complicated

110,000 acres of old-growth redwood forests survive in California and Oregon. That’s 5% of their original range. Photo by Dan Meyers on Unsplash

Cloverly’s customers love trees. Among our categories of carbon offset projects, improved forest management has proven the most popular.

Climate scientists love trees, too. Trees sequester carbon from the atmosphere. A 2019 study in the journal Science estimated that the world could add more than 2.2 million acres of forestland without taking any urban or agricultural land—and that doing so could potentially capture and store 25% of the current atmospheric carbon.

That inspires a deceptively simple question: Are some types of trees better at sequestering carbon?

Here’s the not-so-simple answer: It depends.

Sequestering carbon—pulling it from the air and storing it—really helps fight climate change. An improved forest management project aims to increase the amount of carbon that a particular forest sequesters. Each carbon offset, also known as a carbon credit, represents 1 extra metric ton (2,240 pounds) of carbon sequestered. (For more about offsets, click here.)

Which trees are particularly good at sequestration? Read on for the answer(s):

Redwoods Sequester More Carbon

Oregon arborist Mario Vaden poses with a redwood at Jedediah Smith Redwoods State Park near Crescent City, California. Photo by Mario Vaden/public domain

A 2016 study found that California redwood forests store more carbon per acre than any other forest. Redwoods hit the sequestration trifecta:

  • They’re huge.
  • They’re fast-growing.
  • They’re long-lived.

The world’s tallest tree, at 380.1 feet and still growing an inch or 2 each year, is a California redwood. Even “ordinary” redwoods soar 200 to 240 feet on average. They can achieve a mass of up to 470 tons, second only to giant sequoias.

They can add 2 to 6 vertical feet a year in their first 10 years and can reach 150 feet in 50 years.

Ages of more than 2,200 years have been reliably documented. In a short book published in 1913 by the US Department of the Interior, Yale University geography professor Ellsworth Huntington claimed to have counted the rings of 3 redwoods that were over 3,000 years old and 1 that was 3,150. Redwoods continue growing throughout their lives.

Mountain Ashes Sequester More Carbon

The mountain ash, native to Australia, is the world’s second-tallest tree and tallest flowering plant. Photo by caspar s/CC BY

A different study in 2009 found that Australian mountain ash forests had “the highest known biomass carbon density in the world.” The tree, also commonly known as swamp gum and stringy gum, carries the much more regal scientific name of Eucalyptus regnans—”ruling eucalyptus.” (“Eucalyptus” derives from Greek words meaning “well covered,” a reference to a covering on the flower bud.)

Mountain ash ranks as the world’s second-tallest tree. The current known champion is 330 feet tall. Reports of uncertain reliability have claimed heights of 375 feet, 435 feet, and even 500 feet plus. All of those giants have long since been lost to logging.

The 2009 study found that, in general, temperate moist forests stored much more carbon than tropical or boreal forests. (“Boreal” means “high-latitude” and refers to forests in such northern places as Canada, Alaska, and Siberia.) Temperate forests include those along the US Pacific coast, where the redwoods live, and on the southeast coast of mainland Australia and the Australian island of Tasmania, the native range of mountain ashes.

Are redwoods and mountain ashes better at sequestering carbon than other trees? Not necessarily. Different trees do better in different circumstances. Some trees flourish in certain climates and won’t grow at all in others. Here are 7 other “correct” answers to the question of which trees wear the carbon-sequestration crown:

Larger Trees Sequester More Carbon

The Congress Trail in California’s Sequoia National Park shows off some of the park’s biggest trees. This is the House Group. Photo by Vitto Sommella on Unsplash

Other things being equal, the bigger the tree, the more carbon it stores. Obviously. So we should just plant giant sequoias everywhere, right? At up to 600 tons apiece, they’re Earth’s most massive trees (well, depending on how you define “tree”; more on that in a moment).

Giant sequoias occur naturally only on the western slopes of the Sierra Nevada Mountains in California. They have done well when planted in many other places around the world, especially in the Pacific Northwest, the southern US, warmer regions of Europe, and Australia and New Zealand.

Still, it’s not practical to replace pine forests—or cornfields—with sequoia forests. Sequoias prefer deep, well-drained soil and require plenty of sun. Most significantly, they thrive under conditions of periodic fire.

Fast-Growing Trees Sequester More Carbon

Unfortunately, fast-growing trees, while they do pack on carbon quickly, also tend to be short-lived. When they die and decompose, they emit that carbon right back into the atmosphere. That’s why the timber industry argues that harvesting trees for lumber actually keeps carbon in sequestration much longer than if the trees had died in the forest.

That especially holds true if timber building materials replace concrete and steel, which require a lot of energy to produce. The production of cement, the binding agent in concrete, accounts for about 8% of global carbon dioxide emissions.

Long-Lived Trees Sequester More Carbon

Long-lived trees can keep carbon sequestered for thousands of years. They also tend to grow slowly, meaning that they don’t sequester much in any given year. On the positive side, trees usually do keep growing throughout their lives—adding trunk girth, branches, and roots, if not height.

These bristlecone pines at Cedar Breaks National Monument in southwestern Utah may be thousands of years old. The National Park Service says the oldest known bristlecone pine was 5,065. Photo by KJRSeattle/CC BY-SA

Research published in the journal Nature in 2014 found that “for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire midsized tree.”

That shows why improved forest management is such an important tool for sequestering carbon. Yes, we need to plant more trees (see the next segment)—but it’s crucially important to keep the forests we have, and to manage them in healthy ways.

Young Forests Sequester More Carbon

Older trees may keep growing as they age, but the forest as a whole may shrink in terms of total mass. So it’s not a contradiction to say that both longer-lived trees and younger forests are better at carbon sequestration.

Young beech trees display autumn foliage at The New Forest in Hampshire, United Kingdom. Photo by Richard Loader on Unsplash

In young forests, lots of trees grow close together, collectively grabbing a lot of carbon out of the air. As they age, weaker specimens die off or get crowded out. The surviving trees continue sucking up carbon, but there are far fewer of them. So the forest overall stores less carbon than it did in its youth.

Native Species Sequester More Carbon

Native species are adapted for the local climate, so they generally grow faster and more vigorously than trees brought in from somewhere else. That’s why we can’t solve our climate change problem by turning Kansas into 1 big sequoia forest.

Pest-Resistant Species Sequester More Carbon

Pests weaken trees, slowing their growth. In extreme cases, such pests as elm bark beetles (which spread Dutch elm disease), hemlock woolly adelgids, pine bark beetles, and emerald ash borers kill trees, turning them from carbon sinks into carbon emitters.

Aspen Clones Sequester More Carbon

The Pando clone of quaking aspen in the Fishlake National Forest looks spectacular in the fall. US Forest Service photography by John Zapell/public domain

The North American tree with the greatest range is the quaking aspen. It’s found across Canada and the upper United States from the Atlantic to the Pacific oceans, and in the Western states along the Rocky Mountains south to Mexico.

Depending on what you consider to be a tree, it may also hold the record for the most per-tree mass (much of it underground). Aspens reproduce by seeds and, more commonly, by sprouts from their long lateral roots. They can form very large “clones”—stands of genetically identical trees that all share the same root structure. Each clone is essentially 1 tree with hundreds or thousands of trunks. (Root structure is important for sequestration; trees store carbon in roots as well as in trunks, branches, and foliage.)

Quaking aspens get their name because their leaves flutter in the slightest breeze. The largest stand, the Pando clone in central Utah’s Fishlake National Forest, spreads across 106 acres, includes more than 40,000 individual trees, weighs nearly 6,500 tons, and may be 80,000 years old. Individual aspens generally reach 20 to 80 feet in height, have slender trunks, and usually live no more than 150 years. So a clone has to get pretty large to sequester impressive amounts of carbon.

The term “improved forest management” hints at the real answer to the question of what trees are better at sequestering carbon. The best choice is to work with what we have, and to use the latest research and advice from forestry experts to make it even better. Any tree, no matter the species or size, can contribute.

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Cloverly Team