Old-growth Forests

Many have felt the awe of old-growth forests, even if they weren't aware they were in one.

Old-growth forests are a unique habitat that develops over hundreds of years. They differ in appearance and composition based on region and site, but many old-growth forests share common traits. In the Northern Rockies, this includes:

• AGE: Individual trees at the upper threshold of natural age for their species. For grand fir, this may be 150-200 years old, while for Western red cedar it may 500-1000 years old.
  
  
• COMPLEXITY: The structure of old-growth forests tends to be complex, with canopies of varying heights, trees of varying ages, large standing snags, and dead wood across the forest floor.
  
  
• SPECIES RICHNESS: Old-growth forests tend to be biodiverse, especially for plants, fungi, and insects. Many species of fungi, plants, and wildlife are only found in older forests.
  
  
• NATURAL DISTURBANCES: Insect irruptions, wildfire, tree diseases, and more are a part of life in an old-growth system. Old-growth is the condition of an old forest adapting to constant, small disturbances. Large-scale disturbances can destroy old-growth, like high-intesity fires.
  
  
• CLEAN WATER: In many older forests, water from rain and snow is readily absorbed into the soil and moves below the surface. This "sub-surface" water is often cold and very clean.
  

Forest Carbon 101

Trees, if you were to describe them to an alien, would sound like the ultimate technology. A self replicating organic structure that absorbs carbon and releases oxygen at a planetary scale? How is it even possible?

Lucky for us, it’s possible, simple, and inexpensive to manage forests for carbon storage. The idea of a “forest carbon reserve” isn’t new, but the accelerating costs of climate disorder have brought forest management into mainstream attention.

Unfortunately, corporations have led an enormous effort to green- wash logging as a carbon neutral practice. Knowing how forests absorb carbon is the first step to defang climate grifters and stop runaway global warming.

How does a tree absorb carbon?

During photosynthesis, a tree’s leaves or needles absorb carbon dioxide from the air to convert into sugars and oxygen. Sequestered carbon accounts for around 50% of a tree’s volume¹. The bigger a tree gets, the more carbon it stores, much of it in the trunk.

You can actually see this by looking at the rings on a stump. Every year of growth is bigger than the year before it. Each new ring increases in surface area from the year before, so the volume of carbon needed to cover the previous year goes up and up.

Because of this, old, large-diameter trees store stupefying amounts of Co2. One study² done
on the Blue Mountains of Oregon found that the largest 3% of trees contained nearly 45% of the above ground biomass. Even more astonishing, large-diameter trees absorb more carbon in one year than the total amount of carbon stored in a tree of half their size.

Which forests store the most carbon?

An abundance of water and nitrogen, as well as a lack of disturbances (fires, windstorms, land-slides, or clearcuts) make for carbon-dense forests. Some are hot, like the Amazon, and some are cold, like the taiga, but they are almost all well-hydrated.

The same is true for the Clearwater, where the very largest diameter trees tend to be western redcedars in riparian areas, like in the Aquarius Research Natural Area. Constant water and shelter from wildfires (what is called fire refugia) mean some trees grow for a millennia or more.

The rainforests of southeast Alaska take this to the extreme. Constant rain and a nearly year-long growing season create an environment perfect for giant trees. When trees finally die, cool temperatures slow down the decaying process, so thousands of tons of carbon get trapped in the soil.

This slow decay is the same reason that peat bogs, wetlands, and undisturbed prairies store so much carbon—roots underground or underwater take a very long time to make it to the atmosphere. Roots, soils, litter, and deadwood account for around 60-70% of the biomass of the forest³, often well above living trees.

How do fires impact carbon stores?

Fires, even high-intensity landscape-scale fires, emit a small percentage of a forest’s carbon. One four-year study of a large Californian wildfire⁴ estimated a measly 2% of biomass was lost. This is because needles and branches (which burn easily) make up a very small proportion of the biomass of an adult tree. Black “snag forests” of relatively intact boles decay slowly on the landscape for decades or centuries, effectively delaying emissions as new trees grow up.

How does logging impact forest carbon?

Logging is the largest source of carbon emissions on national forests. Unlike forest fires or windstorms, which leave dead trees, logging takes the whole tree, processing some and burning the rest. You can learn more by reading the graph above.

How could we manage forests for carbon?

Borrowing a great phrase from the Clark Fork Coalition, we need to “Protect the Best and Restore the Rest.” Today’s forests already store millions of tons of carbon. Keeping that carbon “on the ground” is absolutely key. Old-growth forests should be set aside from logging immediately.

Keeping carbon on Earth’s surface is a good start. Getting carbon out of the atmosphere is even better. This is the main difference between “reforestation” and “proforestation”. Clearcutting and replanting keeps carbon stocks very low. Letting forests rewild can increase carbon stocks longterm. A clearcut forest emits carbon for 10-15 years⁵ before it begins to act as a carbon sink again. Giving these degraded areas a century or more to recover would begin to shift the needle on atmospheric Co2.

Going Further

The dangers posed by global warming—summer heatwaves above 120 degrees, energy grid failures, flash droughts, prolonged fire seasons—pose incredible challenges for developed nations and threaten outright collapse for societies in the global south.

Conserving existing forests is the first step. But some of the best potential for carbon capture is actually on private industrial-style timberlands, like the emaciated Potlatch lands west of the Clearwater National Forest. Intensive plantation-style timberlands have already emitted enormous amounts of carbon into the atmosphere. Switching to selective logging or increasing intervals between harvests would recapture some Co2 currently in the atmosphere. Regulating (or incentivizing) the private timber industry is well within the powers of Congress, and should be on the table as a way to help mitigate the greatest ecological crisis of human history.

1. Thomas and Martin, 2012 “Carbon content of tree tissues: a synthesis” ↩︎
2. Mildrexler et al, 2020 “Large Trees Dominate Carbon Storage in Forests East of the Cascade Crest in the United States Pacific Northwest” ↩︎
3. Birdsey and Heath, 1995 “Carbon Changes in US Forests” ↩︎
4. Harmon et al, 2022 “Combustion of Aboveground Wood from Live Trees in Megafires, CA, USA” ↩︎
5. ↩︎