Dead trees in forest streams hold more carbon than expected

When we talk about climate solutions, we often look up – to the canopy of trees that pull carbon from the air. But there’s something unexpected happening below. The logs and branches that tumble into forest streams are quietly locking away carbon, too.

Scientists from the University of Vermont have discovered that large dead trees in upland streams store impressive amounts of carbon. In fact, the amount of carbon stored in this submerged wood is increasing over time.

The study was focused on forested headwaters in the northeastern United States. The researchers also found that big, old trees along streambanks play an important role in feeding this natural carbon reserve.

A natural climate solution

“We know that about 20% of global annual greenhouse gas emissions come from land use and deforestation,” said study co-author Dr. William Keeton, professor at the University of Vermont.

“But we can also use forests and other land cover as what we call a natural climate solution – finding ways to sequester and store more carbon in vegetation.”

Dr. Keeton had long suspected that fallen logs lying in water were storing carbon, but the scale of it surprised even him.

Dead trees store a lot of carbon

Stephen Peters-Collaer is the study’s lead author and a graduate student at the University of Vermont’s Rubenstein School of Environment and Natural Resources.

“Old-growth forests stored four to five times more carbon in the wood lying in the streams than mature forests did,” said Peters-Collaer.

“And in mature forests, this in-stream, dead-wood carbon pool is about 50% to 60% larger than that stored in downed wood in an area of equivalent size on the forest floor.”

The research showed that while streams take up much less space than the forest floor, they hold an outsize share of carbon – especially when they’re filled with large pieces of wood.

Carbon storage isn’t well-quantified

“One of the reasons that we were interested in this question is because there’s been increasing realization in recent years that wood in streams, lakes and other aquatic systems stores carbon, but that carbon storage wasn’t well-quantified,” said Peters-Collaer.

To get answers, the team studied streams in the Hubbard Brook Experimental Forest in New Hampshire and old-growth forests in New York’s Adirondack State Park.

These field sites have long research histories, providing the team with important background and context for their data.

Dead trees boost carbon storage

The researchers focused on large woody debris – logs and branches that fall from nearby trees into streams.

These oversized pieces tend to decompose more slowly, especially when submerged. Their large volume and relatively small surface area make it harder for decomposers to break them down, meaning they can store carbon for much longer.

The logs also do more than sit still. In small upland streams, they often stretch all the way across, creating natural dams. These structures trap additional wood, leaves, and organic debris – boosting the amount of carbon held in place even more.

Small streams, big role

Although they may look minor on a map, headwater streams make up about 70% of total river miles. And because they’re usually in upland areas, they tend to be less disturbed by development.

This means that the streams offer a surprisingly stable and effective place for carbon storage – if the right conditions are met.

Taken together, the findings suggest that fallen wood in forest streams may be a more significant piece of the global carbon budget than previously thought.

Peters-Collaer explained that researchers hadn’t measured this carbon pool in detail before, especially when comparing old-growth to mature forests.

Dr. Keeton believes that part of the oversight comes from how scientists typically work. “Scientists will often study either the streams or the forest, but not both, or they won’t look at the relationships between them.”

“The connection between stream and forest is not static but dynamic. One of our major messages is that we must take a long view, and we must think of these as dynamic systems.”

The study points to a quiet but important insight. The story of carbon storage doesn’t stop when trees die – it just flows in a new direction.

The study is published in the journal Ecosystems.

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