Researchers study the role older, larger trees play in carbon sequestration
By Tim Rowland
Brawny, old-growth trees may not appear to be growing much, but their capacity to store more carbon on an annual basis exceeds that of their younger, faster-growing brethren.
What this means for climate change is that aging forests, such as those increasingly found in the Adirondack Forest Preserve, have more value than is commonly understood in the planet’s attempt to change the ratio of carbon dioxide produced versus what can be soaked up by the world’s oceans and woods.
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Scientists, speaking before a sizable audience last week at Paul Smith’s College, said this fact calls for a rethinking of accepted forestry practices that promote selective cuttings for speedier regeneration. Instead Bill Moomaw, a Nobel Prize-winning climate scientist and professor emeritus at Tufts University, argued some forests should be used for industrial production, while others need to be set aside, untouched, as part of a “global operating system.”
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Old-growth contributions
“Mature and old forests are the most effective forecast-related climate mitigation strategy,” he said. The capacity of old-growth trees to be the workhorse of carbon mitigation, he said, was demonstrated in the Australian island state of Tasmania, where a decision to halt cutting on half its forests tipped its scales to a net negative producer of carbon in seven years.
The power of forests as environmental engines can be stunning and not well understood. In South America, Moomaw said, more of the moisture that reaches the coast is given off by trees than is carried by the Amazon River itself.
Bob Leverett, an engineer and cofounder of the Native Tree Society — who quipped that his wife “pats me on the head” whenever he gets exceedingly excited over a new mathematical formula calculating the exact volume of massive trees — said there’s a lot of misunderstanding and misinformation about forests’ ability to swallow carbon.
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How much carbon do trees take in?
“We are inundated with information about carbon sequestration, but a lot of it is not accurate at all,” he said. Popular websites indicate mature trees capture in the neighborhood of 40 pounds of carbon a year, while the reality is closer to 400, Leverett said.
There is a tendency to assume that young, fast-growing trees are more efficient at capturing carbon than mature trees. People are influenced by wide tree rings of young trees, believing this a sign of their carbon-capturing capacity, in contrast to the tightly bunched rings indicative of slow growth.
But due to their massive size, even a small uptick in the diameter of a mature tree adds more carbon-sequestering volume than the growth spurts common in young trees. “Surprisingly enough, a lot of people just don’t do the math,” he said. “Volume growth is greater at (age) 100 to 150 years — that is really not widely understood.”
The Adirondack Forest Preserve was created in 1885 and has been added to over the decades, meaning its protections play an outsized role in carbon mitigation. But there are some other, more surprising carbon sinks in the Northeast, including Western Massachusetts and southern New Hampshire and Vermont.
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These areas have no special protections, but have simply grown up after farmers abandoned the lands in the 1800s for the less stony soils of the Midwest. “This is the greatest reforestation in the history of the world, and it happened because of benign neglect,” Moomaw said. “Imagine if we did it on purpose.”
Bad news for boreal forests
The consequences of the world’s current carbon trajectory are not encouraging, said Lee Frelich, director of the Center for Forest Ecology at the University of Minnesota. Warm-weather “transitional” trees such as red maple are infiltrating the boreal forests of Minnesota’s Boundary Waters Canoe Area, and on our current path the southern edge of the cold-weather boreal evergreen forest will recede 300 miles to the north.
Heat, drought, winds, fire and insects gain a toehold in this scenario, killing off trees whose decay causes them to release carbon instead of absorbing it. Where wolves don’t keep deer populations in check, Frelich said, hardwood seedlings are eaten and former Minnesota forestland will be at risk of becoming prairie.
Frelich said he got in a bit of trouble for saying it, but it’s true: “We have one perfectly good Kansas now — we don’t need another one.”
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There are two sides to the carbon sequestration of trees. The fact that “matter cannot be created or destroyed “ is often ignored. During daylight trees through photosynthesis take in carbon dioxide and give off oxygen. At night if they keep growing trees take in oxygen and give off carbon dioxide. When the tree dies and rots it gives off some but but not all of the carbon it sequestered. Some of the carbon becomes humus and stays on the floor of the forest.
Peter missed the point of the article.
There is more to carbon sequestration in forests than simple transpiration and photosynthesis. We rarely pay attention to what is going on below ground with the “wood wide web”. We are still learning about the relationships between mycorrhizal fungi and plants that take place largely out of sight. Mycorrhizal fungi can sequester carbon in the soil via plants and store and share it in their hyphae – and healthy forest soils are loaded with hyphae. Leaves and wood are just a portion of the carbon stored in a forest if you consider the forest soils and masses of organisms we never even think about.