What makes older forests so special?

Big trees mean big things for climate change


Julia Dinmore

Cover photo credit: Rob DeGraff via Flickr


“The old-growth forest is as stunning in its elegance of function as in its beauty.”

–Robin Wall Kimmerer, Braiding Sweetgrass (p. 284)

The term “old-growth forest” conjures up images of towering trees surrounded by such bountiful undergrowth as berries, ferns, lichen and wildflowers. I envision wandering through a world of green, replete with blankets of moss spread delicately across vast forest floor. It’s a scene fit for fairies. But this landscape isn’t some mythic ancient land inhabited by fantastical creatures. In fact, all kinds of wildlife currently wander between these behemothic trees—bears, deer, wolves, beetles, foxes, eagles, moose, salmon, and the list goes on. 

And yet, the wildlife sustained by these incredible ecosystems face devastation from logging. Few old-growth forests remain in the United States with an average loss of 95% in the West and 99% in the East. It’s a disheartening statistic, but it doesn’t have to be this way because if you let them, trees grow. And that’s exactly what we are asking the Biden administration for, a new plan that protects our valuable older trees and forests so that they can grow old.

While our old-growth forests have been decimated, the United States still has ample acreage inhabited by mature trees. Mature trees are those with a substantial number of years under their belts – for many species, that’s seventy or eighty years – but not quite enough to be considered “old-growth.” That’s the thing about old-growth forests: they take time to develop. Protecting these mature trees now can replenish our old-growth forests in the years to come.

While old-growth forests are undeniably beautiful, their breathtaking looks cannot be separated from their “elegance of function,” as Robin Kimmerer put it in Braiding Sweetgrass. To understand how function and beauty work together in old-growth forests, we must first answer a foundational question: What is an old-growth forest? 

Credit: Forest Service Alaska Region, USDA via Flickr

Tom Spies, who is a research forest ecologist for Pacific Northwest Research Station, and University of Washington Professor Jerry Franklin developed a definition of old-growth forests for the U.S. Forest Service: “Old-growth forests are ecosystems distinguished by old trees and related structural attributes…that may include tree size, accumulations of large dead woody material, number of canopy layers, species composition, and ecosystem function.”

In more basic terms, you can think of these old trees and their related structural attributes as “the ‘green architecture’ of the forest structure itself,” which Kimmerer describes as “a model of efficiency.” So, what makes this forest structure so ‘green’ and ‘efficient’? 

Forests left free to grow on their own for centuries develop irreplicable ecological functions. Multilayered canopies yield a mosaic of foliage that can both optimize solar energy, which helps rich undergrowth prosper and provides critical habitat for countless species. Even fallen trees have a role to play by feeding fungi, moss, lichen and providing safe havens for crawling critters like mice and salamanders. With so many years to develop, old-growth forests foster interconnected and symbiotic ecosystems that effectively cycle energy and nutrients through the entire forest. Essentially, “if we are looking for models of self-sustaining communities, we need look no further than an old-growth forest.”

Leaving older forests to their own devices means safeguarding vital wildlife habitats and healthy ecosystems that fortify our planet’s overall climate stability. If that isn’t reason enough to keep our remaining big trees standing, then perhaps this will tip the scale: Old-growth forests store vast amounts of carbon. 

The older a tree, the more carbon it stores. A mature tree is at its near-peak of carbon sequestration. It will continue to absorb carbon at this high rate for years with incremental increases as time passes. Even the soil in old-growth forests can store carbon. For example, the Tongass National Forest in Alaska sequesters 44% of total carbon stored across the entire National Forest System—that’s 8% of the carbon stored in all U.S. forests. It’s important to note that 50% of that carbon is held by the top 1% of the Tongass’s oldest trees. On top of this incredible ability to mitigate global warming, old-growth and mature trees are also more resilient than younger forests to drought and wildfires, conditions that are only growing more severe as we journey deeper into the climate crisis. 

In short, big trees mean big things for climate change, and now is the time to capitalize on them. The Biden Administration should act to end logging of mature and old-growth trees and forests right now. Why? Because our forests are disappearing. Each year, Earth loses 5 million hectares of forest. That’s a land area nearly the size of Costa Rica. If we’re going to be serious about avoiding a mass extinction and preventing the worst effects of global warming, we must get serious about letting trees grow. And we need to start now.

Photo credit: Hani Amir via Flickr


Julia Dinmore

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