Why Old-Growth Trees Are Crucial to Fighting Climate Change

Lutz also needed such a vast area to study because he was interested in a relatively rare resident of the forest; to get enough examples of it for his studies to be meaningful, he’d have to cover a lot of land. Lutz’s elusive quarry? Thick, old trees like the big Douglas fir that Bible so admired. These days, old-growth forest is itself a rare find, but even within it, the attrition of centuries means that most trees aren’t actually that old. As Lutz puts it, “to grow a big tree, you need an old tree, which means a tree has to survive”—not just logging but fires and insects and diseases, and anything else that could have come along during its long life and killed it. Old-growth forest is naturally a complicated mix of ages and sizes and structures. But though truly big trees aren’t the most common of the forest’s residents, Lutz has learned that their role in its ability to store carbon is as oversized as they are.

In a 2018 paper looking at 48 different forest plots, including the one in Wind River, he found that the largest 1 percent of trees contain fully half of all the above-ground live biomass, which also means half of all the carbon, since the two are directly correlated. Young trees sequester carbon faster, packing it on in the vigorous growth of their early years, but they can’t begin to compete with what large trees have been able to build into their trunks and branches through years and years of maturation. “You can’t sequester a lot of carbon without big trees,” Lutz says. “You just can’t do it.”

This makes old trees—and even Munger’s much-hated dead trees and logs, which can take centuries to rot in the Northwest—not useless but precious. While a single-age stand would lose 1 percent of its carbon storage if it lost 1 percent of its trees, big trees are so important that a 1 percent loss of individuals in an old forest could reduce its carbon by half. And while old forests eventually begin to reach an equilibrium, at which they’re not adding a lot more carbon than they’re losing through death and decomposition, researchers have found that the old growth in Wind River is still sequestering new carbon each year, adding to the huge amount it already stores. “Even just putting a thin annual growth layer on such a big cylinder is a huge deal,” explains Ben Vierra, who manages NEON’s research in the Pacific Northwest. Bible, deep in the grove, says: “This forest is still putting on forest. Quite a bit actually—it could give a young forest a run for its money.”

There has recently been much discussion of tackling climate change by planting lots and lots of trees, something that Lutz is all for—it’s still carbon, after all. But he’s cautious about how much can really be stored by a lot of willy-nilly new planting, especially if those trees are planted in conditions that will not allow them to thrive and grow old.

Methods for optimizing nature’s ability to store carbon are known as natural climate solutions. The word natural, it turns out, is as key as the word solutions—these are strategies that have a lot of potential, but that also work in complex ways that can be difficult for us to understand and re-create. That program I used to figure out the park’s carbon footprint in Hawaii back in 2008? Its calculations for carbon storage in landscapes were so new, and still so unsophisticated, that some parks didn’t yet bother to report them. (Depending on how I defined the type of forest we had—it was a mix, but in the program I had to choose between “wet tropical” or “dry tropical”—the amount of carbon that the program credited the park with removing from the atmosphere could nearly double.) It’s not that forests and other natural areas can’t store a lot of carbon—they’re currently storing much, much more than is in the atmosphere, including both what’s there naturally and what’s human-added—it’s that carbon moves through them in complicated ways that are hard to measure. How do you account for the natural release of carbon when plants rot? For the widely differing amounts of carbon that different ecosystems hold in soils? For the ways that climate change itself is affecting the way that plants’ biology works and how much carbon they can store?

You may have seen last year’s ecstatic headlines that planting a trillion trees could “stop” climate change (or the recent endorsements of the idea by the World Economic Forum and the Trump administration). In fact, the paper in question simply asserted that many new trees could offset more than 200 gigatons of emissions, and it was followed by a series of responses from other scientists who argued that the authors had, pretty dramatically, overestimated the carbon storage potential of new trees. (The authors of the original paper stand by their results.)