The Eastern United States' powerhouse forests

A study suggests some forests are thriving in our current climate. Others are withering.

Measuring the girth of a pine tree in a Pennsylvania forest

Thousands of scientific papers are published each week, and my goal when skimming the press releases that land relentlessly in my inbox like mosquitoes in summertime is often to find excuses to delete them. But every now and then I see a paper that makes me say, “Wow!”

Most recently, that paper was a study of how fast American forests are growing. Examining several decades’ worth of data, the study authors pieced together a tale of two forests. To put it simply, in the eastern half of the country, forests are booming. In the western half, they’re withering.

While you’ve no doubt read about how wildfire, heat and droughts are devastating forests in the West, the thriving forests in the East are less dramatic, so naturally, they’ve received far less attention — which is a shame.

It’s a shame because it deprives the 180 million of us who live in the eastern U.S. a chance to better know our own forests, and the forces that are helping them thrive. It also deprives us all of a hopeful story at a time when environmental news often seems relentlessly bleak.

So let’s talk about the new study, which was published a couple of weeks ago in the journal PNAS. The authors used the Forest Service’s Forest Inventory and Analysis, a remarkable and unique dataset supported by our tax dollars. Every year, Forest Service field crews, contractors and state partners measure trees in more than 10,000 plots spread across the country; each plot in the inventory is remeasured every five to ten years.

(The lead photo shows people taking a tree diameter measurement, though it’s not from an FIA plot — at least as far as I know. The actual locations of the plots are kept secret.)

To my mind, the FIA is a national treasure, like the James Webb Space Telescope. There’s probably no comparable public dataset in the world, yet unlike the James Webb, most people have never heard of it.

It’s long been known both from FIA and other data that eastern U.S. forests are soaking up carbon dioxide as they rebound from a near-total deforestation that started in the early 1600s and ended only in the early 1900s. One shocking photo I came across recently shows what is now the Brookland neighborhood of Washington, D.C. during the Civil War.

A view of Fort Bunker Hill and a military camp in what is now Washington, D.C.’s Brookland neighborhood. See here for more details. Source: Library of Congress

This desolate scene is hard to square with the lush, green city D.C. is today, especially in outlying neighborhoods like Brookland. Yet around the time of the Civil War, large swaths of the District were apparently nearly as treeless as a western desert — as, indeed, was much of the eastern half of the country.

The scientists found, unsurprisingly, that American forests have bulked up as they rebound from the deforestation that created scenes like the one captured in the photo. While this regrowth will eventually taper off as forests mature, so far it seems to be going strong.

More remarkably, the researchers also found that regrowth alone cannot explain the blistering pace at which our trees are putting on wood. By examining forest growth rates while controlling for age-related differences, the scientists determined that something else is supercharging growth.

And while the study did not directly answer what that something is, the authors highlighted one likely explanation: Trees are gobbling up some of the excess carbon dioxide we’re putting into the atmosphere. Essentially, by burning fossil fuels in our cars, buildings and factories, we are fertilizing nature. And nature is responding.

Many studies have speculated about carbon fertilization using computer models, experiments and theory. It’s clear that all things being equal, plant leaves respond to higher carbon dioxide levels by ramping up photosynthesis (the biochemical process plants use to turn carbon dioxide into sugars), which could cause plants to grow faster and ultimately store more carbon.

But in nature, many things can affect how fast trees grow. Experiments have pumped high levels of carbon dioxide into young forests and found that trees initially grew faster than in unfertilized control plots but eventually leveled off, presumably as nutrient limitations or other factors throttled trees’ growth rates.

The new study is among the first to provide clear evidence that real-world forests over a vast landscape are indeed able to use the extra CO₂ to bulk up. And if our trees are feasting on carbon dioxide, those in other places with moderate temperatures and ample moisture, such as northern Europe and eastern Asia, likely are too.

“CO₂ fertilization is almost certainly happening, and it is almost certainly contributing to the carbon sink in some regions of the world, like the eastern U.S.,” said Jeremy Lichstein, an ecologist at the University of Florida whose group led the research.

Lichstein also said that some forests are growing even more robustly than carbon fertilization can explain. One possible additional explanation is nitrogen deposition from tailpipes and smokestacks — it turns out that carbon is not the only plant fertilizer our industrial society spews into the atmosphere. Another is that warming itself has benefited trees, for example by extending the growing season. Lichstein cautions that scientists have come to mixed conclusions about whether these factors have enhanced forest growth, so take these suggestions with a grain of salt for now.

Intriguingly, some even newer research suggests that regrowing eastern forests have also had a regionwide cooling effect that’s most pronounced in midsummer — and that may reduce heat stress and help those same forests grow faster. I’ll have more to say about that in a future post.

Now, “carbon fertilization” might sound like a wonky scientific abstraction. But for the 180 million of us who live in this part of the world, it has had real, measurable benefits. The trees in our neighborhoods and parks, and even our own yards, have grown faster than they otherwise would have. That means more shade, more storm protection, more wildlife habitat — and, not least of all, more wood. We have all benefited, in multiple ways.

Indeed, given that we’re seeing CO₂ levels unparalleled in human history, everyone alive today has a different experience of tree growth than have humans at any time in the past. If, like me, you’ve marveled at how much wood a young tree is able to put on in one growing season, part of what you’ve marveled at is the impact of our industrialized age. I think that’s pretty profound.

In the western U.S., unfortunately, the data tell a different story. Forests there are growing less and less robustly, as heat and drought limit trees’ ability to benefit from high CO₂ levels. (Interestingly, the Forest Service has reported this for years based on FIA data, but I suspect their reports rarely get read.)

The plight of western forests has gotten plenty of coverage, so I won’t dwell on it here except to note that the fact that some forests are benefiting from high CO₂ levels does not mean we can simply assume that all forests will — and certainly does not suggest we don’t need to worry about climate change.

I asked Lichstein and his colleague Aaron Hogan what implications their research has for natural climate solutions — the idea that natural ecosystems like forests can soak up some of the carbon dioxide we humans emit by burning fossil fuels. Their answer, perhaps surprisingly, was not much. In fact, they said that at the global scale, the strength of the carbon fertilization effect is probably exaggerated in most of the computer models scientists use to forecast climate trends.

In other words, as future warming stresses forests, they will probably absorb a smaller and smaller fraction of our total carbon dioxide emissions. Right now that fraction is one quarter. If the strength of the carbon sink diminishes, we could be in real trouble.

This aligns with my view, based on years of reporting on forest science, that at a broad scale, forests and other ecosystems are probably already doing about as much as we can hope for to slow climate change. The idea that we’re going to jam tons of additional carbon into trees or soil strikes me as more aspirational than realistic.

That’s because natural climate solutions are limited both by physical factors like those the authors found operating on western forests and by human factors. Growing long-lived forests for carbon storage will always be just one of many things humans want to do with land. It so happens that in the U.S., farming proved to be more profitable in the Midwest than in the east, so much of the eastern forestland was able to recover. But elsewhere in the world, forests are still being cleared for agriculture.

Nevertheless, the fact that eastern U.S. forests have done so well over the past century tells us that all is not lost. Even if forests may not be a major global-scale climate solution, the study has another powerful message: Modern, technologically advanced humans are capable of sharing large regions of the Earth with thriving forests.

It’s also worth noting that most of these fast-growing forests are not in parks or preserves, but on private land. That means they can legally be cut down, but it also means that millions of people have a stake in them. The public ownership that’s more common in the West — and that’s often assumed to be more protective — can also be more neglectful, especially when governments don’t have the resources to properly care for vast tracts they’ve been tasked to manage.

While private ownership is not a panacea either, when a lot of people live among forests, there are a lot of people with reasons to keep an eye on them and care for them. Technologically speaking, we could easily cut down every tree in the eastern U.S. — as far fewer, less technologically advanced people once did. Yet instead, we’ve allowed them to grow at the same time that our own population has grown.

This pushes strongly against what I would describe as the prevailing narrative that people are simply bad for trees. This kind of simplification has even been embraced by the august New Yorker, a publication I would expect to do better.

It’s time to ditch simplistic morality tales for a more nuanced and reality-based view of the relationship between trees and humans. After all, we need trees to thrive in places where people actually live. I’ve argued previously that the densely populated Mid-Atlantic could be a climate refuge, in part because our cities and towns are embedded within what, so far at least, appear to be remarkably climate-resilient forests. The new study suggests the same might be said about much of the eastern U.S.

As other parts of the world become less hospitable, we need to do all we can to keep trees growing where the growing is good.

This post has been updated to clarify how FIA data are collected. See here for more information and hat tip to Tom Kimmerer for notifying me that not all data are collected by Forest Service staff.