It rained overnight. A morning walk along the well-worn path in the family’s woodlot is not crunchy and soothing, as it is on many fall mornings. Today it’s a bit sodden, with the last drips from the rain falling off the tall trees along the trail. Perhaps a dog or two scampers ahead, glancing back to assure a human’s presence, ready to dash into the decades-old stands on both sides at any sign of a squirrel.
It’s so familiar that it seems unchanging. But it is not.
Unseen, unheard, belowground processes are behaving differently than they were a generation ago – especially in the layers of leaves and fallen twigs directly underfoot and in the rain-soaked soil just below them.
Scientists call the uppermost layer of the forest’s organic horizon, which is made up of leaf litter and other recently fallen material, the Oi (pronounced “oh-eye”) layer. Below the Oi layer is the Oe (pronounced “oh-ee”) layer – the graveyard, or perhaps better thought of as the composter, of the forest. The Oe layer, where tree roots thrive, is full of partially decomposed leaves. The organic horizon contains microorganisms and fungi that help break down organic matter and contribute to nutrient cycling – and plant health.
In work published in May 2025, Dartmouth College researchers Caitlin Hicks Pries and Sophie von Fromm, along with a team of collaborators, delved in to decades of archived soil samples at the Hubbard Brook Experimental Forest in New Hampshire to learn how these layers are reacting to a roughly 1.4°C increase in temperature in New England over the past 50 years or so.
What they found, reported in Global Change Biology, is that the top layers of soil – the Oi and Oe layers – are cycling through carbon stocks faster than they did 30 years ago. And during each of those cycles, less carbon remains in the soil, while more is released into the atmosphere.
The findings suggest “that the rapidly cycling litter layer at the smaller scale is responding to recent environmental changes,” the researchers write in the report. “Our results highlight the importance of litter carbon as an ‘early-warning system’ for soil responses to environmental change.”
How researchers were able to understand these changes requires a brief trip through time and space – to a small island in the South Pacific more than seven decades ago. An event here connects directly to today’s research into the changing behavior of a woodlot’s organic horizon.
Early in the morning of March 1, 1954, on Bikini Atoll, the United States tested a thermonuclear weapon a thousand times more powerful than previous atomic bombs. It was about 10 years before the first ban on aboveground nuclear testing, and the United States was at the height of its bomb-testing program. This test, code-named Castle Bravo, formed a fireball four-and-a-half miles in diameter. The resulting crater was more than 6,000 feet wide and 250 feet deep. The mushroom cloud reached a height of 130,000 feet. (For comparison, commercial airplanes typically fly at altitudes up to about 35,000 feet.)
This explosion – and the more than 1,000 aboveground atomic weapon tests conducted by the United States and other countries between 1945 and 1963 – spread radioactive material throughout the planet’s atmosphere. Leaves pulled that radiation into themselves during photosynthesis, and the radiation then moved through plants into soils. These weapon tests created what’s called a “bomb spike” in the global soil-carbon record.
Hicks Pries, von Fromm, and many other scientists use that “bomb spike” to date radiocarbon in soils with unusual precision.
Today, it’s not radiation from bombs filling the Earth’s atmosphere; it’s fossil-fuel emissions. Without an effort to curb these emissions akin to the work to ban atmospheric weapons testing, the effects of climate change are impacting our backyards and woodlots as surely as they are driving more powerful hurricanes or deadly extreme-heat events in Pakistan or Texas.
“People who have their woodlots and are out in the forest, they can see the change in the trees. They see the trees are stressed. They see that they’re dropping their leaves early because of the flash drought. They see when there’s a lot of pests; they see the ash dying,” Hicks Pries said. “But maybe they’re not thinking about, well, what’s happening belowground?”
Using long-term data from Hubbard Brook soil samples to drive computer models, this team of researchers examined near-surface soil layers to see what changes, if any, are contributing to the stress trees are exhibiting aboveground. For this team, finding those stressors meant looking at possible changes to the soil’s carbon cycle: when and how carbon comes in, how long it stays, and when it’s replaced. There’s one big obstacle to finding out about changes to that cycle: there is a lot of carbon in the soil layers in a forest.
The 1960s carbon-14 (radiocarbon) bomb spike serves as a timestamp to date carbon’s entry into the soil. Compare a sample from the 1960s with a sample from today and it’s possible to measure how much of that era’s radiocarbon remains as a component of the forest’s carbon cycle. Because carbon-14 only differs from other kinds of carbon in its number of neutrons, the length of time radiocarbon resides in the cycle is comparable to all the carbon in surrounding soil. Tracking those residence times means it’s possible to see any changes in the soil carbon cycle over years or decades.
Ideally, the cycle holds near a steady state, and forest soil layers maintain the same amount of carbon over years or decades or centuries. Trees, after all, can live a long time, and organic carbon is the soil’s fuel and scaffolding. Carbon feeds microbes and fungi, allowing them to knit soil together and help soils store carbon. In the forest’s topsoil – the layer directly below the organic horizon – microbes make carbon-rich sticky compounds as carbon from decomposing leaf litter and roots mixes downward from the Oi and Oe layers to form crumblike structures called aggregates. These aggregate structures protect nutrients such as nitrogen and reduce the amount of nitrates flushed into streams during rain events.
Aggregates also create a pore network that stores plant-available water, which supports seedlings during dry spells and reduces the stress on mature trees during droughts. Aggregates are always forming and breaking in the topsoil. But if there’s enough of a disturbance, such as the ongoing warming of climate change that intensifies wet-dry cycles, the structure of the aggregates can be disrupted and accelerate the microbial respiration of organic matter to CO2.
A steady state carbon cycle is needed to maintain current forest health. The data Hicks Pries and von Fromm collected offer some reassuring findings: across the landscape scale, and deeper down where carbon can remain for centuries (“the oldest carbon data I think I’ve ever received from a soil sample was between 16,000 and 20,000 years old,” Hicks Pries said), researchers did not detect much change in the soil’s carbon cycle.
But measuring radiocarbon’s residence time in forest pools also shows that there is a detectable and somewhat worrisome change to parts of the cycle. While the forest’s deep carbon seems to be strong and enduring, that doesn’t seem to be the case in the Oi and Oe layers, where leaves and twigs are essential to replenishing parts of the carbon cycle. In those top layers, more carbon is going out than is coming back in. “Interestingly,” as von Fromm, et. al. describe it, “at the site scale, and over a shorter period (1998–2023), the Oi/Oe layers showed a decline in carbon stocks and an increase in soil CO2 respiration.”
Driving this recorded change – less carbon and more respiration – are factors such as warmer air and more rain: the oft-reported results of climate change seen across New Hampshire. This small change in the surface carbon cycle that Hicks Pries, von Fromm, and their team identified in a small area of the White Mountains serves as an early warning for possible big changes to how forests’ roots feed and how microbes decompose leaf litter as air temperatures rise and precipitation increases. This destabilization of the carbon cycle may seem insignificant, but in the delicate balance of the forest ecosystems, small changes happening now can indicate big imbalances are ahead.
“It’s warning us that the system we’re studying – in this case, the forest – is responding to changes that we humans are causing,” von Fromm said. “Sometimes it’s not easy to see those changes. Because we have such a huge amount of carbon in the soil, it’s also difficult to detect small changes. And once we detect those changes, we might be too late, because then we may not be able to stop the system from shifting into a new state.”
It’s not that there won’t be trees and forests in New Hampshire; the concern is functional. Organic carbon drives many of the forest’s processes, and if the carbon stocks continue to shrink, as the data suggest is possible, soils could hold less water, further increasing drought stress on saplings and mature trees. Nutrient losses could also rise, along with the threat of nitrogen pollution downstream. The forest will remain, but the processes could function differently, undermining tree vigor and long-term productivity.