Local maple leaves as bioarchives
The Geological Survey of Canada began collecting maple leaves in Gatineau Park in 1961, taking samples once a month during each growing season. Its scientists analyzed the leaves until the agency’s radiocarbon laboratory closed in 2004. The AEL-AMS later took over the analyses and extended the record through 2025, adding 59 annual and seasonal measurements. The collection is ongoing, though leaves gathered in September 2026 are not included in the current study.
“These leaves are a 65-year diary of the atmosphere,” explains Dr. Leonard Wassenaar, Executive Director of the André E. Lalonde National Facility for Accelerator Mass Spectrometry (AMS) at the University of Ottawa. “It took more than 65 years for the extra radiocarbon from weapons testing to disappear from the Ottawa-area biosphere. The effects were still with us decades after the last test.”
Because leaves absorb carbon dioxide that is turned into sugars used for the tree's growth, the radiocarbon in each leaf sample preserves information about the atmosphere at the time. The Gatineau Park collection gave researchers an unusually detailed picture of how atmospheric radiocarbon changed from the height of Cold War weapons testing to present day.
“These leaves are a 65-year diary of the atmosphere”
Dr. Leonard Wassenaar
— Executive Director of the André E. Lalonde National Facility for Accelerator Mass Spectrometry
The rise and fall of the nuclear “bomb pulse”
Above-ground nuclear tests conducted in the late 1950s and early 1960s, mostly by the United States and by the Soviet Union but also by the U.K., France and China, released large amounts of radiocarbon into the atmosphere, roughly doubling global levels. Scientists refer to this sharp increase as the “bomb pulse.”
In leaves from the Ottawa region, the signal peaked in 1964 at 842 parts per thousand above the reference level. By 2025, it had fallen to –13.9 parts per thousand. That decline happened for two main reasons. Oceans gradually absorbed much of the excess radiocarbon, while emissions from burning coal, oil and natural gas diluted it. Fossil fuels are so old that the carbon dioxide they produce contains virtually no radiocarbon.
Direct air measurements show atmospheric radiocarbon dropped below its pre-bomb baseline around 2021–2022 but local maple leaves crossed that threshold in 2024.
New leaves contain traces of older carbon
Researchers often use radiocarbon in leaves and tree rings to reconstruct past atmospheric conditions. These studies usually assume that a sample reflects the atmosphere in the year it grew. But these new findings show that the picture is slightly more complicated. Plants do not build new growth entirely from carbon absorbed that same year. They can also draw on carbon stored during previous growing seasons.
The researchers found that sugar maple leaves closely follow changes in atmospheric radiocarbon, but lag behind by about 2.2 years, with a margin of uncertainty of 0.3 years. The leaves also showed a small positive offset that averaged 7.4 parts per thousand between 1990 and 2019.
“Plants often use older carbon, stored from previous years, to grow new leaves,” adds Dr. Wassenaar. “We didn’t know how much that changed a leaf’s radiocarbon signature, or whether leaves tracked the atmosphere consistently. Now we do.”
These findings give scientists a more accurate way to interpret radiocarbon measurements from preserved leaves, tree rings and other plant material.
“Leaves are a reliable way to reconstruct atmospheric radiocarbon from places and periods we never measured directly,” concludes Dr. Wassenaar. “Once you factor in the two-year delay, our interpretations should be more precise.”
The study, titled “Stored carbohydrates buffer Bomb and Suess radiocarbon effects in maple leaves across a 65-year Ottawa region record”, was published in Nature Scientific Reports. Authorship: Leonard I. Wassenaar, Carley A. Crann & Sarah Murseli. DOI: 10.1038/s41598-026-76700-9
The leaves were collected with permission from the National Capital Commission.
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