A 65-year record of sugar maple leaves from Gatineau Park captures the rise and decline of radiocarbon from Cold War nuclear testing—and shows that leaves do not mirror the atmosphere immediately. A Phys.org report describes a study led by the André E. Lalonde National Facility for Accelerator Mass Spectrometry at the University of Ottawa. Publication details name Leonard I. Wassenaar and colleagues as the study’s authors; it appeared in Scientific Reports.
The Geological Survey of Canada began collecting leaves in the park in 1961, taking samples monthly during each growing season. Its scientists analyzed them until the agency’s radiocarbon laboratory closed in 2004. The University of Ottawa facility later continued the analyses through 2025, adding 59 annual and seasonal measurements. The collection is ongoing, but leaves collected in September 2026 were not part of the study described in the report.
Leaves can act as records of atmospheric carbon because trees take in carbon dioxide and use its carbon to make sugars for growth. Radiocarbon in a leaf therefore carries information about the atmosphere, though the report’s findings show that the signal is not a perfect snapshot of the year the leaf grew.
Above-ground nuclear tests in the late 1950s and early 1960s released large amounts of radiocarbon. This “bomb pulse” roughly doubled global atmospheric levels. In the Ottawa-region leaves, radiocarbon peaked in 1964 at 842 parts per thousand above the reference level. By 2025, the measured value had fallen to –13.9 parts per thousand.
The report attributes the decline to two processes: oceans gradually absorbed much of the excess radiocarbon, and emissions from burning coal, oil and natural gas diluted it. Carbon dioxide from these fossil fuels contains virtually no radiocarbon. Direct air measurements fell below the pre-bomb baseline around 2021–2022; the maple-leaf record crossed that threshold in 2024. The distinction suggests the local leaves preserved a lingering signal after the direct atmospheric record had passed the baseline.
The study also examined how stored carbon affects the timing of the leaf record. Sugar maples can draw on carbohydrates made in previous growing seasons when producing new leaves. The reported analysis found that leaf radiocarbon followed atmospheric changes with an average lag of about 2.2 years, with an uncertainty of 0.3 years. From 1990 to 2019, the leaves also showed a small positive offset averaging 7.4 parts per thousand.
That delay matters for scientists using leaves, tree rings and other plant material to reconstruct past atmospheric conditions. If a plant sample is treated as though it reflects only the atmosphere in the year it grew, the timing or level of the reconstructed signal may be slightly off. The report says accounting for the lag can make such interpretations more precise, including for places and periods without direct air measurements. The reported result concerns sugar maple leaves from one long-running Ottawa-region record; it does not establish that all plants or locations have the same delay.
The source report presents this as a study of radiocarbon records, rather than a direct measurement of nuclear tests themselves. Its central contribution is a correction for how stored carbon can shape the record preserved in new leaves.