A study of tree rings from Swedish forests found that nitrogen-isotope values declined from 1961 to 2018. In the authors’ statistical models, rising atmospheric carbon dioxide (CO₂) was the strongest predictor of that pattern, which they interpret as declining nitrogen availability in boreal forests. The study is observational: it does not show that every conifer forest worldwide is nitrogen-limited.
What the study found
Bassett and colleagues reconstructed nitrogen-isotope patterns from archived tree rings across Sweden’s 23.5-million-hectare forest area. Their chronologies span 1961–2018 and a 1,500-kilometre study region. The authors report declining δ15N values throughout the region, including in the far north, where nitrogen deposition is very low. The Nature paper describes the pattern as evidence of reduced nitrogen availability.
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δ15N is the ratio of the nitrogen-15 isotope to nitrogen-14, expressed relative to a standard. In this study, tree-ring δ15N serves as an indicator researchers use to infer changes in nitrogen availability; it is not a direct measurement of nitrogen limitation in every tree.
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How the researchers compared possible drivers
Nitrogen deposition varied fourfold across the study region, while atmospheric CO₂ rose broadly uniformly across it. The authors compared CO₂ with nitrogen-deposition variables, temperature and forest basal area using linear mixed-effects models. As the paper’s abstract puts it: “Linear mixed-effects models showed that rising CO2 is the strongest predictor of δ15N values, whereas N deposition variables, temperature and forest basal area had lower explanatory power.”
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That result identifies the strongest statistical predictor among the factors tested; it does not, by itself, prove cause and effect. The authors interpret their observational records and models as showing that elevated CO₂ is causing oligotrophication—a decline in nutrient availability—in boreal forests. Their findings are not from a controlled experiment that exposed forests to different CO₂ levels.
What the finding may mean for carbon sinks
Forest growth and carbon storage depend on more than CO₂: nutrient availability can also constrain how forests respond as atmospheric CO₂ rises. The authors say the inferred decline in nitrogen availability has implications for forecasting boreal forests’ future role as carbon sinks. The study does not quantify a specific future loss of carbon uptake, so it supports caution in projections rather than a precise prediction.
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Where the result applies—and where it does not
The evidence comes from archived tree rings in Swedish boreal forests, not measurements across all conifer species or forest regions. The consistent isotope decline across a wide Swedish study area, despite substantial geographic variation in nitrogen deposition, strengthens the authors’ case that CO₂ is an important predictor there. It does not establish that conifers everywhere are struggling to obtain nitrogen.
The paper, “Rising atmospheric CO2 reduces nitrogen availability in boreal forests,” was published in Nature on 18 February 2026. The USGS publication record also lists the study.
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