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A study of Swedish boreal forests found that tree-ring nitrogen-isotope records declined from 1961 to 2018, and rising atmospheric carbon dioxide was the strongest predictor of that pattern among the factors tested. The authors interpret the result as CO₂-related oligotrophication—a decline in nitrogen availability. The evidence comes from archived tree rings and statistical models, not a controlled CO₂ experiment, and it does not show that all conifer forests worldwide are nitrogen-limited.
What the Swedish study found
Bassett and colleagues analyzed archived tree-ring samples to build nitrogen-isotope records, known as δ15N chronologies, for Swedish forests. The records declined across the study region over 1961–2018. The paper, published in Nature on 18 February 2026, reports that rising atmospheric CO₂ was the strongest predictor of the isotope values in the authors’ models. Read the study in Nature.
The authors interpret the declining isotope pattern as evidence that nitrogen availability is falling in boreal forests as atmospheric CO₂ rises. They describe this process as oligotrophication. δ15N is an indicator used to infer changes in nitrogen availability; it is not a direct measurement of nitrogen limitation in every tree.
How broad was the evidence?
The study used archived samples from Sweden’s 23.5-million-hectare forest area and covered a 1,500-kilometer span. Its chronologies run from 1961 through 2018. Nitrogen deposition varied fourfold across the region, while rising atmospheric CO₂ was spatially uniform. The researchers reported declining chronologies throughout Sweden, including the far north, where nitrogen deposition is very low.
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That range allowed the authors to compare a spatially consistent rise in CO₂ with nitrogen deposition patterns that differed across the study area. But the evidence remains specific to Swedish forests and an observational tree-ring record. It is not a global survey of conifers or an experiment that exposed forests to controlled CO₂ levels.
Which factors best tracked the isotope pattern?
The authors compared candidate drivers using linear mixed-effects models. Their abstract states: “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.” The USGS publication record also lists the paper and its publication details.
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- Atmospheric CO₂: strongest predictor of tree-ring δ15N in the reported models.
- Nitrogen deposition variables: lower explanatory power, despite deposition varying across the study region.
- Temperature and forest basal area: also had lower explanatory power than CO₂.
This is a comparison of how well the tested variables tracked the isotope chronology in the models. Because the study is observational, the model ranking alone does not establish experimental proof that CO₂ caused each local change. The authors nevertheless interpret their combined evidence as showing that elevated CO₂ is reducing nitrogen availability in boreal forests.
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Nitrogen is a nutrient that can constrain plant growth. If its availability declines, it may affect how forests grow and how much carbon they can absorb and store. The authors therefore point to possible consequences for forecasting forests’ future role as carbon sinks. The study identifies that concern; it does not, on the evidence summarized here, quantify a specific change in forest carbon uptake.
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What the result does—and does not—say
- It does say: Swedish tree-ring δ15N chronologies declined from 1961 to 2018, and rising CO₂ was the strongest predictor among the tested variables.
- It does not establish: that every conifer forest, or forests worldwide, are nitrogen-limited or will respond in the same way.
- It does not directly measure: nitrogen limitation in each sampled tree. The isotope records serve as an indicator used to infer nitrogen availability.
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