Rising atmospheric CO2 intensifies nitrogen limitation in boreal forests

Kelley R. Bassett; Stefan F. Hupperts; Sandra Jämtgård; Lars Östlund; Jonas Fridman; Steven S. Perakis; Michael J. Gundale

doi:10.1038/s41586-025-10039-5

Rising atmospheric CO2 intensifies nitrogen limitation in boreal forests

Nature

By: Kelley R. Bassett, Stefan F. Hupperts, Sandra Jämtgård, Lars Östlund, Jonas Fridman, Steven S. Perakis, and Michael J. Gundale

https://doi.org/10.1038/s41586-025-10039-5

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Abstract

Anthropogenic nitrogen (N) pollution has been emphasized as a cause of eutrophication globally. However, several recent datasets have suggested widespread oligotrophication may be occurring in some ecosystems, which is suggested to be a response to rising atmospheric carbon dioxide (eCO₂). Plant δ¹⁵N chronologies have served as primary evidence for oligotrophication, however, there has been wide disagreement whether eCO₂or temporal changes in N deposition explain these patterns. We constructed δ¹⁵N tree ring chronologies across Sweden’s 23.5 million hectare productive forest area from the 1950s to 2010s. The study area spans a 1500 km latitudinal distance where N deposition varies four-fold, but where eCO₂ is spatially uniform. Our data revealed negative δ¹⁵N chronologies throughout Sweden, including forests in the far north where atmospheric N deposition rates are very low. Linear mixed effects models showed that eCO₂ was by far the strongest predictor of δ¹⁵N values, whereas N deposition variables, temperature, and forest basal area had much lower explanatory power. Our results clarify debates on the interpretation of previous δ¹⁵N chronologies, and provide clear evidence that eCO₂ is causing oligotrophication in boreal forests, which has implications for predicting their future role as sinks in the global carbon cycle.

Suggested Citation

Bassett, K.R., Hupperts, S.F., Jämtgård, S., Östlund, L., Fridman, J., Perakis, S.S., Gundale, M.J., 2026, Rising atmospheric CO2 intensifies nitrogen limitation in boreal forests: Nature, no. 650, p. 629-635, https://doi.org/10.1038/s41586-025-10039-5.

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Additional publication details
Publication type	Article
Publication Subtype	Journal Article
Title	Rising atmospheric CO2 intensifies nitrogen limitation in boreal forests
Series title	Nature
DOI	10.1038/s41586-025-10039-5
Issue	650
Publication Date	February 18, 2026
Year Published	2026
Language	English
Publisher	Springer Nature
Contributing office(s)	Forest and Rangeland Ecosystem Science Center
Description	7 p.
First page	629
Last page	635
Country	Sweden