How Rising CO2 is Starving Boreal Forests of Nitrogen

For decades, scientists have tracked the dual impacts of human activity on Earth's ecosystems: the enrichment of nitrogen from industrial pollution and the rise of atmospheric carbon dioxide from fossil fuel combustion. While nitrogen pollution has caused well-documented problems of eutrophication in many regions, new research reveals a counterintuitive trend in the world's boreal forests. A comprehensive study of Swedish forests spanning over half a century demonstrates that rising CO2 is actually reducing nitrogen availability—a phenomenon with profound implications for these critical carbon sinks and global climate predictions.

The Nitrogen Paradox: From Enrichment to Scarcity

Since the industrial revolution, human activities have increased reactive nitrogen creation by a factor of ten through the Haber-Bosch process, leguminous crop expansion, and fossil fuel combustion. This nitrogen enrichment has caused widespread eutrophication and acidification problems globally. However, recent datasets have suggested an opposite trend in some ecosystems—declining nitrogen availability, or oligotrophication. The debate has centered on whether this decline results from reduced nitrogen deposition due to environmental policies or from the effects of rising atmospheric CO2.

Researchers have used nitrogen stable isotope ratios (δ15N) in plant tissues as key evidence for tracking changes in nitrogen availability over time. Higher δ15N values generally indicate greater nitrogen availability, while declining values suggest increasing nitrogen limitation. Previous studies showing declining δ15N chronologies have been subject to conflicting interpretations, with some attributing the trend to changing nitrogen deposition patterns rather than CO2 effects.

A Natural Laboratory: Sweden's Forest Gradient

The recent study published in Nature provides compelling evidence to resolve this debate. Researchers analyzed 1,609 tree cores collected between 1961 and 2018 from Sweden's 23.5-million-hectare forest area. This landscape offered an ideal natural laboratory because it spans a 1,500-kilometer latitudinal gradient where nitrogen deposition varies fourfold—from very low in the north to relatively high in the southwest—while rising atmospheric CO2 concentrations are spatially uniform across the entire region.

The research team used archived samples from the Swedish National Forest Inventory, focusing on two dominant tree species: Norway spruce (Picea abies) and Scots pine (Pinus sylvestris). By analyzing decadal growth increments from trees of the same age class, they eliminated confounding factors like tree aging and nitrogen translocation across xylem that have complicated previous studies. This methodological rigor produced δ15N chronologies that were markedly more consistent and sensitive to ecosystem changes than earlier efforts.

CO2 Emerges as the Dominant Driver

The results were striking and consistent across Sweden's diverse forest regions. Linear mixed-effects models showed that atmospheric CO2 concentration was consistently the strongest predictor of δ15N values, with a strongly negative relationship for both tree species. For every part-per-million increase in CO2, δ15N values declined by 0.04‰ for Scots pine and 0.028‰ for Norway spruce.

This relationship persisted even when controlling for other factors including nitrogen deposition variables, temperature, forest basal area, latitude, and longitude. The partial explanatory power of CO2 (partial R²m = 0.176) substantially exceeded that of nitrogen deposition variables (partial R²m ≤ 0.005), providing robust evidence that rising CO2, not changing nitrogen deposition patterns, drives the observed decline in nitrogen availability.

Why Nitrogen Declines as CO2 Rises

The study identifies two non-mutually exclusive mechanisms linking rising CO2 to declining nitrogen availability. First, elevated CO2 increases plant photosynthesis, growth, and nitrogen demand in accumulating biomass, effectively depleting available nitrogen from soil. Second, rising CO2 increases plant carbon-to-nitrogen ratios, leading to production of high C:N litter that stimulates soil microbial nitrogen immobilization during decomposition. Both responses reduce fractionating nitrogen loss from the ecosystem, steering δ15N values more negative over time.

Supporting this interpretation, data from the Swedish National Forest Inventory show long-term growth increases in mesic pine and spruce forests since the 1950s across all regions. The research found a strong negative relationship between temporal changes in forest growth and wood δ15N values—consistent with the prediction that increasing plant nitrogen demand creates a negative feedback on nitrogen availability.

Implications for Boreal Forests and Climate

Boreal forests play a disproportionately important role in the global carbon cycle, covering approximately 17% of terrestrial land area but accounting for about 32% of terrestrial carbon storage. The finding that rising CO2 reduces nitrogen availability in these ecosystems has significant implications for predicting their future role as carbon sinks.

The phenomenon of progressive nitrogen limitation suggests that reduced nitrogen availability may eventually constrain the response of forest growth to future CO2 increases. This aligns with observations that forest growth has leveled off in many parts of Sweden in recent decades and with predictions from some Earth-system models that nitrogen feedbacks will eventually limit terrestrial net primary productivity responses to rising CO2.

Mycorrhizal Fungi: An Additional Mechanism

The study also highlights the potential role of mycorrhizal fungi in the observed patterns. As forest nitrogen demand increases with rising CO2, trees may enhance carbon investment in mycorrhizal fungi to improve soil nitrogen acquisition. Ectomycorrhizal fungi, which dominate in boreal forests, disproportionately retain ¹⁵N in their hyphal mass while passing ¹⁴N to host trees—with greater fractionation occurring as nitrogen availability declines. Thus, declining δ15N chronologies may reflect an increasing proportion of plant nitrogen obtained via mycorrhizal pathways.

Broader Environmental Implications

This research demonstrates that human alterations to the nitrogen and carbon cycles are not changing independently but are interacting in complex ways. While nitrogen pollution remains a serious environmental problem in many regions, the study suggests that declining nitrogen availability in response to rising CO2 may become a more dominant driver of boreal forest dynamics than nitrogen enrichment from atmospheric deposition.

The findings have relevance beyond Sweden, as declining δ15N chronologies have been identified in northern forests across Europe and North America. They also align with other indicators of tightening nitrogen cycles, including declining foliar nitrogen content in European and North American forests, reduced riverine nitrogen exports in central Europe and North America, and declining stream water nitrogen exports in northernmost Sweden where nitrogen deposition is very low.

Conclusion: A New Understanding of Forest Responses

This comprehensive analysis of Swedish forest data provides compelling evidence that rising atmospheric CO2 is causing oligotrophication in boreal forests. By demonstrating that CO2, not changing nitrogen deposition patterns, drives declining δ15N chronologies, the research resolves a key scientific debate and advances our understanding of how these critical ecosystems respond to environmental change.

The implications extend to climate modeling, forest management, and environmental policy. As atmospheric CO2 concentrations continue to rise, understanding how nitrogen limitation may constrain forest growth responses becomes increasingly important for predicting future carbon sequestration and developing effective climate mitigation strategies. The study underscores the complex interplay between human-altered biogeochemical cycles and highlights the need for integrated approaches to managing these interconnected environmental challenges.