Rising CO2 starves Sweden's boreal forests of nitrogen, study finds

Rising CO2 is starving boreal forests of nitrogen

New research shows that rising atmospheric carbon dioxide is causing widespread nutrient decline, or oligotrophication, in Sweden's vast boreal forests. This process threatens to undermine the forests' critical role as a global carbon sink.

The study, published in Nature Ecology & Evolution, analyzed nitrogen isotope data from tree rings across Sweden from 1961 to 2018. It found a consistent decline in nitrogen availability across the entire 23.5-million-hectare forest area, even in remote northern regions with very low atmospheric nitrogen pollution.

Solving a scientific disagreement

Scientists have long debated the cause of declining nitrogen availability in some ecosystems. Some argued it was due to changes in nitrogen pollution deposition, while others pointed to rising CO2 levels as the primary driver.

This study directly tested these competing hypotheses. Sweden's forests provided an ideal natural laboratory because they span a 1,500-kilometer latitudinal gradient where nitrogen deposition varies fourfold, but the increase in atmospheric CO2 is uniform.

"Our data show declining δ15N chronologies throughout Sweden," the authors state. The widespread pattern, occurring even in low-pollution areas, pointed to a cause that was equally widespread.

CO2 is the dominant driver

The researchers used linear mixed-effects models to determine the strongest predictor of the declining nitrogen signal in tree rings. The results were clear.

Rising atmospheric CO2 was the most powerful explanatory factor. Variables like nitrogen deposition, temperature, and forest density had significantly lower predictive power.

This indicates that the fundamental chemistry of plant growth is changing. As CO2 levels increase, plants can incorporate carbon more easily, but this accelerated growth dilutes the available nitrogen in their tissues, creating a nutrient deficit.

Implications for the global carbon cycle

The findings have major consequences for climate models. Boreal forests are one of the planet's largest terrestrial carbon sinks, currently helping to slow atmospheric CO2 accumulation.

Oligotrophication could severely weaken this function. Nitrogen-starved forests grow more slowly and sequester less carbon over time.

The study suggests that predictions which assume forests will continue to absorb more CO2 as levels rise—a process known as CO2 fertilization—may be overly optimistic. The nutrient limitation revealed by this research creates a critical feedback loop.

• Higher CO2 leads to faster but nutrient-poor plant growth.
• This causes oligotrophication (nitrogen decline).
• Nitrogen-starved forests then absorb less carbon in the long term.
• This reduced absorption leaves more CO2 in the atmosphere, accelerating climate change.

The research underscores that the planet's biogeochemical systems are responding to human activity in complex, interconnected ways. Mitigating climate change requires understanding these feedbacks, not just counting carbon inputs and outputs.