Unknown sources fuel boreal forests with nitrogen: "Really surprising"
Wildfires are vital for regrowth in boreal forests. But where does the nitrogen these recovering forests use actually come from? A new study on wildfire recovery sites across the boreal biome led by SLU, shows that the source of this vital nutrient largely lacks explanation.
Boreal forests — the vast green belts stretching across North America, Scandinavia, and Russia — play a major role in regulating Earth’s climate. Wildfires that naturally sweep through these ecosystems constitutes a vital mechanism in binding different nutrients as the forests begin to regrow. And with that recovery comes a vital question: where do the nutrients come from that fuel this growth?
A new international study led by Stefan Hupperts, researcher at the Department of Forest Ecology and Management at SLU, tackled this question by pulling data from 18 wildfire recovery sites across the northern hemisphere. In the paper, published in Global Change Biology, the researchers focused on nitrogen, a key nutrient for plant growth that often limits how effectively forests can capture carbon from the atmosphere.
Majority of nitrogen stocks lack explanation
The results were puzzling. Forests that regrew as deciduous stands (like birch and aspen) after severe, stand-replacing fires accumulated nitrogen at a much faster rate than evergreen forests recovering from lighter burns. But when the researchers added up the known sources of nitrogen — such as rainfall carrying nitrogen from the atmosphere and nitrogen-fixing organisms in mosses and roots — the totals fell far short of what was observed. At most, these sources could only explain a fraction of the nitrogen actually found in recovering forests.
– Overall, we found that more than half of the nitrogen in boreal forests was unexplained by the known sources, and in some cases 90% of the nitrogen was unexplained, which was a really surprising discovery. We were especially surprised to discover how much nitrogen that deciduous forests were accumulating and how little of it was actually explained by the known sources, says Stefan Hupperts.
Understanding how boreal forests accumulate nutrients will matter in the future. As climate change intensifies, researchers expect that boreal regions will see more severe fires and a shift toward deciduous trees. If we don’t fully understand how nitrogen is replenished, we can’t accurately predict how much carbon these forests could lock away.
– It’s very difficult to accurately predict how boreal forests will respond to changing climates if we don’t know how they acquire nitrogen. Finding these ‘missing’ sources of nitrogen will be critical, says Stefan Hupperts.
Source: Hupperts, SF; Nilsson, MC; Gundale, MJ... more (Aug 2025) A Meta-Regression of 18 Wildfire Chronosequences Reveals Key Environmental Drivers and Knowledge Gaps in the Boreal Nitrogen Balance GLOBAL CHANGE BIOLOGY 31(8)DOI10.1111/gcb.70398