Boreal Forest Carbon Storage Gap Is Up to 8 Times Larger Than Official Estimates

AAAS / Science

How much carbon does a forest lose when it becomes a timber plantation? The answer matters enormously for climate policy, because nations around the world are betting that managed forests and forest-derived bioenergy will help them meet emission reduction targets. If those bets are based on the wrong numbers, the entire strategy needs rethinking.

A study published March 19 in Science suggests the numbers have indeed been wrong, substantially so.

Measuring what had never been measured

Didac Pascual and colleagues combined data from the Swedish National Forest Inventory, the Swedish National Forest Soil Inventory, and targeted field surveys to build the most complete picture yet of carbon storage in Sweden's primary (old-growth) forests. They measured carbon across every relevant compartment: living vegetation, dead wood, soils, and even the carbon embodied in harvested wood products like lumber, paper, and biofuel.

Multiple analytical methods were applied to estimate total carbon storage and to compare primary forests with the managed secondary forests that have largely replaced them, particularly since industrial-scale forestry took hold in the 1950s.

9.9 kilograms per square meter, missing

The headline finding: primary forests store about 72% more carbon than managed secondary forests when all carbon pools are considered together. Across Sweden, that translates to 9.9 kilograms of carbon per square meter more in primary forests than in managed ones.

That gap is 2.7 to 8 times higher than earlier estimates. The discrepancy is not a rounding error. It is a fundamental miscalculation that has flowed into climate models, national carbon budgets, and the projections used to justify expanding bioenergy production from northern forests.

Soils account for the largest share of the difference. Boreal forest soils accumulate organic carbon slowly, over centuries, as cold temperatures slow decomposition. When these forests are clear-cut and managed intensively, the soil carbon built up over hundreds of years begins to dissipate. Replanting does not reverse this quickly. The timescale for soil carbon recovery in boreal systems is measured in centuries, not the 60-to-80-year harvest rotations typical of Swedish forestry.

The bioenergy blind spot

Most models used to construct pathways for stabilizing global temperatures assume increased use of northern forests, especially for bioenergy. The logic is that burning wood for energy is carbon-neutral because new trees absorb what was released. But this accounting ignores the soil carbon that vanishes when old-growth forests are converted to managed stands, and it ignores the centuries-long lag before soil carbon recovers.

If managed forests store less than half the carbon of the old-growth forests they replaced, the climate projections underlying bioenergy strategies may be significantly too optimistic. The study does not claim that all forest bioenergy is counterproductive, but it suggests the carbon math needs revision.

Limitations worth stating plainly

This study covers Sweden. Boreal forests span a vast area across Canada, Russia, Scandinavia, and Alaska, and conditions vary. Soil types, tree species composition, management histories, and climate all differ from country to country. Extrapolating these specific numbers globally would be premature without comparable fieldwork elsewhere.

The study also cannot isolate the contribution of individual management practices. Clear-cutting, drainage, fertilization, thinning, and soil preparation all occur in Swedish forestry, often in combination. Understanding which practices cause the most carbon loss would require controlled experiments that this observational study was not designed to provide.

There is also the question of what "primary forest" means in practice. These are forests with minimal or no direct human disturbance, but defining that boundary is inherently somewhat subjective, and the remaining examples in Sweden are increasingly rare.

A recalculation, not a revolution

The study does not argue that forestry should cease. It argues that the carbon costs of converting primary forests to managed stands have been systematically underestimated, and that protecting remaining old-growth forests offers larger climate benefits than previously recognized. For policymakers building climate strategies on forest-based carbon accounting, the message is straightforward: the baseline numbers need updating.