Swedish Study Compares Carbon Storage in Old-Growth and Managed Boreal Forests

Old-Growth Boreal Forests Store Significantly More Carbon Than Managed Counterparts, Study Finds

A recent study by researchers from Lund University and Stanford University has revealed a substantial difference in carbon storage between old-growth and managed boreal forests in Sweden.

The research indicates that primary forests can store between 72% and 89% more carbon than actively managed counterparts, with the majority of this difference attributed to carbon stored in forest soils.

These significant findings suggest critical implications for climate change mitigation strategies and current forest management practices.

Context and Background

Most natural forests in Europe have undergone extensive transformation for agriculture and managed forestry over centuries. The remaining old-growth natural forests therefore offer invaluable insights into historical forest conditions and the long-term impact of human management.

Swedish forests are predominantly boreal, a coniferous woodland ecosystem common in northern regions. Large-scale forest use in Sweden began relatively late compared to other parts of Europe, due to historically low populations.

Modern forest management in Sweden, which emerged in the 20th century, typically involves a cycle of practices. These include clear-cutting, followed by planting or sowing new trees, cleaning, thinning, and disturbing the soil through ploughing and trenching to remove water. This management cycle commonly repeats every 120 years. Globally, boreal forests constitute the largest forest biome and are estimated to absorb approximately 30% of human-caused carbon dioxide emissions.

Study Methodology

The research, initiated eight years ago, focused specifically on mapping lowland old-growth forest remnants in Sweden. Slow-growing mountain forests were deliberately excluded from the analysis to concentrate on landscapes historically used for wood production.

Scientists from Lund University and Stanford University, including lead author Didac Pascual and senior author Rob Jackson, spearheaded the study. Over three years, researchers meticulously collected samples and measured carbon levels at more than 200 forest plots across the country.

The methodology involved digging 220 one-meter deep pits to collect soil samples at various depths for subsequent laboratory analysis. Carbon content in trees and dead wood was also precisely calculated from measurements taken in the field. This comprehensive field data was then combined with information from the Swedish National Forest Inventory (NFI) and the Swedish National Forest Soil Inventory to estimate carbon storage in managed forests for comparison. Carbon was measured across multiple components: vegetation, dead wood, soils, and harvested wood products.

Key Findings on Carbon Storage

The study determined that old-growth forests store between 78% and 89% more carbon than managed forests. When all carbon pools, including carbon stored in harvested wood products, were considered, primary forests stored approximately 72% more carbon than managed secondary forests.

Excluding harvested wood products, this figure rose to 83% more carbon per acre. The findings indicated that primary forests in Sweden store 9.9 kg of carbon per square meter more than managed secondary forests, a difference reported as 2.7 to 8 times higher than earlier estimates.

Old-growth forests were found to store 87% more carbon in trees, 334% more in dead wood, and 68% more in soils compared to managed forests.

Crucially, the majority of this carbon is stored in the soils; old-growth forest soils were observed to store as much carbon as managed forests do across trees, dead wood, and soils combined. In lowland primary forests, the top meter of soils contained approximately 64% of total carbon in an average plot, compared to about 30% in live trees and 6% in dead wood.

Even when accounting for carbon stored in harvested wood products (approximately half of which is burned for energy, 25% used for paper, and 25% for longer-lasting products like houses), primary forests still contained about 70% more carbon than managed forests. The amount of carbon in dead wood in old-growth forests alone was found to surpass the combined carbon in wood products and dead wood from managed forests.

Implications for Climate Change Mitigation and Policy

The observed difference in carbon storage between old-growth and managed forests, even when accounting for harvested wood products, is profound.

This carbon storage disparity has been further quantified as nearly 8 billion tons of carbon dioxide, which is equivalent to 1.5 times all Swedish fossil fuel emissions since 1834, and also to 220 years of current Swedish fossil fuel emissions.

The study suggests that protecting existing natural forests, including those previously logged but retaining characteristics close to untouched states, could facilitate the creation of carbon sinks as large trees regrow and prevent soil carbon losses. Carbon losses resulting from forest management in Sweden were reported as larger than previous estimates. While some wood products replace high-carbon intensity materials, non-wood alternatives for heat, electricity (e.g., heat pumps, solar, wind), and construction exist.

Between 2003 and 2019, Sweden experienced a loss of unprotected old-growth forests to clear-cutting at an annual rate of 1.4%. The rate of old-growth forest loss in Sweden has been reported as five to seven times faster than that observed in the Brazilian Amazon forest.

Although EU regulation provides protection for remaining old-growth forests in Europe, the specific definition of "old-growth" is determined by individual member states. Sweden's proposed definition for old-growth, based primarily on tree age (180 years in the north, 160 in the south), has drawn criticism from conservation organizations. These groups suggest that Sweden's definition may undermine EU nature restoration goals and risk continued logging of unprotected old-growth forests. Protecting and restoring old-growth forests is considered a potential contributor to significant climate change mitigation and biodiversity benefits.

Nations frequently rely on forests to meet climate commitments, often assuming increased use of northern forests, particularly for bioenergy production. The study's findings suggest that if managed forests store less than half as much carbon as the old-growth boreal forests they replace, projections of climate benefits from forest-derived biofuels may be overestimated. Conversely, the value of protecting old-growth forests or improving forest management practices might be significantly underestimated.

Future Research

Ongoing research aims to further understand how specific forest management practices contribute to carbon storage capacity. Researchers are also working to extrapolate these results from Sweden to other boreal regions, including Canada, Russia, and Alaska. Future studies plan to investigate the crucial role of microbes in old-growth forest soils, with the ultimate goal of potentially using microbe-driven mechanisms to enhance carbon sequestration in managed forests.