Moss-covered conifers and fallen deadwood in Sweden's boreal forests confirm that old-growth forests hold more than three times as much carbon in deadwood as managed forests do. That is the conclusion reached by a group of Swedish forest scientists after carrying out a wide-ranging study across the country's forest areas.

Most of Europe's original natural forests have been converted into farmland and managed forests that supply energy, paper and timber. The few remaining areas of "old-growth" natural forest are relics of the past that illustrate what forests would look like without human intervention. They can tell us how people have transformed forests.

Most of Sweden's forests are what are known as boreal forests. This type of coniferous forest ecosystem covers most of the planet's northern regions. These relatively cold regions have historically had small populations, and large-scale forest use began comparatively late.

In Sweden, modern forest management methods appeared in the 20th century. They involve felling most of the trees in a given area — what we call clear-cutting — followed by planting and sowing new trees, tending and thinning, until the mature trees are felled again and the cycle continues. Soil preparation and the digging of trenches and ditches to drain water away from forests are very common practices.

After mapping and measuring Sweden's naturally old forests, we found that they differ from managed forests far more than previously thought, even where some of the managed forests looked old. We discovered that old-growth forests store 78-89% more carbon than managed ones. The difference in carbon storage is greater than Sweden's total emissions from burning fossil fuels since 1834. Our study highlights carbon storage benefits from protecting forests that are far greater than those from using them for bioenergy and wood products.

Eight years ago we began mapping naturally old lowland forests across the country, focusing on the "remnants" of old-growth forest in the areas least attractive for agriculture and forest management. We did not include the slow-growing forests of the Swedish mountains, which store less carbon than the lowland forests used for timber production. For three years we collected samples and measured the carbon content of old-growth forests, comparing them with samples from managed forests. Soil is difficult to study. It stores enormous amounts of carbon, and measuring it is complicated. The main methods for measuring soil carbon have not changed over the past century. We dug 220 pits up to one metre deep and took samples at various depths across the country.

We analysed these soil samples in the laboratory and, on the basis of our measurements, calculated the carbon content of trees and deadwood. We used the extensive Swedish National Forest Inventory (a database compiling the results of annual sample surveys) to calculate carbon stocks in managed forests, which allowed us to compare carbon stocks.

Managed forests are losing carbon

We found an enormous difference in carbon stocks between old-growth forests and managed forests. Old-growth forests store 87% more carbon in their trees, 334% more in deadwood and 68% more in the soil than managed forests. In total, then, Sweden's boreal old-growth forests hold 83% more carbon than managed forests. Most of the carbon is stored in the soil. Old-growth forests store as much carbon in the soil as managed forests store in trees, deadwood and soil combined.

The methods we use to compare old-growth forests with managed forests show the total amount of carbon accumulated in forests over time. This means the differences may be due to carbon losses in managed forests or to greater carbon uptake in old-growth forests. We also took into account how the timber harvested from managed forests was used in wood products — in house building, for example, where it might not reach the atmosphere or drive climate change over the coming decades.

In Sweden, roughly half of the harvested timber or biomass is burned for heating and electricity generation, and about 25% is used to make paper. Only around 25% ends up in products with a relatively long lifespan, such as houses, where it can build up a substantial carbon stock over time. Even counting the carbon in all these products, old-growth forests still stored about 70% more carbon than managed forests. In fact, there is more carbon in the deadwood of old-growth forests than in these wood products and the deadwood of managed forests combined.

Why does this matter?

Carbon losses from forest management in Sweden are far greater than previously estimated. The difference in carbon stocks between old-growth forests and managed forests (including harvested wood products) is equivalent to all of Sweden's fossil fuel emissions since 1834. Of course, if wood products were not used, other materials would be used instead, some of which could be carbon-intensive (steel, for example), which makes the overall greenhouse gas impact hard to assess. There are now many alternatives for heat and electricity that do not involve wood (heat pumps and solar and wind power, for example).

There are extensive areas of natural forest where the largest trees were felled many decades ago; the condition of such forests is likely to be much closer to undisturbed old-growth forest than to the average managed forest. Protecting these forests will create carbon uptake, as the large trees will grow back, and will prevent the soil carbon losses caused by forest management. We have reported previously on the continuing decline of old-growth forests in Sweden. The process is happening five to seven times faster than in Brazil's Amazon region.

EU regulation currently protects all remaining old-growth forests in Europe, with the definitions of such forests left to member states. The definition of old-growth forest proposed in Sweden is based solely on the age of the trees. We believe it is not sufficiently grounded in science and sets a very high threshold: 180 years in the north of the country and 160 years in the south.

Sweden's proposed definitions have been sharply criticised by nature conservation organisations, because they undermine the aim of the EU Nature Restoration Regulation to protect all remaining old-growth forests. If the proposed definition stands, only a small fraction of the remaining unprotected old-growth forests will be protected, and logging in them will most likely continue.

Protecting and restoring Sweden's old-growth forests for carbon storage and the conservation of biodiversity could make a substantial contribution to limiting climate change.

*Anders Ahlström, Associate Professor, Department of Physical Geography and Ecosystem Science, Lund University; Didac Pascual, Postdoctoral Researcher, Department of Earth and Environmental Sciences, Lund University; and Pep Canadell, Chief Research Scientist, CSIRO Environment; Executive Director, Global Carbon Project, CSIRO. This article is republished from The Conversation under licence.