Most of Europe’s original natural forests have been transformed for agriculture and managed forests producing energy, paper and timber. The few remaining “old-growth” natural forests are relics of the past that illustrate how forests would have looked in the absence of human management. They can, therefore, tell us how people have transformed forests.

Most Swedish forests are so-called boreal forests. This type of coniferous woodland ecosystem encompasses most of the northern regions of the planet. These relatively cold regions have historically had low populations. Here, large-scale use of forests began relatively late.

In Sweden, modern forest management emerged in the 20th century. It involves cutting most trees in an area – clear-cutting – followed by planting and sowing of new trees, cleaning and thinning until the trees are clear-cut again up to 120 years later. The soil is also disturbed. It is very common to plough the soil and excavate trenches and ditches to remove water from forests.

After mapping and measuring the most natural old-growth forests in Sweden, we found that they differ much more from managed forests than previously thought, even if some of those managed forests looked old.

We found that old-growth forests store 78-89% more carbon than managed forests do, a difference in carbon storage larger than Sweden’s cumulative emissions from the combustion of fossil fuels since 1834. Our new study underscores the much larger carbon storage benefits that flow from protecting forests than using them to produce bioenergy and wood products.

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Read more:
Sweden has vast ‘old growth’ forests – but they are being chopped down faster than the Amazon

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Eight years ago, we started mapping the most natural lowland old-growth forests across the country. We focused on old-growth forest remnants in the least attractive areas for agriculture and forest management. We excluded these because they are usually slow-growing mountain forests and store less carbon than they would in the broader landscapes used for wood production. We then spent three years collecting samples and measuring the carbon content of the old-growth forests and compared with that of managed forests.

Soils are difficult to study. They store vast amounts of carbon but measuring that is difficult. The main methods to measure soil carbon have not changed in the last century. We dug 220 pits up to one-metre deep and took samples at different depths from across the country.

We analysed those soil samples in a lab and calculated carbon content in trees and dead wood from our measurements. We used the vast Swedish national forest inventory (a database collating annual sample-based survey results) to estimate carbon storage in managed forests and could then compare their carbon storage.

Managed forests are losing carbon

We found a huge difference in carbon storage between old-growth and managed forests. Old-growth forests store 87% more carbon in the trees, 334% more in dead wood, and 68% more in the soils than managed forests do. Overall, this amounts to 83% more carbon in old-growth forests than managed forests in Sweden’s boreal forests.

Most of that carbon is stored in the soils. Old-growth forests store as much carbon in their soils as the managed forests do in trees, dead wood and soils combined.

Our methods of comparing old-growth to managed forests show the sum of the total carbon accumulated in forests over time. This means the differences can be due to the loss of carbon in managed forests or a larger carbon uptake in old-growth forests.

We also took into account how the wood extracted from managed forests was used as wood products (for example, to build a house), which might not reach the atmosphere and produce climate change for decades to come.

In Sweden, around half of the harvested wood (or biomass) is burnt for heating and electricity production, around 25% is used for paper, and only around 25% ends up in products with relatively long lifetimes, such as houses, where they can form a sizeable storage over time.

When including carbon in all these products, primary forests still stored about 70% more carbon than managed forests. Actually, there’s more carbon in dead wood in the old-growth forests than in these wood products and dead wood in managed forests combined.

Why losing old-growth forest matters

The losses of carbon from forest management in Sweden are much larger than previously estimated. The difference in carbon storage between old-growth and managed forests (including harvested wood products) is equivalent to 1.5 times all Swedish fossil fuel emissions since 1834, or 220 years of Sweden’s fossil fuel emissions at current levels.

Of course, if wood products had not been used, other materials would have been used instead, some of which may have high carbon intensity (such as steel). This makes it difficult to estimate the overall effect on atmospheric greenhouse gases. However, there are now plenty of non-wood alternatives for heat and electricity (heat pumps, solar and wind energy, for example).

There are also vast areas of natural forests where the largest trees were logged many decades to a century ago, and they are likely in a state much closer to an untouched old-growth forest than an average managed forest is. Protecting these forests will, therefore, lead to a carbon sink as the large trees grow back, and avoid soil carbon losses from management.

We have previously reported on the ongoing loss of these old-growth forests in Sweden – a loss that is five to seven times faster than the loss of the Brazilian Amazon forest.

EU regulation currently protects all remaining old-growth forests in Europe, but definitions of old-growth forests are left to the member countries. In Sweden, the proposed definition of old-growth forest is based only on tree ages. This definition is not well anchored in science and sets a very high bar: 180 years in the north of the country and 160 years in the south.

These proposed Swedish definitions have been heavily criticised by conservation organisations for undermining the ambition of the EU nature restoration regulation to protect all remaining old-growth forests. If the proposed definition stands, little of the remaining unprotected old-growth forest will be protected and their logging will likely continue.

Protecting and restoring old-growth forests for carbon storage and biodiversity benefits can significantly contribute to limiting climate change in countries like Sweden.