The most effective way to improve the climate benefit of Swedish forests is by harvesting less. This is a brief summary of the current state of knowledge.
Markku Rummukainen, Professor of Climatology at Lund University, has compiled scientific literature and reports from authorities on the climate benefit of forests, primarily in Sweden (1).
There are two main ways to increase the climate benefit of forests. One is to increase their carbon uptake, mainly by reducing deforestation. The other is to use the wood from trees to make longer-lasting products and replace carbon-intensive products.
It is urgent that we limit global heating, and Sweden has binding EU targets to increase the carbon uptake of forests by 2030. Transitioning to new products will take much longer than that. Improving carbon sequestration, on the other hand, provides immediate climate benefit, and is the measure that will have the greatest impact in the next few decades.
These are the main conclusions of Markku Rummukainen’s synthesis, published in June 2024.
Forests act as carbon sinks. A carbon sink absorbs carbon dioxide from the atmosphere and stores the carbon for a period of time. From a global perspective the main carbon sinks are vegetation on land, especially forests, and the oceans.
Carbon dioxide is absorbed by the oceans when it dissolves in water or is used by phytoplankton during photosynthesis. Plants on land also use carbon dioxide for photosynthesis.
But trees and other plants, like most living organisms, also release carbon dioxide into the air during the chemical process of respiration. And carbon dioxide is released from the soil in forests through respiration, by the roots of trees and when dead plants decompose.
When a forest is clear-cut, it reduces photosynthesis more than it reduces respiration, so more carbon is released than absorbed. As the forest starts to grow back, carbon uptake and sequestration increase. After a while, a balance is reached and the forest becomes a net carbon sink for a long period of time.
In the mid-twentieth century, carbon sequestration started to increase as the forests of northern Europe grew. Forest policy, which mainly focused on increasing the supply of timber, contributed to this increase. The draining of wetlands to grow more trees is also a factor. But environmental factors such as nitrogen deposition and climate change have also had an impact on growth. In addition, large areas of forest had already been felled by the beginning of the twentieth century, so some of the increase in carbon uptake was simply part of recovery.
Over the last decade, the annual carbon sequestration of forests in the EU has actually decreased, partly due to increased harvesting. Between 2000 and 2021, EU deforestation increased by around 25 per cent. Net sequestration has also been impacted by reduced reforestation and growth, while forests have been hit by storms, fires and bark beetle infestations.
Markku Rummukainen’s research synthesis shows that the most obvious course of action to reverse this trend is to reduce deforestation.
In Sweden, 97 per cent of productive forest land is managed by clear-cutting.
It is widely accepted that clear-cutting results in net carbon emissions for a number of years after felling. Estimates of how many years emissions remain higher than uptake vary somewhat. The figure also varies from stand to stand. However, current studies show that it is a matter of decades rather than a few years.
One of the studies cited in the synthesis covers five stands in southern Sweden that were clear-cut or wind-felled (2). They produced net carbon emissions for about 10 years after felling, and it took around 20 years for total carbon uptake to balance net emissions.
Another study from northern Sweden covered around 50 stands. In this case it took an average of 18 years for net emissions to be balanced by carbon uptake. However, this time period may be greatly underestimated because the calculations omitted data for the first year after felling (3, 4), when net emissions are highest.
Carbon uptake can also be improved by thinning trees less intensively, protecting areas of forest and allowing trees to grow older before felling.
Until recently there was an incomplete understanding of the balance between carbon dioxide uptake and emissions in really old forests. But the synthesis notes that several studies have now shown that forests remain carbon sinks long after they have passed the age when they are usually felled.
One study showed that the uptake of over 1000 Norwegian coniferous forest stands continued to rise for 50–100 years after the usual felling age (5). Another showed that forests aged between 130 and 200 years old in the north-east US and south-east
Canada remained a significant carbon sink (6). In Sweden, the average final felling age for the country as a whole is just under 100 years.
The second way to increase the climate benefit of forests, alongside reducing deforestation, is to use the harvested wood in a more carbon-efficient way.
Only a fifth of the wood harvested in Sweden is used to make durable wood products. The rest is used to generate bioenergy and make pulp, paper and cardboard which, even when paper is recycled, is burned after a relatively short time, returning carbon to the atmosphere.
Substitution means that wood that is used to make long-lived or short-lived products that replace fossil-intensive materials or fossil energy. For example, cement can be replaced with timber building materials. Coal, oil and fossil gas can be replaced by biofuels.
The climate benefits of substitution are estimated by calculating a “substitution factor” (or displacement factor). This factor is based on a number of assumptions, such as which products are being substituted, how they would be produced and their life-cycle emissions. The climate benefits of substitution should also take into account the value of leaving the carbon sink in the forest instead of extracting the wood at all, as well as alternative uses for the wood.
Different studies are based on different assumptions and it is not always clear which assumptions were made. This makes it difficult to compare studies. However, the research synthesis notes that the substitution factor is generally always positive, which means that when fossil-intensive materials and fossil energy are replaced with wood, some emissions are avoided. But it should also be noted that the substitution factor tends to be lower in more recent studies than in older studies.
It is also uncertain how the estimated climate benefit of a given substitution will hold up in a changing future. It could drop considerably if new production methods and materials are developed, recycling increases, energy systems change or material use decreases.
Overall, the research shows that at current levels of wood usage, greater climate benefit can be achieved by increasing carbon uptake in forests than by increasing felling. The latter would require a large and rapid change in the way that wood is used, which is unrealistic within the next few decades. On the other hand, reducing deforestation would provide an immediate increase in climate benefit. Nevertheless, switching to new products can make a significant contribution to increasing the climate benefit of forests and is therefore an important measure.
The synthesis does not directly address the issue of forest resilience, i.e. resistance to climate-related damage. However, some studies are mentioned that touch on this subject. There are many indications that forest damage has increased in Europe in recent years, and that this is at least partly due to climate change. For example, damage has been caused by bark beetle infestations, storms, forest fires and extreme drought. These lead to increased emissions and reduced carbon sequestration. To maximise climate benefit it is therefore important to use forestry methods that increase resistance to damage. Influential factors include felling method, tree density, species selection, planting material and early detection of pests.
Finally, Markku Rummukainen concludes that it is entirely possible to increase the climate benefits of forests in ways that benefit both the forest owner and society, and hence get closer to climate goals. However, this will require changes in forestry and the forest industry. This in turn requires political decisions, economic and legal instruments and awareness-raising.
“It is reasonable that forest owners should be given the opportunities to improve the climate benefits of forests, for example by receiving compensation for measures that increase carbon uptake. This would also benefit Sweden as a whole by increasing the potential for us to meet our climate commitments, while providing other important benefits in terms of value from the forest,” commented Markku Rummukainen in a press release from Lund University in connection with the publication of the synthesis.
Karin Bergqvist
science writer/journalist
Sources
- Rummukainen M (2024). Forest climate benefits 2.0 – Climate transition next. CEC Synthesis No 8. Centre for Environmental and Climate Science, Lund University.
- Grelle, A. et al. 2023. Agricultural and Forest Meteorology. doi:10.1016/j.agrformet. 2022.109290
- Peichl, M. et al. 2023b. Global Change Biology. doi:10.1111/gcb.16772
- Lindroth, A. 2023. Global Change Biology. doi:10.1111/gcb.16771
- Stokland, J.N. 2021. Forest Ecology and Management. doi:10.1016/j.foreco.2021.119017
- Thom, D. et al. 2019. Global Change Biology. doi:10.1111/gcb.14656
