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Difficult to reach consensus on how climate-smart it is to manage forests

Last changed: 29 March 2023

Should harvesting Swedish of forests increase or decrease for the sake of the climate? That is a question several scientific studies in recent years have examined. However, these studies came to different conclusions. Here is a review of some them, which all used information from the Swedish National Forest Inventory and Heureka, a forest decision support system. Some of the studies argue that harvesting should decrease, while others argue that it is better for the climate to use forests more intensively. Why do the conclusions differ?

To understand this, it is important to know the three main aspects examined in the studies.

The first aspect is the ability of forests to absorb carbon dioxide from the atmosphere via photosynthesis and store it in the form of carbon in the growing treas and soils.

The second aspect is the wood products made from trees. This is important to study because the wood products store the carbon previously absorbed by the growing trees. Depending on which products are made from wood, the carbon is stored for longer or shorter time. The longer the lifespan of the wood product, the longer it takes for the carbon to be released back into the atmosphere in form of carbon dioxide. For example, wood-based construction materials in houses have a long lifespan, while paper-based products or wood used for energy production have a short lifespan.

The third aspect is the extent to which wood products reduce the use of or replace fossil fuels and fossil-intensive products, and how this affects the greenhouse gas balance in the athmosphere. This replacement of products or energy is called substitution and its climate impact is called substitution effect. If a wood product emits less fossil greenhouse gases into the athmosphere than its non-wood counterpart, the substitution can lead to lower emissions and vice versa become a climate burden if the wood product emits more than its non-wood counterpart.

All the studies mentioned here examined all three aspects and their greenhouse gas balance. For two of these aspects, namely (aspect 1) the carbon sequestration capacity of forests and (aspect 2) the carbon storage capacity of wood products, the studies tend to find similar results. That is, the loss of stored forest carbon from harvest is greater than the gain from storing carbon in wood products.

Three simple illustrations of the different aspects. Aspect 1 a tree binding carbon. Aspect 2 a wooden house that store carbon over a longer period time and toilet paper that binds for a shorter time. Aspect 3 a wooden house that store carbon and emits less carbon than its counterpart, a concrete building.

Aspect 1: Trees bind carbon in forest and ground. Aspect 2: Forest products containing carbon with long and short lifespan. Part 3: Wood products that replace usage of fossil material; substitution.

Different assumptions regarding the use of wood products explain the difference
When it comes to aspect 3 however, different assumptions about the substitution effect, i.e., if and to what extent wood products substitute fossil fuels and products, make the studies differ on whether more or less harvesting is good for the climate. On the one hand, this is because there is a multitude of ways to use wood as a product or energy. On the other hand, it is important which fossil product is replaced and which management method was used to get the wood from the forest. For example, forestry with longer rotation times tends to produce larger higher and thicker trees that the construction industry can use more extensively to replace steel and concrete. This in turn leads to less pulpwood and products made from pulpwood, such as toilet paper.

Large uncertainty about the amount of fossil products replaced by wood

A more fundamental question is the amount of wood that replaces fossil products and energy. One option is to assume that all harvested wood leads to the replacement of fossil products as it is supplied to an industry engaged in the production of various wood products and wood-based energy. However, this is a very rough estimate and may lead to overly optimistic substitution effects. A more nuanced approach is instead to calculate the substitution effect only for the final wood products. Then typical "product pairs" can be matched, e.g., paperboard-based vs. petrol-based packaging, which gives more realistic comparisons. Moreover, this approach accounts for wood losses along the value chain, for example the fact that only half of the timber becomes sawn wood products, since by-products end up as energy or pulpwood. Moreover, if a product, such as writing paper, does not have a fossil counterpart or equivalent, then there is no substitution effect. However, even with this more nuanced approach, there is a wide variation in substitution effects depending on which "product pairs" are used and how carbon intensive the fossil products are. The carbon footprint of a fossil product can differ widely depending on which geographical region it originates from, e.g., if the product is produced using fossil or renewable energy.

To conclude, both approaches estimate substitution effects only in a limited way and thus cannot provide clear advice on the most climate-friendly forest management strategy. Both approaches assume that substitution effects arise solely from the supply of harvested wood or wood products and thus follow a "supply" perspective. However, here it is unclear whether wood products actually always replace an opposing fossil product, or whether wood products instead merely complement the market. After all, this is not determined by the supply of products, but by demand.

Demand for wood a better starting point?

An alternative approach is to use the demand for wood products to estimate substitution effects. How much and which wood products will be demanded in the future? Here the analysis does not start with the forest and how much is harvested, followed by the production of wood products, which ultimately leads to substitution. Instead, the direction is the reverse and the analysis starts with an explicit "product pair", e.g., wood frames instead of concrete and steel frames as load-bearing elements in buildings. This means that the substitution effect per product unit is clear from the outset but a possible future scenario is assumed, e.g., an increase in demand for timber frame. The demand for wood in the scenario assuming an increased use of timber frame is then compared with a reference scenario corresponding to the current situation. Thus, it is possible to compare the increase in carbon storage in wood products with the decrease in forest carbon due to increased harvesting (i.e. the biogenic carbon balance) and the impact of substitution on the fossil greenhouse gas balance. This approach thus solves the problems of the "supply” perspective mentioned above as the ability to substitute fossil products and energy with wood alternatives is ultimately dependent on demand decisions. However, the application of this alternative approach following a "demand" perspective still remains an exception.

Time decides what is climate-smart now and later

What the above reviewed studies agree on is that in the short term it is more climate-smart to leave more forests standing, but that in the long term it can be more climate-smart to actively manage forests. When the tipping point for this occurs depends on supply, demand and on how much both the forest- and the other fossil-based industries reduce their emissions.

This article is only concerned with carbon sequestration, storage in wood products and the substitution effect, but there are many other influencing aspects affecting the climate, such as how much sunlight is reflected by the forest. The question of forests and the climate benefits of forestry is an important and complex one with no easy answers. The climate benefits of forests also need to be balanced with other societal goals such as biodiversity, wood product and energy needs and the social values of forests.

Text: Jeannette Eggers, Maximilian Schulte


Studies that the article is based on

Gustavsson, L., S. Haus, M. Lundblad, A. Lundström, C. A. Ortiz, R. Sathre, N. Le Truong, and P.-E. Wikberg. 2017. Climate change effects of forestry and substitution of carbon-intensive materials and fossil fuels. Renewable and Sustainable Energy Reviews, Vol. 67: 612–24. doi: 10.1016/j.rser.2016.09.056. 

Gustavsson, L., T. Nguyen, R. Sathre, and U. Tettey. 2021. Climate effects of forestry and substitution of concrete buildings and fossil energy. Renewable and Sustainable Energy Reviews, Vol. 136: 110435. doi: 10.1016/j.rser.2020.110435. 

Lundmark, T., J. Bergh, A. Nordin, N. Fahlvik, and B. C. Poudel. 2016. Comparison of carbon balances between continuous-cover and clear-cut forestry in Sweden. Ambio, 45 Suppl 2: 203–13. doi: 10.1007/s13280-015-0756-3. 

Petersson, H., D. Ellison, A. Appiah Mensah, G. Berndes, G. Egnell, M. Lundblad, T. Lundmark, A. Lundström, J. Stendahl, and P.-E. Wikberg. 2022. On the role of forests and the forest sector for climate change mitigation in Sweden. GCB Bioenergy, Vol. 14(7): 793–813. doi: 10.1111/gcbb.12943. 

Schulte, M., R. Jonsson, T. Hammar, J. Stendahl, and P.-A. Hansson. 2022. Nordic forest management towards climate change mitigation: time dynamic temperature change impacts of wood product systems including substitution effects. Eur J Forest Res. doi: 10.1007/s10342-022-01477-1. 

Skytt, T., G. Englund, and B. G. Jonsson. 2021. Climate mitigation forestry—temporal trade-offs. Environmental Research Letters, Vol. 16. doi: 10.1088/1748-9326/ac30fa.