Management or protection?

© Lars-Erik Håkansson

The fate of the vast boreal forest belt of the northern hemisphere is crucial for global climate. A new AirClim-report looks into our possibilities to protect and manage these forests for climate mitigation. Reducing paper consumption turns out to be an option.

A new report presents an overview of the relationships between boreal forests, forestry and climate change. It looks exclusively at climatic aspects, which does not mean that other aspects, such as socioeconomics or biodiversity, are seen as less important. The sole reason for this limitation is the wish for clarity.

A central assumption is that +2°C is a critical threshold for global warming, and that severe reductions in greenhouse gas emissions are needed over the next few decades to avoid exceeding this threshold. Analysis of the importance of boreal forests and the effects of various courses of action should therefore not be limited to a long-term perspective (100 years or more).

The amount of carbon stored in boreal forests is greater than that of any other land ecosystem, and almost twice that stored in tropical forests. This huge accumulation of carbon makes the boreal forest a key factor in future climate.

About half of the world's boreal forests are old-growth forests, mostly or entirely unaffected by forestry. They represent a very large share of the total carbon stored. These forests could continue to act as carbon sinks for centuries (figure). However, continued global warming could transform old-growth boreal forests into a source of carbon source as the result of an increase in natural disturbances, such as fires and insect infestation. We can already see clear trends in this direction. If warming exceeds a critical level (3–5°C is suggested) heat stress and water scarcity could lead to widespread forest death in the boreal region. A large proportion of the stored carbon will then be released into the atmosphere, further driving global warming in an irreversible and self-sustaining process.

Figure: How stored carbon changes with time in an Alaskan black spruce forest. Carbon stored in biomass reaches a peak when the forest is around 200 years old, while the total carbon stored continues to rise even after 500 years, thanks to carbon uptake by the soil. The graph is based on estimates and shows idealised trends. (After Kasischke et al 1995).

Logging or managing these forest in order to avoid such a development is not an option. Turning old-growth boreal forest into managed forest has a negative impact on climate in the short and medium term, as some of the vast amount of stored carbon is released into the atmosphere during harvesting. It takes a long time – 100 years or more – for new forests to bind the corresponding amount of carbon, which means that the felling of old-growth forest further accelerates global warming when seen in this short-term perspective.

In managed boreal forests, there are greater opportunities to influence the movement of greenhouse gases through forest practices and use of the harvested biomass. Once again it is important to take into account the need for rapid restrictions on emissions when we weigh up alternatives course of action. A solution that may appear optimum over a span of 100 or 200 years may be counterproductive when seen in the light of what we need to achieve in the next few decades.

More intensive forestry, which enables higher production and higher yields through widespread fertilisation, for example, poses risks to the climatic. Aside from the risks, it is not clear that these alternatives are positive, particularly in the short term. For example, if we were to start the widespread uprooting of stumps in Swedish forests to increase the yield of forest fuel it would create a carbon deficit – in other words be negative from the climate viewpoint – for at least the next 30 years.

Interesting opportunities are offered by forest management strategies that have other goals than maximising production and timber yield. Extending rotation periods in Scandinavian forestry has, for instance, been shown to have positive climatic effects, particularly in spruce forests, even after taking into account substitution effects (see below). This is mainly due to an increase in the sawn timber share of yield. Eliminating clear-cutting would also have immediate positive effects in relation to the impact of forestry on climate, since clear-cutting creates carbon sources

Wood products can replace fossil fuels, both directly through combustion and indirectly, by replacing materials with high embodied energy, such as steel and concrete. Over time, the estimated substitution effects can be high, since the eliminated emissions are cumulative with each forest generation. In the short term (a few decades), it is however questionable whether the climatic benefits of substitution justify investing in increased forest logging or production.

In substitution studies it is often assumed that increased timber yield is used to substitute for materials or fuel, or a combination of both. This differs markedly from the actual situation in boreal forestry. In Sweden, less than one fifth of the timber yield is used for long-lived structural timber, and about half is used for papermaking. The manufacture and use of paper is on the whole negative from a climate perspective (although the effects are likely to differ between various paper products). Reducing consumption of paper and using more of the harvested wood for timber and fuel would thus benefit the climate.
 
Roger Olsson

To manage or protect? Boreal forest from a climate perspective, AirClim report #26 by Roger Olsson