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Countries have set themselves the objective of becoming carbon neutral: their residual emissions will have to be balanced by equivalent removals through carbon sinks on their territory. While forests will obviously have a role to play, the definition of this role is the subject of intense debate. For the World Forest Day, Julia Grimault revisits the uncertainties surrounding the forest carbon sink and calls for localized and no-regrets actions to act against climate change.

Forests, a multitasking asset of carbon neutrality

Forests, and more precisely the forest-based sector, has this almost unique capacity to play on the two aspects of carbon neutrality: on one hand, it allows CO2 to be removed from the atmosphere and, on the other hand, it helps to decarbonise the other sectors.

Through the process of photosynthesis, forests are able to absorb CO2 from the atmosphere and store it in the biomass and soils. Every year,European forests absorb nearly 400MtCO2  more than they emit: this is known as the forest carbon sink, which in Europe represents nearly 11% of emissions. Wood products, especially those with a long lifespan (beams, frames…), also make it possible to store carbon for several years or decades. They extend the storage period of part of the forest carbon. Moreover, the use of wood instead of more energy-intensive materials such as concrete or steel avoids CO2 emissions in the industrial or construction sectors. We talk about material substitution. And we talk about energy substitution when wood is used instead of fossil fuels such as coal or fuel, and helps to decarbonise the energy sector.

These different levers are key, but unfortunately we cannot maximize them all at the same time… and within the same time horizon. An increase in the use of wood in construction, for example, is generally based on an increase in forest harvests. In the short term, the emissions from logging exceed the emissions avoided by substitution. In the long term, this initial carbon debt is repaid thanks to the cumulative substitution effects and reforestation efforts. Depending on the type of forest stands, pedoclimatic conditions, the type of harvest, and the use of products, the climate payback time can range from a few decades to more than a century!


The debate that is stirring society: where to draw the line between increasing harvests and doing nothing?

What can be done in the sector to contribute to the climate change mitigation? Should we do nothing and keep the carbon stocks in place? Should we harvest more to help decarbonise our economy? This debate crystallizes the oppositions.

The choice between these two strategies, conservation or harvesting, first depends, as we have seen, on the time horizon in which we want to act. Within the framework of climate policies, we are forced to think in terms of a shorter time horizon than that of the forest: while some forest stands may have rotation durations of more than a hundred years, the time horizon in which we must achieve neutrality is about thirty years. It is therefore important not to lose sight of short- to medium-term impacts, in a sector where we are accustomed to thinking in the very long term.

The trade-off between conservation and increased harvesting also depends on the anticipated impact of climate change on forests (Valade et al., 2017). If a significant impact on forest stands is anticipated (wildfires, dieback, etc.), there is a high risk that the sequestered carbon will be released back into the atmosphere. In this case, an increase in removals can be seen as a way of accelerating forest adaptation while improving the effects of storage and substitution by products. Conversely, if a lesser impact of climate change is anticipated, the conservation of existing sinks, the ‘do nothing’ strategy, may be the best option in the medium term.

The problem, once again, is that there are many uncertainties about the evolution of the forest sink. Over the past 70 years, climate change has contributed to boosting forest growth and thus the carbon sink (Bellassen, Luyssaert, 2012). However, it is not certain that this will last: by 2100, the models’ predictions diverge. Some models predict that this favourable imbalance will increase, while others anticipate a reversal of the trend and see forests becoming net emitters of CO2. Furthermore, we have all witnessed the massive fires in Brazil, Australia and North America, during which several million hectares of forest disappeared. And in France, for several summers now, we have seen significant dieback linked to a combination of droughts, pest and diseases (bark beetles, chalarose…). However, the tree could hide the forest: disasters are indeed much more visible and publicized than the diffuse “doping” effect of environmental changes.

Solutions must be sought locally.

In summary, many uncertainties remain about the forestry strategy to adopt to maximize the contribution of forests to climate change mitigation. Is this terrible news? Perhaps not that bad. Indeed, the importance of large-scale forest management change as a massive lever for climate change mitigation needs to be put into perspective. While recent French studies (Roux et al., 2017; Valade et al., 2017 ; Valade & Bellassen, 2020) agree on a  negative impact of increased harvesting over the next 30 years in most scenarios, this effect remains modest: between 0% and 4% of current national emissions. In any case, we cannot count on a strong growth of the forest carbon sink to massively offset our emissions. Emissions will have to be drastically reduced in all sectors of the economy, bearing in mind that wood will have a role to play.

Does this mean that there is nothing to be done in the forest-based sector? No, because, locally, changes in forest management can have a very positive impact on climate. For example, restoring declining stands allows us to break a forestry deadlock and simultaneously restore forest removals and the substitution of fossil fuels. Actions of this type, where forest management change confers a climate benefit before 2050, could concern several million hectares. But how can they be identified and financed?

This is precisely what “research-led” economic instruments, such as the French Low Carbon Standard and other domestic carbon standards in Europe, make possible. Open to all innovative solutions, the French standard help ensure that they have a positive climate impact by 2050 where they are implemented. The reconstitution of degraded forest stands (following storms, wildfires or intense dieback) already benefits from this instrument where it is useful. Other promising practices such as orientation towards longer-life wood products, conversion of coppice to high forest stands or silvicultural practices that preserve soil carbon could follow…

The identification of these actions goes of course far beyond carbon certification standards.  In contrast to the energy sector where renewable energy brings climate benefits in all circumstances, there are no forest wind turbines. For the forest-wood sector, it is therefore necessary to design “bottom-up” instruments that make it possible to finance management changes where they are effective and adapted to local conditions. This integration of the climate strategy in a local context is all the more important since forests also have the capacity to influence climate via biophysical effects (albedo, evapotranspiration, etc.), and also provide us with multiple services, well beyond carbon sequestration alone.

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