Droughts, fires… Forests could absorb less carbon

Due to the drought, in the Vosges forest (here in Sewen, in 2019), fir trees and spruces are dying.

Forests are the safest way to store carbon today. By absorbing large amounts of atmospheric carbon through photosynthesis, their role as carbon sinks is earning them increasing attention in the name of nature-based solutions. But a study published in Science September 2 shows that, disrupted by climate change, forests may well not play the role expected of them.

To assess the risks facing the world’s forests in XXIe century, the international study combines the results of several scientific approaches, from modeling carbon flows to direct observations of the evolution of forests. The synthesis shows that some approaches are more optimistic than others. Results on carbon storage [à partir de modèles basés sur les échanges de carbone] are probably overly optimistic. Methods based on satellite observations tend to show greater risks »comments the study’s first author William RL Anderegg, a researcher at the University of Utah.

The American researcher explains to Reporterre that the former overstate the benefits of increased atmospheric carbon. A higher concentration of CO2 promotes plant growth, and thus helps to increase carbon storage. But on condition that the forests are in good condition. However, droughts, heat peaks and fires directly threaten the world’s forests. Damage poorly taken into account by these first models.

A significantly higher level of uncertainty »

Other approaches, on the other hand, look at how forests will evolve under the effect of warming and the increase in extreme events. Scientists know, for example, that the change in the biological rhythm of plants, with longer growing seasons which contribute to increasing water stress, weakens vegetation. The disappearance of certain species is expected in regions where the climatic conditions will become too far from their current habitat. A study published in Nature last February thus showed the evolution of the distribution of forest species on the planet under the effect of global warming.

The drier regions of the Amazon are at high risk. Here in the region of Manaus (Brazil), in 2005. Wikimedia Commons/CC BY 3.0/James Martins

Another key finding of the study Sciencethe sometimes contradictory results of the models. The level of uncertainty — especially between different approaches — is significantly higher than I expected »recognizes William RL Anderegg. A conclusion which shows that knowledge to predict the consequences of climate change on forests remains incomplete.

However, some regions—the North American forests, the drier parts of the Amazon, and the southern boreal forests—are at particularly high risk over the coming decades, regardless of methods and scenarios. These areas, already located at the edge of large distribution areas, are particularly vulnerable to global warming. Western North American forests are particularly dry, which has already led to significant tree mortality as a result of climate change »underlines the researcher.

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