Hot weather in cold forests

In recent years, millions
of hectares of boreal forests have been ravaged by major wildfires, and scientists are wondering how
these ecosystems will evolve under the impact of global warming. Knowing that
the evolution of this gigantic natural carbon sink will have repercussions
on the earth's climate system.

Large fires are nothing new. For millennia, these fires have performed essential functions in boreal forests, promoting tree regeneration and nutrient cycling. But over the past decade, the scale and frequency of these fires have increased. In Canada, several million hectares of boreal forest burned in the Northwest Territories in 2014, then in British Columbia in 2017 and 2018. In 2023, the country will set a new record, with more than 18 million hectares up in smoke. Record fires are also ravaging the boreal forests of Siberia. In the summer of 2021, more than 18 million hectares of taiga burned down(Les forêts froides brûlent, 22/07/2021, Université de Montpellier). These mega-fires are affecting the health of boreal forests throughout the Northern Hemisphere, and increasing carbon dioxide emissions into the atmosphere. Scientists are seeking to anticipate how these boreal ecosystems will evolve under the impact of global warming.

Look in the terrestrial archives

"Cold forests account for a third of the world's forests. The future of this enormous carbon reservoir, accumulated mainly in peatlands over several millennia, will have repercussions on the planet's climatic equilibrium", emphasizes Adam Ahmed Ali, of the Montpellier Institute of Evolutionary Sciences(ISEM).¹. The paleoecology researcher has joined the international Cold Forests network, based at the Université du Québec en Abitibi-Témiscamingue. Paleoecology allows us to look into the earth's archives to see how climate has influenced these ecosystems over the millennia. Looking into the sediments at the bottom of the great lakes of boreal Quebec, Adam Ahmed Ali and his team measured the quantity of charcoal for each climatic period of the Holocene. And their results - published in the journal PNAS in 2012 - are counter-intuitive.

"We would have expected to find large fires, especially during the climatic optimum of the Holocene, 6,000 years ago, when the climate was warmer and drier than today (+2°C). On the contrary, they are concentrated during the 'cold and wet' period of the Holocene, i.e. at -4,000, during the Neoglacial period", explains the researcher. This pattern can be explained by recurrent episodes of exceptional spring and summer drought around the year 1000, during the Medieval Warm Period. The increased occurrence of large forest fires during this climatic redoubling was recorded at various locations in Canada's boreal forest. "These data show that the processes controlling fire regimes, particularly large forest fires, are complex. Does the year 2023 foreshadow conditions similar to those recorded during the medieval warming anomaly?"

The chicken-and-egg paradox

Many uncertainties remain before such a scenario can be confirmed. Firstly, the speed of warming is unprecedented, and "an average increase in annual temperatures of 3 to 4°C, as suggested by certain predictions, has never been recorded in the sedimentary archives of the Holocene", explains Adam Ahmed Ali. In addition, precipitation trends are uncertain (Climate determined the fire cycle and humans influenced fire frequency). Climate models are unable to predict whether boreal regions will receive more or less rainfall than they do today. More generally, climatologists still have more questions than answers about the future climate of the Anthropocene era, such as the frequency and scale of extreme events, or the evolution of ocean currents, which are powerful climate regulators.

Another interesting fact to consider, according to the researcher, is the increase in flammable conifers such as black spruce (Picea mariana) and jack pine (Pinus banksiana), during the cold Neoglacial period (An 8500-year history of climate-fire-vegetation interactions in the bioclimatic domain of black spruce and eastern Maritime moss). Was it the climate that changed the vegetation that subsequently changed the fire regime, or was it the fire regime that changed the vegetation? " The famous chicken-and-egg paradox," sums up Adam Ahmed Ali. The nature of tree species is indeed a crucial question in understanding the evolution of fires. In particular, the proportion of hardwoods in the boreal forest, which are less sensitive to fire. However, warming will allow hardwoods to migrate from temperate to boreal forests, and should therefore lead to a reduction in forest fires. While these factors make it difficult to anticipate the long-term scale of forest fires, scientists are unanimous in saying that major fires will become more frequent in the decades to come.

1. Isem (CNRS, UM, IRD)
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