Wild forest fire danger not limited in climate variations

A wildfire is a fire in an area of combustible vegetation that occurs in the countryside or rural area. These are common in summer in the temperate boreal forest, rarer at high altitudes, and unheard of in an ice age at high altitudes, until now.

According to a new study, evidence of wildfires dating back 20,000 years were discovered in the heart of the French Alps, 2,240 metres above sea level.

A second area located further north, in the Massif de la Vanoise (Auvergne-Rhone-Alpes) where sediment accumulated during the Last Glacial Maximum, showed no trace of vegetation.

No evidence of wildfires was found in this area, because without vegetation to burn, fire couldn't spread.

Blarquez said, "This discovery is not trivial."

Blarques further added, "It echoes the recent wildfires in the Arctic tundra, where [the presence of] trees have become increasingly common. The situation has drawn the attention of the scientific community because of its significant impact on the [Earth's] carbon cycle. Changes in high mountain forest cover due to global warming, and especially the abandonment of agricultural land, risk exacerbating the spread of wildfires in the coming years."

During research, Blarquez and Carcaillet reconstructed wildland fire frequency and forest composition of over past 20,000 years, including the Last Glacial Maximum, when ice sheets were at their greatest extension.

The study suggests that there was a tree glacial refugium during this period, when wildfires were able to break out.

Also, they consider the complex long-term interactions between fires, vegetation and climate.

Carcaillet, co-director of the Laboratoire international associe franco-canadien MONTABOR shared, "Wildfires spread when fuel is available and the climate is dry."

"It is therefore counterintuitive to imagine wildland fires in periglacial, subpolar or mountain areas. And yet [the discovery of] high mountain lacustrine sediments revealed just that. Wildfires were indeed rare, but the presence of wood charcoal confirmed that they did occur, even during glacial and postglacial periods."

In the Queyras, Swiss stone pines and larches formed an isolated tree glacial refugium, "like an island in the middle of an ocean of ice," according to the study.

Carcaillet noted, "This study demonstrates that a periglacial climate does not preclude wildfires."

Continuing, "Trees - in this case, Swiss stone pines - are necessary for fires to burn in high mountains. The climate affects the frequency of fires, and in return, fires affect tree diversity."

The study was published in journal New Phytologist.