Sedimentary charcoal pattern in a karstic underground lake (Vercors massif, Alps, France) : implications for paleo-fire history

International audienceKnowledge on processes of charcoal transportation is crucial for fire reconstruction based on sedimentary charcoal. Charcoal is susceptible to long-distance transport by water. A lake basin with a large and long catchment area is likely to accumulate charcoal from many fires, not only those produced by fires nearby the lakeshore. Here we test the potential of charcoal transportation by analysing sedimentary charcoal accumulated in an underground lake within a karstic massif. Fires cannot spread around the lake, nor within the karstic massif. Organic materials, including charred particles, are generated several kilometres from the lake on the karstic plateau above. The pattern of sedimentary charcoal shows that the underground lake records continuously produced charcoal by wild fires or human-made biomass burning (slash-and-burn, charcoal kilns) over centuries and millennia, but also stored charcoal from eroded soils. Although the charcoal series shows a certain high variability signal, fire frequency reconstruction cannot be performed owing to chronological uncertainties. The charcoal accumulation corresponds to a more or less regular background input. Such background input is empirically well described in palaeo-fire reconstruction, but was never experimentally displayed. This study provides evidence that the pattern (surface, length, slope, etc.) of catchment areas is crucial for interpreting sedimentary charcoal series. Large catchment areas draining long rivers are not suitable for high-resolution and spatially precise fire reconstructions

Spatial variability of fire history in subalpine forests: From natural to cultural regimes

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Holocene Soil and Landscape Dynamics Reconstructed by Sediment Analysis in the French Alps.

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Control of the multimillennial wildfire size in boreal North America by spring climatic conditions

International audienceWildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire–climate–vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca. 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America

Local versus regional processes: can soil characteristics overcome climate and fire regimes by modifying vegetation trajectories?

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Paléostructures de végétation à la limite supérieure des forêts dans les Alpes françaises internes.

International audienceLa comparaison de six diagrammes polliniques des Alpes françaises permet de reconstituer les changements passés de la structure de la végétation à la limite supérieure de l'étage subalpin. Des peuplements denses de Pinus cembra s'y sont développés entre 6500 et 2400 cal. BP, à la fois dans les Alpes du Sud et du Nord. Les Alpes du Sud semblent toutefois caractérisées par des limites altitudinales plus élevées, comme en témoigne le développement passé de sapinières à 2080 m dans la vallée de l'Ubaye. Cette étude souligne l'importance de la prise en compte des paramètres locaux dans les synthèses régionales ou continentales