Fire regime vegetation and climate
Fire is the most ubiquitous disturbance affecting terrestrial ecosystems, most prominent in savannas, Mediterranean woodlands, and boreal forests. It is an important component of the Earth system affecting the vegetation, the carbon cycle and the radiative forcing through fire emissions. It has the potential therefore to amplify or reduce initial climatic changes through its different feedbacks on climate. However, tackling its exact influence on climate still remains challenging as fire varied in the past both at temporal and spatial scales, responding to different climatic variability over different boundary conditions.
To go beyond these issues, we aim at identifying and quantifying the different controls of biomass burning (fire) through time. Climatic conditions are the primary control of the incidence of fire, but fire is also influenced by the nature of the vegetation. Vegetation on the other hand depends itself of climate changes on timescales from interannual (vegetation productivity) to multi-millennial (vegetation dynamics and distribution). Another superimposed control on fire is natural (by lightning storms) or anthropogenic ignition. Humans have been put forward to explain both increased and reduced periods of fire through fire use intensification for ecosystem management. This can lead to an increase of fire, as well as fire suppression by fragmenting landscapes and reducing fuel load.
We examine how key climatic and vegetation variables governed biomass burning in regions that are today sensitive to fire, in particular the Mediterranean region and south-western Africa. We analyse microcharcoal particles along with pollen grains preserved in long and continuous deep-sea sedimentary sequences, focusing on orbital and millennial time scales. This approach is worth to draw on changes in biomass burning directly in relation with vegetation and climate at regional scale.
Figure illustrating the different feedbacks of fire on climate
Figure illustrating natural controls of biomass burning
Relationship between biomass burning (microcharcoal concentration) and fuel (index of biomass) over the last glacial period in southwestern Iberia (Daniau et al., PloSONE, 2010). Microcharcoal particles were counted in the deep-sea core MD95-2042 - in addition to pollen. It is currently believed that increase of biomass burning represent an increase of dryness periods. This study showed however that during the last glacial period biomass burning was controlled by availability of fuel. Increase in biomass burning during Greenland Interstadials (warm phases, green color) was linked to the development of Mediterranean vegetation promoting fire, while low semi-desert biomass limited fire spread during Greenland and Heinrich Stadials (cold phases, blue and orange colors). Both Neanderthals and Modern Humans (MH) are associated either with periods of relatively high biomass burning and fuel or periods of low biomass burning and fuel. This implies that the observed regional scale variations in biomass burning were not determined by the population replacement. This suggests that MH who colonised Western Europe did not used fire in an extensive way to disturb the natural fire regime variability.