Holocene vegetation responses to fire events in the inner French Alps (Queyras Massif): data from charcoal and geomorphological analysis of travertine sequences

Travertines are calcareous deposits that result from water stream saturation in bicarbonate; they are extremely sensitive to environmental variations. Homeostatic conditions, such as a good development of vegetation and regular river flow, induce the formation of homogenous travertine facies enclosing plant imprints. Significant climatic changes and/or human perturbations may disturb the carbonate deposit favouring an increase in detrital material often rich in charcoal remains. Travertine systems located in the Queyras Massif (Aigue Agnelle and Peynin valley) provided abundant charcoal material; its analysis allowed us to identify five vegetation phases between c. 8400 and 1709 cal. yr BP, and to ascertain Holocene vegetation responses to fire perturbation. An ecological dynamic based on forestfire events is proposed. During periods of reduced fire frequencies (long-term fire cycle), Pinus cembra dominated the Queyras Massif vegetation. The predominance of travertine facies during these phases corroborates this interpretation. Periods of increased fire frequency triggered the development of Larix decidua/Picea abies (short-term fire cycle). This vegetation change was concomitant with the deposition of thick detrital layers resulting from slope erosion, as a consequence of recurrent fire perturbations. The permanent settlement in this region of human communities since c. 4400 cal yr BP favoured the development of Larix decidua/Picea abies. Finally, our results suggest that the collapse of agropastoral activities since AD 1950 might progressively lead to the regeneration of Pinus cembra.