Jomelli 2004 b

Référence bibliographique complète

Abstract: Lichens were measured on 25 avalanches boulder tongues in the Massif des Ecrins to elucidate the Littele Ice Age history of avalanche activity. Results have shown an increase of lichen size from the median to the distal zone of deposits, a decrease from the edge to the centre, and 3 types of lichen settlement. From the uppermost to the median zone, lichens are absent because avalanche activity is high. Down-slope, lichens occur in 2 different zones: the median zone is colonized by 5-20 mm size lichens on sides of blocks protected from the abrasional action of avalanches, while in the distal zone lichen diameters are largest (more than 30mm) and occur on all sides of the blocks. The spatial distribution of the lichens and their size according to elevation make it possible to distinguish different phases during which avalanche activity has increased. At high elevation, the avalanche activity was at a maximum before 1650 and between 1730 and 1830. During these 2 periods avalanches had sufficient magnitude to reach the basal zone of the deposits. At low elevation since 1650 the magnitude and frequency of avalanches have declined.

Organismes / Contact

CNRS Laboratoire de Géographie Physique, UMR 8591, Meudon Bellevue jomelli@cnrs-bellevue.fr
Université Paris 1, Panthéon Sorbonne

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

Massif / Secteur

Site(s) d'étude

Période(s) d'observation

(1) - Modifications des paramètres atmosphériques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(2) - Impacts du changement climatique sur le milieu naturel

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(3) - Impacts du changement climatique sur l'aléa

No links with climate change have been provided.

Down-slope, the frequency of avalanches reaching 9/10 of the profile is very low, so lichens are older than on the other parts of the deposit. Avalanches geomorphic action is stronger on the central axis. Lichenometry shows a diachronicity in sedimentation on avalanche boulder tongues during the LIA. According to the lichen patterns, an irregular decrease of avalanche run-out distances has occurred for these last centuries. However, a distinction must be made according to the elevation.

At high elevation, it seems that the activity of avalanches was restricted to the distal zone between 1650 and 1700 (if an extrapolation of the lichen growth curve is made). Before 1650 the frequency of avalanches was sufficiently high to prevent lichens from colonizing the deposits. Between 1650 and 1730, avalanche activity decreased, allowing free development of lichens on the surface of snow avalanche fans. Between 1730 and 1830, the activity seems to have increased, only occasional lichens located on protected sides of blocks surviving this erosive activity. From 1830 to 1850, according to the mean of the 5 largest lichens, the frequency reaching the distal zone was extremely low. It may reflect a decrease in the quantity of debris transported by avalanches, reducing their erosive power. Between 1830 and present, 2 contrasting periods can be distinguished. Because lichen size decreases from the base to the median part of all deposits, the magnitude of avalanches has decreased to 30% between 1830 and 1950 (lower frequency). From 1950, magnitude is low but frequency remains high in the upper part of the deposits.

At low elevation, the variation of the avalanche activity during the LIA is different. The lack of a clear hiatus in the distribution of the lichens suggests that there was a maximum in avalanche activity around 1600-1650 (according to the lichen growth curve extrapolation). Since around 1700, colonization in the distal zone is observed, which is due to a decrease in activity of avalanche process. The phase observed around 1830, during which maximum run-out distances at high elevation occurred, also exists but the activity was less intense.

Paramètre de l'aléa

Sensibilité du paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Avalanche frequency

Measurements of Rhizocarpon sp.lichens were perfomed during two fields campaigns on 25 deposits for which geometrical and sedimentological characteistics have been studied previously.

The location of lichens on boulders, in relation to the supposed trajectories of avalanches was also noted. Between 15 and 70 measurements (one measuremet on each block) were made within each sample area. To compare the lichen patterns between deposits, the lenght deposit was measured over a constant segment of 10 m. To date the avalanche activty, a lichenometric growth curve was used.

(5) - Syntèses et préconisations

Pays / Zone	Massif / Secteur	Site(s) d'étude	Exposition	Altitude	Période(s) d'observation
French Alps	Massif des Ecrins (45°00’N, 6°30’E)			1800-2450 m asl	~1600-2000

(1) - Modifications des paramètres atmosphériques
Reconstitutions
Observations
Modélisations
Hypothèses

(2) - Impacts du changement climatique sur le milieu naturel
Reconstitutions
Observations
Modélisations
Hypothèses

(3) - Impacts du changement climatique sur l'aléa
Reconstitutions	No links with climate change have been provided. Down-slope, the frequency of avalanches reaching 9/10 of the profile is very low, so lichens are older than on the other parts of the deposit. Avalanches geomorphic action is stronger on the central axis. Lichenometry shows a diachronicity in sedimentation on avalanche boulder tongues during the LIA. According to the lichen patterns, an irregular decrease of avalanche run-out distances has occurred for these last centuries. However, a distinction must be made according to the elevation. At high elevation, it seems that the activity of avalanches was restricted to the distal zone between 1650 and 1700 (if an extrapolation of the lichen growth curve is made). Before 1650 the frequency of avalanches was sufficiently high to prevent lichens from colonizing the deposits. Between 1650 and 1730, avalanche activity decreased, allowing free development of lichens on the surface of snow avalanche fans. Between 1730 and 1830, the activity seems to have increased, only occasional lichens located on protected sides of blocks surviving this erosive activity. From 1830 to 1850, according to the mean of the 5 largest lichens, the frequency reaching the distal zone was extremely low. It may reflect a decrease in the quantity of debris transported by avalanches, reducing their erosive power. Between 1830 and present, 2 contrasting periods can be distinguished. Because lichen size decreases from the base to the median part of all deposits, the magnitude of avalanches has decreased to 30% between 1830 and 1950 (lower frequency). From 1950, magnitude is low but frequency remains high in the upper part of the deposits. At low elevation, the variation of the avalanche activity during the LIA is different. The lack of a clear hiatus in the distribution of the lichens suggests that there was a maximum in avalanche activity around 1600-1650 (according to the lichen growth curve extrapolation). Since around 1700, colonization in the distal zone is observed, which is due to a decrease in activity of avalanche process. The phase observed around 1830, during which maximum run-out distances at high elevation occurred, also exists but the activity was less intense.
Observations
Modélisations
Hypothèses