Tree-ring based assessment of rockfall frequency on talus slopes at Solà d'Andorra, Eastern Pyrenees

The determination of the frequency of rockfall events continues to challenge quantitative hazard assessments in most mountain areas. Dendrogeomorphological analysis was used to assess rockfall frequency on talus slopes at Solà d'Andorra (Eastern Pyrenees, Andorra). Rockfall events were dated at two sites: one at the outlet of a chute, and the other below a rock wall. The impact wounds visible on the tree surface were analyzed with a seasonal temporal resolution. At each site, trees were sampled in three forest strips 15 to 30 m wide located at different heights on the talus. All the trees with visible injuries in the strips were sampled.Rockfall frequency cannot be assessed by a simple analysis of time series of tree damage. A satisfactory assessment requires a prior interpretation of the location of the damaged trees in relation to a number of rockfall events. At Solà d'Andorra, the rockfall chronology was reconstructed for the last 25 years by dating visible wounds. For older periods, the tree wound record of rockfalls is incomplete owing to progressive closure of wounds. Wounds exceeding 40 years in age had already disappeared from the tree surface in the study area. Our results show a clear reduction of the rockfall frequency down the talus, a noticeable lateral change in frequency, and an influence of source morphology (chute or wall) on rockfall activity on the talus.