Seasonal timing of rockfall activity on a forested slope at Täschgufer (Swiss Alps) - a dendrochronological approach

Summary. Dendrochronology was used to study 25 years of rockfall activity on a forested slope at Taschgufer, Tasch (Swiss Alps). We introduce a new approach by evaluating the initiation of callous tissue and resin duct formation after rockfall occurrence to determine the seasonal timing of events. Results from 270 stem discs of 18 Larix decidua Mill. trees show distinct seasonal differences in rockfall activity and indicate that, except for the years with anthropogenic activity on the slope, the occurrence of rockfall proves to be mostly restricted to the dormancy and is rather uncommon during the vegetation period. Only 12% of the injuries occur within the vegetation period, which locally lasts from early June through mid October. In contrast, some 88% of the scars occur in the dormant season between mid October and end of May. Direct observations on the slope confirm these findings, indicating that rockfall activity is highest in April and May, when global insolation on the west-facing Taschgufer slope gradually rises and ice lenses formed from meltwater slowly disappear in the joints and fissures of the rockfall source areas. According to nearby meteorological data, rockfall at Taschgufer seems to be neither influenced by thunderstorms in summer nor abundant rainfall in autumn.