Possibilities and limitations of dendrogeomorphic time-series reconstructions on sites influenced by debris flows and frequent snow avalanche activity

Past debris-flow and snow avalanche activity was assessed for the Reiselehnrinne (Tyrol, Austria) using growth disturbances in growth-ring series of 372 Norway spruce (Picea abies (L.) Karst.) trees. Determination of events was performed by analyzing (a) the number and (b) intensity of growth disturbances within tree-ring series and (c) the spatial distribution of affected trees. Differentiation of debris flow from snow avalanche events was based on the intra-annual position of scars, callus tissues or tangential rows of traumatic resin ducts, and on the spatial distribution of trees with simultaneous reactions in the tree-ring series. We introduce a weighting factor to substantiate the dating of past process activity in a comprehensive way and to compare individual events as to their intensity and total number of tree-ring responses. The accuracy of the dendrogeomorphic assessment was then evaluated by comparing the reconstructed event frequency with chronologies available for the Reiselehnrinne. Comparison of tree-ring with historical data demonstrated clearly that the reconstructed event frequency contains the majority of past debris flow and snow avalanche events in the Reiselehnrinne, but that dating of events is not always possible, especially if they are clustered in time or have a limited spread on the cone

Frequency and spread of debris floods on fans: A dendrogeomorphic case study from a dolomite catchment in the Austrian Alps

Growth disturbances in tree-ring series have been regularly used to date debris-flow events in mountain environments. In contrast, no studies are available to date that have reconstructed debris floods by means of dendrogeomorphology. Therefore, the aim of this study was to determine the event frequency and the spread of debris floods in the Gratzental (Tyrol, Austria). The analysis of growth disturbances in the tree-ring series of 227 Picea abies (L.) Karst. and Larix decidua Mill. allowed the reconstruction of 37 events for the last 200 years. The lateral spread and preferable avulsion locations of reconstructed debris-flood events were assessed based on the dating of the events and the spatial position of trees affected by an event on the fan. Results show that the Gratzentalbach preferentially avulsed to the east, but affected trees were evenly spread over the fan. Reconstructed data illustrates the high potential of dendrogeomorphology for hazard assessment of debris floods