Dendrogeomorphic reconstruction of past debris‐flow activity using injured broad‐leaved trees

Tree‐ring records from conifers have been regularly used over the last few decades to date debris‐flow events. The reconstruction of past debris‐flow activity was, in contrast, only very rarely based on growth anomalies in broad‐leaved trees. Consequently, this study aimed at dating the occurrence of former debris flows from growth series of broad‐leaved trees and at determining their suitability for dendrogeomorphic research. Results were obtained from gray alder (Alnus incana (L.) Moench), silver birch and pubescent birch (Betula pendula Roth and Betula pubescens Ehrh.), aspen (Populus tremula L.), white poplar, black poplar and gray poplar (Populus alba L., Populus nigra L. and Populus x canescens (Ait.) Sm.), goat willow (Salix caprea L.) and black elder (Sambucus nigra L.) injured by debris‐flow activity at Illgraben (Valais, Swiss Alps). Tree‐ring analysis of 104 increment cores, 118 wedges and 93 cross‐sections from 154 injured broad‐leaved trees allowed the reconstruction of 14 debris‐flow events between AD 1965 and 2007. These events were compared with archival records on debris‐flow activity at Illgraben. It appears that debris flows are very common at Illgraben, but only very rarely left the channel over the period AD 1965–2007. Furthermore, analysis of the spatial distribution of disturbed trees contributed to the identification of six patterns of debris‐flow routing and led to the determination of preferential breakout locations of events. The results of this study demonstrate the high potential of broad‐leaved trees for dendrogeomorphic research and for the assessment of the travel distance and lateral spread of debris‐flow surges. Copyright © 2010 John Wiley & Sons, Ltd.