Abstract. The understanding of geomorphic processes and knowledge of past events are important tasks for the assessment of natural hazards. Tree rings have on varied occasions proved to be a reliable tool for the acquisition of data on past events. In this review paper, we provide an overview on the use of tree rings in natural hazards research, starting with a description of the different types of disturbances by geomorphic processes and the resulting growth reactions. Thereafter, a summary is presented on the different methods commonly used for the analysis and interpretation of reactions in affected trees. We illustrate selected results from dendrogeomorphological investigations of geomorphic processes with an emphasis on fluvial (e.g., flooding, debris flows) and mass-movement processes (e.g., landslides, snow avalanche), where lots of data have been generated over the past few decades. We also present results from rockfall and permafrost studies, where data are much scarcer, albeit data from tree-ring studies have proved to be of great value in these fields as well.Most studies using tree rings have focused on alpine environments in Europe and North America, whereas other parts of the world have been widely neglected by dendrogeomorphologists so far. We therefore challenge researchers to focus on other regions with distinct climates as well, to look on less frequently studied processes as well and to broaden and improve approaches and methods commonly used in tree-ring research so as to allow a better understanding of geomorphic processes, natural hazards and risk.

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Following mechanical injury, stems of many conifers produce tangential rows of traumatic resin ducts (TRDs), the distribution of which has been used to date geomorphic events. However, little is known about how far TRD formation extends tangentially and axially from the point of injury or what the time course of TRD appearance is. We analyzed 28 injuries in eight Larix decidua Mill. tree stems resulting from debris flows in October 2000 and November 2004. Injuries occurred outside the period of cambial activity, and TRD formation occurred in the first layers of the growth ring formed in the year following that of injury. The axial extent of TRD formation averaged 74 cm and was greater above the injury than below it. At the height of the wound center, TRDs extended horizontally to a mean of 18% of the stem circumference excluding that portion where the cambium had been destroyed. In subsequent growth rings, TRDs, if present, were confined mainly to the height of the center of injury. Both the vertical and horizontal extent of TRD formation was related to the injury size. Within growth rings, the position of TRD formation changed with increasing distance from the wound progressing from early earlywood to later portions of the growth ring.

Dendrogeomorphological analyses of trees affected by debris flows have regularly been used to date past events. However, this method has always been limited to forested cones where trees registered the impact of previous events. The minimum age dating of trees growing in the debris deposits can, in contrast, provide information on the latest possible moment of past activity. In this paper, we report on results obtained from a combination of these two approaches on a forested cone in the Valais Alps (Switzerland). A detailed geomorphic map in a scale of 1:1000 served as a basis for the sampling strategy. Disturbed Larix decidua Mill. and Picea abies (L.) Karst. trees growing in the deposits allowed reconstruction of 49 events between AD 1782 and 2005 as well as the determination of the spatial extent of events. In the debris-flow channels where survivor trees are missing, we selected the oldest post-event trees and assessed their age by counting their growth rings. Missing rings due to lack of center as well as to sampling height were added so as to determine real tree age. The combination of the dendrogeomorphological event reconstruction with the assessment of germination dates of successor trees allowed realistic approximation of the minimum time elapsed since the last debris-flow activity in 23 of the 29 channels present on the current-day cone surface. In general, channels in the northern part of the cone and those close to the currently active channel generally show signs of (sub-) recent activity with one last overbank sedimentation event in the 1980s, whereas signs of debris-flow activity are absent from the channels in the outermost part since the late 19th century. As a consequence of the deeply incised channel and the stabilization measures undertaken along the banks, signs of debris flows are missing in the tree-ring record for the past two decades.