Mass movements and tree rings: A guide to dendrogeomorphic field sampling and dating

Abstract

Trees affected by mass movements record the evidence of geomorphic disturbance in the growth-ring series, and thereby provide a precise geochronological tool for the reconstruction of past activity of mass movement. The identification of past activity of processes was typically based on the presence of growth anomalies in affected trees and focused on the presence of scars, tilted or buried trunks, as well as on apex decapitation. For the analyses and interpretation of disturbances in tree-ring records, in contrast, clear guidelines have not been established, with largely differing or no thresholds used to distinguish signal from noise. At the same time, processes with a large spatial footprint (e.g., snow avalanches, landslides, or floods) will likely leave growth anomalies in a large number of trees, whereas a falling rock would only cause scars in one or a few trees along its trajectory.Based on the above considerations, we examine issues relating to the interpretation and dendrogeomorphic dating of mass movements. Particular attention is drawn to sampling in terms of sample distribution across a study site, the actual selection of trees as well as to sample size (i.e., number of trees sampled). Based on case studies from snow avalanche, debris flow, and landslide sites, we demonstrate that thresholds can indeed improve dating quality and, at the same time, minimize noise in time series. We also conclude that different thresholds need to be used for different processes and different periods of the reconstruction, especially for the early stages of the reconstruction when the number of potentially responding trees will be much smaller. This paper seeks to set standards for dendrogeomorphic fieldwork, analysis, and interpretation for different processes of mass movements.