Heterogeneity of complex landslides from the perspective of dendrogeomorphology

Abstract

Complex landslides are characterised by the presence of zones with different depths and mechanisms of movement. From the perspective of dendrogeomorphic research, however, these landslides are generally treated as homogeneous units, which can, however, introduce critical scatter and ambiguity in the results. This limitation is particularly serious because tree-ring based approaches are currently considered the most accurate tool for dating landslides in temperate climates. Thus, this study focused on distinguishing the specificity of landslide movement behaviour in two contrasting zones within a complex landslide area through dendrogeomorphic methods. Three complex landslides were studied using data from 746 increment cores of disturbed trees. Within each area, two different landslide types (zone of earthflow and zone of rotational landslide) were defined and analyzed separately. Dendrogeomorphic analysis revealed major differences between the two zones in several parameters. Trees in the two zones responded differently to the ratio of the two types of growth disturbances. Rotational landslides and earthflows showed different frequencies of occurrence and sizes of reactivated areas. In addition, magnitude-frequency analyses suggested different physical mechanisms for the two processes. The difference in potential triggers was found to be crucial. Earthflows zones responded to short-term precipitation extremes after a long dry period, whereas rotational landslide zones responded to medium-term extremes after several years of above-average precipitation. Thus, the results of the study clearly indicate the necessity of strict separation of the contrast landslide types within complex landslides in dendrogeomorphic research for higher accuracy of results with minimization of their scatter.