Understanding complex slope deformation through tree-ring analyses

Dendrogeomorphic methods are frequently used for the analysis of past landslide behaviour and have become the standard approach used to date landslide activity. Unfortunately, many questions related to the application of tree-rings to landslide analysis remain unsolved. This study points to the significance of dividing a large complex slope deformation area into homogenous zones to obtain as much relevant chronological data as possible and to help with a more precise landslide hazard assessment. The multidisciplinary approach included geophysical measurement and geomorphic mapping. The clay mineral content in weathered slope sediments was analysed to verify the presence of expanding minerals contributing to slope instability. Furthermore, 713 samples from 271 trees and 18 tree roots were analysed to create event chronologies for each zone. The results provided evidence about the different behaviours of each zone. The intensity of tree responses to slope movement significantly changed even within one isolated zone. Chronological data were used to identify landslide triggers using rainfall characteristics and indices for climate variability and extremes. The results suggested a significant contribution of spring rainfall to landslide activity. In addition, shallow landslides movement was dependent on above-average rainfall characteristics compared to the movement of deep-seated blocks. Tree-ring-based chronologies of individual zones were used to characterise the general concept of complex slope deformation development.