Assessment of forested shallow landslide movements coupling tree ring records from stems and exposed roots
Résumé
The purpose of this study is to explore and illustrate the potential of exposed roots to reconstruct larger-scale landslide activity and thus to complement tree-ring data gathered from stems to reconstruct spatio-temporal patterns of landslide reactivation. Work was undertaken in a forested area of the Davids-Bas landslide, Barcelonnette (Southeastern French Alps) and based on growth disturbances (GD) from 48 stems and on anatomical changes (decrease of cell lumina) in 20 exposed root sections of heavily affected Mountain pine (Pinus uncinata Mill. ex Mirb.) trees growing on the scarps and upon the landslide body. A total of 95 GD and 20 anatomical changes were identified in the samples pointing to 7 movements of the landslide body since AD 1977. The study demonstrates that reconstructions of landslide reactivations obtained from exposed roots samples are not significantly different from those gathered from stems, but that the inclusion of exposed roots permits realization of frequency maps in sectors which could not be documented with classic dendrogeomorphic approaches focusing on tree-ring records from stems (scars, cracks). In addition, and even more importantly, the inclusion of exposed roots allowed assessment of the geomorphic evolution of the landslide at the local scale and to detect precursor signals of major reactivations in the form of crack widening before the main movement was registered in the tree stems. In that sense, the combined approach presented in this paper can be considered as a valuable tool for land-use planners and emergency cells in charge of forecasting future events and in protecting people and their assets from the negative effects of landslides.