The potential of tension wood for the detection of landslide signals in the tree ring series of Fagus sylvatica L.: The extension of dendrogeomorphic tools for landslide analysis

The knowledge of past landslide movement chronology is crucial for estimating future landslide development or for detailed analysis of triggers. A very precise approach to landslide history detection is based on tree ring analysis (dendrogeomorphology). The detection of landslide movements is based on dating growth responses induced by slope movements. To date, a relatively limited spectrum of tree growth disturbances has been used in landslide research. This study provides new evidence about the possibilities of using tension wood (a growth response to stem tilting) in Fagus sylvatica L. for detection of past landslide movements. Twenty-three disturbed trees growing on a landslide with a known origin (July 1997) were analysed. Parameters of the tension wood and tree rings (vessel lumen area (VLA) and its change, the duration of tension wood, and tree ring eccentricity and its change) were compared with each other and with tree parameters (tilting degree, age, and stem diameter). Tension wood was readily macroscopically identified on the prepared surfaces of increment cores using a stereoscope. Significant relationships were found between the degree of stem tilting and tension wood duration and the magnitude of VLA change. Thus, these two parameters of tension wood can be viewed as potential indicators of landslide-influenced intensity. Tension wood formation seems to be very sensitive to tree stem tilting, as all studied trees with various degrees of stem tilting contained it in the entire tree ring width after 1997. Moreover, the onset of tension wood in 1997 always started in the second part of the tree ring width. This signals the immediate growth response to landslide movements and the possibility of using tension wood for dating with sub-annual (seasonal) resolution. Tree ring eccentricity correlated with the stem tilting degree as well; nevertheless, no relationship with tension wood parameters was found. The findings of this study with the discussed limits provide support for the very effective use of tension wood in the dendrogeomorphology of landslides.