Dendrogeomorpholigical analysis of a slope near Lago, Calabria (Italy)

The dendrogeomorphological analysis has been used to investigate the period s of disturbance on a slope affected by deep-seated gravitational movements . The method proved to be of great help in determining the temporal sequenc e of diffused slope movement in the study area, and, though to a lesser ext ent, to find out the possible causes of triggering the mass-movement. In ge neral, leaning trees indicate that the movement is active. The visual growt h analysis indicates that anomalies consisted of sudden decreases of the gr owth (suppression of tree-rings) induced by stress consequent on ground dis turbance, followed in some cases by sudden increases of tree-ring width ind uced by the higher moisture content in the landslide body. By anomaly analy sis, an increase of the anomaly index (It) occurred between 1840 and 1860; subsequently, a period of strongly oscillating values of It occurred that l evelled off around 1950. It also appears that mass-movements began to affec t this zone soon after 1850s, thus, we can tentatively assume that they hav e been the cause of the growth anomalies, with a maximum influence in the p eriod between 1860 and 1895. As regards the causes for mass-movement, we inquired about timing of extrem e meteorological events and earthquakes. The meteorological data obtained f rom raingauging stations are not so well related to mass-movement reactivat ion as the seismic data are. However, only a minority of extreme meteorolog ical events may produce such a disturbance that can be recorded in the tree -ring record. Indeed, only 30% of anomalies can be explained in terms of ex treme events. On the other hand, the continuous creeping of the sackung mig ht irregularly trigger the movement of shallower landslides in non extreme- events years. We obtained, instead, a higher degree of coincidence between disturbing causes and anomalous tree growth using archive reports on extrem e rainfall periods. (C) 1999 Elsevier Science B.V. All rights reserved.