Contribution of dendrogeomorphology to the dating of secondary processes on dormant rockslides

Rockslides in the Variscan orogenic belt of the Central Europe are a rare and poorly studied phenomenon. These relatively stable features have recently been shaped by secondary rockfall, toppling, sliding, or slumping. On afforested slopes, such processes can be efficiently analysed and dated by dendrogeomorphic methods. We performed detailed analyses of 355 increment cores from 81 Picea abies (L.) Karst. trees growing on two dormant rockslides in northeastern Czechia to reconstruct the activity of rock block movements and rockfalls. For the event determination we used standard event-response (I_t) index and a semi-quantitative approach involving logical spatial position of disturbed trees during a three-year period. Furthermore, climate preparatory and triggering factors were analysed to investigate possible main drivers of recent secondary processes. Overall, four periods of certain block reactivations at the Prudký site since 1940 and seven periods of certain block reactivations at the Rudohorský site since 1834 were reconstructed. Most of the events can be dated to the period 1960–2000, but our data do not indicate any high-magnitude activity. This paper also demonstrates the results of tree eccentric growth not only in the main supposed direction of stem tilting but also in the direction perpendicular to the main direction when ca. 40% of all trees growing on rock blocks recorded the movements in both analysed axes of stem tilting, suggesting possible complex deformation and different directions of block movements over time. It also appeared that the periods with greatest activity of secondary movements were characterised by a significantly higher rain-on-snow factor (p = 0.007 and 0.026 at the Prudký and the Rudohorský site, respectively) thus indicating block detachments during periods of rapid snowmelt.