The spatio-temporal analysis of the rockfall activity of the past two centuries through dendrogeomorphic reconstruction in Cozia Massif (Southern Carpathians, Romania)

Rockfalls are geomorphic processes characteristic of steep mountain environments, which pose a threat to buildings, transport routes, tourism activities and infrastructure, causing material damage, injuries and even loss of human life. The mitigation of these negative effects involves, on the one hand, reconstruction past rockfall activity and, on the other hand, conducting hazard zonation,—along with estimating—evolution patterns—on different temporal scales (e.g. decadal). In the present study, the spatio-temporal analysis of rockfall activity using dendrogeomorphic methods was carried out in the Poarta de Piatră geomorphosites (1400 m a.s.l.), which is developed on gneiss and located in Cozia Massif (Southern Carpathians). The dendrogeomorphic reconstruction spans over 200 years (1807–2022). By analysing—100 increment cores and 70 stem discs sampled at varying heights on the stems of 40 Picea abies (L.) Karst trees spread, over an area of 0.19 hectares, we were able to date 1137 growth anomalies caused by rockfalls. Among these, 928 were identified as distinct events, exhibiting a mean recurrence interval of 5.4 years. The results of our research indicate the manifestation of processes with high frequency (at the beginning and end of the past century) and low magnitude of rockfalls, without large-scale events being recorded, as well as the main directions of rock fragments release from the source area. Our study emphasizes that the temporal reconstruction patterns of past rockfall activity strongly depend not only on the depth and size of the sample but also on the individual characteristics of each sampled tree, including their age, and the sensitivity of the cambium.