Recent increase in debris flow activity in the Tatras Mountains: Results of a regional dendrogeomorphic reconstruction

Debris flows are very frequent geomorphic agents that form relief of the High Tatras Mountains, which are the highest mountain range in the Carpathian arc. The knowledge of their history is based on lichenometric dating, historical orthophotos and incomplete archival records. Nevertheless, complete chronologies of debris flows at an annual resolution for the High Tatras Mountains have not yet been reconstructed. In this paper, we present results of dendrogeomorphic research in the southern region of the mountains (the Great Cold Valley and Small Cold Valley). The research was undertaken to reconstruct regional year-to-year chronologies of debris flows, which would present unique perspectives on the debris flow activity. A total of 474 scars on the stems and branches of 414 Pinus mugo were dated with a seasonal precision (the best tool for distinguishing debris flow scars from snow avalanche scars) in 12 debris flow tracks. 22 debris flows over the last circa 30years (the valid chronological range) in six event years were reconstructed. A significant increase in debris flow activity (verified using historical orthophotos) has occurred since AD 2007 (21 of 22 events occurred during the last decade). Two basic spatial patterns of debris flow activity were identified: regional events with at least five debris flows in one or both valleys and local events with a maximum of two debris flows per year.Based on reports by several authors, increase in extreme precipitation events during the last decade has been observed, but debris flows have not occurred even during the most extreme precipitation events in the study area. Preparatory factors must play an important role at this stage. A significant increase in summer and autumn temperatures at the turn of the 21st century has been reported in the High Tatras Mountains. This could accelerate (dis)continuous permafrost thawing in the source zones, increase the amount of material for the next transport and influence the increase in debris flow activity.