Debris-flow activity in the Japanese Alps is controlled by extreme precipitation and ENSO - Evidence from multi-centennial tree-ring records

Japanese mountains are recurrently affected by mass movements and sediment-related disasters. Large efforts have therefore been undertaken to enhance understanding of the impacts and triggering of recent disasters, but ancient and continuous records to put recent events into perspective are still lacking. Here, we present a multi-centennial tree-ring-based debris-flow reconstruction in two size-contrasting torrential catchments of the Japanese Alps. We analyzed 197 affected trees growing on debris-flow fans and along torrential channels, which allowed us to identify 62 past events (38 and 24 in each catchment) from the late 19th century to today. The new chronology shows a non-stationary behavior of debris flows in the long-term, and an upward trend in debris flow activity over the last decades. Analysis of the 3-day rainfall legacy explains the occurrence of debris flows best, whereas rainfall thresholds show a positive trend linked to an increase in the occurrence of extreme precipitation over the last decades. Our results clearly support a link between debris flows with the passage of typhoons as well as La Ni & ntilde;a events. The unprecedented length of the debris-flow record provided here opens new avenues for climate change and sediment-disaster research in Japan. Thus, our findings have important implications for the understanding of past, recent, and near-future debris-flow trajectories under climate change scenarios.