Fig. 3. At Multetta, debris-flow frequency and magnitude do not show any trend linked to climate variability and warming.

A Annual number of living trees (i.e. sample depth, gray area) and growth disturbances (GD, black bars) for the period 1600–2020. B Reconstructed debris flows (circles) and their relative magnitude shown as XS, S, M, L, XL and XXL (yellow-to-black color gradient). Completeness analyses suggest that the reconstruction is free of biases after 1687 or 1750 (vertical dotted lines), depending on the approach used. The black line depicts the cumulative number of debris flows; we do not find any breakpoint (see Methods) pointing to a change in debris-flow frequency over time. This absence of anomalies is confirmed at decadal timescales (green bars) where the frequency of events ranges from 0 to 0.3 (mean = 0.16, horizontal dashed line). No clear control of temperature fluctuations on debris-flow activity can be evidenced during the coldest period of the Little Ice Age (blue rectangles) or as a result of the ongoing and accelerating warming climate (red rectangle). The purple surface shows the period spanning from the 1860s to the 1910s during which changes in debris-flow activity are similar between the Multetta (this study) and the Ritigraben (Western Swiss Alps) catchments.