Defining sample size and strategy for dendrogeomorphic rockfall reconstructions

Pauline Morel, Daniel Trappmann, Christophe Corona and Markus Stoffel

Optimized sampling strategies have been recently proposed for dendrogeomorphic reconstructions of mass movements with a large spatial footprint, such as landslides, snow avalanches and debris flows. Such guidelines have been missing for rockfalls and cannot be transposed owing to the sporadic nature of this process and the occurrence of individual rocks and boulders. Based on a dataset of 314 European larch (Larix deciduaMill.) trees (64 trees/ ha), growing on an active rockfall slope, this study bridges this gap and proposes an optimized sampling strategy for the spatial and temporal reconstruction of rockfall activity. Spatially, our results demonstrate that the sampling of only 6 representative trees/ha can be sufficient to yield a reasonable mapping of the spatial distribution of rockfall frequencies on a slope, especially if the oldest and most heavily affected individuals are included in the analysis. Temporally, we demonstrate that at least 40 trees/ha are needed to obtain reliable rockfall chronologies.

2016/1 356-369