Spatial and temporal rockfall activity in a forest stand in the Swiss Prealps—A dendrogeomorphological case study

Abstract

Rockfall is a major threat to settlements and transportation routes in large parts of the Alps. While protective forest stands in many locations undoubtedly reduce rockfall risk, little is known about the exact frequency and spatial distribution of rockfall activity in a given place or about how these parameters can be assessed. Therefore, the objective of the present study was to reconstruct rockfall events with dendrogeomorphological methods and to analyse the spatial and temporal rockfall activity in a subalpine forest stand. The study site is located in the transit zone of frequently passing, rather small rockfall fragments (mean diameter of 10 to 20 cm). In all, 33 stem discs from previously felled Picea abies trees found at the foot of Schwarzenberg in Diemtigtal (Swiss Prealps) were sampled, and a total number of 301 rockfall events were dated to between A.D. 1724 and 2002.Results showed that the spatial distribution of rockfall changed slightly with time, and that rockfall activity increased considerably over the last century. In contrast, rockfall magnitude presumably remained on a comparable level. The seasonal occurrence of rockfall showed a clear peak during the dormant season of trees, most probably in early spring. Furthermore, on a 10-year moving average basis, rockfall rates were positively correlated with mean annual as well as summer and winter temperatures. This means that higher temperatures resulted in increased rockfall activity. On the other hand, no correlation with annual or seasonal precipitation totals was revealed. Overall, this study provides an appropriate method for the detailed assessment of spatial and temporal variations in rockfall activity in a given place.

Tree-ring based reconstruction of the seasonal timing, major events and origin of rockfall on a case-study slope in the Swiss Alps

“…In contrast to other mass-movement processes (e.g., Shroder, 1980;Braam et al, 1987;Wiles et al, 1996;Solomina, 2002), tree-ring analyses have only occasionally been 204 D. M. Schneuwly and M. Stoffel: Tree-ring based reconstruction of rockfall in the Swiss Alps used to study past rockfall activity, mainly to determine sedimentation rates on scree slopes (Lafortune et al, 1997) or to reconstruct rates or the spatial occurrence of rockfall (Stoffel et al, 2005a;Perret et al, 2006).…”

“…While callus tissue only occurs in the wood segment bordering the injury (Stoffel et al, 2005a, b;Perret et al, 2006), TRD can be even observed at some distance from the wound as well (Bollschweiler et al, 2007a;. Bollschweiler et al (2008) indicate a mean lateral TRD extension of 19% of the stem's circumference.…”

Debris flows on forested cones – reconstruction and comparison of frequencies in two catchments in Val Ferret, Switzerland

“…3): 1) A passing surge can cause abrasion at the tree stem. The tree reacts to this type of disturbance with changes in its cell structure and the formation of callus tissue as well as tangential rows of traumatic resin ducts (TRD; Stoffel et al, 2005b, c;Perret et al, 2006). 2) The unilateral pressure of the flow can lead to the inclination of the stem and the formation of compression wood on the downslope side of the trunk (Braam et al, 1987;Fantucci and SorrisoValvo, 1999;Stefanini, 2004).…”

A Review of Studies Dealing with Tree Rings and Rockfall Activity: The Role of Dendrogeomorphology in Natural Hazard Research

“…However, they suffer from the limited number of available trees or cross-sections (since trees cannot normally be felled in protection forests and stumps of felled trees are not always readily available). With the exception of possibly Perret et al (2005), the term 'frequency' should, thus, be read with caution in these studies. .…”

“…Following the procedures described above, Perret et al (2005) analyzed the seasonal timing of events in growth rings of Norway spruce trees on a rockfall slope in the Swiss Prealps (Diemtigtal, Bernese Oberland). Here, seasonal frost prevails and there is no permafrost.…”