Assessment of the recurrence intervals of rockfall through dendrogeomorphology and counting scar approach: A comparative study in a mixed forest stand from the Vercors massif (French Alps)

Robin Mainieri,Jérôme Lopez-Saez,Christophe Corona,Markus Stoffel,Franck Bourrier,Nicolas Eckert

doi:10.1016/j.geomorph.2019.05.005

Abstract

As direct observations of rockfalls are rare and difficult to obtain over long timescales, stem injuries and their dating with dendrogeomorphic techniques have been applied frequently in the past to reconstruct process activity. However, the analysis of tree-ring samples requires considerable temporal efforts, which can be detrimental in applied research and expert opinions. To compensate for this shortcoming, several studies have lately explored the potential of the counting of visible scars as an alternative method to dendrogeomorphology as it requires much less time and human efforts to reconstruct spatial patterns of rockfall activity. Yet, to date, both approaches have not been compared at the same site. In this paper, based on the extensive analysis of 278 conifer and broadleaved trees (from which 1097 tree-ring cores were extracted) from a mixed forest plot of the Vercors Massif (French Alps), we demonstrate that both methods provide similar spatial patterns of rockfall activity with a strong increase of recurrence intervals down the talus slope and a clear lateral zonation of activity. Despite this apparent convergence between both approaches, our study also evidences that rockfall frequencies strongly differ with tree species and diameter: the visual inspection of conifer stems and the analysis of tree-ring signals in large-diameter broadleaved trees typically lead to an overestimation of recurrence intervals. Based on these findings, we recommend giving priority to the scar-counting approach on small diameter (<15 cm) broadleaved stems and to restrict tree-ring analysis to old conifers that are more susceptible – through the identification of tangential rows of traumatic resin ducts – to provide reliable estimations of long recurrence intervals (as generally observed in the lower portions of the slope). These guidelines should help to improve the reliability of recurrence interval maps of rockfall process activity in forests further, but also to improve cost-benefit ratios of future studies.

Highlights

Rockfalls are widespread natural hazards in mountain regions where they threaten inhabited areas and transport corridors, causing each year significant economic and human losses (Volkwein et al, 2011)
To date, the two approaches have not been compared systematically at a single site and for a common dataset. In this context, based on a procedure that includes (i) an extraction of a representative number of trees from an exhaustive forest inventory, (ii) the estimation of tree ages from allometric curves and increment cores as well as (iii) the quantification of rockfall-related growth disturbances based on the visual inspection of stems and tree-ring analyses, this paper aims at comparing recurrence interval maps derived from both approaches so as to evaluate their respective reliability as well as their advantages and inconveniences
On forested slopes, the frequency of recent rockfalls can be determined either through (i) the analysis of rockfall injuries remaining visible on the tree surface or (ii) the dating of characteristic growth disturbances in tree-ring series

Summary

Introduction

Rockfalls are widespread natural hazards in mountain regions where they threaten inhabited areas and transport corridors, causing each year significant economic and human losses (Volkwein et al, 2011). They are commonly defined as the detachment of a rock fragment from a steep rock wall (Selby, 1993) which travels some distance down the slope by bouncing, falling or rolling (Varnes, 1978). Despite their limited volumes, rockfalls pose a significant hazard, due to their rapid evolution, high velocity and impact energy, but their unpredictable occurrence hinders detailed investigation of their dynamics and drivers under natural conditions. Major efforts are still required to quantify rockfall activity in terms of frequency and magnitude at the local scale, which is crucial for risk assessment, the choice of appropriate mitigation measures, and for the validation of output from simulation runs

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