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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)

Mainieri, Robin, Lopez-Saez, Jérôme, Corona, Christophe, Stoffel, Markus, Bourrier, Franck, Eckert, Nicolas
Geomorphology 2019 v.340 pp. 160-171
applied research, broadleaved trees, conifers, cost benefit analysis, dendroclimatology, expert opinion, forest stands, geomorphology, growth rings, guidelines, mixed forests, resin canals, rockfalls, stems, talus, Alps region, France
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.