Preliminary quantification of the erosion of sandy-gravelly cliffs on the island of Porquerolles (Provence, France) through dendrogeomorphology, using exposed roots of Aleppo pine (Pinus halepensis Mill.)

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This study is a first attempt to specify the geodynamic processes leading to the erosion of detrital quaternary cliffs on the island of Porquerolles, and to quantify the rate of erosion by means of dendrogeomorphology. The island is located in the Mediterranean Sea, off the coast of Provence (France). This method identifies the roots of Aleppo pine (Pinus halepensis Mill.) that have been unearthed by cliff retreat. The year of denudation is revealed by the net change in tracheid anatomy in tree rings, so that the retreating rates of cliffs can be established and the ratio between the distance of loosening and the number of years since denudation can be assessed. 13 root samples were used to determine that the cliff retreat is due to small rockfalls, at an average rate of 2.5 cm/a. This value is compared to other rates of erosion quantified on rocky or sandy shores. A review of the method and a comparison with methods based on photogrammetry and micrometrics are proposed.