A scanning electron microscopy (SEM) study of sediments from Lake Cristol, southern French Alps, with special reference to the identification of Pinus cembra and other Alpine Pinus species based on SEM pollen morphology

Abstract

The scanning electron microscopy (SEM) pollen morphology of the major Alpine Pinus species was established, then the sediment cores from Lake Cristol (2248 m a.s.l.), southern French Alps, were palynologically studied with the help of SEM. SEM was used in order to distinguish Pinus cembra pollen grains from other Alpine Pinus species with higher reliability, and served to reconstruct the Holocene vegetation history near the site with more accuracy. Recent pollen grains of P. cembra, P. sylvestris, and P. uncinata were examined by SEM in an attempt to establish the diagnostic characteristics of P. cembra pollen. The cappa exine surface of P. cembra is smooth to slightly rugulate, whereas that of the other two species is verrucate to rugulate and deeply sculptured. This difference in ornamentation is so significant that it is possible to identify the fossil pollen grains of P. cembra with SEM. Pollen analyses were carried out on the cores from Lake Cristol using both light microscopy (LM) and SEM. Then both the macro remains in the cores and the fossil trunks in the lake were 14C dated. Pollen curves of P. cembra obtained with LM and SEM showed considerably different trends. SEM results explained the age distribution of the fossil trunks better than LM results, showing the higher reliability of P. cembra identification by SEM. The arrival of the species to the site is estimated around 5600 yr BP (late Atlantic). Its abrupt disappearance is probably correlated with the deforestation around Roman times. Between those two epochs (from late Atlantic to early Subatlantic), fluctuation of P. cembra in the vegetation around the site is represented by its curve in the pollen diagram and the age distribution of the fossil trunks. Comparison of the local history of P. cembra with other nearby studied sites implies the SE to NW migration of the species during its settlement into the Alpine region.