LATE HOLOCENE ENVIRONMENTAL EVOLUTION OF THE UPPER VALLE DEL GALLO (CENTRAL ALPS): AN INTERDISCIPLINARY STUDY

Abstract

Through an interdisciplinary study based on the application of geomorphological, stratigraphic, dendrochronological, pedological and
phytosociological methods, the late Holocene environmental evolution of the valley bottom of the upper Valle del Gallo (upper Valtellina / upper Valle dello Spöl, Central Italian Alps), an area high in the mountains characterized by a strong environmental dynamism, has been reconstructed. The geomorphological survey and some stratigraphic sections prove that the studied area was occupied, probably from the Late-glacial period, by a lacustrine basin. Laminites, microturbidites, fluvial, deltaic and debris flow deposits formed by the progradation of the surrounding fans deposited in the lake and buried it before the Middle Holocene, originating a flat primitive valley bottom. This original surface was subsequently carved by the regressive erosion of the main river and its tributaries which isolated, especially since the Middle Ages, numerous surfaces from the more recent debris flow activity. Some fans were partially dismantled while in some depressed sectors a succession of silty deposits formed by sheetfloods accumulated. The adaptation and the reaction of the arboreal vegetation to the debris flow activity allowed us to apply dendrogeomorphological techniques thanks to which some events were dated and the recent morphologic evolution of one of the fans reconstructed. The ecological succession takes place on two types of carbonatic substratum characterising the study area: sandy gravel deriving from debris flows, silt and fine sands deriving from sheetfloods. The soils and vegetation of the upper Valle del Gallo are generally scarcely developed. The vegetation is dominated by Pinus montana forests at various degrees of evolution, following pioneer communities in which the pine plays a key role. The pedological study shows that pine forests develop on Leptosols, poor and dry grounds rich in rock fragments of the fans, only locally susceptible of evolving into Cambisols. These surfaces are frequently buried by debris flows that cover the grounds thus inducing regressive stages in the vegetational series. Yet trees often survive these events since they are able to heal their scars, straighten themselves up and to develop adventitious roots from the buried base of the stems and branches. On the contrary Fluvisols develop on sheetfloods silty deposits, where peculiar herbaceous communities predominate, adapted to a light periodic burial that does not kill the vegetation. Last but not least must have been the influence of the anthropic deforestation, that has probably contributed in a considerable way to the present landscape setting.