Influence of human impact and bedrock differences on the vegetational history of the Insubrian Southern Alps

Vegetation history for the study region is reconstructed on the basis of po llen, charcoal and AMS C-14 investigations of lake sediments from Lago del Segrino (calcareous bedrock) and Lago di Muzzano (siliceous bedrock). Late- glacial forests were characterised by Betula and Pinus sylvestris. At the b eginning of the Holocene they were replaced by temperate continental forest and shrub communities. A special type of temperate lowland forest, with Ab ies alba as the most important tree, was present in the period 8300 to 4500 B.P. Subsequently, Fagus, Quercus and Alnus glutinosa were the main forest components and A. alba ceased to be of importance. (Castanea sativa and Ju glans regia were probably introduced after forest clearance by fire during the first century A.D. On soils derived from siliceous bedrock, C. sativa w as already dominant at ca. A.D. 200 (A.D. dates are in calendar years). In limestone areas, however, C. sativa failed to achieve a dominant role. Afte r the introduction of C. sativa, the main trees were initially oak (Quercus spp.) and later the walnut (Juglans regia). Ostrya carpinifolia became the dominant tree around Lago del Segrino only in the last 100-200 years thoug h it had spread into the area at ca. 5000 cal. B.C. This recent expansion o f Ostrya is confirmed at other sites and appears to be controlled by human disturbances involving especially clearance. It is argued that these forest s should not be regarded as climax communities. It is suggested that under undisturbed succession they would develop into mixed deciduous forests cons isting of Fraxinus excelsior, Tilia, Ulmus, Quercus and Acer.