Résumé
This study addresses the timing and causes of the Holocene population expansion of Fagus sylvatica at two sites in the southern Prealps (Italy): Lago di Fimon and Lago Piccolo di Avigliana. At both sites pollen and microcharcoal have been analysed at high temporal resolution. The impact of humans and of fire on the forest dynamics is tested by means of time-series analysis and the influence of climatic change has been inferred from summer temperature and precipitation reconstructions in the Alps. The time intervals during which the population expansion of F. sylvatica occurred (ie, phases during which population doubling times were shortest) is determined by fitting linear regressions through the ln-transformed Fagus pollen-accumulation rate (ln(PAR)). At Lago di Fimon, the population expansion of F. sylvatica occurred at 7300-6400 cal. yr BP (FIM I). After a marked decline the population re-expanded at 5500-4700 cal. yr BP (FIM II). At Lago Piccolo di Avigliana F. sylvatica expanded c. 5300-4600 cal. yr BP (AVP I). Time-series analyses show that Fagus expanded after forest fires decreased during FIM I, and human impact likely contributed to the F. sylvatica population expansions at the two sites during FIM II and AVP I. The comparison with independent climatic records suggests that favourable climatic conditions (eg, cool and wet) were a determining factor. Our study suggests that the expansion of F. sylvatica populations has been triggered by more than one single factor alone.