Pollen and plant macrofossil analyses of radiocarbon dated mid-Holocene profiles from two subalpine lakes in the Rila Mountains, Bulgaria

摘要

Pollen analysis, plant macrofogsil determination and radiocarbon dating were performed on profiles of Holocene age obtained from two subalpine lakes (2320-2340 m a.s.l.) in the Central Rila Mountains, Bulgaria. The palaeovegetational reconstruction dates back to the mid-Atlantic (6500-6700 cal. BP). The plant macrofossil record revealed that groups of firms mugo and Juniptrus sibirica were distributed in the subalpine zone. The treeline was shaped by Pinus sylvestris and Pinus pence and was higher (up to 2200-2300 m a.s.l.) compared with the present-day. A change to cooler summers and warmer winters after 6700 cal. BP, characterized by a rise in humidity and precipitation, stimulated the formation of a coniferous vegetation belt dominated by Pinus sylvestris, Pinus peuce and Abies alba. The pollen assemblages also indicated a dominance of deciduous trees (Quercetum mixtum-Corylus phase) at low altitudes until c. 6000 cal. BP. The sub-Boreal was characterized by a wider distribution of Abies alba in the lower part of the coniferous belt and a gradual enlargement of the areas occupied by Fagus sylvatica and Picea abies after 3800-3500 cal. BP. The macrofossil finds of Picea abies testify to its expansion after 2800 cal. BP when average temperatures dropped and precipitation increased. Traces of human disturbance in the subalpine area are continuously registered in the pollen and plant macrofossil records since 3770 cal. BP, indicating livestock-grazing, burning of dwarf-pine (Pinus mugo) for new pasture land and lowering of the treeline.