Holocene forest development along the Setesdal valley, southern Norway, reconstructed from macrofossil and pollen evidence

The Setesdal valley in South Norway runs north to south for 200 km, from alpine vegetation at 1200 m, passing the tree-line at around 1000 m, through Boreal forests, to Nemoral forest at sea level. The Holocene vegetation history and its altitudinal differentiation were reconstructed using pollen percentages and influx and plant macrofossil concentration records from four lakes along an altitudinal transect. During the early Holocene (c. 10500-8000 cal b.p.) Betula pubescens, Pinus sylvestris, Alnus, and Corylus expanded in the lowlands. Only Pinus and B. pubescens reached 1000 m asl (Lille Kjelavatn). Only B. pubescens reached Holebudalen (1144 m asl) at about the same time as it arrived in the lowlands. Between c. 8000-3000 cal b.p. mixed deciduous forest developed around Dalane (40 m asl) and to a lesser extent around Grostjorna (180 m asl), birch woodland with pine surrounded Lille Kjelavatn and birch woodland occurred at Holebudalen. From c. 3000 cal b.p. to present, the vegetation at Dalane hardly changed except for slight human impact and the immigration of Picea abies. At Grostjorna Pinus expanded. At Lille Kjelavatn Pinus disappeared and Betula became sparse as at the tree-line today. Betula retreated from Holebudalen thus leaving it above the tree-line in low-alpine vegetation. The strengths and weaknesses of pollen and plant macrofossil data were assessed for forest reconstructions. Where local pollen production is low, as near the tree-line, percentages of long-distance tree pollen can be misleadingly high. Pollen influxes of Betula and Pinus were much smaller near their altitudinal limits than at lower altitudes, although their macrofossils were equally abundant. The limited dispersal capacity of macrofossils documents the local presence of species and the character of the local vegetation, although macrofossils of some tree taxa are rarely found. Pollen and plant macrofossil evidence complement each other to provide a more complete reconstruction of Holocene tree-limits and tree-lines and hence climate changes, than either form of evidence alone.