Holocene history of the forest–alpine tundra ecotone in the Scandes Mountains (central Sweden)



The Holocene history of the forest-alpine tundra ecotone in Central Sweden (Scandes Mountains) is inferred from radiocarbon analyses of subfossil wood remains. Pinus sylvestris was the dominant subalpine tree species during the early Holocene, when it ascended almost 200 m higher than currently. A short climatic episode (less than 100 years) is postulated to have triggered erosional processes around 6300 n.p., and extinguished the upper part of the subalpine pine woodland. Subsequently, a subalpine belt of Betula pubescens ssp. tortuosa and Alnus incana developed. A Holocene thermal optimum occurred around 6100 b.p., when the birch/alder belt nourished and the tree-limits probably reached their highest levels during the Holocene. Shortly after 6000 b.p., a long-term pine forest retrogression started and the birch/alder belt was disrupted by expanding snow-beds. Pine receded slightly at its tree-limit, but the uppermost belt of closed pine forest (presently dominated by birch) remained intact until c. 3300 b.p., when a severe climatic deterioration occurred. The present-day subalpine belt of pure birch forest developed successively and increased in vertical extent after c. 5300 b.p., when summer temperature declined. The evolution of the birch belt is postulated to have been ultimately a response to decreased seasonally, which favoured birch at the expense of pine. Because of the ‘inertia’ characterizing the highest pine forest, the birch belt was relatively narrow until a major thermal decline e. 3300 b.p., when it made a massive downslope expansion. The latest phase of pine recession was during the Little Ice Age, 800–300 b.p.