A coherent postglacial tree-limit chronology (Pinus sylvestris L.) for theSwedish scandes: Aspects of paleoclimate and "recent warming" based on megafossil evidence

A coherent elevational tree-limit chronology, based on megafossil wood of P inus sylvestris L. (Scots pine), is presented for the entire Holocene perio d in the Swedish Scandes. The chronology is argued, on the basis of a close modern tree-limit/climate relationship, to describe quite sensitively the secular-millennial course of summer temperature variations. Pinus had immig rated to the Scandes by about 11,700 BP (ca. 13,800 cal BP), which is much earlier than previously believed. The highest pine limit position, relative to the present day, was attained about 9400 BP (ca. 10,700 cal BP). Subseq uently, it has descended gradually by ca. 500 m up to the present. This per manent state of unidirectional flux compares well with the Milankovitch mod el of orbital climate forcing, prescribing an early-Holocene thermal optimu m, successively turning into a stage with cooler summers. Prior to ca. 8000 BP the rate of tree-limit recession was higher than later on, which is exp lained by additional cooling originating from particularly large glacioisos tatic land uplift before this break point. Apart from the smooth longterm t hermal decline and a few minor excursions (brief warmings and coolings), th e Holocene climate appears to have been fairly stable. There are no indicat ions of millennial-scale and large-amplitude climatic cycles. The virtually unbroken postglacial cooling, consistent with the predictive mechanism of orbital forcing, makes us hypothesize that this trend could be projected in to the future, given that only natural climate mechanisms were operative. I n such a perspective, the warm 20th century (and its high pine limit) stand s out as an anomaly. Possibly, it signals some anthropogenic climate forcin g, although contributory natural mechanisms cannot be precluded. The earlie st megafossil dates originate from high mountain peaks during a period when traditional glacial models envisage that the Scandes and large areas to th e east, south, and north were still covered by the extensive and entirely c ontinuous Weichselian ice sheet. Thus, it appears that the biogeographic an d deglaciation history of the Scandes is more complicated than previously u nderstood.