A Coherent Postglacial Tree-limit Chronology (Pinus sylvestris L.) for the Swedish Scandes: Aspects of Paleoclimate and “Recent Warming,” Based on Megafossil Evidence

Abstract

A coherent elevational tree-limit chronology, based on megafossil wood of Pinus sylvestris L. (Scots pine), is presented for the entire Holocene period 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 immigrated 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). Subsequently, it has descended gradually by ca. 500 m up to the present. This permanent state of unidirectional flux compares well with the Milankovitch model of orbital climate forcing, prescribing an early-Holocene thermal optimum, 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 explained by additional cooling originating from particularly large glacioisostatic land uplift before this break point. Apart from the smooth long-term thermal decline and a few minor excursions (brief warmings and coolings), the Holocene climate appears to have been fairly stable. There are no indications 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 into the future, given that only natural climate mechanisms were operative. In such a perspective, the warm 20th century (and its high pine limit) stands out as an anomaly. Possibly, it signals some anthropogenic climate forcing, although contributory natural mechanisms cannot be precluded. The earliest megafossil dates originate from high mountain peaks during a period when traditional glacial models envisage that the Scandes and large areas to the east, south, and north were still covered by the extensive and entirely continuous Weichselian ice sheet. Thus, it appears that the biogeographic and deglaciation history of the Scandes is more complicated than previously understood.