On the presence of late‐glacial trees in western Norway and the Scandes: a further comment

Abstract

A response to L. Kullman (2005) On the presence of late-glacial trees in the Scandes. Journal of Biogeography32, 1499–1500, a reply to H.H. Birks, E. Larsen & H.J.B. Birks (2005) Did tree-Betula, Pinus and Picea survive the last glaciation along the west coast of Norway? A review of the evidence, in light of Kullman (2002). Journal of Biogeography, 32, 1461–1471. The primary aim of the paper by Birks et al. (2005b), as indicated by its title, was to examine the inferences of Kullman (2002) that Betula, Pinus, and Picea survived the last glaciation along the west coast of Norway. We highlighted and discussed the incompatibility between Kullman's inferences, drawn from dated late-glacial tree megafossil records from the Swedish Scandes, and other lines of palaeoecological evidence actually from western Norway. We regard the latter to be equally as valid as Kullman's evidence, and in no way to be dismissed as ‘merely’ negative evidence (Kullman, 2005). Late-glacial radiocarbon-dated plant and animal records positively demonstrate the local occurrence of arctic and high alpine plants, chironomids, oribatid mites, and beetles along the Norwegian west coast, consistent with the inference of a climate unsuitable for tree growth (e.g. Birks & Ammann, 2000; Birks et al., 2005a). This contradicts Kullman's biogeographical inference of late-glacial tree refugia there, which is based on pollen evidence alone, with no supporting macrofossil or megafossil evidence. To strengthen his interpretations, Kullman (2005) quotes his record of megafossils that suggest early Holocene tree growth at a high altitude in Swedish Lapland (Kullman, 1999). He says (Kullman, 2005) that this is fully compatible with the lack of tree macrofossils in the sediment record in the same lake (Barnekow, 1999). However this situation is puzzling rather than compatible, as the sediment record of Barnekow (1999) (which includes Salix polaris) seems to be ecologically incongruous with the simultaneous occurrence of Pinussylvestris and Alnus incana remains reported by Kullman. No explanation has been offered by either Kullman or Barnekow for this conundrum. Birks et al. (2005b) also documented the glaciological data that provide rational and valid evidence of an inhospitable habitat for trees at 1360 m a.s.l. during the late-glacial, an altitude not reached by trees even at the present time in the Scandes. Kullman (2005), however, vigorously defends his radiocarbon dates on megafossils and takes them as irrefutable evidence of tree growth on Mt Åreskutan in the late-glacial. Thus another incongruity is highlighted and this particular puzzle also remains unexplained and unsolved. Our paper discussed these points with the aim of stimulating further research. It is to be hoped that we, Kullman, and others can adopt a positive approach and try to find scientific explanations for the apparently incompatible evidence. For example, what is the pollen and macrofossil representation of a few isolated trees in an otherwise treeless landscape? Surface samples and modelling approaches could be used to investigate this (Birks, 2005). Is the pollen and macrofossil production of trees such as Betula, Pinus, and Picea reduced by marginal growing conditions at or near the tree line, and, if so, by how much? How could trees grow at 1360 m in the Swedish mountains in the Allerød and Younger Dryas? We have highlighted the palaeoecological incongruities. We hope that they can now be used as starting points to investigate matters further, utilizing a variety of techniques and approaches. Editor: Mark Bush