Changes in Betula in the Holocene record from Iceland—a palaeoclimatic record or evidence for early Holocene hybridisation?

Abstract

Tree birch in its mountain form is the only woodland-forming tree found in the Holocene record of Iceland. Given the close relationship between tree-lines and summer temperature in Fennoscandia, it should therefore provide a valuable proxy temperature indicator for the Holocene in Iceland. Following a review of issues relating to the taxonomy of mountain birch, and the identification of tree birch in the palynological record, data are presented from the Tröllaskagi area of northern Iceland relating to the development of birch woodland in the early Holocene. It is argued that following the development of communities in which dwarf birch, Betula nana L., was important, true woodland communities only became established after a period of relative instability in the vegetation cover. Data from morphometric analyses of Betula pollen from this period are interpreted as representing probable hybridisation between the different forms of birch, rather than reflecting a clear climatic signal. The question is also raised of whether the progenitor of tree birch in Iceland was an immigrant form of mountain birch, (Betula pubescens Ehrh. ssp. tortuosa (Lebed.) Nyman as defined in Scandinavia), or whether Icelandic mountain birch developed by hybridisation in Iceland between Betula nana L. and Betula pubescens Ehrh. during the Holocene. The use of birch as a palaeoclimatic indicator, especially prior to the establishment of birch woodland at suitable locations and altitudes, should not therefore be accepted unless accompanied by other independent climatic evidence.