| Summary: | The introgressive hybridisation between downy birch, Betula pubescens Ehrh., and dwarf birch, B. nana L., has been confirmed in Iceland but limited knowledge on the extent or timing of such hybridisation exists. The present study focuses on hybridisation in the Holocene, its frequency and scope, and the environmental factors initiating hybridisation. The history of Betula in Iceland is reviewed; possible means of dispersion from NW-Europe and development of woodlands in the Holocene are discussed. This work is centred on three studies published in attached papers III, IV and V which all rely on the results from papers I and II. The first two papers (I and II) describe the size and shape of Betula pollen, sampled from B. nana, B. pubescens and triploid hybrids in ten existing woodlands. The main study (III, IV and V) was conducted on subfossil pollen sampled from peat at three locations in Iceland. Pollen was prepared with heated NaOH, HCl, HF and acetolysis with conventional methods. The pollen samples were analysed with special emphasis on Betula pollen, which was measured for size, and any deviations from normal structure were noted for identification of hybrid pollen. Species proportions were calculated from size distributions. The three sites revealed different climatic conditions as reflected in the abundance of Betula species pollen, other pollen and spores, as well as in meteorological data existing from the 20th century. Proportion of Betula species fluctuated with climate changes and periods of hybridisation were detected at all three sites, especially connected to warming climate and tree birch advances near the Holocene thermal maximum. Kynblöndun ilmbjarkar, Betula pubescens Ehrh., og fjalldrapa, B. nana L., og genaflæði milli tegundanna er þekkt en lítið er vitað um umfang blöndunarinnar né hvenær hún hefur orðið. Þessi rannsókn beinist að tegundablöndun birkis á nútíma, þ.e. síðustu tíu þúsund árum, tíðni blöndunar og umfangi jafnframt því sem leitað er að svörum við því hvers konar aðstæður ýta ...
|
|---|