Vegetation responses to rapid warming and to minor climatic fluctuations during the Late-Glacial Interstadial (GI-1) at Gerzensee (Switzerland)

High-resolution pollen analyses made on the same samples on which the ratios of oxygen isotopes were measured that provided the time scale and a temperature proxy after correlation to NorthGRIP. (1) A primary succession: The vegetation responded to the rapid rise of temperatures around 14,685 yr BP, with a primary succession on a decadal to centennial time scale. The succession between ca 15,600 and 13,000 yr BP included: (1.1.) The replacement of shrub-tundra by woodland of Juniperus and tree birch (around 14,665 yr BP) (1.2.) The response of Juniperus pollen to the shift in oxygen isotopes in less than 20 yr, (1.3.) A sequence of population increases of Hippophaë rhamnoides (ca 14,600 yr BP), Salix spp. (ca 14,600 yr BP), Betula trees (ca.14,480 yr BP), Populus cf. tremula (ca. 14,300 yr BP), and Pinus cf. sylvestris (ca. 13,830 yr BP). (2) Biological processes: Plants responded to the rapid increase of summer temperatures on all organisational levels: (2.1) Individuals may have produced more pollen (e.g. Juniperus); (2.2) Populations increased or decreased (e.g. Juniperus, Betula, later Pinus), and (2.3) Populations changed their biogeographical range and may show migrational lags. (2.4) Plant communities changed in their composition because the species pools changed through immigration and (local) extinction. Some plant communities may have been without modern analogue.These mechanisms require increasing amounts of time. (2.5) Processes on the level of ecosystems, with species interactions, may involve various time scales. Besides competition and facilitation, nitrogen fixation is discussed. (3) The minor fluctuations of temperature during the Late-Glacial Interstadial, which are recorded in δ18O, resulted in only very minor changes in pollen during the Aegelsee Oscillation (Older Dryas biozone, GI-1d) and the Gerzensee Oscillation (GI-1b). (4) Biodiversity: The afforestation at the onset of Bølling coincided with a gradual increase of taxonomic diversity up to the time of the major Pinus expansion.