Paleodistribution of the alerce and cypres of the Guaitecas during the interstadial stages of the Llanquihue Glaciation: Llanquihue and Chiloé provinces, Los Lagos Region, Chile.

Abstract

The purpose of this study is to contribute with new paleobotanical knowledge of the Seno de Reloncaví and the adjacent eastern and northern coast of Isla Grande de Chiloé. The authors have collected at eight different sites subfossil tree trunks of alerce (Fitzroya cupressoides (Mol.) Johnst.) and Guaitecas cypress (Pilgerodendron uviferum (D. Don) Florin). The tree trunks were in situ and very well preserved. According to six preliminary radiocarbon dates showing finite ages between 42.600 and 49.780 ¹⁴C yr BP, the deposits with the tree trunks would correspond to interstadial stages of middle Llanquihue glaciation (isotopic stage 3). The authors have analyzed the stratigraphy and fossil pollen content at two of these sites, Isla Tenglo in the Llanquihue Province (41°30'S) and Punta Pirquén, along the eastern coast of Isla Grande de Chiloé (42°13'S). Conifer tree trunks at both sites are associated with organic silt and peat underlying a thick succession (2-4 m) of laminated grey silt (lacustrine rhytmite) and capped by fluvioglacial sand and gravel which correspond to the Last Glacial Maximum. The pollen spectra of the tree-containing organic deposits at the base of the succession are dominated mainly by arboreal forest taxa: Nothofagus dombeyi-type and several conifers, Fitzroya cupressoides (Mol.) Johnst., Pilgerodendron uviferum (D. Don) Florin, Saxe-gothaea conspicua Lindl., Podocarpus nubigena Lindl. and Lepidothamnus fonkii (Phil.) Ball. The spectra suggest a relatively warm and wet interstadial during middle Llanquihue glaciation. Conifer forests probably had wider and more continuous distribution than today, occupying lowland sites in the Central Depresion of Llanquihue and Chiloé provinces before the onset of the cold stadials of late Llanquihue glaciation (30.000-14.000 ¹⁴C yr BC, isotopic stage 2). These forests have a disjunct distribution at present, with populations present on the summits and high elevation sites of both mountain ranges (Andes and Coastal) in the Lake Region, and small isolated populations present at lower elevations. This montane distribution is attributed to increasing temperatures since the late glacial and during the Holocene, and hence, the populations at lower elevations would be relicts from glacial stages, as it is suggested by a greater degree of genetic divergence present in these relicts as compared to the high elevation populations.