Читать книгу Biogeography in the Sub-Arctic - Группа авторов - Страница 38

Iceland is a volcanic island with several intrabasaltic plant‐bearing sedimentary rock formations. The oldest sedimentary rocks are found in Northwest Iceland (ca. 15 Ma) and East Iceland (ca. 14 Ma) and are of Miocene age (Figures 1 and 2). In a simplified version the strata become progressively younger towards the volcanic zones crossing the centre of the island from southwest to northeast (for a recent review of the geology of Iceland see contributions in Sigmundsson et al. 2008). Sedimentary rock formations occur in Miocene to Pleistocene strata and often contain plant fossils. The palaeo‐floras of Iceland have been comprehensively studied in recent years (e.g. Denk et al. 2011; see also Denk et al. 2005; Grímsson et al. 2005, 2008; Grímsson and Símonarson 2006, 2008a, 2008b).

The middle Miocene plant assemblages of Iceland (15–12 Ma) record vegetation thriving under a warm and moist climate. Wetlands and riparian vegetation of the lowlands was characterized by warmth‐loving taxa, such as taxodiaceous Cupressaceae, Magnoliaceae (Magnolia, Liriodendron), Lauraceae (Sassafras), Platanaceae (Figure 4E and F) and others, whereas the well‐drained vegetation of the hinterland comprised forests dominated by Fagus with evergreen trees and shrubs in the understorey (Rhododendron, Ilex). The endemic linden tree, Tilia selardalense Grímsson, Denk and Símonarson (Figure 4B) was confined to these forests (Grímsson and Denk 2005; Grímsson et al. 2007a, 2007b; Denk et al. 2011). The oldest floras of Iceland share a few taxa with the Paleogene floras of Greenland and/or Svalbard: Glyptostrobus, Platanus, Fagus, and Tilia.

A major change is seen in the early Tortonian (10 Ma) floras of Iceland, both in the palynological and the macrofossil record. Whereas herbaceous taxa did not play a significant role in the older floras, they amount to 30% of all recorded plant taxa in the 10 Ma floras. This increase in herbaceous plants is accompanied by the first occurrence of small‐leaved Ericaceae typical of the modern tundra vegetation in Iceland (Vaccinium, Arctostaphylos) and boreal conifers such as Larix. Nevertheless, several warmth‐loving elements persisted and new elements are recorded (Ginkgo; Denk et al. 2005, 2011). Floras preserved in strata between 10 and 3.6 Ma reflect stepwise cooling; Fagus persisted until 7–6 Ma and Quercus until 5.5 Ma, whereas the evergreen, large‐leaved Rhododendron aff. ponticum L. ranges from the oldest to the 3.8–3.6 Ma floras. At 4.4–3.6 Ma, small‐leaved Salicaceae occur for the first time. The second major reorganization of the vegetation is recorded in floras from the Pliocene–Pleistocene transition. Temperate woody elements are not found in any of the Pleistocene floras, which are essentially similar to the modern flora of Iceland (Denk et al. 2011).