Elstu flórur Íslands
Fiðgeir Grímsson, Leifur A. Símonarson og Thomas Denk
Bls: 85–106 2.–4. hefti 75. árg. 2007

ÁGRIP
Í greininni er lýst eins nákvæmlega og unnt er plöntusamfélögum úr elstu setlagasyrpum á Íslandi, en þær eru taldar 15 og 13,5 milljón ára. Allmargar plöntutegundanna sem hér verður fjallað um hafa ekki verið greindar áður úr íslenskum jarðlögum og sagt er frá nýrri tegund af linditré sem nýlega var nafngreind og fékk latneska nafnið Tilia selardalense1, en við höfum kosið að nefna arnarlind á íslensku. Þegar bornir eru saman 15 milljón ára gamlir plöntusteingervingar frá Selárdal í Arnarfirði og Botni í Súgandafirði kemur í ljós að á þessum tíma þrifust hér mismunandi gerðir af plöntusamfélögum og geta þau gefið okkur töluverðar upplýsingar um ýmsa umhverfisþætti sem hér voru ríkjandi. Plöntuleifarnar í Selárdal og setlögin sem þær eru varðveittar í bera vott um frekar orkuríkt hálendissvæði markað af eldvirkni, en plöntuleifarnar og setlögin í Botni endurspegla hins vegar plöntusamfélag á láglendi. Plöntusamfélagið í setlögunum í Selárdal einkennist af lauftrjám og algengasta tegundin þar er arnarbeyki (Fagus friedrichii), en barrtré sem þrífast best í mýrum og fenjum, og einnig á þurrari svæðum, setja svip sinn á samfélagið í Botni. Plöntuleifarnar sem fundist hafa ofan Ketilseyrar í Dýrafirði eru yngri, eða um 13,5 milljón ára, og þar er flóran einhæfari og svipar meira til plöntusamfélagsins í Selárdal en í Botni. Litlar breytingar virðast hins vegar hafa orðið á samsetningu flórunnar á tímabilinu frá því fyrir 15 milljón árum og þar til fyrir 13,5 milljón árum. Íslensk plöntusamfélög á þessum tíma voru allrík af plöntum náskyldum núlifandi tegundum sem einkenna heittempruð miðjarðarsvæði, meðal annars vatnafuru (Glyptostrobus), magnólíu (Magnolia) og hjartartré (Cercidiphyllum). Benda plöntusamfélögin eindregið til þess að hér hafi verið frekar heitt og rakt heittemprað loftslag (svokallað Cfa-loftslag samkvæmt flokkunarkerfi Köppens).2 Rannsóknir á dreifiháttum plöntuhópa úr elstu setlögum landsins sýna að sumir þeirra, eins og kastanía (Aesculus) og beyki (Fagus), hefðu ekki getað numið hér land ef frum-Ísland hefði verið aðskilið frá meginlöndum af víðáttumiklu hafi. Þar að auki dreifast flestar aðrar tegundir sem þarna finnast með vindi frekar stutt frá móðurplöntunni. Rannsóknir sýna að þegar plöntur námu land á frum-Íslandi var það enn tengt við meginland Evrópu eða Norður-Ameríku á einhvern hátt. Þetta landsvæði var hluti af Grænlands- Skotlands þverhryggnum, en vitað er að stór hluti hans var ofan sjávarmáls alveg frá upphafi nýlífsaldar og til loka ólígósentíma og ef til vill með einhverjum hætti allt fram á miðbik míósentíma.
SUMMARY
Late Langhian to early Serravallian floras of Iceland

The oldest Miocene floras known today are found on the Northwestern Peninsula, in the 15 Ma (late Langhian) sediments on Mount Þórishlíðarfjall, Selárdalur, and in the Botn sediments, Súgandafjörður. The sediments are referred to as the Selárdalur-Botn Formation. The second oldest flora is found in the 13.5 Ma (early Serravallian) sediments on Mount Tafla close to the Ketilseyri farm and in Lambadalur, referred to as the Dufansdalur-Ketilseyri Formation.

The most characteristic feature of the 15 Ma Selárdalur flora is the dominance of Fagus (> 90% of macrofossils). Components of the Selárdalur flora are typical representatives of hardwood forests with a humid warm temperate appearance as found today in eastern North America (Appalachians), western Eurasia (northern and eastern Black Sea, southern Caspian Sea), and East Asia (Japan, central and eastern China). Typical taxa are, among others, Tilia and Aesculus. The Selárdalur flora represents broadleaved deciduous and evergreen (beech) forests found mainly on welldrained slopes. Only few taxa have been recorded from the Botn sediments. The most prominent ones are Glyptostrobus and Sequoia, which are represented by vegetative and fruiting twigs, whereas Fagus is mostly represented there by cupules and nuts and only very few fragmentary leaves. It is likely that Glyptostrobus and partly Sequoia where elements of floodplains adjacent to lakes. While Glyptostrobus tolerates high ground water table, Sequoia may have grown in slightly more elevated areas (hummocks), intermixed with some other hardwood taxa, such as Fagus. This lowland type of vegetation is likely to have merged into a hardwood forest similar to the one known from Selárdalur.

The pollen and macrofossil data from the Selárdalur-Botn Formation (15 Ma) give the impression of a broadleaved deciduous and evergreen forest with an admixture of conifers that covered mountain slopes and canyons. These forests were dominated by Fagus friedrichii, Tilia selardalense, Aesculus sp., Ulmus sp., Cercidipyllum sp., Platanus leucophylla, Magnolia sp., Rhododendron sp., and Lonicera sp. At the bottoms of valleys elements such as Alnus sp. and Salix sp. became more prominent as the groundwater level rose. In areas with high groundwater table, in valleys, around lakes and rivers, on floodplains, and in delta regions, conifers, mostly Glyptostrobus europaeus, and Alnus sp., dominated the floral assemblages. On hummocks, alluvial plains and well-drained lowland sites Sequoia abietina may have been more prominent. The pollen and macrofossil data from the Dufansdalur- Ketilseyri Formation (13.5 Ma) do not indicate significant changes in the broadleaved forests except for the sudden decrease of Tilia type pollen that was so conspicuous in the older (15 Ma) formation. A more prominent shift is seen in the conifers where the amount of Taxodiaceae pollen decreases considerably and Picea pollen become prominent. Most of the taxa recognized from the 15 Ma formation have diaspores that are dispersed over short distances by wind (Coniferales, Cercidiphyllum, Platanus, Ulmus), while only few may be transported over long distances (Betula and Rhododendron). Diaspores of the remaining taxa are dispersed by animals over short distances (Fagus and Aesculus by mammals), or long distances in various ways (Magnolia and Lonicera by birds, endozoochory; Platanus by mammals or birds, exozoochory). Terrestrial mammals from Icelandic sediments are extremely rare and only a few bones belonging to a small deer have been recovered from Pliocene sediments (3.5–3.0 Ma). Plant fossils indicate the presence of terrestrial vertebrates long before that time and it seems that the lack of terrestrial vertebrate fossils is mainly due to the unfavourable conditions for calcareous bones to be preserved.

The dispersal mechanisms encountered in late Langhian (15 Ma) plants from Iceland strongly indicate that colonization took place via land or over short seaways. At least Fagus and Aesculus have no potential for long–distance dispersal and taxa such as Ulmus, Fraxinus, and Tilia have a restricted dispersal radius. Only Betula and Rhododendron would have had the possibility to reach Iceland crossing extensive water masses. This indicates the presence of an almost continuous land connection to either Greenland/North America or the Faeroe Islands/Europe when these taxa colonized proto-Iceland. Most of the elements of the early floras from Iceland do not indicate a particular source area (North America/Greenland versus Faeroe Islands/ Europe), but belong to a widespread Northern Hemispheric element (all taxodiaceous taxa, Cercidiphyllum, Magnolia, Platanus, Ulmus, Lonicera, Rhododendron) and could have reached proto–Iceland both from the east and west. Interestingly, a considerable number of higher taxa (Glyptostrobus, Cercidiphyllum, Aesculus, Platanus, Ulmus, Magnolia etc.) were also part of the Brito–Arctic Igneous Province (BIP) floras, although these floras are at least 20 million years older. All species found in Icelandic Miocene sediments differ from the ones in the BIP floras, with the exception of Glyptostrobus europaeus.

Palaeobotanical data from Icelandic sediments indicate migration of plants over the proto-Iceland region long after the accumulation of the BIP floras. It is a matter of speculation, however, how long these migration routes were open during the Neogene. Considering a subaerial Greenland-Scotland Transverse Ridge in this region long before 16 Ma (the oldest rocks in Iceland) it cannot be ruled out that at least parts of this flora arrived much earlier in proto-Iceland and persisted until the accumulation of the 15 Ma Selárdalur and Botn sediments.

Comparison of fossils to modern living taxa indicates that the mean annual temperature in Iceland 15–13.5 Ma was between 9.3°C and 10.5°C.