Between the mountains and the sea: Late Holocene Caspian Sea level fluctuations and vegetation history of the lowland forests of northern Iran

Pollen analysis and radiocarbon dating of a core from northern Iran enable reconstructing the past three millennia of vegetation dynamics of the Caspian lowlands and Alborz foothills of central northern Iran as a function of Caspian Sea level oscillations, climate change and human impact. The high values of Alnus pollen and a diverse assemblage of non-pollen palynomorphs indicate that during most of the time span covered by the record a wet eutrophic to mesotrophic alder (Alnus cf. glutinosa) carr was present at the coring site. Only at the onset of the 2600-BP highstand event tree growth was hampered because of inundation. Our findings (including calcareous gyttja with remains of Charophytes; remnants of molluscs, freshwater snails, Foraminifera, Ostracoda, Algae and sponges) indicate that the highstand extended to and culminated in the period 2100 to 1900 calBP. Wingnut (Pterocarya fraxinifolia), which was a major component in the vegetation composition of the Caspian lowland from the beginning of the record, strongly decreased after ca. 1200 calBP, most probably due to a drier or colder climate. Artemisia and Chenopodiaceae and Amaranthaceae (C-A) show fluctuating and largely parallel values throughout the record, suggesting the natural co-occurrence and similar ecology of their corresponding plants in the coastal areas under the influence of Caspian Sea transgressions. The steep slopes south of the mire were probably most of the time covered by a species-rich deciduous forest predominated by hornbeam (Carpinus betulus) and beech (Fagus orientalis) accompanied by oak (Quercus castaneifolia), elms (Ulmus glabra/U. carpinifolia), Persian ironwood (Parrotia persica), maples (Acer velutinum/A. cappadocicum), box tree (Buxus hyrcana), grape (Vitis vinifera), ash (Fraxinus excelsior), and lime (Tilia platyphyllos). However, climate change and human impact triggered significant vegetational changes both in the lowland and the hilly upland. The first signal of human activity in the area can be inferred from findings of pollen of Juglans regia at ca. 2350 calBP and slightly later of Fagopyrum esculentum. A dramatic decline of tree pollen over the last centuries suggests that tree cover of both the lowland and upland landscape surrounding the mire declined and forests were increasingly replaced by open vegetation most likely as a result of intensified human activities.