From the coldest ice age to green carpets of beauty: A 20,000-year vegetation history from the Hyrcanian forest refugium of northern Iran

Abstract

The Hyrcanian region is a biogeographic entity of high biodiversity and a centre of Arcto-Tertiary relict flora. A pollen record from the mid-elevation of the Alborz Mountains (northern Iran) reaching back to 20,000 cal BP reveals the Late Quaternary vegetation history of this globally important forest ecosystem. For the period 20,000–14,419 cal BP the pollen record indicates the presence of beech, oak, and birch associated with elm and hornbeam, which indicates the presence of a Weichselian refugium for deciduous tree species. The simultaneous presence of abundant pollen attributable to Artemisia, Amaranthaceae, Apiaceae, and Hippophaë rhamnoides suggests the presence of a dry and cold steppe environment at higher elevations. For the period 16,746–14,419 cal BP, the pollen record indicates a steppe vegetation around the studied peatland (with a predominance of Artemisia, Amaranthaceae and Apiaceae species, along with Eremurus), a slight expansion of oak, and a strong decrease of beech. The period 14,419–12,993 cal BP started with a substantial rise in the values of pollen attributable to Quercus, Carpinus, and Ulmus with the near absence of pollen of dry-growing herbaceous vegetation. For 12,993–11,745 cal BP, a drastic decline of arboreal pollen types, pronounced peaks of NAP types, and constant values of pollen of Ephedra persuasively point to a dry period in the studied region. Towards the end of this period, when the climate apparently became somewhat warmer, deciduous trees expanded around the study site earlier than in other regions, probably because of the site’s proximity to the refugium. After 8600 cal BP, oak declined in importance whereas beech and hornbeam expanded, showing the establishment of the contemporary temperate climate and vegetation. A pronounced expansion of alder and wingnut took place around 4000 cal BP. A remarkable decline of Pterocarya pollen at 1060 cal BP may relate to human impact and/or a cooling/drying climate.