Late Holocene vegetation history suggests natural origin of steppes in the northern Mongolian mountain taiga

Abstract

Insular occurrences of steppe vegetation are a common feature of the northern Mongolian mountain taiga. Steppe vegetation is limited here to southern slopes, whereas northern slopes and valley bottoms are principally wooded with light and dark taiga forests. In a case study in the valley of the river Eroo at Khonin Nuga Research Station in the western Khentey Mountains, we searched for evidence of an anthropogenic versus natural origin of steppe vegetation on the southern slopes. Pollen data of three profiles covering the last 2500 years showed continuous presence of steppe throughout the late Holocene with human influence restricted to the recent past. Virtual absence of charcoal in the soil on and beneath three steppe slopes suggested that the present steppe grasslands are not replacing former forests burnt by humans or lightning. The floodplains in the center of the Eroo valley were recently deforested. This is suggested by the pollen analysis and by interviews with local people on landuse history. Steppe grasslands of the study area have probably never been used as pastures. Pastoral nomads traditionally avoided the Eroo valley near Khonin Nuga because of difficult access and high densities of wolves and bears. All our data suggest that the present vegetation pattern of the western Khentey with steppes (and single small Ulmus pumila trees) on south-facing slopes occurring as islands in the mountain taiga is driven by climate and relief and is not the result of human activities as suggested for other regions of Asia.