干旱区生态环境知识资源中心

The Tibetan highlands host one of the world’s largest pastoral ecosystems, but the evolutionary impact of the Tibetan nomadic livestock economy on the environment has not yet been investigated. Despite this grazing impact, the vegetation of the Tibetan highlands is widely believed to be natural. Our ecological approach to reconstructing the making of a pastoral environment uses the present composition of the plant cover as a baseline. Today’s prevailing plant functional types are grazing weeds highly adapted to grazing. The first pollen record of grazing weeds can thus be assumed to mark the onset of pastoralism, supposedly with goats and sheep introduced from the Middle East and the endemic large bovid of the yak, domesticated in the Tibetan highlands. This study represents the first attempt to determine the age of pastoralism with the help of palynomorphs using the indicator-species approach. This is independently corroborated 1) by the synchronous occurrence of pollen clumps indicating disturbance effects from trampling and 2) the precipitous decline of forest pollen on the eastern declivity of the highland during the mid-Holocene climatic optimum. As all pollen core sites currently have a climatic potential of forest as demonstrated by fruiting and progenitive forest relicts, it is suggested that early livestock holders continued to burn these forests to obtain pastures. The charcoal record supports this conclusion. It is hypothesized that the making of a pastoral environment in the Tibetan highlands started around 8.8 ka cal BP during the mid-Holocene climatic optimum. As the pattern of arboreal pollen decline at these sites contrasts with delta(13)C, delta(18)O TOC content, C/N ratio. and ostracod assemblages that are independent of rangeland management decisions of early pastoralists, we may conclude that pastoralists took advantage of the mid-Holocene climatic optimum to convert forests into high-yield pastures. Using pollen clumps as a proxy for herbivore load suggests a tenfold higher amount of livestock than of wildlife before the introduction of pastoralism. In contrast to pastoralism in and environments of the Old World’s desert belt, pastoralists in the eastern Tibetan highlands created their own environment transforming forests and tall grassland into the present golf course-like pastures. (C) 2009 Elsevier B.V. All rights reserved.