Palaeoecological and experimental evidence of former forests and woodlands in the treeless desert pastures of Southern Tibet (Lhasa, A.R. Xizang, China)

Abstract

Palaeoecological and palaeopedological investigations around Lhasa (3650 m, 29°40′N/91°04′E, A.R. Xizang, China) provide evidence of human induced environmental changes which have occurred over the past 4600 years. The present desert pastures of Southern Tibet most probably replaced forests of Juniperus convallium, J. tibetica and Cupressus gigantea along with Prunus mira and Buddleja crispa in the understorey. Paleosol investigations and determined charcoal indicate a likeliness of forest destruction followed by increased erosion and sedimentation of colluvial soils. For the first time cereal pollen types could be clearly distinguished in southern Tibet. Juniperus pollen has been proved to have only a short dispersal range. The presence of cereal pollen and other human indicator pollen proves that it is more likely that human activity rather than climatic changes caused the forest decline during the younger Holocene. Successful non-irrigated reforestation trial experiments since 1997 on southern slopes around Lhasa with indigenous Cupressaceae have demonstrated that desertification of southern Tibet is a reversible process. As Lhasa receives 443 mm annual precipitation and has summer temperatures of above 10 °C it follows that the natural vegetation should be a forest, as opposed to the desert status quo that presides at present.