Holocene treeline shifts and monsoon variability in the Hengduan Mountains (southeastern Tibetan Plateau), implications from palynological investigations

摘要

The upper 8.45 m of a sediment core from Lake Naleng were analysed for its pollen and charcoal content to detect vegetation, climate, and human activity changes on the southeastern Tibetan Plateau (TP) since the Pleistocene-Holocene transition (11.7 cal kyr BP). The application of pollen indicator taxa, pollen sums, biomisation, and multivariate analysis helped to reconstruct environmental conditions during the Holocene. Furthermore, 64 surface samples from the southeastern TP and its foreland were analysed to investigate the relationship between modern pollen assemblages and vegetation. The modern vegetation was recorded in the surface pollen assemblages by pollen indicator types, although sediments also contain pollen spectra extra-regional pollen transported from lower vegetation belts, which is seen as a consequence of steep elevation gradients at the southeastern margin of the TP. The relation between different vegetation belts and temperature from the modern sample sites was used to infer quantitative temperature trends for the Holocene record. Spreading of forests at the beginning of the Holocene marks the transition to warmer conditions. From 10.7 to similar to 4.4 cal kyr BP open Abies-Betula forests were found at Lake Naleng probably reflecting a stronger-than-present summer monsoon. Temperature range reconstructions suggest a temperature that was about 2-3 degrees C warmer and a treeline position that was 400-600 m higher than today. A severe climate deterioration with temperatures about 1-2 degrees C colder than before is documented by a retreating forest between 8.1 and 7.2 cal kyr BP. Forests retreated while subalpine Rhododendron shrubs spread around Lake Naleng indicating colder conditions with temperatures 1-2 degrees C below early and mid Holocene level after 4.4 cal kyr BP. The forest decline at 4.4 cal kyr BP is considered to reflect climatic conditions as low concentrations of charcoal particles found in this period exclude human-induced fires as reason for the forest retreat. However, grazing indicators imply human influence on the environment since 3.4 cal kyr BP around Lake Naleng. We therefore consider the rapid treeline shift seen at 3.4 cal kyr BP as partly generated by human activity and partly by climatic conditions.