Significance of pollen assemblages for the vegetationcomposition of alpine shrub meadow in theQinghai-Tibetan Plateau, China

Abstract

The Qinghai-Tibetan Plateau is a fragile, globally significant ecosystem which responds sensitively to global climate change and regional human activity. Modern observations show that the ecological environment of the Qinghai-Tibetan Plateau has changed substantially, with obvious regional differences in vegetation composition, since the 1980s. Studying the history of vegetation succession and changes in the spatial distribution of vegetation on the Qinghai-Tibetan Plateau can provide a scientific basis for accurate assessment and prediction of future changes. For this purpose, we selected Zeku Basin, in the northeastern Qinghai-Tibetan Plateau. The study area is open and flat and it belongs to the alpine and semi-humid climate zone with distinct seasonality. The annual average temperature ranges from −3−3°C, and the annual precipitation ranges from 400−700 mm and is concentrated during June-September. The regional vegetation in Zeku Basin includes alpine meadow and alpine shrub, which is mainly composed of herbs and shrubs tolerant of cold and humid conditions. The alpine meadow is dominated by Kobresia and Carex , accompanied by Compositae, Ranunculaceae, Poaceae and Potentilla , which are mainly distributed in basin areas and on the sunlit slopes of the mountains. The alpine shrubs are dominated by Potentilla fruticosa , with occasional Hippophae and Spiraea alpina , mostly distributed on the shaded slopes of the mountains. The extent of the study area was determined according to the degree of vegetation regionalization with reference to Google satellite imagery of Qinghai Province. 31 representative sites were selected within the study area, using the random layout method. At each site surface moss samples were collected and a vegetation inventory conducted. We analyzed the pollen composition of the surface samples and determined the characteristics of the pollen assemblages and the pollen source area in alpine shrub meadow. The principal results are as follows: (1) The pollen assemblages are dominated by Cyperaceae (29.75%), Aster (21.86%), Ranunculaceae (16.99%), Rosaceae (7.84%) and Poaceae (5.1%), which mainly originate from the vegetation within 40 km of the sites (the proportion of total pollen loading reached was 83%). There is the minor representation of the pollen of arboreal plants, such as Betula , Pinus and Picea , which was wind-transported over long distances. The vegetation composition varied substantially between different sites within distances of 0–100 m of each site, resulting in a poor relationship with the pollen assemblages. However, the pollen assemblages and the vegetation composition were more similar over the distance of 0–3 km from each site, suggesting that the regionally-homogenized vegetation has a better relationship with the pollen assemblages. (2) We suggest that the surface pollen assemblages from lakes in the Qinghai-Tibetan Plateau may be greatly affected by extra-regional pollen or intrazonal vegetation in the shrub meadow. High percentages of Artemisia and Chenopodiaceae in pollen assemblages do not reflect the coverage of these taxa in the corresponding regional vegetation. Therefore, within shrub meadow environments of the Qinghai-Tibetan Plateau, these taxa should be excluded from lake-sediment-based pollen studies of regional vegetation succession and climate change.