The Surface Pollen Assemblages And Their Relationships With Climate In The Middle And Lower Reaches Of The Yalung Zangbo River

In order to investigate the relationship between the surface pollen and modern vegetation, surface soil samples collected from41sites in the middle and lower reaches of the Yalung Zangbo River were analyzed, and then the pollen assemblages of every zone were obtained. It is found that pollen assemblages of surface soil samples generally reflect the whole features of the modern vegetation, and they are basically similar in the spatial arrangement, types and the main and dominant genera. From west to east, The original vegetations of the region are mainly as follows:Subalpine grassland, Alpine meadow, Shrub, Quercus aquifolioides forest, Pinus densata forest, Coniferous-broad-leaf mixed forest, Evergreen coniferous forest and Coniferous-broad-leaf mixed forest mainly composed of coniferous trees(Abies georgei var. Smithii, Sabina wallichiana), Evergreen broad-leaved forest and Coniferous-broad-leaf mixed forest mainly composed of broadleaf trees, Coniferous forest or Coniferous-broad-leaf mixed forest mainly composed of Coniferous trees. Correspondingly, pollen assemblages were identified:Artemisia-Gramineae pollen assemblage, Cyperaceae pollen assemblage, Artemisia-Hippophae pollen assemblage, Pinus-Quercus-Artemisia pollen assemblage, Pinus pollen assemblage, Pinus-Abies-Artemisia pollen assemblage, Abies-Pinus-Artemisia pollen assemblage, Alnus-Betula-Tsuga pollen assemblage and Pinus pollen assemblage.Different vegetation types show different characters in the combination of pollen in surface soil. In non-forest communities, herbs pollen percentages are also superiority which higer than50%while the content of arboreal pollen percentages are less than30%, AP/NAP<1, In the forest area, the content of arboreal pollen is on the are more than42%, and pinus and Taxodiaceae are the dominant in the pollen assemblages; in the mixed coniferous and broad-leaved communities, the conifer pollen percentages are dominant position which higher than50%while the pollen of herbs is less than45%, AP/NAP>1.The ratio of A/Cy(the ratio between percentage of Artemisia and Cyperaceae) is useful to significantly identify the vegetation condition between Steppe and Alpine meadow. The result shows that the modern vegetation has higher ratio(A/C>1) and the Steppe vegetation has lower ratio(A/C<1). The study of pollen presentation and indication to vegetation shows that the taxa of Pinus, Picea, Abies, Betula, Artemisia are over-representative, Lauraceae, Quercus, Gramineae, Leguminosae are under-represented. Picea, Abies, Cyperaceae indicate plant community very well.This study focus on the comparing between the percentages of Pinus and Abies pollen from surface samples and they distribution altitude in the middle and lower reaches of the Yalung Zangbo River, then found that its altitude instruction significance. The altitude (2200-3500m) occurred Pinus densata and Pinus armandii consistent with the samples altitude (2500-3200m) over50-60%percentages of Pinus. Considerating of the vegetation coverage (50-60%), the percentage of Pinus with50-60%is a relible index that can indicate the altitude of Form. The elevation between the distribution of Abies georget var.smithii and samples of Abies percentage over60%(or50%) is consistent, and the pollen percentage also can represent vegetation coverage (50-70%). In addition, due to the differences in weight and productivity of Pinus and Abies, pinus pollen is more easily spread to high altitude regions, while most of Abies pollen is more landing below the distribution area of Form. Abies georgei var. smithii.The distribution of the study area surface pollen assemblages has a close relation with climate.600mm annual mean precipitation (MAP) line can distinguish between surface pollen assemblages of forest communities and Non-forest communities at the horizontal direction. Altitude due to temperature change is also an important factor affecting the spatial distribution of surface pollen. The herb and arboreal pollen percentages are significantly related to MAP, and the former has a negative correlation, the latter has a positive correlative with MAP. Spores, pollen concentration showed a weak positive correlation with MAP. There is no obvious correlation between the spores, pollen concentration and annual mean temperature (MAT). Pollen taxa and environment variable correlation analysis shows that pinus, Abies, Picea, Cyperaceae, Caryophyllaceae, Taraxacum type pollen percentages are positively related with MAP. Pinus, Tsuga, Cyperaceae, Caryophyllaceae are positively related with MAT.