Pollen Analysis Of The Surface Sediments In Northwest Of Yunnan

The northwest of Yunnan province is affected by the alternation of the southwest monsoon blowing from the Indian Ocean and the westerly circulation,characterized by dry and wet seasons and small annual temperature difference.It is an ideal area for researching changes of the southwest monsoon and the westerly circulation by using the pollen data to rebuild paleovegetation of late Quaternary.The study of the relationship between modern pollen assemblages and the vegetation environment is the basis for the interpretation of paleovegetation and paleoclimate via fossil pollen spectra.In this study,the pollen analyses and the brief analyses of charcoal and pediastrum are made on the basis of sample collections in as many as 67 lake and reservoir surfaces in the four vegetation areas in Northwestern Yunnan,namely the Picea and Abies forest and wormwood shrub meadow area of alpine and plateau in the northwestern part,the Pinus yunnanensis forest and Picea and Abies forest area in the alpine and plateau of middle mountain in the middle eastern part,Pinus yunnanensis forest and Quercus aquifolioides forest in the middle mountain and gorge in the southeastern part,the Cyclobalanopsis glaucoides forest,Castanopsis orthacantha forest as well as Pinus yunnanensis forest in the plateau and basin valley in the southern part of northwestern Yunnan.All samples are identified as sporopollen species with 46 families and 66 genera,including dominant species of the four vegetation areas,such as Pinus,Picea,Abies,Quercus,Lithocarpus,and so on.Among the 67 lake surface samples,the cluster analyses of the pollen spectrum are made,except for one abnormally big amount of pollen in Asteraceae,which is later treated as an abnormal sample.Cluster analysis classifies the pollen spectra into four categories: the first category consists of 33 samples,mainly from the Pinus yunnanensis forest as well as Picea and Abies forest in the alpine and plateau of middle mountain in the middle eastern part of northwestern Yunnan,Pinus yunnanensis forest and Quercus aquifolioides forest in the middle mountain and gorge of southeastern part of northwestern Yunnan.The pollen spectra are mainly composed of Pinus,Quercus E and Poaceae,which reflects that the Pinus yunnanensis forest and Quercus aquifolioides are the dominant vegetation.The second category consists of seven samples,from Pinus yunnanensis forest as well as Picea and Abies forest in the alpine and plateau of middle mountain in the middle eastern part of northwestern Yunnan and Pinus yunnanensis forest as well as Quercus aquifolioides forest in the middle mountain and gorge of southeastern part of northwestern Yunnan.The pollen spectrum is mainly composed of Pinus,Picea,Quercus,Quercus E and Alnus,which basically reflects the forest vegetation in this region.The high content of Alnus in the pollen spectra may be due to the large number of Alnus planting in the process of returning farmland to forest in recent decades,because Alnus can maintain soil and water and improve the condition of soil.The third category consists of six samples,from Pinus yunnanensis forest as well as Quercus aquifolioides forest in the southeastern part of northwestern Yunnan.The pollen spectra are mainly composed of Pinus,Quercus E and Alnus,and there are a large number of herbaceous plants like Poacea.Arboreal pollen basically reflects the forest vegetation of the regional vegetation,and the relatively large proportion of Alnus pollen comes from the dominant tree species in this region after the deforestation.The high content of Poaceae is related to a wide range of crops in the source area of pollen.The fourth category consists of 20 samples,from Pinus yunnanensis forest as well as Quercus aquifolioides forest in the southeastern part of northwestern Yunnan.Although pollen spectra are mainly composed of arboreal pollen like Pinus and Quercus E,the herbaceous pollen like Poaceae and Chenopodiaceae not only reflects that Pinus yunnanensis forest and Quercus aquifolioides forest are the dominant regional forest vegetation but also shows that there is a wide range of crop area in the source areas of pollen.In order to explore the environmental factors influential on the above-mentioned pollen spectra,the principal component analysis of the pollen spectra composed of 20 species with high ecological significance and high content is made.The results show that the eigenvalues of the first two principal component axes are 4.10 and 2.91 respectively,indicating 20.5% and 14.5% of the total variance of the pollen spectra.In the first principal component axis,the positive load is mainly composed of the herbaceous plants such as Poaceae,Chenopodiaceae,Asteraceae,and the cold-tolerant Hippopheae,the negative load is mainly composed of moist-oriented forest species such as Picea,Abies,Tsuga,Quercus,and Betula.Thus,the first principal component axis that reflects the main environmental factors affecting the current pollen spectra is the rain.On the second principal component axis,there are mainly Pinus which adapt to the warm weather(mainly Pinus yunnanensis)and arbors which adapt to the cold weather such as Quercus E(mainly hard leaf Quercus aquifolioides),shrub(like Rosaceae)and meadow(like Cyberaceae),they locate in the positive and negative ends of this axis respectively,reflecting the temperature on the modern pollen spectra.The charcoal assemblages in northwestern Yunnan reflect the intensity and frequency of modern fires to a large extent,as well as the activities related to human production and life.Therefore,it can provide an important basis for the reconstruction of ancient forest fires.There is no obvious relationship between the concentration of Pediastrum and the lake habitat(lake area,PH value and water depth)in the northwestern Yunnan,indicating that the mechanism of different types of lakes on the growth of Pleurotus nebrodensis is different and there is no uniform regular pattern.