Modern Pollen Rains And Their Relationships With Climate In Central Yunnan

Modern pollen rains are of great importance for the interpretation and quantitative reconstruction of paleovegetation and paleoclimate using fossil pollen data.The diverse vegetation types and numerous lakes in central Yunnan provide us good materials to study modern pollen rains.We collected surface sediment samples from 157 lakes(include natural and semi-natural lakes as well as man-made lakes such as reservoirs and ponds)in central Yunnan.Pollen/charcoal analysis of these samples was conducted.We also collected observational data of climate and available vegetation data.Modern pollen,charcoal,climate and vegetation data were processed using ordination analysis,cluster analysis,correlation analysis,and others to qualitatively and quantitatively reveal relationships among pollen,chaocoal,vegetation and climate,and to determine major climatic factors controlling the distribution of modern pollen rains and the spatial characteristics of main pollen taxa in central Yunnan.The following main results were obtained in this study:1.Modern pollen rains of central Yunnan were dominated by arboreal pollen.Dominant pollen taxa of pollen rains exhbitated obvious regional differences,which were associated with modern vegetation types.In the moist rainforest and monsoon evergreen broad-leaved forest area,pollen rains were dominated by pollen of evergreen and deciduous trees such as Lithocarpus/Castanopsis,Cyclobalanopsis,Quercus,Alnus,and Castanea,with low Pinus pollen.In the evergreen broad-leaved forest and Pinus yunnanensis forest area,pollen rains mainly contained Pinus,Alnus,evergreen Quercus and Betula pollen with absolute dominant Pinus pollen,and the average of AP/NAP(Arboreal pollen/Nonarboreal pollen)average of values was as high as 2.71.In the semi-humid evergreen broad-leaved,sclerophyllous forest and Pinus-Quercus mixed forest area,pollen rains were evenly shared by AP and NAP with an average of 1.01 of AP/NAP values,and the main arboreal pollen included Pinus,Alnus,Ulmaceae and Betula,while major nonarboreal pollen contained Poaceae,Polygonaceae and Cruciferae.2.The results of principal component analysis(PCA)on pollen percentages showed that the first and second principal component axes with eigenvalues of 0.56 and 0.21 explained 55.7% and 20.7% of the total variation of pollen data,respectively.Among them,dry-tolerant Pinus was on the positive half axis of the first principal component,while hypophytic AP pollen such as Salix,Corylus/Carpinus and NAP pollen such as Cyperaceae and Thalictrum were on its negative half axis,indicating the difference of effective moisture.On the positive axis of the second principal component were Alnus,evergreen Quercus,Pterocarya,and Lithocarpus/Castanopsis,while on the negative axis were Picea,Tsuga and Thalictrum pollen,reflecting the difference of temperature.Three pollen sample groups determined by cluster analysis were also distinguished by the first and second axes of PCA,and these groups corresponded to three different vegetation types.However,some samples might not be well classified due to the influence of non-zonality natural factors and human activities.3.The ordination analysis of modern pollen and climate data were conducted.The result of redundancy analysis(RDA)showed that the first and second ordination axes accounted for 27.2% of species information and 97.0% of species-environment variable information.The first axis indicated regional humidity,while the second axis reflected temperature status.P-dry,MAT and MAP are main climatic factors controlling the characteristics of modern pollen rains in central Yunnan.The results of correlation analysis showed that the major arboreal pollen except deciduous Querus and Juglandaceae as well as AP/NAP were significantly correlated with these main climatic factors,indicating that modern pollen rains reflected the changes of temperature and precipitation,and AP/NAP could be a proxy for precipitation in central Yunnan.4.The spatial analysis of major arboreal pollen data showed that different pollen taxa were mainly distributed in different temperature and precipitation ranges,i.e.,different hydrothermal configurations.In central Yunnan,the annual precipitation isohyte of 900 mm can be used as the limit of the distribution of dominant arboreal pollen taxa.Among them,the distribution of Pinus was greatly affected by temperature and precipitation,whereas Castanea,evergreen Quercus,Cyclobalanopsis,Euphorbiaceae and Lithocarpus/Castanopsis were mainly distributed in regions south to the 900 mm precipitation isohyte with relatively high temperature.Picea pollen was mainly distributed in the region with relatively low precipitation,while Alnus pollen appeared in the region with relatively high precipitation.The deciduous Quercus pollen was often seen in the whole Central Yunnan,and it had no obvious relationship with temperature and precipitation.