Records Of Airborne Pollen Deposition And Its Relationship With Meteorological Factors In Xianlin District,Nanjing

Introduction and AimsThe modern process of pollen,including dispersion,transportation and deposition,are one of the key aspects of Quaternary palynology research.Modern pollen can reflect the surrounding vegetation to some extent,which may provide quantitative basis for interpretation of Quaternary pollen data.This work aims to present the distribution rules of airborne pollen and its relationships with meteorological factors in Nanjing.This information will be helpful to offer quantitative basis for Quaternary palynology paleovegetation research of this area.LocationNanjingMethodsHere we present the results of airborne pollen distribution during two consecutive years(2013-2014)using a Burkard pollen trap.Among them,samples during 2014 was identified every other day.In the meantime,surface pollen samples and Tauber pollen samples were collectted as reference for comparsion.Statistical analysis has been used to distinguish the relationships between pollen concentrations of the main pollen types and the meteorological elements.We traced the source area of Quercus on the basis of Hybrid Single-Particle Lagrangian Integrated Trajectory(HYSPLIT)Model combined with the location of oak forests.ResultsA total of 103 taxa were determined in the study area.Out of these,66 belonged to the arboreal and shrubs,28 to the herbs and 8 to the ferns.Two pollen concentration peaks occurred during one year,i.e.spring and autumn.Apart from these peaks,wetland herbs and ferns dominated in summer while in winter appeared in spring and the other happened in early autumn there is uncomment instance of Artemisia,Chenopodiaceae and Pinus.The seasonal variation of pollen types correspond to their flowering periods,especially for Platycarya,Typha,Cupressaceae,Morus and Humulus.A decreasing trend of average pollen concentration towards 2014 was noted,which probably due to the increasingly raining days during the flowering periods.Correlation analysis between pollen concentrations of the main pollen types and the meteorological elements obtaining from the same meteorology station indicated that pollen concentrations of arboreal taxa(i.e.Quercus,Platycarya,Morus,Pinus)show a negative relation with temperature and relative humidity,while data from the Artemisia and Humulus show a significant positive relation with temperature and a negative relation with relative humidity.The source area of oak pollen during its peak period was investigated on the basis of HYSPLIT trajectory model.The air mass at 50m a.s.l.was first influenced by the wind from the northwest and then the easterly winds,which passing through Inner Mongolia Plateau,the Northeast Plain,the Yellow Sea and finally arrived at Nanjing.According to the overlaying out of oak forest and the back trajectory information,we found that some oak pollen were mainly from the northeast Changbai Mountains.Results indicated that main pollen types were Pinus?Quercus?Pterocarya?Ulmus?Rosaceae?Compositae?Gramineae,and the pollen assemblages generally reflect the whole features of the surrounding vegetation.Principal component analysis(PCA)produced a good distribution of samples from different sampling zones,i.e.lawns,farmlands and forested areas.Farmland has the lowest average pollen concentration(4964.28 grains/g),which may caused by ploughing and the microbial environment.Average pollen concentration and pollen influx are the high in the forest(29176.4 grains/g and 49358.4 grains/cm2/yr respectively).The decrease of pollenconcentration may caused by an increase in human activities.When investigating the human activities in this area during historical days,we can perform a comprehensive analysis of pollen concentration and pollen taxa related to agriculture.Proportions of arboreal pollen types in each Tauber trap samples are lower than their corresponding surface soil samples,particularly for Pinus.In terms of pollen spectra from the three sampling media,the percentage of upland herbaceous pollen is higher in Tauber trap samples,while the Burkard pollen trap is more likely to capture arboreal taxa.Pollen data from the surface samples show similar patterns to local vegetation zones.The main pollen types derived from the three sampling methods are quite similar,while the Burkard pollen trap can capture some taxa with a weak persistence in the soil(e.g.Cupressaceae),as well as pollen taxa releasing from a long distance.Pollen assemblages from Burkard pollen trap may reflect various pollen spectra with a larger spatial scale.Main conclusions1.The annual sums of daily airborne pollen concentrations vary in different seasons.2.A certain amount of airborne pollen would be perished by persistent rainfall in that the peak periods of airborne pollen can only appear under such favorable conditions as higher temperature,lower relative humidity and continuous fine weather.Airbrone pollen spectra are more sensitive to the change of climate than the vegetation consititution.3.While the rise of temperature in last winter and spring is an incentive for the advance of the flowing stage,the key factor in determining the pollen concentration is the change in rainfall during the flowing periods.4.The combination of HYSPLIT and source locations provides a new method to trace the source area of pollen.However,long-term studies need to be conducted to further validate these results.