Study On The Relationship Between Topsoil Pollen Assemblages And Modern Vegetation In The Peak Cluster Depression Of Northwest Guangxi

There still exists the rocky desertification phenomenon in karst area of southwestern China,which profoundly restricts the sustainable development of the region and directly threatens the ecological security of the Yangtze River and Pearl River basins.Although human activities have a significant impact on the occurrence of rocky desertification,further evidence is needed to determine the specific degree of impact.Palynological analysis of geological layers is one of the important methods for exploring ancient vegetation and climate,and the analysis of the relationship between topsoil pollen and modern vegetation is a prerequisite for interpreting palynological data.The northwest of Guangxi is a sloping zone between the southeastern edge of the Yunnan-Guizhou Plateau and the Guilin karst plain,where the cluster of peaks and valleys is the most typical and severe region of karst landform and rocky desertification.The slope-valley system in this area is also an important site of erosion and deposition.Based on this,84 samples were selected for the study,Which from11 different land use types in a typical karst Peak-cluster depression area in southern Huanjiang,Guangxi.The results showed that:(1)A total of 76 families/genera of pollen types were identified,and conifers were dominant in the pollen assemblages,with an average pollen content of 57.83%,among which Pinus and Cupressaceae were dominant.Fern spore followed,with an average content of38.36%,dominated by Dicranopteris,Pteris,Polypodiaceae and Nephrolepis.Then there is herbaceous pollen(32.46%),among which the main pollen taxa are Poaceae、Amaranthaceae、Chenopodaceae、Asteraceae and Araceae.Broad-leaved trees had the lowest content,with an average of 9.71%.Chestnut、Euphorbiacea、Quercus and Fabaceae were the main species.The topsoil pollen assemblage can basically reflect the general characteristics of vegetation around the depression.(2)The pollen assemblages of different land use types were significantly different,and the herbaceous pollen content was the highest in paddy field(62.52%),mainly in rice-type Poaceae(≥40μm).Coniferous pollen was the main type in the topsoil of mulberry forest,and the content reached 70.66%.The content of herbaceous pollen in maize field was 45.55%,mainly Asteraceae and Zea.In the secondary forest,71.25% conifer pollens were dominant,the broad-leaved trees pollens increased to 11.12%,and the fern spore content reached the highest 57.44%.The pollen of broad-leaved trees in orange forest increased to 18.99%,but the pollen of Rutaceae was less.The abandoned land was dominated by coniferous pollen and fern spores.At the same time,because the abandoned land was an outcrop of groundwater,the content of concentricystes was abundant.The herb pollen content in the sweet potato field is 60.65%,but because it is close to the paddy field,the herbaceous pollen type is similar to the paddy field,and Batatas pollen is rarely seen.The pollen content of coniferous is higher.The pollen assemblage of cultivated forest is similar to that of secondary forest,mainly coniferous and ferns.(3)PCA analysis can distinguish paddy field,maize field and paper mulberry well.It can basically distinguish secondary forest,cultivated tree forest,orange forest,grassland and abandoned land.However,sweet potato field,soybean field and mulberry forest can not be well distinguished,which may be related to pollen transmission mode,sampling time,pollen preservation conditions,etc.(4)The concentration of pollen in non-agricultural land is significantly higher than that in agricultural land,The moss samples was significantly higher than that of soil samples.which may be related to it’s upper vegetation cover and difference in topsoil disturbance.