Relationship between topsoil pollen and modern vegetation at the Huanjiang Karst Observation Station of Guangxi

HAO Xiudong; LU Yaxian; OUYANG Xuhong; LI Lixue; QIN Linjuan; WEI Jiasheng; WANG Ailan; HUANG Linqian; LIN Chunyu

doi:10.11932/karst2025y017

Abstract：This study takes topsoil pollen samples from different land use methods/vegetation types at the Huanjiang Observation and Research Station for Karst Ecosystems,Chinese Academy of Sciences(hereinafter referred to as the Huanjiang Station)as its research object.The Huanjiang Station is located in the southern part of Huanjiang Maonan Autonomous county,Hechi City,Guangxi and represents typical karst peak-cluster depression ecosystem in China.It is situated in the subtropical monsoon climate zone,with a mild climate and abundant rainfall throughout the county.The forest communities are primarily composed of evergreen broadleaf forests,followed by some deciduous broadleaf and mixed evergreen coniferous and broadleaf forests. This study aims to explore the characteristics of topsoil pollen assemblages under different land use methods/vegetation types at the Huanjiang Station.It seeks to establish a correlation between topsoil pollen and modern vegetation to provide theoretical references for ecological restoration,Quaternary paleoenvironmental and paleoclimate reconstruction,and historical research on human interference activities in the study area and comparable karst regions.All 24 topsoil and moss samples were prepared at the Key Laboratory of Environment Change and Resources Use in Beibu Gulf(Nanning Normal University),Ministry of Education.Standard conventional acid-base methods were then used to extract the pollen.About 30 g of dry topsoil samples and about 3 g of dry moss samples were weighted.HCL was added to dissolve calcareous minerals,and HF was used to digest silica,followed by several rinses with distilled water.Finally,a 7-μm nylon sieve was employed to remove unwanted small particles,and the pollen was collected to a 5-ml centrifuge tube.A tablet containing Lycopodium spores(10,315 grains per tablet)was added to each sample to calculate pollen concentration.The percentage of each pollen taxon is based solely on the total terrestrial pollen sum,whereas the percentage of spores is calculated from the combined sum of both pollen and spores.Pollen and spores were identified with the use of a Leica DM4000 B microscope at 200/400x magnification.More than 300 pollen grains of per sample were counted.The program Tilia(Version 1.7.16)was used to construct the pollen diagrams.Principal Component Analysis(PCA)was applied to the data pollen percentage with the use of Canoco(Versions 5.0)software. The results show:(1)A total of 49 pollen taxa(genera or family)were identified from 24 topsoil(or moss)samples collected at Huanjiang Station.Among these,25 taxa belonged to trees and shrubs,mainly including Pinus,Cupressaceae/Taxodiaceae,Fabaceae,Euphorbiaceae,Magnoliaceae,and Moraceae.Pinus had the highest content,averaging 56.8％and reaching a maximum of 87.62％.Additionally,20 taxa were Herbaceous,mainly Poaceae,Asteraceae,and Araceae.Fern spores were mainly from Nephrolepis,Dicranopteris,and Pteris.Freshwater algae(Concentricystis)spores were also present in the samples.(2)There was a large variation in the concentration of topsoil pollen among different land use methods/vegetation types in the depression of Huanjiang Station.The lowest pollen concentration was 4,131 grains·g-1 in topsoil samples from secondary growth forest,while the highest was 416,781 grains·g-1 in planted restoration forest.The average pollen concentration was relatively low,at 54,432 grains·g-1.A significant difference was observed between the average concentrations of topsoil samples(27,371 grains·g-1)and moss samples(189,738 grains·g-1).Among all samples,Pinus pollen had the highest concentration,reaching 19,232 grains·g-1,followed by Asteraceae(2,280 grains·g-1),Poaceae(1,535 grains·g-1),and Euphorbiaceae(1,461 grains·g-1).In addition,fren spore concentration was relatively high,with an average of 6,353 spores·g-1.(3)PCA results showed that the pollen assemblage was well separated and heterogeneous,allowing for rough classification.Among different land use methods/vegetation types,corn fields corresponded clearly with Zea,and coniferous woodland corresponded clearly with Moraceae,enabling effective differentiation.Secondary growth forest and planted restoration forest were relatively well grouped. The conclusion are as follows:(1)The topsoil pollen composition corresponds well with the vegetation types of each land use method/vegetation type,and the combined topsoil pollen assemblage can basically reflect the overall characteristics of vegetation surrounding the depression.(2)PCA analysis can effectively distinguish grassland,planted restoration forest,coniferous woodland,secondary growth forest,and cornfields,indicating that topsoil pollen from different land use methods/vegetation types exhibits clear differentiation.However,mulberry forests cannot be well distinguished,which may be related to the timing of Morus sampling and pollen preservation conditions.(3)Pollen concentrations of different land use methods/vegetation types follow the order:planted restoration forest＞mulberry forest＞cornfield＞secondary growth forest＞grassland＞coniferous woodland.This pattern may be closely related to vegetation composition,soil disturbance,and pollen preservation conditions.