Vegetation Changes And Response Regimes Since The Last Deglacial In Mountain Area Of Central Inner Mongolia: Based On Pollen Record Of A Lake Core

Holocene vegetation pattern and possible forcing and response regimes in marginal monsoon area of north China attract scientific interests because it can provide long term perspectives to understand the dynamics of modern vegetation,which is useful for modern forest preservation and prediction of forest response in the further warming.In this paper,based on the well-dated and high-resolution fossil pollen record of Wulanhushao lake core(WLHS core)in Chahar Right Back Banner,Inner Mongolia,vegetation history during past 18,520 years was reconstructed in the mountain area of the northern Ulanqab city.Furthermore,multi-statistical analysis including redundancy analysis(RDA)and sensitivity analysis(Pearson correlation)were performed on WLHS fossil pollen data and potential influencing variables,such as Asian winter monsoon(WM),Asian summer monsoon(SM),mean annual temperature(MAT),mean summer temperature(MST),and human impact index(HII),to disclose possible relationships in hope that potential forcing and response regimes can be figured out.In addition,13 lacustrine fossil pollen sequences in marginal monsoon area of northern China were synthesized and primary vegetation biomes were reconstructed for each site based on calculated biome scores and thresholds estimated on published pollen representation.Then,spatial and temporal pattern of vegetation were plotted considering geographic locations,climate and elevations,and potential mechanisms on vegetation migration,refugia,resist and resilience,etc.were discussed for forcing and response regimes.The primary conclusions are as follows:(1)The vegetation reconstructed by pollen percentages and pollen based biome scores experienced eight stages since 18.52 cal.kyr BP.Prior to 17.47 cal.kyr BP,desert steppe extensively prevailed in study area without any forest;steppe replaced desert steppe between 17.47 and 16.31 cal.kyr BP;from 16.31 to 10.73 cal.kyr BP,sparse birch forest or small patches came into being in shady slopes of high elevation,and most area was still occupied by steppe,thus a woodland steppe landscape prevailed;temperate deciduous forest steppe dominated the study area during 10.73 to 8.75 cal.kyr BP,with oak and elm in the middle elevation area and birch forest in high elevation area,and steppe vastly in low elevation and basins;cool deciduous forest prevailed in the high altitude area during 8.75 to 7.48 cal.kyr BP,with temperate deciduous forest and eurythermal coniferous forest in the middle and lower elevation area,and steppe expanded in the lowland;mixed temperate forest steppe developed during 7.48 to 5.70 cal.kyr BP,when birch forest retreated in high elevation areas,pine forest expanded perhaps at the cost of oak forest in the middle to lower elevation area;sparse pine woodland steppe dominated during 5.70 to 2.40 cal.kyr BP,in which time birch forest in high elevation and oak forest in middle and lower elevation greatly retreated while pine forest possibly sustained,steppe expanded extensively;steppe absolutely dominated the study area while all types of forest exterminated during the since 2.40 cal.kyr BP.(2)Differentiations can be observed for vegetation response regimes across pollen types,biomes,times and influencing variables.The result of RDA suggest that the MAT has the most influences on pollen data during 11.0 to 6.0 cal.kyr BP and the SM has the most influences during 6.0 – 0 cal.kyr BP followed by HII.Sensitivity analysis suggest that the forest biomes(cool deciduous forest,CDF;temperate deciduous forest,TDF;eurythermal conifer forest,ECF)sensitively responded to MAT during 11.0 to 6.0 cal.kyr BP and to SM during 6.0 to 0 cal.kyr BP.(3)Though the RDA and Pearson analysis suggest a possible impact of HII on vegetation during past 6000 years,the comparison of our pollen based biomes with the history of population and land reclamation suggest that the HII had little impact on vegetation until past several hundred years,when massive migration and land reclamations were carried out.(4)The reviews and synthesis of the 13 fossil pollen sequences in marginal monsoon area of northern China yield following conclusions.(1)The CDF began to develop at the mountain area to the north at ~10.0 cal.kyr BP and retreated overall at ~6.0 cal.kyr BP,suggesting that the refuge of the CDF might have been located in the local area with favorable water and heat conditions during the climate deterioration,and the overall shrink of the CDF might be a sensitive response to moisture decline.(2)The TDF began to develop synchronously in the north and the south of the marginal monsoon area.However,differences can be observed across elevations,with late beginning and early retreat in high elevation area.Hence,we argued that the refugia of TDF might also be located at the low elevation area of mountain itself.It is the high resist and resilience inherent with mountain morphology that are responsible for the insensitive response of TDF to SM during the first half of the Holocene.(3)The ECF established early in the south and late in the north,possibly supporting the viewpoint that the refugia of ECF was in the far south area.However,we did not observe the phenomena that the ECF retreated early in the north and late in the south.Perhaps this might be attributed to the insensitive response of ECF to SM.(4)The steppe biome,which competed with forest in our reviewed area,display sensitive response to SM throughout the whole Holocene.