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Earth-Science Reviews 97 (2009) 242–256
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Earth-Science Reviews
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Vegetation response to Holocene climate change in monsoon-influenced regionof China
Yan Zhao a,⁎, Zicheng Yu b, Fahu Chen a,⁎, Jiawu Zhang a, Bao Yang c
a MOE Key Laboratory of Western China's Environmental System, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, Chinab Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015, USAc Cold and Arid Regions Environment and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China
⁎ Corresponding author. Tel.: +86 931 891 2337; faxE-mail addresses: [email protected] (Y. Zhao), fhc
0012-8252/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.earscirev.2009.10.007
a b s t r a c t
a r t i c l e i n f oArticle history:Received 26 April 2009Accepted 16 October 2009Available online 26 October 2009
Keywords:Holocenevegetation changeclimate changemonsoonal Chinafossil pollenhuman activity
Fossil pollen records from 31 sites with reliable chronologies and high-resolution data in the monsoonalregion of China were synthesized to document Holocene vegetation and climate change and to understandthe large-scale controls on these changes. The reconstruction of moisture histories was based on a four-classordinal wetness index at 200-year time slices at individual sites. The vegetation experienced diverse changesover the Holocene in different regions: (1) between tropical seasonal rain forest and more open forest intropical seasonal rain forest region; (2) from mixed evergreen and deciduous broadleaved forest to moredeciduous or Pinus-dominated forest in subtropical region; (3) from mixed evergreen and deciduousbroadleaved forest to deciduous forest in temperate deciduous forest region; (4) from deciduousbroadleaved forest to conifer–deciduous forest in conifer–deciduous mixed forest region; (5) from steppeforest to steppe in temperate steppe region; and (6) from steppe forest/meadow to meadow/steppe inhighland meadow/steppe region. Despite various vegetation sequences in different regions, our synthesisresults show that a humid climate generally characterized the early and middle Holocene, and a drier climateprevailed during the late Holocene, with an abrupt shift at ca. 4.5 ka (1 ka=1000 cal yrBP). Abruptpalynological changes based on a squared-chord distance of pollen assemblages occurred at 11–10, 6–5 and2–1 ka from most sites. The synthesized pattern of moisture change is similar to the ones inferred from otherindependent climate proxies; however, gradual vegetation changes in the early Holocene lagged about1000yr behind the summer monsoon maximum as indicated by speleothem isotope records from Donggeand Sanbao caves. Human activities likely affected vegetation change greatly during the late Holocene, butthe magnitude and precise timing are less clear and require further investigation.
: +86 931 891 [email protected] (F. Chen).
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© 2009 Elsevier B.V. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2432. Data sources and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243
2.1. Study region and site selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2432.2. Moisture index and statistical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
3. Pollen records of Holocene vegetation and climate changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2463.1. Tropical monsoonal rain forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2463.2. Subtropical evergreen and deciduous forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2463.3. Temperate deciduous forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2483.4. Temperate mixed conifer–hardwood forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2483.5. Temperate steppe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2483.6. Highland meadow and steppe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2494.1. Temporal and spatial patterns of Holocene vegetation and climate change. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2494.2. Vegetation responses to climate oscillations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2524.3. Human disturbance during the late Holocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
5. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255
243Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255
1. Introduction
The East Asian monsoon system is among the most dynamiccontinent–ocean–atmosphere systems in the world. Emerging evidencesuggest that the summer monsoon intensity followed the summerinsolation trend, with the maximum monsoon occurring in the earlyHolocene and decreasing afterwards (Kutzbach, 1981; COHMAP, 1988;Morrill et al., 2003; Ruddiman, 2008). This trend has been documentedin various proxy records in monsoonal China, including speleothemisotopic records from Dongge Cave (Yuan et al., 2004; Dykoski et al.,2005; Wang et al., 2005), Sanbao Cave (Shao et al., 2006; Wang et al.,2008) and Heshang Cave (Hu et al., 2008). Other attempts were alsomade to study the Holocene climate pattern in monsoon-influencedregion of China. Shi et al. (1992) proposed that Holocene Megathermalperiod happened at 8500–300014CyrBP (ca. 9.4–3.1 ka; 1 ka=1000 calyrBP) on the basis of pollen data, fossil fauna, paleosol, lake level, glacialremains, and archaeological data. An et al. (2000) proposed that theHolocene East Asian summer monsoon precipitation reached amaximum at different periods in different regions of China, with thetrend of frontal migration paralleling the trend of summer insolation.However, this time-transgressive Holocene optimum was only sup-ported by some coarse resolution records, especially from south China.Heet al. (2004) reviewedmulti-proxypaleoclimatic records across chinaand found that the Holocene optimum in eastern China was at ca. 6.5–5.5 ka. Herzschuh (2006) assessed the late-Quaternarymoisture changein Central Asia (including the Tibetan Plateau, northwest and north-central China and Mongolia) using recent pollen data and otherindependent proxies. Her review suggested that the Holocene climateoptimumwith high precipitation occurred during the early Holocene inthe Indian monsoon region, but possibly occurred during the mid-Holocene in the East Asian region. Nonetheless, the high-resolutionspeleothem isotope records with reliable chronology shed light on thegeneral pattern of climate change during theHolocene in themonsoonalChina region. However, the major patterns of temporal and spatialvariation in past vegetation are far less clear than climate in this region.Pollen data would provide biological responses as well as climatechange, in a widespread network compared to cave records, and wouldalso help further test the Holocene pattern by using a unified synthesismethodology.
Fossil pollen data from lakes and wetland deposits have beenfrequently used in vegetation and climate reconstructions, as they tendto reflect vegetation and climate changes at a regional scale. As a result,fossil pollen has become one of the most widely used and availablepaleoclimatic proxies. Liu (1988) reviewed late-Quaternary pollen datafrom temperate forests of China and found that the Holocene pollenstratigraphies suggest a tripartite division, with a period of maximumwarmth, the Hypsithermal, in the mid-Holocene. Sun and Chen (1991)briefly reviewed the Holocene pollen data in China and indicated thatvegetation change was sudden and remarkable in the early Holoceneand late Holocene. Ren and Zhang (1998) and Ren and Beug (2002)presented Holocene pollen maps of China north of Yangtze River for sixtime slices at 2000-year intervals, revealing large changes in Holocenevegetation, especially since 6–4 ka. Yu et al. (1998) used a biomizationapproach tomappollendata and their reconstructions showed the shiftsin biome distributions, implying significant changes in climate since6000yr ago. These syntheses are based on a large collection of Holocenepollen records of various time spans and data quality available in China;however, the chronologies are limited by datingmaterial (most on bulkorganic matter) and coarse dating control, and data sensitivity is alsolimited by the coarse sampling intervals and by the discontinuousnature of the sediment, including fluvial, coastal, or marine sediments,
used for pollen analysis (Liu, 1988). In addition, in the past couple ofdecades after these syntheses, many more high-resolution pollenrecords with relatively good chronology control have been available.Herzschuh (2006) assessed the late-Quaternary moisture change inCentral Asia based on recent pollen data and other independent proxies,but her review evaluates very few records from eastern China. Hopeet al. (2004) reviewedover1000marineand terrestrial pollen records toreconstruct the vegetation history in the Austrial-Asian region,including the monsoonal China; however, their analysis was mostly toexamine vegetation change at broad regional and coarse temporalscales. There are no syntheses of pollen data specifically in the entireeastern monsoonal China for understanding the vegetation pattern andresponse to climate change. Therefore, there is a need to synthesizepaleoclimatic information about monsoonal China based on recentpublished fossil pollen records.
In this review, we used 31 fossil pollen records from easternmonsoonal China (see location in Fig. 1 and site information in Table 1).Our review and synthesis differ from earlier published reviews (e.g., Liu,1988; Sun and Chen, 1991; Yu et al., 1998; An et al., 2000; Ren and Beug,2002; Herzschuh, 2006), as we focused on monsoonal China, applied auniform set of site-selection criteria, and usedmostly recent pollen data.The objectives of this paper were to document regional vegetationpatterns by synthesizing fossil pollen records from monsoonal China,and to evaluate and understand vegetation response to climate changesand the large-scale controls. This synthesis would not only bringtogether existing high-resolution data but also would assist the designof paleoecological and paleoenvironmental studies in the reviewedregion.
2. Data sources and methods
2.1. Study region and site selection
The geographical region considered in this review covers easternmonsoonal China (Fig. 1). Altitude ranges from near sea level to about5000 m above sea level (Table 1). The Asian summer monsoon plays asignificant role in controlling the effectivemoisture of the region. Underthe influence of the monsoon systems, annual precipitation decreasessharply from the southeast to the northwest from >1800mm to ca.400 mm. At most of the sites the precipitation of the three summermonths (JJA) (June, July, and August) usually accounts for more than80% of the total annual precipitation, leavingwinter and spring dry (Renand Beug, 2002). The study sites are located in six vegetation zones (seedetails in Table 2; Fig. 2; Wu, 1980; Hou, 2001): 1) tropical monsoonalrain forest; 2) subtropical evergreen and deciduous forest; 3) temperatedeciduous forest; 4) temperate conifer–deciduous mixed forest; 5)temperate steppe; and 6) highland meadow and steppe.
Abundant pollen records of various time spans and data quality arenow available in this region. However, many records tend to have lowtemporal resolution and can be discontinuous. In this study, weselected pollen sites based on three criteria: (1) a reliable chronologywith a minimum of 4 dating control points over the Holocene; (2)high sampling resolution with a minimum 200yr per sample; and (3)continuous record covering most of the Holocene without documen-ted depositional hiatus. A total of 26 lake and peat sites met thesecriteria in this study. There are no satisfactory records in thetemperate deciduous forest region. Therefore, we included tworecords with lower resolution in this region (the Yellow River Deltaand Maohebei) when we discuss the general vegetation and climatehistory. In addition, three other sites (Poyang Lake, Taishizhuang andTianchi Lake) that have high resolution pollen records but only for the
Fig. 1. Map showing the location of fossil pollen sites in monsoonal China reviewed in this paper (see Table 1 for site information and references).
244 Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
late Holocene were also used to evaluate possible human disturbanceon vegetation change. These three sites were not used in climatesynthesis analysis owing to their limited time span. Our site-selectioncriteria make our review and synthesis different from earlierpublished reviews of pollen records (e.g., Liu, 1988; Sun and Chen,1991; Ren and Zhang, 1998; Ren and Beug, 2002). Moreover, oursynthesis includes the latest research results since 2000, coveringmore than 60% of the 31 pollen records used.
Radiocarbon dating was the geochronological technique for allprofiles used in this synthesis. All ages for the reviewed profiles havebeen calibrated or recalibrated to calendar years before the present(BP=AD 1950) using the latest IntCa04 calibration dataset (Reimeret al., 2004). Calibrated ages are used in compiling effective moisturecurves and throughout the text (expressed as cal yrBP, or ka; 1 ka=1000 cal yrBP).
For the convenience of discussion, we organized the sites by sixmodern vegetation zones as described above. We present and discusseight summary pollen diagrams that are representative of vegetationchanges in these regions of different vegetation types. Pollen datawere obtained from the original authors or digitized from pollendiagrams in the publications. At some sites, we also briefly describethe modern pollen-rain results, if available, to help evaluate fossilpollen interpretations.
2.2. Moisture index and statistical analysis
The main pollen indices we used here include tropical tree pollenpercentages, evergreen tree pollen percentages, broad leaved treepollen percentages, and tree pollen percentage. We used these pollenindices to make a semi-quantitative estimate of relative moistureconditions, which is controlled by both precipitation and temperature(through influence on evaporation). The monsoonal region of China,including tropical monsoonal rain forest, subtropical evergreen forestand deciduous forest regions, often has high relative humidity insummer monsoon season, so evaporation would not increasesignificantly with temperature increase. Even in the steppe regionnear the limit of monsoon influence where evaporation might be animportant factor for effectivemoisture, vegetation is more sensitive tominor change in effective moisture than the eastern parts of our studyregion. Therefore the monsoon-induced precipitation variability is
more important than evaporation variability in understanding thevariation in effective moisture in the monsoon region of China, as alsoindicated by An et al. (2000). As shown in other geological andbiological records, effective moisture has a strong influence onvegetation in east-central China (An et al., 2000). We therefore usedthese pollen indices from 26 sites (excluding the two sites from thetemperate deciduous forest region and three late Holocene sites) toestimate dry–wet climate fluctuations semi-quantitatively at individ-ual sites during the Holocene. The fourmoisture classes were assignedfrom the wettest (score 4) to the driest period (score 1) at eachindividual site. The justification for designating an ordinal wetnessscale is that pollen index types and values at each individual site maynot be directly comparable to the ones at other sites, owing todifferent geographic locations, different dominant vegetation andhydrological settings. So the wetness scales are relative, in a semi-quantitative sense, on the basis of individual sites.
HereweuseQinghai Lake as an example to illustrate howwe assignedrelative wetness values to pollen records. We use total tree pollen (%) atQinghai Lake as the index to infer effectivemoisture,with high tree pollenrepresenting high tree dominance under awet climate (Shen et al., 2005).Fossil pollen assemblageswith total tree pollen percentages of <10%, 10–20%, 20–30%, and 30–40%, receive wetness scores of 1, 2, 3 and 4,respectively. Although the score assignment is semi-subjective, itsatisfactorily shows the general moisture trend at individual sites.
Synthesized wetness curves for each individual region (vegetationzone) and the entire study region of monsoonal China during theHolocene were generated by averaging the wetness scores at 200-yeartime intervals for all 26 sites, with error bars as standard errors. Due topotentially different sensitivity to climate change at individual sites, therange andmagnitude of percentage pollen change at individual sites canbe very large (e.g., from<10 to>90% for total tree pollen), so averagingthese highly variable values may not be reasonable and acceptable.Ordinal indices could smooth the range of variations for each site, thusproviding more reasonable averaged values than original pollen data.This averaging procedure based on wetness scales assumes that eachsite is equally sensitive to climate change and equally well dated, andthat there are nogradients in response across the region. In reality, somerecords were obviously better dated, at higher resolution, or highersensitivity to climate change than others. Unfortunately, with informa-tion available in the original publications, it is not practical for the
Table 1List of fossil pollen sites from eastern Asian monsoon China used in this review.
Site no. Site name Location Elevation Precip. Sample resolution Dating material Number ofdate
Type ofarchives
Reference(m a.s.l.) (mm) (yr)
A1 Shuangchi Maar Lake E110°11′,N19°57′
75 1670 <100 Bulk organic matter 6 Lake core Zheng et al. (2003)
A2 Huguang Maar Lake E110°17′,N21°9′
255 1600 170 Bulk organic matter/leaves 7 Lake core Wang et al. (2007)
B1 Dahu E115°,N24°41′
250 1600 240 Plant macrofossils 6 Peat section Xiao et al. (2007);Zhou et al. (2004)
B2 Bajiaotian E110°20′,N25°48′
1900 1814 250 Bulk organic matter 6 Peat core Li et al. (1993)
B3 Chao Lake E117°16′54″,N31°25′28″
20 1000 <200 Bulk organic matter /charcoal/shell/ plant macrofossil
10 Fuvial + lakesediment
Chen et al. (2009)
B4 Poyang Lake E116°15′,N28°52′
14.1 1528 170 Bulk organic matter 9 Lake core Jiang and Piperno (1999)
B5 Longquan Lake E112°1′,N30°52′
100 1100 <150 Bulk organic matter 6 Lake core Liu et al. (1993)
B6 Dajiuhu E109°59′45″,N31°29′27″
1760 1500 120 Peat cellulose 7 Peat section Zhu et al. (2006)
B7 Dianchi E 103°,N25°
1886 1010 160 Bulk organic matter 7 Lake core Sun et al. (1986)
B8 Erhai Lake E 100°05′,N25°36′
1974 900 <100 Bulk organic matter 6 Lake core Shen et al. (2006)
B9 Shayema Lake E101°35′,N28°05′
2400 1090 130 Bulk organic matter 5 Lake core Jarvis (1999)
C1 Yellow River Delta E118°54.3′,N37°47.8′2
4.9 750 <1000 Mollusc shell 9 Delta borehole Yi et al. (2003)
C2 Maohebei E119°12′,N39°32′
2 650 >300 Bulk organic matter 4 Peat section Li and Liang (1985)
C3 Taishizhuang E115°49.5′,N40°21.5′
500 420 <100 Bulk organic matter 6 Peat section Tarasov et al. (2006)
D1 Jinchuan E126°22′,N42°20′
662 700 210 Bulk organic matter 4 Peat section Jiang et al. (2008)
D2 Gushantun E126°10′,N42°30′
600 630 300 Bulk organic matter,charcoal
6 Peat section Liu (1989)
D3 Qindeli E133°15′,N48°00′
52 600 280 Bulk organic matter 5 Peat section Xia (1988)
E1 Haoluku E116°45.42′,N42°57.38′
1295 370 200 Bulk organic matter 4 Lake section Liu et al. (2002)
E2 Bayanchagan E115.21°,N41.65°
1355 400 130 Bulk organic matter 7 Lake core Jiang et al. (2006)
E3 Diaojiao Lake E112°21′N41°18′
1800 421 85 Bulk organic matter 4 Lake core Shi and Song (2003);Song et al. (1996)
E4 Daihai Lake E112°33′,N40°29′
1221 423 <100 Bulk organic matter 8 Lake core Xiao et al. (2004)
E5 Chasuqi E111°08′,N40°40′
1000 400 70 Bulk organic matter 4 Peat section Wang and Sun (1997)
E6 Midiwan E108°37′,N37°39′
1400 395 100 Bulk organic matter 23 Peat section Li et al. (2003)
E7 Tianchi Lake E106°19′,N35°16′
2430 615 85 Plant macrofossil 19 Lake core Y. Zhao (unpublished data)
E8 Dadiwan E105°54′,N35°01′
1400 400 <50 Bulk organic matter 3 Marsh section An et al. (2003)
F1 Zoige Basin E103°25′,N32°20′
3492 710 190 Bulk organic matter 8 Peat section Yan et al. (1999)
F2 Qinghai lake E99°36′,N36°32′
3200 350 60 Bulk organic matter 7 Lake core Shen et al. (2005)
F3 Hidden Lake E92°48′,N29°49′
4980 450 110 Plant macrofossils 7 Lake core Tang et al. (2000)
F4 Qongjiamong Co E92°22.37′,N29°48.77′
4980 450 <200 Flies/charcoal/macrofossil 20 Lake core Shen (2003)
F5 Zigetang Lake E90.9°′,N32.0°
4560 320 160 Bulk organic matter 5 Lake core Herzschuh et al. (2006)
F6 Selin Co E88°31′,N31°34′
4530 290 200 Bulk organic matter 5 Lake core Sun et al. (1993)
245Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
purpose of this synthesis to assign weightings to each individual site.Nonetheless, these synthesized time series help us document andunderstand variations of effective moisture, though with someuncertainties as shown in the error bars for the synthesis curve.
We choose the squared-chord distance (SCD), which has provenespecially satisfactory to reveal regionally synchronous palynologicalchanges from pollen-frequency data (e.g. Overpeck et al., 1985;Grimm and Jacobson, 1992; Faison et al., 2006). SCD is calculated as:
dij=σk(pik1/2−pjk1/2)2, where dij is the SCD between two multivariate
samples i and j, and pik is the proportion of species k in sample i. Foreach pollen time series (site) used in this synthesis, the pollen datawere interpolated at 200-year intervals by using the programAnalySeries 2.0.4 (Pailard et al., 1996). The SCD between the adjacent200-year samples was used to calculate the rate of palynologicalchange per 100yr (Grimm and Jacobson, 1992). Although most of theselected pollen records have only 5–10 radiocarbon dates, the age
Table 2Modern environmental settings (Wu and Zhang, 1992).
Vegetation type MAT (°C) MAP (mm) Dominant plant taxa
Tropical monsoonalrain forest
22 to26.5
1200–3000 Terminalia hainanensis, Lannea coromandelica, Ficus altissima, Chukrasia tabularis, Vatica astrotricha, Madhucasubquincuncialis, Altingia chinenesis, Dacrydium pierrei, Lithocarpus fenzelianus, and Adinandra hananensis.
Subtropical evergreenand deciduous forest
14 to 22 >1000 Quercus variabilis, Q. serrata, Q. fabri, Castanopsis sclerophylla, Castanea sequinii, Platycarya strobilacea, Lithocarpuscleistocarpus, Betula luminifera, and Liquidambar formosana.
Temperate deciduousforest
9 to 14 500–900 Quercus mongolica, Q. liaotungensis, Q. aliena, Q. acutissina, Q. variabilis, Robinia pseudoacacia, Salix matsuduna,and Populus simorill.
Temperate conifer–deciduous mixed forest
2 to 8 500–800 Pinus koraiensis, P. sylvestris, Abies nephrolepis, Picea jezoensis, Larix gmelinii, L. olgensis and Quercus mongolica, Tiliaamurensis, Ulmus propinqua, and Betula platyphylla.
Temperate steppe −3 to 8 200–450 Artemisia spp., Stipa spp., Carex spp., Rosaceae and Fabaceae. Pinus, Quercus, Betula, Ulmus, Salix, Corylus and Populus are themain tree taxa on nearby hills.
Highland meadow andsteppe
−2 to 0 300–400 Highland meadow: Cyperaceae (e.g. Kobresia pygmaea), Poaceae (such as Achnatherum splendens, Stipa breviflora, S. gobica).Asteraceae, Rhododendron, Gentianaceae, Fabaceae, Saxifragaceae and Polygonum; highland steppe: more Poaceae (e.g.Achnatherum splendens, Stipa breviflora, S. gobica) and Artemisia frigida, A. scoparia than alpine meadow.
MAT = mean annual temperature; MAP = mean annual precipitation.
246 Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
controls should be sufficient to detect abrupt changes at multi-centennial or millennial scales. Our site-selection criterion ofcontinuous record covering most of the Holocene without documen-ted depositional hiatus also makes the analysis justifiable. Theanalysis does not reveal the qualitative nature of climate change,but it identifies times of rapid change regardless of the direction.Moreover, the average for a large region will not identify time-transgressive change, but it reveals abrupt, synchronous, geograph-ically widespread change (Huntley, 1992).
3. Pollen records of Holocene vegetation and climate changes
Belowwewill describe the synthesis results of vegetation and climatechange from each of six main vegetation regions in monsoonal China.
3.1. Tropical monsoonal rain forest
Two high-resolution pollen records are available from the tropicaland southern subtropical regions of southeast China. One record isfrom Shuangchi Maar Lake (site A1) on Hainan Island, spanning thelast ca. 9000yr (Zheng et al., 2003). The pollen assemblages were
Fig. 2. Dominant biomes (vegetation
dominated by tropical trees, mainly including Mallotus and Caseariaat 8.9–4.3 ka, suggesting a warm and wet climate. After 4.3 ka, herband fern pollen increased at the expense of tree pollen, and totalpollen concentration was very low, indicating a drying climate.
The other pollen diagram from Huguangyan Maar (site A2) has asampling resolution of ca. 170yr and is divided into 3 zones (Fig. 3A;Wang et al., 2007). The percentage of tropical tree pollen is highduring 11.6–7.8 ka, mainly including Moraceae, Mallotus, Ficus andAporosa. The tropical vegetation represented by pollen suggests awarm and wet climate. From 7.8 ka, tropical tree pollen started todecrease, especially after 4.2 ka.
3.2. Subtropical evergreen and deciduous forest
More well dated high-resolution pollen records are available fromthis region. A peat profile at Dahu peatland (site B1) provided a welldated Holocene pollen record (Fig. 3B; Zhou et al., 2004; Xiao et al.,2007). The pollen diagram indicates a major change dominance ofAlnus to evergreen tree pollen of Castanopsis/Lithocarpus at 10.4 ka,signifying a warming and wetting climate. Castanopsis/Lithocarpus issclerophyllous taxa that are adapted to spring drought, compared to
types) in China (after Wu, 1980).
Fig. 3. Summary percentage pollen diagrams from eastern monsoonal China. (A) Huguangyan Maar Lake (A2), South China (redrawn from Wang et al. (2007)); (B) Dahu peatland (B1), South China (redrawn from Xiao et al. (2007));(C) Dajiuhu peatland (B6), Central China (redrawn from Zhu et al. (2006)); (D) Shayema Lake (B9), southwestern China (redrawn from Jarvis (1993); (E) Maohebei peatland (C2), north China (redrawn from Li and Liang (1985)); (F) Qindelipeatland, northeastern China (redrawn from Xia (1988)); (G) Bayanchagan Lake (E2), northwestern China (redrawn from Jiang et al. (2006)); (H) Qinghai Lake (F2), northeastern Tibetan Plateau (redrawn from Shen et al. (2005)). All thepercentages were calculated based on terrestrial pollen sums.
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many mesic deciduous forest taxa; however, we interpret its presenceas indicating warm/wet condition as it requires more moisture thanAlnus and herbs that are abundant in these pollen records. At 10.4–6 ka,Castanopsis/Lithocarpus remains high, reaching amaximumof near30%. Cyclobalanopsis pollen increases gradually to a value of 19% duringthat period. This period marks the Holocene optimum period (warmerand wetter climate), as represented by a subtropical evergreen broad-leaved forest. After 6 ka, the pollen record shows a rapid increase inferns and herbs at the expense of tree pollen, suggesting a drier climate.
Pollen record fromCaohu Lake (B3),west to the Yangtze River Delta,indicated an evergreen and deciduous mixed broadleaved forestdominated by Cyclobalanopsis and Quercus after 10.5 ka, especiallybetween 8.2 and 7.5 ka, suggesting a warm and wet climate. At 7.5–3.7 ka, there are noticeablefluctuations inmain pollen taxa, inparticulara general decline in Cyclobalanopsis and other arboreal pollen and anincrease in terrestrial herbs. After 3.7 ka, the broadleaved forest treeslargely gave way to terrestrial herbs. Pinus continued to increasealongside with most herbs until 2 ka (Chen et al., 2009).
Pollen record at ca. 120-year sampling resolution from the Dajiuhupeatland (site B6), in Shennongjia Mountains in central China (middlereach of Yangtze River), shows a progressive increase in percentages ofevergreen tree pollen after 11 ka (Fig. 3C; Zhu et al., 2006). During 11–6 ka, pollen assemblages are marked by a continuous increase inevergreen components as well as some deciduous tree pollen, such asCyclobalanopsis/Castanopsis,Betula,Carpinus,Quercus and Juglans.Modernpollen assemblages from surface samples in the Shennongjia region aredominated by deciduous broadleaved trees and represent regional pollensource (Liu et al., 1993). There are good relationships between pollen andclimate variables, with higher evergreen tree pollen percentages atwarmand wet sites (Liu et al., 2000; Zhu et al., 2008). The fossil pollenassemblages imply a mixed forest of deciduous and evergreen broad-leaved trees under a warming and wetting climate during the early andmid-Holocene. After 4.5 ka, coniferous tree pollen increased at theexpense of evergreen tree pollen, suggesting mixed forests of coniferoustreeswithdeciduousbroadleaved trees causedby aweakeningof the EastAsian summer monsoon. Another pollen record from Longquan Lake incentral China (site B5; Li et al., 1993) similarly revealed that evergreenbroadleaved — deciduous broadleaved forest (Castanea, Hamamelis, Ke-terleeria, and Tsuga) dominated from 10.7 to 4.6 ka, while herb becamedominant at the expense of tree pollen after 4.6 ka.
The pollen record from Shayema Lake (site B9) in southwesternChina spans the last 13,000yr (Fig. 3D; Jarvis, 1993).Modernpollen-rainsamples were collected from surface sediments at 13 lakes along anelevational transect between 1800 and 3900 m above sea level (Jarvisand Clay-Poole, 1992). The results show that a significant number ofimportant taxa in the region are insect-pollinated and produceminimalpollen grains. To mitigate the difficulty of working with such a largenumber of underrepresented taxa, pollen percentage values for lowpollen-producing taxa of similar autoecological features (e.g., mesicdeciduous, sclerophyllous evergreen broadleaved) were summed andexamined together, by giving particular weight to the presence of keypollen types representative of a particular climatic regime. The fossilpollen record shows several changes in the vegetation and climate since13 ka (Jarvis, 1993). From 12.7 to 10.6 ka, cold-tolerant species, such asAbies, Betula, and deciduous oaks (Quercus), dominated the vegetation.Between 10.6 and 7.9 ka, the abundance of deciduous oaks decreased,while evergreen oaks increased, as did Tsuga and mesic deciduousspecies, suggesting a warm climate with probably increased precipita-tion. During the period 7.9–4.7 ka, spring drought-adapted sclerophyl-lous taxa (such as Lithocarpus/Castanopsis) increased at the expense ofmesic deciduous taxa, indicating that precipitation was becoming moreseasonal. This is in a great contrast to the climate in the early Holocene,with a reduced seasonality in precipitation and an increased seasonalityin temperature, when winters were colder and spring moisture wasprobably more abundant. The drought-adapted tree taxa continued tobe the dominant components of the vegetation until ca. 1 ka when the
effects of human disturbance are noted in the pollen record (Jarvis,1993). Evidence for deforestation is supported by an increase in Alnus,Artemisia, Poaceae, Pteridium, and Pteris and the minimal amounts ofTsuga and Picea.
Other palynological records from southwestern China (Dianchi:site B7, Sun et al., 1986; Erhai: site B8, Shen et al., 2006) also suggestthat prior to 10 ka the climate was colder and drier relative to today asinferred from dominant conifer pollen (Pinus, Abies). From the early tomiddle Holocene, pollen assemblages show a gradual increase inabundance and diversity of mesic deciduous and evergreen trees(Quercus, Tsuga, Castanopsis/Lithocarpus). Vegetation was reflected byspring drought-adapted sclerophyllous taxa relative to mesic decid-uous taxa during the late mid- to late Holocene. At Erhai Lake, Pinusincreased significantly, along with pollen of Plantago, Artemisia,Chenopodiaceae and Poaceae, during late Holocene, suggestinghuman disturbance (Shen et al., 2006).
3.3. Temperate deciduous forest
There are few pollen records with good chronology and highresolution from this region. Palynological analyses of two boreholesfrom the Yellow River Delta (site C1) provide a vegetation and climatehistory of the last 12 ka, but only on millennial scale (Yi et al., 2003).From 9.8 to 4.5 ka evergreen and broadleaved deciduous forest thrive(indicated by high Quercus, Carpinus/Ostrya, Ulmus/Zelkova pollenvalues), suggestingwarmer andwetter climatic conditions. A significantreduction in deciduous Quercus pollen and an increase in conifer Pinuspollen at ca. 4 ka, together with the first appearance of buckwheat(Fagopyrum) pollen at 1.3 ka, probably reflect widespread humandisturbance of the natural vegetation and intensive cultivation.
Another record with >300-year sampling resolution from Mao-hebei peatland (site C2) spanned from 12.5 to 3 ka (Fig. 3E; Li andLiang, 1985). The pollen record shows that deciduous tree pollen(represented by Quercus, Tilia, Ulmus, Carpinus, Corylus and Betula)peaks at ca. 11.5–8.5 ka, corresponding to warm and humid periods,and the subsequent interval of lower percentages of these trees,corresponding to a dry climate interval (Li and Liang, 1985).
3.4. Temperate mixed conifer–hardwood forest
Three pollen records frompeat profiles (Jinchuan: siteD1, Jiang et al.,2008; Qindeli Bog: siteD3, Xia, 1988; GushantunBog: siteD2, Liu, 1989)in themixed conifer–hardwood forest of northeast China span theentireHolocene. Pollen record at ca. 280-year resolution from Qindeli showsthat from 12.5 to 10 ka broadleaved trees, including Betula, dominatedthe vegetation (Fig. 3F). At 10–5.5 ka, deciduous broadleaved foresttrees, including Ulmus, Quercus, Salix, Carpinus, and Corylus, thrived,indicating a high effective moisture during the growing season. After5.5 ka, Pinus and Abies/Picea increased at the expense of deciduousbroadleaved tree pollen, suggesting a conifer-dominated forest and thusa colder anddrier climate. This trendwas accentuated after 2.5 or 2 ka asindicated by further expansion of the boreal conifers. At Gushantun Bog,Betula increased at ~11.3 ka, and from 11.3 to 4.4 ka, broadleaved treepollen from Quercus, Ulmus, Alnus, Juglans and Corylus had high values.Since 4.4 ka, Pinus increased while deciduous broadleaved treesdecreased, especially after 2.2 ka. The Jinchuan profile has a similarpattern as Qindeli, with a steadily high broadleaved pollen (Quercus,Ulmus and Juglans) from 11.5 to 6/5 ka, suggesting an interval of higheffective moisture.
3.5. Temperate steppe
Jiang et al. (2006) presented a pollen record at ca. 130-year samplingresolution from Bayanchagan Lake in Inner Mongolia (site E2; Fig. 3G).The pollen assemblages show that vegetation around Bayanchagan Lakechanged from a steppe at 12.5–9.2 ka, through a Betula/Pinus-dominated
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steppe woodland at 9.2–6.7 ka, back to steppe after 6.7 ka. Jiang et al.(2006) used standard modern analogue technique to reconstructchanges in paleoclimatic parameters based on modern pollen datasets(including 211 surface pollen assemblages from northern China) andcorresponding climate data. The vegetation sequence and paleoclimaticreconstruction suggest that a relatively humid climate during the earlymid-Holocene at 9.2–6.7 ka was favorable for the development ofwoodland.
Pollen assemblages at other sites (Haoluku: site E1, Liu et al., 2002;Daihai Lake: site E4, Xiao et al., 2004; Diaojiao Lake: site E3, Shi andSong, 2003; Chasuqi: site E5, Wang and Sun, 1997) in eastern InnerMongolia are generally dominated by Betula, Picea, and Ulmus duringthe early Holocene, while by Artemisia with some tree pollenincluding Betula, Quercus, Ulmus, and Pinus in the middle Holocenefrom 7 to 5 ka. After 5 ka or 4 ka, pollen assemblages were dominatedby Artemisia with some tree pollen. Vegetation changed from steppeat the beginning of Holocene, to forest steppe in the late part of earlyHolocene and mid-Holocene, to steppe in the late Holocene. Thesevegetation sequences suggest that climate changed generally from awet climate in the early andmid-Holocene to drier conditions after ca.5 ka.
Dadiwan marsh section (site E8), located in the NW Loess Plateau,has a high sampling pollen resolution of ca. 50yr (An et al., 2003). Thepollen assemblages show that vegetation around Dadiwan changedfrom a desert steppe at 10–8.5 ka, through a Pinus-dominated steppewoodland at 8.5–6.4 ka, to back to desert steppe after 6.4 ka. Thevegetation sequence suggests that a relatively humid climate duringthe mid-Holocene at 8.5–6.5 ka was favorable for the development ofwoodland. Midiwan (site E6; Li et al., 2003), another site on the NWLoess Plateau, shows a different pattern of Holocene vegetation andclimate change. Vegetationwas sparsely wooded grassland, consistingmainly of Thalictrum, Cyperaceae, Betula, and Quercus at 11.5–8.5 ka,indicating a warm and humid early Holocene.
3.6. Highland meadow and steppe
Six pollen diagrams from this region (from sites F1 to F6) were usedfor this synthesis. The pollen diagram from Qinghai Lake (site F2;Fig. 3H; Shen et al., 2005) can be divided into three pollen assemblagezones. Before 10.6 ka, pollen assemblages were characterized by Arte-misia (~60%), tree pollen (~20%), Poaceae (~10%), and Chenopodiacece(~10%). During 10.6 and 4.2 ka, Artemisia decreased to <40%, Pinusincreased up to ~30% and Betula was up to 20%. The highest pollenconcentrations occurred in this zone. After 4.2 ka, Artemisia increasedagain (up to 80%) while Pinus decreased to 15–0% (Shen et al., 2005).Vegetation around Qinghai Lake changed from steppe before theHolocene, through steppe forest in the early and mid-Holocene at10.6–4.2 ka, to Artemisia-dominated steppe in the late Holocene. Thisvegetation sequence suggests a dry climate before the Holocene, a wetclimate in the early and mid-Holocene, and a dry climate in the lateHolocene.
Pollen assemblages from other sites in the Tibetan Plateau (ZoigeBasin: F1, Yan et al., 1999; Hidden Lake: F3, Tang et al., 2000; CoQongjiamong: F4, Shen, 2003; and Selin Co: F6, Sun et al., 1993) weredominated by Artemisia and Cyperaceae, with relatively high Pinus/PiceaandBetulapollen percentages during the early andmiddleHolocene from11.5 to 6 or 5 ka, while dominated by Artemisia, Cyperaceae and Poaceaeduring the lateHolocene. The vegetation sequence suggests a humid earlyand mid-Holocene and a dry late Holocene. However, at Zigetang Lake(F5), pollen assemblages are consistently dominated by Artemisia andCyperaceae (Herzschuh et al., 2006). Pollen assemblages suggest adominance of temperate steppe vegetation during the first half of theHolocene, while alpine steppes with desert elements tend to dominatethe second half. The vegetation sequence indicates a general cooling anddrying trend throughout the Holocene.
4. Discussion
4.1. Temporal and spatial patterns of Holocene vegetation andclimate change
The major vegetation changes at individual sites from variousvegetation zones showdifferent types and patterns during theHolocene(Fig. 4). In the tropicalmonsoonal rain forest region, vegetation changedfrom seasonal rain forest dominated by tropical trees (e.g., Fig. 5A) tomore open forest. In the subtropical region, vegetation changed fromevergreen broadleaved forest dominated by evergreen trees Quercusand Castanopsis/Lithocarpus (e.g., Fig. 5B, C and D), to evergreen-deciduous broadleaved forest dominated by Quercus, Betula, Carya, andUlmus or mixed forest dominated by Pinus and deciduous trees withmore herbs during the late Holocene. In the temperate deciduous forestregion, vegetation shifted from deciduous broadleaved forest duringearly andmid-Holocene (e.g., Fig. 5E), to diverse broadleaved deciduousforest with some herbs (Artemisia, Poaceae) during the late Holocene. Inthe conifer–hardwood mixed forest region, vegetation changed fromdeciduous broadleaved forest (Quercus-dominated) to conifer–deciduousmixed forest dominated by conifer and other trees (Pinus, Picea,Abies, Quercus and Betula) (e.g., Fig. 5F). In the temperate steppe region,steppe (Artemisia, Poaceae), forest steppe (Betula, Pinus, and Artemisia),and steppe dominated the vegetation over the Holocene in eastern InnerMongolia (e.g., Fig. 5G). Vegetation in the northwestern Loess Plateauchanged between desert steppe (Artemisia, Chenopodiaceae, and Poa-ceae), forest steppe (Pinus, Artemisia, Poaceae, and Asteraceae) andsteppe. In the highland meadow and steppe region, vegetation ischaracterized by meadow/steppe (Artemisia, Poaceae, and Cyperaceae)and steppe forest (mainly Pinus, Picea and Abies) in the early and mid-Holocene (e.g., Fig. 5H) and meadow/steppe during the late Holocene.
The semi-quantitative moisture classes obtained from interpretedfossil pollen records in the six regions reveal generally synchronouswet–dry climate changes during the Holocene (Fig. 6), though theirvegetation showsdifferent types andpatterns. Awet climate occurred inthe early and mid-Holocene before 6–5 ka, with the maximum moistperiod at 9.5–6 ka. All the records show a consistently dry climateduring the late Holocene, with an abrupt shift at ca. 4.5 ka (Fig. 6G). Dueto the relative lack of fossil pollen records in the regions of tropicalmonsoonal rainforest and temperate deciduous forest, the pattern inthese regions at the beginning of Holocene is not as clear as in otherregions. Vegetation change at individual sites (Fig. 4) and themoderateerror of the moisture curve (Fig. 8E) together indicate that the analysisresults of generally synchronous patterns in different regions are not anartifact that could be caused by averaging approach. An et al. (2000)proposed that the Holocene optimum, as defined by maximum EastAsian summer monsoon precipitation, was asynchronous in differentregions, at ca. 10–8 ka in northeastern China, at 10–7 ka innorth-centraland north east-central China, at ca. 7–5 ka in the middle and lowerreaches of theYangtzeRiver, at ca. 3 ka in southernChina, and at 11 ka insouthwestern China. However, our synthesis based on pollen data andother evidence from recently published records does not appear tosupport this hypothesis (also see Feng et al., 2006). The differentconclusions from these syntheses are partly due to the fact that An et al.(2000) used some records with low sampling resolution and poordating controls, especially those from southern China.
Dissimilarity analysis of pollen data using SCD measures for theentire eastern monsoonal China shows that times of significantpalynological change occurred at 11–10 ka, 6–5 ka and 2–1 ka (Fig. 7).The regional average curve, which smoothes out site-specific changes,can reveal times of regionally synchronous palynological changes in alarge geographic region. Our result generally agrees with that inferredfrom T-test analysis based on multi-proxy records in Asian monsoonregion (Morrill et al., 2003).At the start of theHolocene, the average rateof change is high atmost sites due to the transition from the late-glacialperiod to the Holocene. The significant changes during this time result
Fig. 4. Summary of vegetation history derived from fossil pollen records in eastern monsoonal China. See Table 1 for site information and references.
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Fig. 5. Summary of major vegetation taxa change in various regions arranged from south to north in easternmonsoonal China. (A) Tropical tree pollen (%) at HuguangMaar Lake (Wang et al., 2007); (B) evergreen tree pollen (%) at Dahu (Zhouet al., 2004); (C) evergreen tree pollen (%) at Dajiuhu (Zhu et al. (2006)); (D) sclerophyllus tree pollen (%) at Shayema Lake (Jarvis, 1993); (E) deciduous tree pollen (%) at Maohebei peatland (Li and Liang, 1985); (F) conifer tree pollen (%) atQindeli peatland (Xia, 1988); (G) tree pollen (%) at Bayanchagan Lake (Jiang et al., 2006); (H) tree pollen (%) at Qinghai Lake (Shen et al., 2005).
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Fig. 6. Synthesized time series of relative moisture changes as inferred from fossil pollen data across eastern monsoonal China. (A) Tropical monsoonal rain forest (n=2);(B) subtropical evergreen forest (n=8); (C) temperate forest (n=2); (D) temperate mixed forest (n=3); (E) temperate steppe (n=7); (F) highland meadow and steppe (n=6);(G) easternmonsoonal China (n=26). Themoisture conditionswere averaged at every 100yr and coded by four classes: 1— dry; 2—moderately dry; 3—moderately wet; and 4—wet.
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mostly from expansions of various tree populations in easternmonsoonal China. Between 10 and 8 ka, the vegetation becamerelatively stabilized. The climate change was probably too slow or toosmall (Fig. 8) to cause synchronous regional response to vegetation.Then major changes were initiated at many sites beginning at about6–5 ka, owingmostly to tree decline (Fig. 4). Maximum rate of summerinsolation change occurring during the mid-Holocene (Berger andLoutre, 1991) might have caused monsoon intensity shift (Wang et al.,2005) and then strong vegetation changes during that time (seediscussion in Section 4.2). Another high peak of SCD occurred at around2–1 ka, due to peak values of Pinus and herbs at many sites.Deforestation could have caused relatively large changes in vegetationover the last 2 ka (Fig. 9), as discussed in Section 4.3.
Fig. 7. Average rate of palynological change during the Holocene from
4.2. Vegetation responses to climate oscillations
Various proxy data from speleothem, lake sediments and peatcores show similar patterns in monsoon intensity change during theHolocene in monsoonal China (Fig. 8; Hong et al., 2003; Wang et al.,2005, 2008; Hu et al., 2008). At Dongge Cave, oxygen isotope valuesshow amajor negative shift at the end of the Younger Dryas at 11.5 ka,reaching a minimum at ca. 9 ka, followed by a gradual long-termincrease. Isotope has been used to track past summer monsoonintensity (Wang et al., 2005). Therefore the oxygen isotope at DonggeCave shows a strong Asian monsoon period in the early Holocene,followed by a gradual weakening trend since 7 ka. Further north atSanbao Cave and Heshang Cave, oxygen isotopes show the similar
all the fossil pollen records (n=26) in eastern monsoonal China.
Fig. 8. Comparison of synthesized Holocene effectivemoisture evolutionwith other selected proxy records from easternmonsoonal China. (A) Oxygen isotope at Dongge Cave (Wanget al., 2005); (B) oxygen isotope at Sanbao Cave (Shao et al., 2006; Wang et al., 2008); (C) oxygen isotope from ostracode shells at Qinghai Lake (Liu et al., 2007). (D) carbon isotopefrom Hongyuan peat; (E) pollen-based moisture index in monsoonal China, with error bars as standard errors; (F) summer insolation at 30°N latitude (Berger and Loutre, 1991).
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general trend as Dongge Cave during the Holocene. Additional lines ofevidence of isotope from lake sediment (at Qinghai Lake) (Liu et al.,2007) and peat (at Hongyuan and Hani sites) (Hong et al., 2003, 2005)and lake level records (Xue and Yu, 2000) support the conclusionsdrawn from the cave records, indicating a marked stronger summermonsoon phase in the first half of the Holocene and a gradual decreaseduring the second half, until the weakest summer monsoon duringthe late Holocene. The general trend of Holocene Asian monsoonhistory has been attributed to changes in summer insolation at lowlatitudes (Kutzbach, 1981; COHMAP, 1988; Wang et al., 2005). Strong
Fig. 9. Sites with evidence for human activity from fossil p
summer insolation in the Northern Hemisphere during the earlyHolocene (Berger and Loutre, 1991) induced strong land–oceanpressure and temperature gradients and increased onshore moist airflow in the summer, causing an enhanced Asian summer monsoon(COHMAP, 1988). The gradual weakening of the Asian summermonsoon since the mid-Holocene was in response to the orbitally-induced decrease in summer insolation (e.g., Gupta et al., 2003),enhanced by the feedbacks from changes in vegetation cover and soilmoisture as was the case in North Africa (Kutzbach et al., 1996;Ganopolski et al., 1998).
ollen data. Numbers indicate the ages of records (ka).
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The climate pattern inferred from fossil pollen records in easternmonsoonal China generally correlates with the monsoon intensityhistory as discussed above, namely moist phase in the first half of theHoloceneand agradual decrease in effectivemoistureduring the secondhalf (Fig. 8). At 9.5–6 ka, moisture based on pollen data in easternmonsoonal China reached the maximum. Therefore our synthesisdemonstrates that changes in vegetation closely correlate withindependently documented changes in climate revealed by otherproxies. However, during the first two millennia in the Holocene from11.5 to 9.5 ka, effective moisture inferred from fossil pollen recordsshowonly a slow and gradual increase, whilemost independent climaterecords, for example from Dongge Cave and Sanbao Cave, tend to showan abrupt onset of the maximum monsoon. Several reasons might beresponsible for that apparently gradual response or 1000-year time lagin vegetation. (1)Vegetation, especially tree-dominated forests, tends toshow 100–200yr lag, as mature trees are less sensitive to climate thanseedlings (Williams et al., 2002). However, it is difficult to explain the1000-year lag, considering that this region was not glaciated, sovegetation immigration delay often-documented in North Americaand Europe is not applicable here (Davis, 1986; Lang, 1994; Williamset al., 2002). (2) A mismatch of favorable moisture and temperatureconditions could cause that delay, if temperature was not favorable inthe early Holocene (Shi et al., 1992), even though precipitation is high(Wang et al., 2005). (3) Age uncertainty of pollen records from lake andpeat sediments could also cause that apparent time lag or gradualresponse.
Effective moisture inferred from pollen data started to decreasefrom 6 ka, with an abrupt shift at ca. 4.5 ka. Rates of palynologicalchange also demonstrate an abrupt change at around 6–5 ka. Climatechange may have been responsible for this vegetation shift. Firstly,independent isotope records from Dongge Cave (Fig. 8A; Wang et al.,2005) and Sanbao Cave (Fig. 8B; Shao et al., 2006; Wang et al., 2008)show that since 6 ka or 5 ka monsoon intensity decreased, suggestingthat vegetation change around that time was likely forced by naturalfactors. These cave records also show an abrupt lowering of Asianmonsoon intensity at ca. 4.4 ka over several decades. In addition to theindication by other independent climatic proxies as discussed above,climate drying was also supported by the evidence of the culturalresponses to prolonged drought during the late Holocene (particu-larly around 4.5–4 ka) in Asia and Africa, including populationdislocations, urban abandonment and state collapse (deMenocal,2001), for example, the collapse of Neolithic culture around theCentral Plain in China (Wu and Liu, 2004). Secondly, pollen evidenceof clear human disturbance is lacking before 2 ka, except at the sitesfrom the Yangtze River drainage basin (Fig. 9). For example, atTaishizhuang (site C3), located in temperate deciduous forest region,both archaeological and paleoenvironmental data support theconclusion that changes in pollen composition between 5.7 and2.1 ka reflect natural variation in precipitation but not deforestationcaused by humans (Tarasov et al., 2006). Archaeological records from100 sites show the habitation of northeastern China during theprehistorical and early historical periods since ca. 8.2 ka, but do notprovide evidence for the use of wood resources or for widespreadfarmlands that would influence the regional vegetation developmentand leave traces in the pollen assemblages.
4.3. Human disturbance during the late Holocene
Although we argue that insolation-induced change in monsoonintensity and precipitation was responsible for vegetation changesduring the late Holocene, human disturbance also played a major roleand contributed to the abrupt vegetation shift over the last severalmillennia, especially over the last 2000yr. In China, there lacks aregionally distinct, broadly synchronous, and clearly defined pollenmarker horizon for human disturbance (Liu and Qiu, 1994), orsettlement comparable to the Ambrosia-rise in North America
(McAndrews, 1988), or the Ulmus decline in western Europe (Behre,1981). Fig. 9 indicated the timing of some sites in eastern monsoonalChina that are exposed to human activities inferred from fossil pollenrecords. It shows that there is some pollen evidence indicating humandisturbance on vegetation in the south part of monsoonal China.Atahan et al.'s (2008) work in the lower Yangtze River region showedthat substantial human impacts are evident at 4.7 ka, 4.1 ka and 2.4 kaat three sites in that region, as suggested by wild rice (Oryza pollenand Oryza phytoliths). Detailed evidence of pollen, algal, fungal sporeand micro-charcoal data from sediment at Kuahuqiao from theYangtze Delta reveal the precise cultural and environmental contextof rice cultivation at this earliest known Neolithic site in eastern Chinaat 7.7 ka (Zhong et al., 2007). At Poyang Lake in the subtropical region,strong human influence over the last 2 ka was inferred from high-resolution pollen record during the late Holocene, indicated bysignificant reduction of arboreal pollen and an increase of herbaceouspollen (Jiang and Piperno, 1999). Two high-resolution pollendiagrams (Shayema Lake and Dianchi Lake) from southwest Chinashow that the impact of human settlement and agriculture was onlyclearly expressed in the sediment and pollen records after 1.5–1 ka byan increase in clastic contents and the occurrence of disturbanceindicators (Phyllanthus, Plantago, Poaceae, Pteridium) and cultigens(Cannabis, Fagopyron). Another pollen record from Erhai Lake insouthwest China shows a pronounced phase of deforestation since2.2 ka, characterized by a rapid decline in Pinus and a marked increasein Poaceae along with other disturbance taxa, including Epilobiumherbs (Shen et al., 2006). At Tianchi Lake (E7) from the southwesternLoess Plateau, pollen records show that vegetation changed fromforest to steppe-like vegetation at ca. 2 ka and Poaceae pollen andmicroscopic charcoal both show sharp increase since then (Y. Zhao,unpublished data), suggesting that human activities over the last2000yr have significantly accelerated deforestation that was initiatedby a drying climate since the mid-Holocene at Tianchi Lake. Mostpollen records reviewed here are from lake sediments and reflectbroad regional vegetation changes. Human activities might haveaffected vegetation at some local sites early in the Holocene, but thereis no evidence showing consistent vegetation change at regionalscales induced by human activities at that time. In any case, the fossilpollen records revealed synchronous deforestation most likelythrough slash-and-burn practice for agriculture development orwood resource at least around 2 ka (Jiang et al., 2008).
In addition to agriculture and wood usage, grazing is anotherimportant human activity that affects vegetation in northern China.Some studies (e.g., Miehe et al., 2008; Schlütz and Lehmkuhl, 2009) onnomadic influence on vegetation have been undertaken on the TibetanPlateau that was previously thought to be less influenced by humanactivities. Miehe et al. (2008) proposed that the presently degradedpastures of the northeastern Tibetan Plateau largely originate from forestssince at least 8 ka based on pollen indicators. Schlütz and Lehmkuhl(2009) found thatfirst signs of nomadic presence appear as early as 7.2 kaand the Poaceae-rich natural vegetation was transformed by nomadicgrazing to Kobresia-pastures at 5.9–2.7 ka on the Nianbaoyeze Mountainin the eastern margin of the Tibetan Plateau, based on a suite ofgeomorphological and palynological data. However, these data from thenortheastern and eastern Tibetan Plateau are from soil profiles withoutrobust age controls and represent localized vegetation signals, which arenot suitable for detecting large-scale vegetation change induced byhumanactivity as from lake sediments.Nevertheless,muchmore researchwith focus on the grazing history is needed to establish a coherentmodel.
In general, the pollen records so far available frommonsoonal China,can only suggest that human disturbance becomes an increasinglyimportant factor in vegetation during the last 2000yr. At most sites inmonsoonal China, particularly in the north, the pollen evidence forhuman impacts on vegetation before 2 ka and how the anthropogenicsignal can be distinguished from the climatic signal are still yet to beexplored (Fig. 9; Liu and Qiu, 1994; Ren and Beug, 2002). More fossil
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pollen records with reliable chronology at high sampling resolution areneeded from monsoonal China to investigate the degree of humaninfluence on natural vegetation.
5. Concluding remarks
1. Fossil pollen data show clear changes in natural vegetation during theHolocene in all vegetation regions that are affected by the summermonsoon, including tropical seasonal rain forest, subtropical forest,temperate deciduous forest, conifer–deciduous mixed forest, temper-ate steppe and highland meadow/steppe. Despite various vegetationsequences in different regions, our synthesis results show a generallyhumid climate during the early and middle Holocene and a drierclimate during the late Holocene.
2. Effective moisture inferred from fossil pollen records in easternmonsoonal China correlates with the summer monsoon intensitypatternas indicated fromother independent climate records.However,effective moisture inferred from fossil pollen records shows only agradual increase at 11.5–9.5 ka, with ca. 1000-year delay after theprecipitation maximum as indicated by monsoon intensity recordsfrom Dongge Cave and Sanbao Cave. Pollen-inferred moisture changeshows a decreasing trend from 6 to 5 ka, with an abrupt shift at ca.4.5 ka, probably due to the decrease in monsoon intensity.
3. Rateof changeanalysis of fossil pollendata showsthat rapidvegetationchange occurred at 11–10 ka, 6–5 ka and 2–1 ka, likely in response toshifts in summermonsoon strengths for thefirst twoperiods butmorelikely induced by human disturbance for the latest interval.
4. Human activities could be a very important factor affecting naturalvegetation at large scale during the last 2 ka. More palynologicaland archaeological data will be needed to evaluate the relativecontributions from climatic and anthropogenic factors to vegeta-tion changes during the late Holocene.
5. High-resolution records with robust chronology for vegetation andclimate reconstructions are still lacking, particularly in tropicalmonsoonal rain forest, temperate deciduous forest and conifer–deciduous mixed forest regions.
Acknowledgements
We thank the following individuals for providing the originalpollen data: ShuyunWang and Houyuan Lu (HuguangyanMaar Lake),Hongyan Liu (Haoluku), Ulrike Herzschuh (Zigetang Lake), QinghaiXu and Jule Xiao (Daihai Lake), Ji Shen (Qinghai Lake), Wenying Jiang(Bayanchagan Lake), and Zhaodong Feng and Chengbang An (Dadi-wan); and Yuting Hou and Xiaoli Guo for digitizing some pollen data.We thank two anonymous reviewers for their helpful comments andsuggestions that improved the manuscript. This project was sup-ported by the National Natural Science Foundation (NSFC grant no.40771212), NSFC Innovation Team Project (no. 40721061) and MOEProgram for New Century Excellent Talents in University.
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