Earth-Science Reviews 97 (2009) 242–256
Contents lists available at ScienceDirect
Earth-Science Reviews
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e a r s c i r ev
Vegetation response to Holocene climate change in monsoon-influenced region
of China
Yan Zhao a,⁎, Zicheng Yu b, Fahu Chen a,⁎, Jiawu Zhang a, Bao Yang c
a
b
c
MOE Key Laboratory of Western China's Environmental System, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China
Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015, USA
Cold and Arid Regions Environment and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e
i n f o
Article history:
Received 26 April 2009
Accepted 16 October 2009
Available online 26 October 2009
Keywords:
Holocene
vegetation change
climate change
monsoonal China
fossil pollen
human activity
a b s t r a c t
Fossil pollen records from 31 sites with reliable chronologies and high-resolution data in the monsoonal
region of China were synthesized to document Holocene vegetation and climate change and to understand
the large-scale controls on these changes. The reconstruction of moisture histories was based on a four-class
ordinal wetness index at 200-year time slices at individual sites. The vegetation experienced diverse changes
over the Holocene in different regions: (1) between tropical seasonal rain forest and more open forest in
tropical seasonal rain forest region; (2) from mixed evergreen and deciduous broadleaved forest to more
deciduous or Pinus-dominated forest in subtropical region; (3) from mixed evergreen and deciduous
broadleaved forest to deciduous forest in temperate deciduous forest region; (4) from deciduous
broadleaved forest to conifer–deciduous forest in conifer–deciduous mixed forest region; (5) from steppe
forest to steppe in temperate steppe region; and (6) from steppe forest/meadow to meadow/steppe in
highland meadow/steppe region. Despite various vegetation sequences in different regions, our synthesis
results show that a humid climate generally characterized the early and middle Holocene, and a drier climate
prevailed during the late Holocene, with an abrupt shift at ca. 4.5 ka (1 ka = 1000 cal yr BP). Abrupt
palynological changes based on a squared-chord distance of pollen assemblages occurred at 11–10, 6–5 and
2–1 ka from most sites. The synthesized pattern of moisture change is similar to the ones inferred from other
independent climate proxies; however, gradual vegetation changes in the early Holocene lagged about
1000 yr behind the summer monsoon maximum as indicated by speleothem isotope records from Dongge
and Sanbao caves. Human activities likely affected vegetation change greatly during the late Holocene, but
the magnitude and precise timing are less clear and require further investigation.
© 2009 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . .
Data sources and methods . . . . . . . . . . . . . . . .
2.1.
Study region and site selection . . . . . . . . . . .
2.2.
Moisture index and statistical analysis . . . . . . .
Pollen records of Holocene vegetation and climate changes .
3.1.
Tropical monsoonal rain forest . . . . . . . . . . .
3.2.
Subtropical evergreen and deciduous forest . . . . .
3.3.
Temperate deciduous forest . . . . . . . . . . . .
3.4.
Temperate mixed conifer–hardwood forest . . . . .
3.5.
Temperate steppe . . . . . . . . . . . . . . . . .
3.6.
Highland meadow and steppe . . . . . . . . . . .
Discussion . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Temporal and spatial patterns of Holocene vegetation
4.2.
Vegetation responses to climate oscillations . . . . .
4.3.
Human disturbance during the late Holocene . . . .
Concluding remarks. . . . . . . . . . . . . . . . . . . .
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and
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⁎ Corresponding author. Tel.: +86 931 891 2337; fax: +86 931 891 2330.
E-mail addresses: [email protected] (Y. Zhao), [email protected] (F. Chen).
0012-8252/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.earscirev.2009.10.007
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Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
The East Asian monsoon system is among the most dynamic
continent–ocean–atmosphere systems in the world. Emerging evidence
suggest that the summer monsoon intensity followed the summer
insolation trend, with the maximum monsoon occurring in the early
Holocene and decreasing afterwards (Kutzbach, 1981; COHMAP, 1988;
Morrill et al., 2003; Ruddiman, 2008). This trend has been documented
in various proxy records in monsoonal China, including speleothem
isotopic 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 also
made to study the Holocene climate pattern in monsoon-influenced
region of China. Shi et al. (1992) proposed that Holocene Megathermal
period happened at 8500–3000 14C yr BP (ca. 9.4–3.1 ka; 1 ka = 1000 cal
yr BP) on the basis of pollen data, fossil fauna, paleosol, lake level, glacial
remains, and archaeological data. An et al. (2000) proposed that the
Holocene East Asian summer monsoon precipitation reached a
maximum at different periods in different regions of China, with the
trend of frontal migration paralleling the trend of summer insolation.
However, this time-transgressive Holocene optimum was only supported by some coarse resolution records, especially from south China.
He et al. (2004) reviewed multi-proxy paleoclimatic records across china
and found that the Holocene optimum in eastern China was at ca. 6.5–
5.5 ka. Herzschuh (2006) assessed the late-Quaternary moisture change
in Central Asia (including the Tibetan Plateau, northwest and northcentral China and Mongolia) using recent pollen data and other
independent proxies. Her review suggested that the Holocene climate
optimum with high precipitation occurred during the early Holocene in
the Indian monsoon region, but possibly occurred during the midHolocene in the East Asian region. Nonetheless, the high-resolution
speleothem isotope records with reliable chronology shed light on the
general pattern of climate change during the Holocene in the monsoonal
China region. However, the major patterns of temporal and spatial
variation in past vegetation are far less clear than climate in this region.
Pollen data would provide biological responses as well as climate
change, in a widespread network compared to cave records, and would
also help further test the Holocene pattern by using a unified synthesis
methodology.
Fossil pollen data from lakes and wetland deposits have been
frequently used in vegetation and climate reconstructions, as they tend
to reflect vegetation and climate changes at a regional scale. As a result,
fossil pollen has become one of the most widely used and available
paleoclimatic proxies. Liu (1988) reviewed late-Quaternary pollen data
from temperate forests of China and found that the Holocene pollen
stratigraphies suggest a tripartite division, with a period of maximum
warmth, the Hypsithermal, in the mid-Holocene. Sun and Chen (1991)
briefly reviewed the Holocene pollen data in China and indicated that
vegetation change was sudden and remarkable in the early Holocene
and late Holocene. Ren and Zhang (1998) and Ren and Beug (2002)
presented Holocene pollen maps of China north of Yangtze River for six
time slices at 2000-year intervals, revealing large changes in Holocene
vegetation, especially since 6–4 ka. Yu et al. (1998) used a biomization
approach to map pollen data and their reconstructions showed the shifts
in biome distributions, implying significant changes in climate since
6000 yr ago. These syntheses are based on a large collection of Holocene
pollen records of various time spans and data quality available in China;
however, the chronologies are limited by dating material (most on bulk
organic matter) and coarse dating control, and data sensitivity is also
limited by the coarse sampling intervals and by the discontinuous
nature of the sediment, including fluvial, coastal, or marine sediments,
243
255
255
used for pollen analysis (Liu, 1988). In addition, in the past couple of
decades after these syntheses, many more high-resolution pollen
records with relatively good chronology control have been available.
Herzschuh (2006) assessed the late-Quaternary moisture change in
Central Asia based on recent pollen data and other independent proxies,
but her review evaluates very few records from eastern China. Hope
et al. (2004) reviewed over 1000 marine and terrestrial pollen records to
reconstruct the vegetation history in the Austrial-Asian region,
including the monsoonal China; however, their analysis was mostly to
examine vegetation change at broad regional and coarse temporal
scales. There are no syntheses of pollen data specifically in the entire
eastern monsoonal China for understanding the vegetation pattern and
response to climate change. Therefore, there is a need to synthesize
paleoclimatic information about monsoonal China based on recent
published fossil pollen records.
In this review, we used 31 fossil pollen records from eastern
monsoonal 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 a
uniform set of site-selection criteria, and used mostly recent pollen data.
The objectives of this paper were to document regional vegetation
patterns by synthesizing fossil pollen records from monsoonal China,
and to evaluate and understand vegetation response to climate changes
and the large-scale controls. This synthesis would not only bring
together existing high-resolution data but also would assist the design
of paleoecological and paleoenvironmental studies in the reviewed
region.
2. Data sources and methods
2.1. Study region and site selection
The geographical region considered in this review covers eastern
monsoonal China (Fig. 1). Altitude ranges from near sea level to about
5000 m above sea level (Table 1). The Asian summer monsoon plays a
significant role in controlling the effective moisture of the region. Under
the influence of the monsoon systems, annual precipitation decreases
sharply from the southeast to the northwest from >1800 mm to ca.
400 mm. At most of the sites the precipitation of the three summer
months (JJA) (June, July, and August) usually accounts for more than
80% of the total annual precipitation, leaving winter and spring dry (Ren
and Beug, 2002). The study sites are located in six vegetation zones (see
details in Table 2; Fig. 2; Wu, 1980; Hou, 2001): 1) tropical monsoonal
rain forest; 2) subtropical evergreen and deciduous forest; 3) temperate
deciduous 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 are
now available in this region. However, many records tend to have low
temporal resolution and can be discontinuous. In this study, we
selected pollen sites based on three criteria: (1) a reliable chronology
with a minimum of 4 dating control points over the Holocene; (2)
high sampling resolution with a minimum 200 yr per sample; and (3)
continuous record covering most of the Holocene without documented depositional hiatus. A total of 26 lake and peat sites met these
criteria in this study. There are no satisfactory records in the
temperate deciduous forest region. Therefore, we included two
records with lower resolution in this region (the Yellow River Delta
and Maohebei) when we discuss the general vegetation and climate
history. In addition, three other sites (Poyang Lake, Taishizhuang and
Tianchi Lake) that have high resolution pollen records but only for the
244
Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
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).
late Holocene were also used to evaluate possible human disturbance
on vegetation change. These three sites were not used in climate
synthesis analysis owing to their limited time span. Our site-selection
criteria make our review and synthesis different from earlier
published reviews of pollen records (e.g., Liu, 1988; Sun and Chen,
1991; Ren and Zhang, 1998; Ren and Beug, 2002). Moreover, our
synthesis includes the latest research results since 2000, covering
more than 60% of the 31 pollen records used.
Radiocarbon dating was the geochronological technique for all
profiles used in this synthesis. All ages for the reviewed profiles have
been calibrated or recalibrated to calendar years before the present
(BP = AD 1950) using the latest IntCa04 calibration dataset (Reimer
et al., 2004). Calibrated ages are used in compiling effective moisture
curves and throughout the text (expressed as cal yr BP, or ka; 1 ka =
1000 cal yr BP).
For the convenience of discussion, we organized the sites by six
modern vegetation zones as described above. We present and discuss
eight summary pollen diagrams that are representative of vegetation
changes in these regions of different vegetation types. Pollen data
were obtained from the original authors or digitized from pollen
diagrams in the publications. At some sites, we also briefly describe
the modern pollen-rain results, if available, to help evaluate fossil
pollen interpretations.
2.2. Moisture index and statistical analysis
The main pollen indices we used here include tropical tree pollen
percentages, evergreen tree pollen percentages, broad leaved tree
pollen percentages, and tree pollen percentage. We used these pollen
indices to make a semi-quantitative estimate of relative moisture
conditions, which is controlled by both precipitation and temperature
(through influence on evaporation). The monsoonal region of China,
including tropical monsoonal rain forest, subtropical evergreen forest
and deciduous forest regions, often has high relative humidity in
summer monsoon season, so evaporation would not increase
significantly with temperature increase. Even in the steppe region
near the limit of monsoon influence where evaporation might be an
important factor for effective moisture, vegetation is more sensitive to
minor change in effective moisture than the eastern parts of our study
region. Therefore the monsoon-induced precipitation variability is
more important than evaporation variability in understanding the
variation in effective moisture in the monsoon region of China, as also
indicated by An et al. (2000). As shown in other geological and
biological records, effective moisture has a strong influence on
vegetation in east-central China (An et al., 2000). We therefore used
these pollen indices from 26 sites (excluding the two sites from the
temperate deciduous forest region and three late Holocene sites) to
estimate dry–wet climate fluctuations semi-quantitatively at individual sites during the Holocene. The four moisture classes were assigned
from the wettest (score 4) to the driest period (score 1) at each
individual site. The justification for designating an ordinal wetness
scale is that pollen index types and values at each individual site may
not be directly comparable to the ones at other sites, owing to
different geographic locations, different dominant vegetation and
hydrological settings. So the wetness scales are relative, in a semiquantitative sense, on the basis of individual sites.
Here we use Qinghai Lake as an example to illustrate how we assigned
relative wetness values to pollen records. We use total tree pollen (%) at
Qinghai Lake as the index to infer effective moisture, with high tree pollen
representing high tree dominance under a wet climate (Shen et al., 2005).
Fossil pollen assemblages with 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, it
satisfactorily shows the general moisture trend at individual sites.
Synthesized wetness curves for each individual region (vegetation
zone) and the entire study region of monsoonal China during the
Holocene were generated by averaging the wetness scores at 200-year
time intervals for all 26 sites, with error bars as standard errors. Due to
potentially different sensitivity to climate change at individual sites, the
range and magnitude of percentage pollen change at individual sites can
be very large (e.g., from <10 to >90% for total tree pollen), so averaging
these highly variable values may not be reasonable and acceptable.
Ordinal indices could smooth the range of variations for each site, thus
providing more reasonable averaged values than original pollen data.
This averaging procedure based on wetness scales assumes that each
site is equally sensitive to climate change and equally well dated, and
that there are no gradients in response across the region. In reality, some
records were obviously better dated, at higher resolution, or higher
sensitivity to climate change than others. Unfortunately, with information available in the original publications, it is not practical for the
Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
245
Table 1
List of fossil pollen sites from eastern Asian monsoon China used in this review.
Site no.
Site name
A1
Shuangchi Maar Lake E110°11′,
N19°57′
Huguang Maar Lake E110°17′,
N21°9′
Dahu
E115°,
N24°41′
Bajiaotian
E110°20′,
N25°48′
Chao Lake
E117°16′54″,
N31°25′28″
Poyang Lake
E116°15′,
N28°52′
Longquan Lake
E112°1′,
N30°52′
Dajiuhu
E109°59′45″,
N31°29′27″
Dianchi
E 103°,
N25°
Erhai Lake
E 100°05′,
N25°36′
Shayema Lake
E101°35′,
N28°05′
Yellow River Delta
E118°54.3′,
N37°47.8′2
Maohebei
E119°12′,
N39°32′
Taishizhuang
E115°49.5′,
N40°21.5′
Jinchuan
E126°22′,
N42°20′
Gushantun
E126°10′,
N42°30′
Qindeli
E133°15′,
N48°00′
Haoluku
E116°45.42′,
N42°57.38′
Bayanchagan
E115.21°,
N41.65°
Diaojiao Lake
E112°21′
N41°18′
Daihai Lake
E112°33′,
N40°29′
Chasuqi
E111°08′,
N40°40′
Midiwan
E108°37′,
N37°39′
Tianchi Lake
E106°19′,
N35°16′
Dadiwan
E105°54′,
N35°01′
Zoige Basin
E103°25′,
N32°20′
Qinghai lake
E99°36′,
N36°32′
Hidden Lake
E92°48′,
N29°49′
Qongjiamong Co
E92°22.37′,
N29°48.77′
Zigetang Lake
E90.9°′,
N32.0°
Selin Co
E88°31′,
N31°34′
A2
B1
B2
B3
B4
B5
B6
B7
B8
B9
C1
C2
C3
D1
D2
D3
E1
E2
E3
E4
E5
E6
E7
E8
F1
F2
F3
F4
F5
F6
Location
Elevation Precip. Sample resolution Dating material
(m a.s.l.) (mm) (yr)
75
1670
< 100
255
1600
250
Number of Type of
date
archives
Reference
Bulk organic matter
6
Lake core
Zheng et al. (2003)
170
Bulk organic matter/leaves
7
Lake core
Wang et al. (2007)
1600
240
Plant macrofossils
6
Peat section
1900
1814
250
Bulk organic matter
6
Peat core
Xiao et al. (2007);
Zhou et al. (2004)
Li et al. (1993)
20
1000
< 200
14.1
1528
170
100
1100
< 150
1760
1500
1886
Bulk organic matter /charcoal/ 10
shell/ plant macrofossil
Bulk organic matter
9
Fuvial + lake
sediment
Lake core
Chen et al. (2009)
Bulk organic matter
6
Lake core
Liu et al. (1993)
120
Peat cellulose
7
Peat section
Zhu et al. (2006)
1010
160
Bulk organic matter
7
Lake core
Sun et al. (1986)
1974
900
< 100
Bulk organic matter
6
Lake core
Shen et al. (2006)
2400
1090
130
Bulk organic matter
5
Lake core
Jarvis (1999)
4.9
750
<1000
Mollusc shell
9
Delta borehole Yi et al. (2003)
2
650
> 300
Bulk organic matter
4
Peat section
Li and Liang (1985)
500
420
< 100
Bulk organic matter
6
Peat section
Tarasov et al. (2006)
662
700
210
Bulk organic matter
4
Peat section
Jiang et al. (2008)
600
630
300
6
Peat section
Liu (1989)
52
600
280
Bulk organic matter,
charcoal
Bulk organic matter
5
Peat section
Xia (1988)
1295
370
200
Bulk organic matter
4
Lake section
Liu et al. (2002)
1355
400
130
Bulk organic matter
7
Lake core
Jiang et al. (2006)
1800
421
85
Bulk organic matter
4
Lake core
1221
423
< 100
Bulk organic matter
8
Lake core
Shi and Song (2003);
Song et al. (1996)
Xiao et al. (2004)
1000
400
70
Bulk organic matter
4
Peat section
Wang and Sun (1997)
1400
395
100
Bulk organic matter
23
Peat section
Li et al. (2003)
2430
615
85
Plant macrofossil
19
Lake core
Y. Zhao (unpublished data)
1400
400
<50
Bulk organic matter
3
Marsh section
An et al. (2003)
3492
710
190
Bulk organic matter
8
Peat section
Yan et al. (1999)
3200
350
60
Bulk organic matter
7
Lake core
Shen et al. (2005)
4980
450
110
Plant macrofossils
7
Lake core
Tang et al. (2000)
4980
450
< 200
20
Lake core
Shen (2003)
4560
320
160
Bulk organic matter
5
Lake core
Herzschuh et al. (2006)
4530
290
200
Bulk organic matter
5
Lake core
Sun et al. (1993)
purpose of this synthesis to assign weightings to each individual site.
Nonetheless, these synthesized time series help us document and
understand variations of effective moisture, though with some
uncertainties as shown in the error bars for the synthesis curve.
We choose the squared-chord distance (SCD), which has proven
especially satisfactory to reveal regionally synchronous palynological
changes from pollen-frequency data (e.g. Overpeck et al., 1985;
Grimm and Jacobson, 1992; Faison et al., 2006). SCD is calculated as:
Flies/charcoal/macrofossil
Jiang and Piperno (1999)
1/2 2
dij = σk(p1/2
ik − pjk ) , where dij is the SCD between two multivariate
samples i and j, and pik is the proportion of species k in sample i. For
each pollen time series (site) used in this synthesis, the pollen data
were interpolated at 200-year intervals by using the program
AnalySeries 2.0.4 (Pailard et al., 1996). The SCD between the adjacent
200-year samples was used to calculate the rate of palynological
change per 100 yr (Grimm and Jacobson, 1992). Although most of the
selected pollen records have only 5–10 radiocarbon dates, the age
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Table 2
Modern environmental settings (Wu and Zhang, 1992).
Vegetation type
MAT (°C) MAP (mm)
Dominant plant taxa
Tropical monsoonal
rain forest
Subtropical evergreen
and deciduous forest
Temperate deciduous
forest
Temperate conifer–
deciduous mixed forest
Temperate steppe
22 to
26.5
14 to 22
>1000
9 to 14
500–900
2 to 8
500–800
− 3 to 8
200–450
− 2 to 0
300–400
Terminalia hainanensis, Lannea coromandelica, Ficus altissima, Chukrasia tabularis, Vatica astrotricha, Madhuca
subquincuncialis, Altingia chinenesis, Dacrydium pierrei, Lithocarpus fenzelianus, and Adinandra hananensis.
Quercus variabilis, Q. serrata, Q. fabri, Castanopsis sclerophylla, Castanea sequinii, Platycarya strobilacea, Lithocarpus
cleistocarpus, Betula luminifera, and Liquidambar formosana.
Quercus mongolica, Q. liaotungensis, Q. aliena, Q. acutissina, Q. variabilis, Robinia pseudoacacia, Salix matsuduna,
and Populus simorill.
Pinus koraiensis, P. sylvestris, Abies nephrolepis, Picea jezoensis, Larix gmelinii, L. olgensis and Quercus mongolica, Tilia
amurensis, Ulmus propinqua, and Betula platyphylla.
Artemisia spp., Stipa spp., Carex spp., Rosaceae and Fabaceae. Pinus, Quercus, Betula, Ulmus, Salix, Corylus and Populus are the
main tree taxa on nearby hills.
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.
Highland meadow and
steppe
1200–3000
MAT = mean annual temperature; MAP = mean annual precipitation.
controls should be sufficient to detect abrupt changes at multicentennial or millennial scales. Our site-selection criterion of
continuous record covering most of the Holocene without documented depositional hiatus also makes the analysis justifiable. The
analysis 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 timetransgressive change, but it reveals abrupt, synchronous, geographically widespread change (Huntley, 1992).
3. Pollen records of Holocene vegetation and climate changes
Below we will describe the synthesis results of vegetation and climate
change 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 tropical
and southern subtropical regions of southeast China. One record is
from Shuangchi Maar Lake (site A1) on Hainan Island, spanning the
last ca. 9000 yr (Zheng et al., 2003). The pollen assemblages were
dominated by tropical trees, mainly including Mallotus and Casearia
at 8.9–4.3 ka, suggesting a warm and wet climate. After 4.3 ka, herb
and fern pollen increased at the expense of tree pollen, and total
pollen concentration was very low, indicating a drying climate.
The other pollen diagram from Huguangyan Maar (site A2) has a
sampling resolution of ca. 170 yr and is divided into 3 zones (Fig. 3A;
Wang et al., 2007). The percentage of tropical tree pollen is high
during 11.6–7.8 ka, mainly including Moraceae, Mallotus, Ficus and
Aporosa. The tropical vegetation represented by pollen suggests a
warm and wet climate. From 7.8 ka, tropical tree pollen started to
decrease, especially after 4.2 ka.
3.2. Subtropical evergreen and deciduous forest
More well dated high-resolution pollen records are available from
this region. A peat profile at Dahu peatland (site B1) provided a well
dated Holocene pollen record (Fig. 3B; Zhou et al., 2004; Xiao et al.,
2007). The pollen diagram indicates a major change dominance of
Alnus to evergreen tree pollen of Castanopsis/Lithocarpus at 10.4 ka,
signifying a warming and wetting climate. Castanopsis/Lithocarpus is
sclerophyllous taxa that are adapted to spring drought, compared to
Fig. 2. Dominant biomes (vegetation types) in China (after Wu, 1980).
Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
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) Qindeli
peatland, 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 the
percentages were calculated based on terrestrial pollen sums.
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Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
many mesic deciduous forest taxa; however, we interpret its presence
as indicating warm/wet condition as it requires more moisture than
Alnus and herbs that are abundant in these pollen records. At 10.4–
6 ka, Castanopsis/Lithocarpus remains high, reaching a maximum of near
30%. Cyclobalanopsis pollen increases gradually to a value of 19% during
that period. This period marks the Holocene optimum period (warmer
and wetter climate), as represented by a subtropical evergreen broadleaved forest. After 6 ka, the pollen record shows a rapid increase in
ferns and herbs at the expense of tree pollen, suggesting a drier climate.
Pollen record from Caohu Lake (B3), west to the Yangtze River Delta,
indicated an evergreen and deciduous mixed broadleaved forest
dominated by Cyclobalanopsis and Quercus after 10.5 ka, especially
between 8.2 and 7.5 ka, suggesting a warm and wet climate. At 7.5–
3.7 ka, there are noticeable fluctuations in main pollen taxa, in particular
a general decline in Cyclobalanopsis and other arboreal pollen and an
increase in terrestrial herbs. After 3.7 ka, the broadleaved forest trees
largely gave way to terrestrial herbs. Pinus continued to increase
alongside with most herbs until 2 ka (Chen et al., 2009).
Pollen record at ca. 120-year sampling resolution from the Dajiuhu
peatland (site B6), in Shennongjia Mountains in central China (middle
reach of Yangtze River), shows a progressive increase in percentages of
evergreen tree pollen after 11 ka (Fig. 3C; Zhu et al., 2006). During 11–
6 ka, pollen assemblages are marked by a continuous increase in
evergreen components as well as some deciduous tree pollen, such as
Cyclobalanopsis/Castanopsis, Betula, Carpinus, Quercus and Juglans. Modern
pollen assemblages from surface samples in the Shennongjia region are
dominated by deciduous broadleaved trees and represent regional pollen
source (Liu et al., 1993). There are good relationships between pollen and
climate variables, with higher evergreen tree pollen percentages at warm
and wet sites (Liu et al., 2000; Zhu et al., 2008). The fossil pollen
assemblages imply a mixed forest of deciduous and evergreen broadleaved trees under a warming and wetting climate during the early and
mid-Holocene. After 4.5 ka, coniferous tree pollen increased at the
expense of evergreen tree pollen, suggesting mixed forests of coniferous
trees with deciduous broadleaved trees caused by a weakening of the East
Asian summer monsoon. Another pollen record from Longquan Lake in
central China (site B5; Li et al., 1993) similarly revealed that evergreen
broadleaved — deciduous broadleaved forest (Castanea, Hamamelis, Keterleeria, and Tsuga) dominated from 10.7 to 4.6 ka, while herb became
dominant at the expense of tree pollen after 4.6 ka.
The pollen record from Shayema Lake (site B9) in southwestern
China spans the last 13,000 yr (Fig. 3D; Jarvis, 1993). Modern pollen-rain
samples were collected from surface sediments at 13 lakes along an
elevational transect between 1800 and 3900 m above sea level (Jarvis
and Clay-Poole, 1992). The results show that a significant number of
important taxa in the region are insect-pollinated and produce minimal
pollen grains. To mitigate the difficulty of working with such a large
number of underrepresented taxa, pollen percentage values for low
pollen-producing taxa of similar autoecological features (e.g., mesic
deciduous, sclerophyllous evergreen broadleaved) were summed and
examined together, by giving particular weight to the presence of key
pollen types representative of a particular climatic regime. The fossil
pollen record shows several changes in the vegetation and climate since
13 ka (Jarvis, 1993). From 12.7 to 10.6 ka, cold-tolerant species, such as
Abies, 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 deciduous
species, suggesting a warm climate with probably increased precipitation. During the period 7.9–4.7 ka, spring drought-adapted sclerophyllous taxa (such as Lithocarpus/Castanopsis) increased at the expense of
mesic deciduous taxa, indicating that precipitation was becoming more
seasonal. This is in a great contrast to the climate in the early Holocene,
with a reduced seasonality in precipitation and an increased seasonality
in temperature, when winters were colder and spring moisture was
probably more abundant. The drought-adapted tree taxa continued to
be 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 of
Tsuga and Picea.
Other palynological records from southwestern China (Dianchi:
site B7, Sun et al., 1986; Erhai: site B8, Shen et al., 2006) also suggest
that prior to 10 ka the climate was colder and drier relative to today as
inferred from dominant conifer pollen (Pinus, Abies). From the early to
middle Holocene, pollen assemblages show a gradual increase in
abundance and diversity of mesic deciduous and evergreen trees
(Quercus, Tsuga, Castanopsis/Lithocarpus). Vegetation was reflected by
spring drought-adapted sclerophyllous taxa relative to mesic deciduous taxa during the late mid- to late Holocene. At Erhai Lake, Pinus
increased significantly, along with pollen of Plantago, Artemisia,
Chenopodiaceae and Poaceae, during late Holocene, suggesting
human disturbance (Shen et al., 2006).
3.3. Temperate deciduous forest
There are few pollen records with good chronology and high
resolution from this region. Palynological analyses of two boreholes
from the Yellow River Delta (site C1) provide a vegetation and climate
history 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 pollen
values), suggesting warmer and wetter climatic conditions. A significant
reduction in deciduous Quercus pollen and an increase in conifer Pinus
pollen at ca. 4 ka, together with the first appearance of buckwheat
(Fagopyrum) pollen at 1.3 ka, probably reflect widespread human
disturbance of the natural vegetation and intensive cultivation.
Another record with >300-year sampling resolution from Maohebei peatland (site C2) spanned from 12.5 to 3 ka (Fig. 3E; Li and
Liang, 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 from peat profiles (Jinchuan: site D1, Jiang et al.,
2008; Qindeli Bog: site D3, Xia, 1988; Gushantun Bog: site D2, Liu, 1989)
in the mixed conifer–hardwood forest of northeast China span the entire
Holocene. Pollen record at ca. 280-year resolution from Qindeli shows
that from 12.5 to 10 ka broadleaved trees, including Betula, dominated
the vegetation (Fig. 3F). At 10–5.5 ka, deciduous broadleaved forest
trees, including Ulmus, Quercus, Salix, Carpinus, and Corylus, thrived,
indicating a high effective moisture during the growing season. After
5.5 ka, Pinus and Abies/Picea increased at the expense of deciduous
broadleaved tree pollen, suggesting a conifer-dominated forest and thus
a colder and drier climate. This trend was accentuated after 2.5 or 2 ka as
indicated 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 tree
pollen from Quercus, Ulmus, Alnus, Juglans and Corylus had high values.
Since 4.4 ka, Pinus increased while deciduous broadleaved trees
decreased, especially after 2.2 ka. The Jinchuan profile has a similar
pattern as Qindeli, with a steadily high broadleaved pollen (Quercus,
Ulmus and Juglans) from 11.5 to 6/5 ka, suggesting an interval of high
effective moisture.
3.5. Temperate steppe
Jiang et al. (2006) presented a pollen record at ca. 130-year sampling
resolution from Bayanchagan Lake in Inner Mongolia (site E2; Fig. 3G).
The pollen assemblages show that vegetation around Bayanchagan Lake
changed from a steppe at 12.5–9.2 ka, through a Betula/Pinus-dominated
Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
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 reconstruct
changes in paleoclimatic parameters based on modern pollen datasets
(including 211 surface pollen assemblages from northern China) and
corresponding climate data. The vegetation sequence and paleoclimatic
reconstruction suggest that a relatively humid climate during the early
mid-Holocene at 9.2–6.7 ka was favorable for the development of
woodland.
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 and
Song, 2003; Chasuqi: site E5, Wang and Sun, 1997) in eastern Inner
Mongolia are generally dominated by Betula, Picea, and Ulmus during
the early Holocene, while by Artemisia with some tree pollen
including Betula, Quercus, Ulmus, and Pinus in the middle Holocene
from 7 to 5 ka. After 5 ka or 4 ka, pollen assemblages were dominated
by Artemisia with some tree pollen. Vegetation changed from steppe
at the beginning of Holocene, to forest steppe in the late part of early
Holocene and mid-Holocene, to steppe in the late Holocene. These
vegetation sequences suggest that climate changed generally from a
wet climate in the early and mid-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. 50 yr (An et al., 2003). The
pollen assemblages show that vegetation around Dadiwan changed
from a desert steppe at 10–8.5 ka, through a Pinus-dominated steppe
woodland at 8.5–6.4 ka, to back to desert steppe after 6.4 ka. The
vegetation sequence suggests that a relatively humid climate during
the mid-Holocene at 8.5–6.5 ka was favorable for the development of
woodland. Midiwan (site E6; Li et al., 2003), another site on the NW
Loess Plateau, shows a different pattern of Holocene vegetation and
climate change. Vegetation was sparsely wooded grassland, consisting
mainly 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 used
for this synthesis. The pollen diagram from Qinghai Lake (site F2;
Fig. 3H; Shen et al., 2005) can be divided into three pollen assemblage
zones. Before 10.6 ka, pollen assemblages were characterized by Artemisia (~60%), tree pollen (~20%), Poaceae (~10%), and Chenopodiacece
(~10%). During 10.6 and 4.2 ka, Artemisia decreased to <40%, Pinus
increased up to ~30% and Betula was up to 20%. The highest pollen
concentrations occurred in this zone. After 4.2 ka, Artemisia increased
again (up to 80%) while Pinus decreased to 15–0% (Shen et al., 2005).
Vegetation around Qinghai Lake changed from steppe before the
Holocene, through steppe forest in the early and mid-Holocene at
10.6–4.2 ka, to Artemisia-dominated steppe in the late Holocene. This
vegetation sequence suggests a dry climate before the Holocene, a wet
climate in the early and mid-Holocene, and a dry climate in the late
Holocene.
Pollen assemblages from other sites in the Tibetan Plateau (Zoige
Basin: F1, Yan et al., 1999; Hidden Lake: F3, Tang et al., 2000; Co
Qongjiamong: F4, Shen, 2003; and Selin Co: F6, Sun et al., 1993) were
dominated by Artemisia and Cyperaceae, with relatively high Pinus/Picea
and Betula pollen percentages during the early and middle Holocene from
11.5 to 6 or 5 ka, while dominated by Artemisia, Cyperaceae and Poaceae
during the late Holocene. The vegetation sequence suggests a humid early
and mid-Holocene and a dry late Holocene. However, at Zigetang Lake
(F5), pollen assemblages are consistently dominated by Artemisia and
Cyperaceae (Herzschuh et al., 2006). Pollen assemblages suggest a
dominance of temperate steppe vegetation during the first half of the
Holocene, while alpine steppes with desert elements tend to dominate
the second half. The vegetation sequence indicates a general cooling and
drying trend throughout the Holocene.
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4. Discussion
4.1. Temporal and spatial patterns of Holocene vegetation and
climate change
The major vegetation changes at individual sites from various
vegetation zones show different types and patterns during the Holocene
(Fig. 4). In the tropical monsoonal rain forest region, vegetation changed
from seasonal rain forest dominated by tropical trees (e.g., Fig. 5A) to
more open forest. In the subtropical region, vegetation changed from
evergreen broadleaved forest dominated by evergreen trees Quercus
and Castanopsis/Lithocarpus (e.g., Fig. 5B, C and D), to evergreendeciduous broadleaved forest dominated by Quercus, Betula, Carya, and
Ulmus or mixed forest dominated by Pinus and deciduous trees with
more herbs during the late Holocene. In the temperate deciduous forest
region, vegetation shifted from deciduous broadleaved forest during
early and mid-Holocene (e.g., Fig. 5E), to diverse broadleaved deciduous
forest with some herbs (Artemisia, Poaceae) during the late Holocene. In
the conifer–hardwood mixed forest region, vegetation changed from
deciduous broadleaved forest (Quercus-dominated) to conifer–deciduous
mixed 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 Inner
Mongolia (e.g., Fig. 5G). Vegetation in the northwestern Loess Plateau
changed between desert steppe (Artemisia, Chenopodiaceae, and Poaceae), forest steppe (Pinus, Artemisia, Poaceae, and Asteraceae) and
steppe. In the highland meadow and steppe region, vegetation is
characterized by meadow/steppe (Artemisia, Poaceae, and Cyperaceae)
and steppe forest (mainly Pinus, Picea and Abies) in the early and midHolocene (e.g., Fig. 5H) and meadow/steppe during the late Holocene.
The semi-quantitative moisture classes obtained from interpreted
fossil pollen records in the six regions reveal generally synchronous
wet–dry climate changes during the Holocene (Fig. 6), though their
vegetation shows different types and patterns. A wet climate occurred in
the early and mid-Holocene before 6–5 ka, with the maximum moist
period at 9.5–6 ka. All the records show a consistently dry climate
during the late Holocene, with an abrupt shift at ca. 4.5 ka (Fig. 6G). Due
to the relative lack of fossil pollen records in the regions of tropical
monsoonal rainforest and temperate deciduous forest, the pattern in
these regions at the beginning of Holocene is not as clear as in other
regions. Vegetation change at individual sites (Fig. 4) and the moderate
error of the moisture curve (Fig. 8E) together indicate that the analysis
results of generally synchronous patterns in different regions are not an
artifact that could be caused by averaging approach. An et al. (2000)
proposed that the Holocene optimum, as defined by maximum East
Asian summer monsoon precipitation, was asynchronous in different
regions, at ca. 10–8 ka in northeastern China, at 10–7 ka in north-central
and north east-central China, at ca. 7–5 ka in the middle and lower
reaches of the Yangtze River, at ca. 3 ka in southern China, and at 11 ka in
southwestern China. However, our synthesis based on pollen data and
other evidence from recently published records does not appear to
support this hypothesis (also see Feng et al., 2006). The different
conclusions from these syntheses are partly due to the fact that An et al.
(2000) used some records with low sampling resolution and poor
dating controls, especially those from southern China.
Dissimilarity analysis of pollen data using SCD measures for the
entire eastern monsoonal China shows that times of significant
palynological 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 a
large geographic region. Our result generally agrees with that inferred
from T-test analysis based on multi-proxy records in Asian monsoon
region (Morrill et al., 2003). At the start of the Holocene, the average rate
of change is high at most sites due to the transition from the late-glacial
period to the Holocene. The significant changes during this time result
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Fig. 4. Summary of vegetation history derived from fossil pollen records in eastern monsoonal China. See Table 1 for site information and references.
Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
Fig. 5. Summary of major vegetation taxa change in various regions arranged from south to north in eastern monsoonal China. (A) Tropical tree pollen (%) at Huguang Maar Lake (Wang et al., 2007); (B) evergreen tree pollen (%) at Dahu (Zhou
et 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 (%) at
Qindeli 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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Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
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) eastern monsoonal China (n = 26). The moisture conditions were averaged at every 100 yr and coded by four classes: 1 — dry; 2 — moderately dry; 3 — moderately wet; and 4 — wet.
mostly from expansions of various tree populations in eastern
monsoonal China. Between 10 and 8 ka, the vegetation became
relatively stabilized. The climate change was probably too slow or too
small (Fig. 8) to cause synchronous regional response to vegetation.
Then major changes were initiated at many sites beginning at about
6–5 ka, owing mostly to tree decline (Fig. 4). Maximum rate of summer
insolation change occurring during the mid-Holocene (Berger and
Loutre, 1991) might have caused monsoon intensity shift (Wang et al.,
2005) and then strong vegetation changes during that time (see
discussion in Section 4.2). Another high peak of SCD occurred at around
2–1 ka, due to peak values of Pinus and herbs at many sites.
Deforestation could have caused relatively large changes in vegetation
over the last 2 ka (Fig. 9), as discussed in Section 4.3.
4.2. Vegetation responses to climate oscillations
Various proxy data from speleothem, lake sediments and peat
cores show similar patterns in monsoon intensity change during the
Holocene in monsoonal China (Fig. 8; Hong et al., 2003; Wang et al.,
2005, 2008; Hu et al., 2008). At Dongge Cave, oxygen isotope values
show a major 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-term
increase. Isotope has been used to track past summer monsoon
intensity (Wang et al., 2005). Therefore the oxygen isotope at Dongge
Cave shows a strong Asian monsoon period in the early Holocene,
followed by a gradual weakening trend since 7 ka. Further north at
Sanbao Cave and Heshang Cave, oxygen isotopes show the similar
Fig. 7. Average rate of palynological change during the Holocene from all the fossil pollen records (n = 26) in eastern monsoonal China.
Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
253
Fig. 8. Comparison of synthesized Holocene effective moisture evolution with other selected proxy records from eastern monsoonal China. (A) Oxygen isotope at Dongge Cave (Wang
et 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 isotope
from 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).
general trend as Dongge Cave during the Holocene. Additional lines of
evidence 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 conclusions
drawn from the cave records, indicating a marked stronger summer
monsoon phase in the first half of the Holocene and a gradual decrease
during the second half, until the weakest summer monsoon during
the late Holocene. The general trend of Holocene Asian monsoon
history has been attributed to changes in summer insolation at low
latitudes (Kutzbach, 1981; COHMAP, 1988; Wang et al., 2005). Strong
summer insolation in the Northern Hemisphere during the early
Holocene (Berger and Loutre, 1991) induced strong land–ocean
pressure and temperature gradients and increased onshore moist air
flow in the summer, causing an enhanced Asian summer monsoon
(COHMAP, 1988). The gradual weakening of the Asian summer
monsoon since the mid-Holocene was in response to the orbitallyinduced decrease in summer insolation (e.g., Gupta et al., 2003),
enhanced by the feedbacks from changes in vegetation cover and soil
moisture as was the case in North Africa (Kutzbach et al., 1996;
Ganopolski et al., 1998).
Fig. 9. Sites with evidence for human activity from fossil pollen data. Numbers indicate the ages of records (ka).
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The climate pattern inferred from fossil pollen records in eastern
monsoonal China generally correlates with the monsoon intensity
history as discussed above, namely moist phase in the first half of the
Holocene and a gradual decrease in effective moisture during the second
half (Fig. 8). At 9.5–6 ka, moisture based on pollen data in eastern
monsoonal China reached the maximum. Therefore our synthesis
demonstrates that changes in vegetation closely correlate with
independently documented changes in climate revealed by other
proxies. However, during the first two millennia in the Holocene from
11.5 to 9.5 ka, effective moisture inferred from fossil pollen records
show only a slow and gradual increase, while most independent climate
records, for example from Dongge Cave and Sanbao Cave, tend to show
an abrupt onset of the maximum monsoon. Several reasons might be
responsible for that apparently gradual response or 1000-year time lag
in vegetation. (1) Vegetation, especially tree-dominated forests, tends to
show 100–200 yr lag, as mature trees are less sensitive to climate than
seedlings (Williams et al., 2002). However, it is difficult to explain the
1000-year lag, considering that this region was not glaciated, so
vegetation immigration delay often-documented in North America
and Europe is not applicable here (Davis, 1986; Lang, 1994; Williams
et al., 2002). (2) A mismatch of favorable moisture and temperature
conditions could cause that delay, if temperature was not favorable in
the early Holocene (Shi et al., 1992), even though precipitation is high
(Wang et al., 2005). (3) Age uncertainty of pollen records from lake and
peat sediments could also cause that apparent time lag or gradual
response.
Effective moisture inferred from pollen data started to decrease
from 6 ka, with an abrupt shift at ca. 4.5 ka. Rates of palynological
change also demonstrate an abrupt change at around 6–5 ka. Climate
change 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, suggesting
that vegetation change around that time was likely forced by natural
factors. These cave records also show an abrupt lowering of Asian
monsoon intensity at ca. 4.4 ka over several decades. In addition to the
indication by other independent climatic proxies as discussed above,
climate drying was also supported by the evidence of the cultural
responses to prolonged drought during the late Holocene (particularly around 4.5–4 ka) in Asia and Africa, including population
dislocations, urban abandonment and state collapse (deMenocal,
2001), for example, the collapse of Neolithic culture around the
Central Plain in China (Wu and Liu, 2004). Secondly, pollen evidence
of clear human disturbance is lacking before 2 ka, except at the sites
from the Yangtze River drainage basin (Fig. 9). For example, at
Taishizhuang (site C3), located in temperate deciduous forest region,
both archaeological and paleoenvironmental data support the
conclusion that changes in pollen composition between 5.7 and
2.1 ka reflect natural variation in precipitation but not deforestation
caused by humans (Tarasov et al., 2006). Archaeological records from
100 sites show the habitation of northeastern China during the
prehistorical and early historical periods since ca. 8.2 ka, but do not
provide evidence for the use of wood resources or for widespread
farmlands that would influence the regional vegetation development
and leave traces in the pollen assemblages.
4.3. Human disturbance during the late Holocene
Although we argue that insolation-induced change in monsoon
intensity and precipitation was responsible for vegetation changes
during the late Holocene, human disturbance also played a major role
and contributed to the abrupt vegetation shift over the last several
millennia, especially over the last 2000 yr. In China, there lacks a
regionally distinct, broadly synchronous, and clearly defined pollen
marker horizon for human disturbance (Liu and Qiu, 1994), or
settlement 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 monsoonal
China that are exposed to human activities inferred from fossil pollen
records. It shows that there is some pollen evidence indicating human
disturbance on vegetation in the south part of monsoonal China.
Atahan et al.'s (2008) work in the lower Yangtze River region showed
that substantial human impacts are evident at 4.7 ka, 4.1 ka and 2.4 ka
at three sites in that region, as suggested by wild rice (Oryza pollen
and Oryza phytoliths). Detailed evidence of pollen, algal, fungal spore
and micro-charcoal data from sediment at Kuahuqiao from the
Yangtze Delta reveal the precise cultural and environmental context
of rice cultivation at this earliest known Neolithic site in eastern China
at 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 highresolution pollen record during the late Holocene, indicated by
significant reduction of arboreal pollen and an increase of herbaceous
pollen (Jiang and Piperno, 1999). Two high-resolution pollen
diagrams (Shayema Lake and Dianchi Lake) from southwest China
show that the impact of human settlement and agriculture was only
clearly expressed in the sediment and pollen records after 1.5–1 ka by
an increase in clastic contents and the occurrence of disturbance
indicators (Phyllanthus, Plantago, Poaceae, Pteridium) and cultigens
(Cannabis, Fagopyron). Another pollen record from Erhai Lake in
southwest China shows a pronounced phase of deforestation since
2.2 ka, characterized by a rapid decline in Pinus and a marked increase
in Poaceae along with other disturbance taxa, including Epilobium
herbs (Shen et al., 2006). At Tianchi Lake (E7) from the southwestern
Loess Plateau, pollen records show that vegetation changed from
forest to steppe-like vegetation at ca. 2 ka and Poaceae pollen and
microscopic charcoal both show sharp increase since then (Y. Zhao,
unpublished data), suggesting that human activities over the last
2000 yr have significantly accelerated deforestation that was initiated
by a drying climate since the mid-Holocene at Tianchi Lake. Most
pollen records reviewed here are from lake sediments and reflect
broad regional vegetation changes. Human activities might have
affected vegetation at some local sites early in the Holocene, but there
is no evidence showing consistent vegetation change at regional
scales induced by human activities at that time. In any case, the fossil
pollen records revealed synchronous deforestation most likely
through slash-and-burn practice for agriculture development or
wood resource at least around 2 ka (Jiang et al., 2008).
In addition to agriculture and wood usage, grazing is another
important human activity that affects vegetation in northern China.
Some studies (e.g., Miehe et al., 2008; Schlütz and Lehmkuhl, 2009) on
nomadic influence on vegetation have been undertaken on the Tibetan
Plateau that was previously thought to be less influenced by human
activities. Miehe et al. (2008) proposed that the presently degraded
pastures of the northeastern Tibetan Plateau largely originate from forests
since at least 8 ka based on pollen indicators. Schlütz and Lehmkuhl
(2009) found that first signs of nomadic presence appear as early as 7.2 ka
and the Poaceae-rich natural vegetation was transformed by nomadic
grazing to Kobresia-pastures at 5.9–2.7 ka on the Nianbaoyeze Mountain
in the eastern margin of the Tibetan Plateau, based on a suite of
geomorphological and palynological data. However, these data from the
northeastern and eastern Tibetan Plateau are from soil profiles without
robust age controls and represent localized vegetation signals, which are
not suitable for detecting large-scale vegetation change induced by
human activity as from lake sediments. Nevertheless, much more research
with focus on the grazing history is needed to establish a coherent model.
In general, the pollen records so far available from monsoonal China,
can only suggest that human disturbance becomes an increasingly
important factor in vegetation during the last 2000yr. At most sites in
monsoonal China, particularly in the north, the pollen evidence for
human impacts on vegetation before 2 ka and how the anthropogenic
signal can be distinguished from the climatic signal are still yet to be
explored (Fig. 9; Liu and Qiu, 1994; Ren and Beug, 2002). More fossil
Y. Zhao et al. / Earth-Science Reviews 97 (2009) 242–256
pollen records with reliable chronology at high sampling resolution are
needed from monsoonal China to investigate the degree of human
influence on natural vegetation.
5. Concluding remarks
1. Fossil pollen data show clear changes in natural vegetation during the
Holocene in all vegetation regions that are affected by the summer
monsoon, including tropical seasonal rain forest, subtropical forest,
temperate deciduous forest, conifer–deciduous mixed forest, temperate steppe and highland meadow/steppe. Despite various vegetation
sequences in different regions, our synthesis results show a generally
humid climate during the early and middle Holocene and a drier
climate during the late Holocene.
2. Effective moisture inferred from fossil pollen records in eastern
monsoonal China correlates with the summer monsoon intensity
pattern as indicated from other independent climate records. However,
effective moisture inferred from fossil pollen records shows only a
gradual increase at 11.5–9.5 ka, with ca. 1000-year delay after the
precipitation maximum as indicated by monsoon intensity records
from Dongge Cave and Sanbao Cave. Pollen-inferred moisture change
shows 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. Rate of change analysis of fossil pollen data shows that rapid vegetation
change occurred at 11–10 ka, 6–5 ka and 2–1 ka, likely in response to
shifts in summer monsoon strengths for the first two periods but more
likely induced by human disturbance for the latest interval.
4. Human activities could be a very important factor affecting natural
vegetation at large scale during the last 2 ka. More palynological
and archaeological data will be needed to evaluate the relative
contributions from climatic and anthropogenic factors to vegetation changes during the late Holocene.
5. High-resolution records with robust chronology for vegetation and
climate reconstructions are still lacking, particularly in tropical
monsoonal rain forest, temperate deciduous forest and conifer–
deciduous mixed forest regions.
Acknowledgements
We thank the following individuals for providing the original
pollen data: Shuyun Wang and Houyuan Lu (Huguangyan Maar Lake),
Hongyan Liu (Haoluku), Ulrike Herzschuh (Zigetang Lake), Qinghai
Xu and Jule Xiao (Daihai Lake), Ji Shen (Qinghai Lake), Wenying Jiang
(Bayanchagan Lake), and Zhaodong Feng and Chengbang An (Dadiwan); and Yuting Hou and Xiaoli Guo for digitizing some pollen data.
We thank two anonymous reviewers for their helpful comments and
suggestions that improved the manuscript. This project was supported by the National Natural Science Foundation (NSFC grant no.
40771212), NSFC Innovation Team Project (no. 40721061) and MOE
Program for New Century Excellent Talents in University.
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