Earth-Science Reviews 113 (2012) 1–10
Contents lists available at SciVerse ScienceDirect
Earth-Science Reviews
journal homepage: www.elsevier.com/locate/earscirev
Vegetation response to Holocene climate change in East Asian
monsoon-margin region
Yan Zhao a,⁎, Zicheng Yu b
a
b
MOE Key Laboratory of Western China's Environmental System, Research School of Arid Environment and Climate Change, Lanzhou University, Lanzhou 730000, China
Department of Earth and Environmental Sciences, Lehigh University, 1 West Packer Avenue, Bethlehem, PA 18015, USA
a r t i c l e
i n f o
Article history:
Received 31 March 2011
Accepted 1 March 2012
Available online 9 March 2012
Keywords:
East Asian monsoon
Fossil pollen
Holocene
Climate change
Vegetation response
Spatial complexity
a b s t r a c t
Fossil pollen records from 20 sites with reliable chronologies and high-resolution data in the East Asian
monsoon margin region were synthesized to document Holocene vegetation and climate change and to
understand the large-scale controls. The vegetation experienced different changes over the Holocene in
various sub-regions. (1) Near the boundary between modern forest and temperate steppe in Northeast
China, forest showed clear expansion in the middle Holocene. (2) In central China near the boundary
between steppe/forest and desert, vegetation showed various patterns at different sites. (3) Further west
on the Tibetan Plateau near the boundary between highland meadow/steppe and semi-desert/desert, forest
expanded at most sites during the early and middle Holocene. Our synthesis indicates that climate in the
margin region was slightly moist in the early Holocene, wettest in the middle Holocene, and dry in the late
Holocene, though there are regional differences as reflected by vegetation change. This general pattern is
very different from either monsoon- or westerly-dominated regions. The maximum moisture occurred during the early Holocene in the monsoon region, while the arid central Asia dominated by the westerlies was
driest in the early Holocene and wettest in the mid-Holocene. The interplay of the Asian summer monsoon,
westerlies, topography and regional vegetation factors might have contributed to this spatial complexity.
© 2012 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Study region, data sources and analysis methods . . . . . . . . . . . . . . . . . . . . .
Temporal and spatial patterns of Holocene vegetation and moisture shifts . . . . . . . . . .
3.1.
Transitional zone between forest and temperate steppe vegetation . . . . . . . . . .
3.2.
Transitional vegetation zone between temperate forest/steppe and desert . . . . . .
3.3.
Transitional vegetation zone between highland meadow/steppe and semi-desert/desert
3.4.
General spatial patterns of Holocene vegetation and moisture changes . . . . . . . .
4.
Possible mechanisms of complex regional vegetation and climate responses . . . . . . . . .
4.1.
Interacting controls of low- and high-latitude atmospheric circulations . . . . . . . .
4.2.
Topography-mediated regional climate changes in the northeastern Tibetan Plateau .
4.3.
Possible land-surface and vegetation feedbacks to regional climate changes . . . . .
5.
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A.
Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
⁎ Corresponding author at: Research School of Arid Environment and Climate
Change, Lanzhou University, China. Tel.: + 86 931 8912329; fax: + 86 931 8912330.
E-mail address: [email protected] (Y. Zhao).
0012-8252/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.earscirev.2012.03.001
The East Asian monsoon is believed to have begun ca. 7 million
years ago (An et al., 2000) and has displayed strong variations on
many time scales (Liu and Ding, 1998; Wang et al., 2001; Yuan
2
Y. Zhao, Z. Yu / Earth-Science Reviews 113 (2012) 1–10
et al., 2004; Wang et al., 2005a, 2005b). At the orbital time scale,
monsoon intensity follows the precession-driven change in seasonal
insolation (Kutzbach, 1981; COHMAP, 1988; Ruddiman, 2008); this
insolation-driven monsoon hypothesis has been confirmed by
many paleoclimate proxy records (e.g., Winkler and Wang, 1993;
An et al., 2000; Morrill et al., 2003), including the new Chinese cave
record spanned over the last 224,000 years (Wang et al., 2008). During the Holocene, various proxy records show that both Indian Summer Monsoon and East Asian monsoon intensities increased sharply
at the onset of the Holocene, but reached maximum monsoon
intensity at 8–9 ka (1 ka = 1000 cal yr BP) (Fleitmann et al., 2003;
Dykoski et al., 2005; Wang et al., 2005a, 2005b; Shao et al., 2006).
This time lag has been attributed to possible response of monsoon intensity to multiple forcing, including changes in precession-driven insolation, obliquity-driven insolation and ice volume (Clemens and
Prell, 2007).
Further inland on Eurasian continent in the westerly-dominated
central Asia, our recent synthesis indicates that its Holocene moisture
history is very different from the monsoon region (Chen et al., 2008).
A synthesized moisture index curve from 11 lakes in arid central Asia
shows a consistently dry climate in the early Holocene, an abrupt shift
to wet climate at 8 ka, a regional maximum moisture condition at 6 ka
and then decreasing moisture trend for the rest of Holocene (Chen
et al., 2008). This general pattern is clearly shown in paleoclimate
records reviewed in the above paper, including Bosten Lake
(Wuennemann et al., 2003; Huang et al., 2009), Issyk-Kul (Ricketts
et al., 2001). The relatively large between-site variability of reconstructed moisture conditions during the mid- and late Holocene suggests the various regional or site-specific controls of either regional
climate or individual proxy records. Most lakes in northwest China
and elsewhere in central Asia were either not filled up or completely
dried up before 8 ka, but abruptly changed to wet condition after 8 ka
across continental interior in central Asia (e.g., Ricketts et al., 2001;
Wuennemann et al., 2003; Feng et al., 2005). This extremely dry climate in central Asia during the early Holocene has been interpreted
as caused by delayed increase of sea-surface temperatures (SSTs) in
the North Atlantic Ocean (Chen et al., 2008) and by the enhanced
subsidence of dry air masses induced by maximum summer insolation in the early Holocene (Broccoli and Manabe, 1992; Zhao et al.,
2007). Cool SSTs due to the presence of waning ice sheets in the
early Holocene reduced the moisture transport from the Atlantic to
Eurasian continent. Also, the maximum insolation in the early Holocene and heating on the Tibetan Plateau would have induced strong
subsidence of dry air masses in surrounding areas north of the
plateau. However, the timing and even directions of moisture changes
during the mid- and late Holocene are highly variable for large variability of reconstructed moisture conditions in the mid- and late
Holocene and appear to lack a coherent spatial pattern across the
region (Rhodes et al., 1996; Chen et al., 2008).
In semi-arid north and northwest China near the limit of East
Asian summer monsoon influences, climate is likely influenced and
mediated by the mid-latitude westerlies and regional features,
such as topography and vegetation, in addition to the summer monsoons. As a result, the regional climate changes during the Holocene
in the monsoonal margin are complex, due to the interactions of
these large-scale and regional controlling factors. The general trends
of climate changes have been broadly documented in East and
central Asia over the Holocene, especially at orbital- and multimillennial time scales (Herzschuh et al., 2006; Chen et al., 2008).
However, the major patterns of temporal and spatial variation in
past vegetation and climate in the monsoon margin are still poorly
documented and understood. Here we review available fossil pollen
records, in the monsoonal margin from the east in Northeast of
China to the west on the northeastern Tibetan Plateau. Our objectives were to document patterns of Holocene vegetation and climate
change at millennial timescales and to understand the causes of
climate change and ecosystem responses in the East Asian monsoon
margin.
2. Study region, data sources and analysis methods
East Asian monsoonal margin is located at the transition between
the monsoon- and westerly-dominated regions; however, it is not a
clearly-defined boundary but a broad transitional zone. Due to the
large latitudinal and altitudinal extents, this transitional region represents various climatic and vegetation zones. As a result, the vegetation across the transitional region from east to west is characterized
by the ecotones between major biomes, including the ecological
boundaries between forest and temperate steppe, between temperate
steppe and desert, and between highland meadow/steppe and semidesert/desert (Fig. 1B).
Many paleoclimatic records from monsoonal margin have been
published. However there is a large disparity in sample resolution
and age controls. In this study we used 20 selected pollen records
from lakes or peatlands, with an exception of Dunde ice cap, that
have an analysis resolution of b200 years and multiple age controls
of >4 dating determinations at each site (Table 1).
Radiocarbon dating was the geochronological technique used to
date all the records used in this paper, except for Dunde ice core,
which was dated by ice layer counting and ice flow modeling. Bulk organic matter, pollen or plant macrofossils were used for 14C dating
from lacustrine and peat deposits. We have followed the original
publications and made old carbon corrections of the measured 14C
dates at Bayanchagan, Sanjiaocheng, Qinghai Lake, Hurleg Lake and
Zigetang Lake, on the basis of the dating of surface sediments at
these sites (Table 1). We did not make correction for other sites
owing to the lack of such information. In this paper, all radiocarbon
dates were calibrated to calendar years before present (BP =
1950 AD) using Calib. 5.0.1 based on IntCal04 calibration dataset
(Reimer et al., 2004). The chronology at most sites was established
based on linear interpolation, except at Hurleg Lake (site #18) and
Tangke (site #12) with 3rd polynomial curve.
Each of these fossil pollen sequences were resampled into 1000year bins by averaging tree pollen percentages or Artemisia/Chenopodiaceae (A/C) ratio, Artemisia/Cyperaceae ratio (A/Cy), Ephedra
distachya/Ephedra fragilis (Ed/Ef) ratio or total pollen concentration
of all samples within that binned time interval. The binned tree pollen
percentages (or other pollen indices) were then standardized to zero
mean and unit standard deviation. All standardized curves were averaged to generate the composite regional tree pollen record (Z-score),
with standard error of the means as an estimate of variations among
sites. We carried out these data synthesis analysis on each sub-region
and for all sites.
3. Temporal and spatial patterns of Holocene vegetation and
moisture shifts
3.1. Transitional zone between forest and temperate steppe vegetation
This zone is located at the boundary between conifer forest and
temperate steppe in northeastern China (Fig. 1B). We here first
describe two pollen records that are representative of vegetation in
this region, and then discuss the general pattern from all records. At
Daihai Lake in Inner Mongolia (site #6 in Table 1), pollen record indicates a steppe forest (with tree pollen up to ~60%) at 8–3 ka, suggesting a wet climate, bracketed by steppe vegetation before 8 ka and
after 3 ka (Xiao et al., 2004; Fig. 2A), and lake-level reconstruction
from the same lake also indicates low and fluctuating lake level at
11–8 ka, high and stable lake level at 8–3 ka and decreasing and low
level after 3 ka (Sun et al., 2009; Fig. 2A). The pollen assemblages
from Bayanchagan Lake (site #3; Fig. 2D) just north of Beijing show
that vegetation around the lake changed from a steppe at 12.5–
Y. Zhao, Z. Yu / Earth-Science Reviews 113 (2012) 1–10
3
Fig. 1. A. Digital elevation model of southeast Eurasian continent (mostly China) showing the monsoonal margin transect and paleoclimate sites in the monsoon- and westerlydominated regions: a. Dongge Cave; b. Sanbao Cave; c. Guliya ice cap; d. Bosten Lake; e. Wulun Lake; f. Issyk-Kul. B. Vegetation map of China showing major biomes (Hou,
2001). Also shown are sites along the monsoonal margin: 1. Hulun Lake; 2. Haoluku; 3. Bayanchagan; 4. Diaojiao Lake; 5. Chasuqi; 6. Daihai Lake; 7. Dadiwan; 8. Qingtu Lake; 9.
Sanjiaocheng; 10. Eastern Juyan; 11. Zoige Basin; 12. Tangke;. 13. Hongyuan; 14. Koucha; 15. Qinghai Lake; 16. Dalianhai; 17. Dunde; 18. Hurleg Lake; 19. Zigetang Lake; 20.
Selin Co.
9.2 ka, through a Betula/Pinus-dominated steppe woodland at 9.2–
6.7 ka, back to steppe after 6.7 ka (Jiang et al., 2006). Jiang et al.
(2006) used this pollen sequence to reconstruct climate changes
using modern analogue technique on the basis of 211 surface pollen
samples from northern China. 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.
All sites in this sub-region, except at Haoluku, show the high tree
pollen between 8 and 4 ka with peaks at ~6 ka, which is clearly
reflected in composite tree pollen curve (Fig. 2G). These changes in
pollen assemblages reflected shifts between forest and steppe
vegetation, in response to change in precipitation caused by change
in the intensity of the Asian summer monsoon. However, during the
early Holocene, the sites from this region show complex vegetation
and moisture patterns. For example, at Bayanchagan, tree pollen
shows gradual increase in the early Holocene and reaches the maximum value at 6 ka before declining in two steps to the lowest values
in the last 2 ka (Jiang et al., 2006). At Daihai Lake, tree pollen shows
slightly increase (Xiao et al., 2004). Haoluku (site #2) shows high
tree pollen in the early Holocene, but this is a site that has the lowest
sampling resolution (Liu et al., 2002; Fig. 2E). At Diaojiao Lake (site
#4), and Hulun Lake (site #1), tree pollen shows fluctuations (Shi
and Song, 2003; Wen et al., 2010; Fig. 2C and F).
4
Y. Zhao, Z. Yu / Earth-Science Reviews 113 (2012) 1–10
Table 1
List of fossil pollen sites used in this review from the Asian monsoon margin region of China.
Precipitation
(mm/yr)
Sample
resolution
(years)
Number
of datesa
Dating material
Old carbon
correction
(14C yr)*
Type of
archives
Reference
545
333
N/A
13
Bulk organic matter
–
Lake core
1295
370
200
4
Bulk organic matter
–
Lake section
Bayanchagan
E116°45.42′,
N42°57.38′
E115.21°, N41.65°
Wen et al.,
2010
Liu et al., 2002
1355
400
130
7
Bulk organic matter
570
Lake core
4
Diaojiao Lake
E112°21′, N41°18′
1800
421
85
4
Bulk organic matter
–
Lake core
5
Chasuqi
E111°08′, N40°40′
1000
400
70
4
Bulk organic matter
–
Peat section
6
Daihai Lake
E112°33′, N40°29′
1221
423
b100
8
Bulk organic matter
–
Lake core
7
8
9
Dadiwan
Qingtu Lake
Sanjiaocheng
E105°54′, N35°01′
E103°40′, N39°03′
E103°20′, N39°00′
1400
1309
1325
400
115
100–500
b50
150
b100
3
3
9
matter
matter
matter
–
–
540
Marsh section
Lake section
Lake section
10
Eastern Juyan
E101.85°, N41.89°
892
480
140
5
Bulk organic
Bulk organic
Bulk organic
charcoal
Bulk organic
matter
–
Lake core
11
Zoige core RM
E102°21′, N33°57′
3401
705
150
3
Bulk organic matter
–
Peat core
12
E103°25′, N32°20′
3492
70
190
8
Charcoal
–
Peat core
13
Tangke
ZB08-C1
Hongyuan
E102°31′, N32°47′
3505
700
175
31
Bulk organic matter
–
Peat section
14
Koucha Lake
E97.2°, N34.0°
4540
470
200
4
Pollen
–
Lake core
15
Qinghai lake
E99°36′, N36°32′
3200
350
60
7
Bulk organic matter
1039
Lake core
16
17
Dalianhai
Dunde
E100°24′, N36°12′
E99°36′, N36°32′
2850
5325
250
400
80
1–1000
–
–
Lake core
Ice core
18
Hurleg Lake
E96°54′, N37°19′
2809
100
100
7
Plant macrofossil
Ice layer counting
and model
Plant macrofossils
2758
Lake core
19
Zigetang Lake
E90.9°′, N32.0°
4560
320
160
5
Bulk organic matter
2010
Lake core
20
Selin Co
E88°31′, N31°34′
4530
290
200
5
Bulk organic matter
–
Lake core
Site no.
Site name
Latitude,
longitude
1
Hulun Lake
E112°00′, N48°31′
2
Haoluku
3
Elevation
(m a.s.l.)
10
-
Jiang et al.,
2006
Shi and Song,
2003
Wang and Sun,
1997
Xiao et al.,
2004
An et al., 2003
Li et al., 2009
Chen et al., 2006
Herzschuh
et al., 2004
Shen and Tang,
1996
Zhao et al.,
2011
Zhou et al.,
2010
Herzschuh
et al., 2009
Shen et al.,
2005
Cheng, 2006
Liu et al., 1998
Zhao et al.,
2007
Herzschuh
et al., 2006
Sun et al., 1993
– indicates no old carbon correction.
a
Corrections for too old date as shown.
Fig. 2. Holocene paleoclimate records in monsoon margin region A (between forest and steppe): A. total tree pollen percentage and lake-level reconstructions at Daihai Lake, Inner
Mongolia (tree pollen in green: Xiao et al., 2004; lake depth in black: Sun et al., 2009); B. total tree pollen percentage at Chasuqi (Wang and Sun, 1997); C. total tree pollen percentage at Diaojiao Lake (Shi and Song, 2003); D. total tree pollen percentage at Bayanchagan, Inner Mongolia (Jiang et al., 2006); E. total tree pollen percentage at Haoluku
(Liu et al., 2002); F. total tree pollen percentage at Hulun Lake, Inner Mongolia (Wen et al., 2010); G. synthesized standard curve for tree pollen percentages.
Y. Zhao, Z. Yu / Earth-Science Reviews 113 (2012) 1–10
3.2. Transitional vegetation zone between temperate forest/steppe and
desert
This zone is located at the boundary between temperate steppe
and desert, with temperate forest in the highlands, in northwestern
China (Fig. 1B). Dadiwan (site #7) is representative of the pollen
sites in this region with a high sampling resolution of ca. 50 years
(An et al., 2003; Fig. 3D). The pollen data indicate that vegetation
around Dadiwan changed from a desert steppe at 12–8.5 ka, through
Pinus-dominated steppe woodland (steppe forest) at 8.5–6.5 ka, to
desert with sparse steppe after 6.5 ka. This 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, while a
relatively dry early and late Holocene allowed arid open vegetation
to develop.
Fossil pollen data from this region reveal a complicated pattern,
and the composite curve of tree pollen and other pollen index show
larges error bars during the early and middle Holocene (Fig. 3E). At
Eastern Juyan (site #10), pollen assemblages at 10.7 ka-5.4 ka are
characterized by highest values of Chenopodiaceae, Ephedra fragilistype and other desert plants, suggesting a dry climate (Herzschuh
et al., 2004). Most favorable conditions are reconstructed between
5.4 ka and 3.9 ka on the basis of relative increase in abundance of
Artemisia pollen. The pollen diagram from Sanjiaocheng suggests
that the region was covered by steppe vegetation at 11.6–7 ka, desert
or desert steppe at 7–3.8 ka, and desert after 3.8 ka, indicating a wet
early Holocene, a dry mid-Holocene and variable late Holocene
(Chen et al., 2006; Fig. 3B). Pollen concentration from Qingtu Lake
shows a maximum during the middle Holocene and rather dry during
the mid-Holocene (Li et al., 2009; Zhao et al., 2008; Fig. 3C).
3.3. Transitional vegetation zone between highland meadow/steppe and
semi-desert/desert
This zone is located at the boundary between highland meadow/
steppe and semi-desert/desert in the Tibetan Plateau (Fig. 1B). Shen
et al. (2005) presented a high-resolution pollen record from Qinghai
5
Lake (site #14) on the northeastern Tibetan Plateau and found gradual increases in total pollen concentration and in tree pollen
(especially Betula) since 11 ka, reaching maximum at 7–6 ka (>40%
tree pollen; Fig. 4F). Tree pollen gradually decreases after 6 ka and
reach a minimum after 2 ka. An oxygen isotope record from Qinghai
Lake shows that a positive water balance occurred earlier than indicated by pollen (Lister et al., 1991), perhaps suggesting different responses of lake hydrology and landscape vegetation (Wei and
Gasse, 1999; Colman et al., 2007). While at Hurleg Lake (site #18),
the only freshwater lake in the Qaidam Basin on the northeastern
Tibetan Plateau, both Artemisia-to-Chenopodiaceae (A/C) pollen
ratios, a proxy of landscape effective moisture, and sediment carbonate content, an indicator of groundwater/river water inputs and lake
levels, show high moisture conditions at ~8.5–6.5 ka, driest period
at 6.5–3 ka, and increasing moisture after 3 ka (Zhao et al., 2007,
2009a, 2009b, 2009c; Fig. 4C). This pattern has been confirmed by
other geochemical proxy records (ostracode shell isotopes and trace
elemental ratios) from the same lake (Zhao et al., 2009a, 2009b,
2009c, 2010).
Fossil pollen data from this region reveal uniform pattern, with
only a few exceptions, and the composite curve of tree pollen and
other pollen index show small error bars during the Holocene
(Fig. 4H). Along the south to north regional transect on the eastern
Tibetan Plateau, tree pollen abundance at all sites increased in the
early Holocene and reached the maximum abundance in the midHolocene, suggesting a wetter climate; however, tree-pollen abundance varied from >80% in Hongyuan peatland (site #13), to less
than 30% at our study site in the central Zoige Basin (Tangke ZB08c1; site #12) and 80% in the northern Zoige Basin (Zoige core RM;
site #11) (Shen and Tang, 1996; Zhou et al., 2010; Zhao et al.,
2011), to b60% at Qinghai Lake and Dalianhai (Shen et al., 2005;
Cheng, 2006; Fig. 4). At Koucha Lake (site #14), to the west of these
sites above-mentioned, pollen assemblages are dominated by Cyperaceae and Artemisia (Herzschuh et al., 2009), and the vegetation
around the lake region changed from steppe before 6.6 ka to highalpine meadow after then, suggesting that climate became cooler
and wetter during the Holocene. Pollen concentration at Dunde
Fig. 3. Holocene paleoclimate records in monsoon margin region B (between forest/steppe and sesert): A. Artemisia-to-Chenopodiaceae (A/C) pollen ratios and Ephedra distachyato-Ephedra fragilis (Ed/Ef) ratios at Eastern Juyan Lake, Inner Mongolia (Herzschuh et al., 2004); B. total tree pollen percentage at Sanjiaocheng, (Chen et al., 2006); C. total pollen
concentration at Qingtu Lake, Hexi Corridor (Li et al., 2011); D. total tree pollen percentage at Dadiwan Lake, Loess Tibetan Plateau (An et al., 2003); E. synthesized standard curve
for pollen index.
6
Y. Zhao, Z. Yu / Earth-Science Reviews 113 (2012) 1–10
Fig. 4. Holocene paleoclimate records in monsoon margin region C (between forest/highland meadow/steppe and highland semi-desert and desert): A. total tree pollen percentage
at Selin Co, Tibetan Plateau (Sun et al., 1993); B. Artemisia-to-Chenopodiaceae (A/C) pollen ratios at Zigetang, Tibetan Plateau (Herzschuh et al., 2006); C. Artemisia-to-Chenopodiaceae
(A/C) pollen ratios at Hurleg Lake, Qinghai (Zhao et al., 2007); D. pollen concentration at Dunde Icecap, Tibetan Plateau (Liu et al., 1998); E. total tree pollen percentage at Dalianhai,
Qinghai (Cheng, 2006); F. total tree pollen percentage at Qinghai Lake, Qinghai (Shen et al., 2005); H. total tree pollen percentage at Zoige peatlands, Sichuan (Zhao et al., 2011);
G. synthesized standard curve for pollen index.
icecap (site #17) west to these sites shows high values during both
the early and middle Holocene (Liu et al., 1998; Fig. 4C). In central
Tibetan Plateau, A/C pollen ratio from Zigetang Lake (site #19) has
highest value at 6.4–4.2 ka (Herzschuh et al., 2006; Fig. 4B). However,
Selin Lake (site #20), another site from central Tibetan Plateau, shows
the highest tree pollen abundance during the early Holocene (Sun
et al., 1993; Fig. 4A).
3.4. General spatial patterns of Holocene vegetation and moisture
changes
These pollen records along the west–east transect in the semi-arid
monsoonal margin region show variable but coherent shifts in
regional vegetation (Fig. 5), reflecting changes in effective moisture.
However, it appears that major vegetation shifts occurred at ~8 ka
and at ~5 ka at most of these sites in the monsoonal margin region
of China. The changes in effective moisture show different spatial patterns in different regions along this geographic transect. Most sites in
the transitional zone between forest and temperate steppe vegetation
show the high forest values and the most moist climate between
8 and 4 ka but peak at ~ 6 ka (Fig. 5B). In the transitional vegetation
zone between temperate forest/steppe and desert, fossil-pollen data
from this region reveal a complicated vegetation pattern, and the
composite curve of tree pollen and other pollen indices have large error
bars during the early and middle Holocene (Fig. 5C). Generally the sites
in the transitional vegetation zone between highland meadow/steppe
and semi-desert/desert reveals relatively uniform change indicated by
the pollen composite curve, showing wet climate during the early
Fig. 5. A. Synthesized moisture history of 11 lakes (relative moisture index: 0–4 representing from dry to wet) in arid central Asia (modified from Chen et al., 2008; two gray lines
represent errors in standard deviations); B. synthesized standard curve for pollen index in Region A; C. synthesized standard curve for pollen index in Region B; D. synthesized
standard curve for pollen index in Region C; E. synthesized standard curve for pollen index in all monsoon margin region; F. pollen-based moisture index in monsoonal China
(Zhao et al., 2009a, 2009b, 2009c); G. oxygen isotopes (‰ relative to VPDB) at Dongge Cave, Guizhou (in green; Wang et al., 2001) and summer insolation (in red; Berger and
Loutre, 1991).
Y. Zhao, Z. Yu / Earth-Science Reviews 113 (2012) 1–10
and middle Holocene and dry climate during the late Holocene, with
relatively small error (Fig. 5D).
The synthesis curve of vegetation indices from all sites shows the
maximum moisture during the middle Holocene at 8–4 ka and the
driest period in the last 4 ka (Fig. 5E). This general pattern is very
different from either the monsoon or westerly regions. In the monsoon region, the maximum moisture occurred during the early
Holocene (Fig. 5F and G; Wang et al., 2005a, 2005b; Zhao et al.,
2009a, 2009b, 2009c), whileas in arid central Asia, which is dominated by the westerlies, the early Holocene was driest and the midHolocene was wettest (Fig. 5A; Chen et al., 2008).
4. Possible mechanisms of complex regional vegetation and
climate responses
Climate in the transitional zone is influenced by multiple largescale climate controls and regional land surface factors. These controls include subtropical monsoon circulation as a regional expression
of Hadley Cell (Webster, 2005), the mid-latitude westerlies,
topography-induced vertical air motion around the Tibetan Plateau,
and potential regional vegetation feedbacks. As a result of these interacting and competing factors, Holocene climate changes of this region
were complex and regional heterogeneity as shown above. In addition to multiple large-scale climate controls and regional land surface
factors above-mentioned, human activity might have also influenced
the Holocene vegetation change in the monsoon margin region. Zhao
et al. (2009a, 2009b, 2009c) reviewed the fossil pollen data in the
monsoon region and their results showed that human activities
could be an important factor affecting natural vegetation at a large
scale during the last 2 ka. However, we argue that human activity is
not a major player in the vegetation and climate change at multimillennial scale during most part of the Holocene.
4.1. Interacting controls of low- and high-latitude atmospheric
circulations
Interacting controls of low- and high-latitude atmospheric circulations might have contributed to the inconsistency of the vegetation
and moisture change in the transition vegetation zone between
temperate forest, steppe and desert. The onset and intensity of monsoons have generally been attributed to contrasts in the thermal
properties between land surface and oceans, caused by summer insolation change and radiative heating (Kutzbach, 1981; Webster
and Fasullo, 2003). However, recent studies have reexamined this
conventional idea and attribute that the onset of monsoon circulations is in response to large shifts in Intertropical Convergence
Zone (ITCZ) (Chao and Chen, 2001) and is caused by interactions
of tropical overturning atmospheric (Hadley) circulation and extratropical eddies (Bordoni and Schneider, 2008). This new understanding may shed light on reconciling long-term monsoon history
and dynamics during the Holocene. If monsoons are not primarily
controlled by low-latitude insolation-driven heating of the continents, then we may not necessarily expect to see a close match
between the maximum monsoon intensity and summer insolation
maximum. This may explain the delay in the timing of monsoon
maximum in related to peak summer insolation as documented
in many low-latitude paleo-records, such as at Dongge and Sanbao
caves. This delayed response in monsoon precipitation may be
expected farther north, away from the center of monsoon influences,
even just considering the subtropical monsoon circulation. For example, maximum moisture occurred during the mid-Holocene at
the most sites in the transition zone between forest and temperate
steppe, and maximum moisture occurred during the mid-Holocene
or late early Holocene in the transition zone between highland
meadow/steppe and semi-desert/desert. The timing difference
of maximum moisture conditions occurring in the two zones, is
7
probably in response to rapid and intense heating of the Tibetan
Plateau during summer insolation maximum.
At mid-latitude regions, such as near the northern limit of monsoon influences, however, the mid-latitude westerlies likely also
play a major role in mediating the influences of monsoon circulations.
The intensity and moisture properties of the westerlies depend on
large-scale boundary conditions. Farther inland in the mid-latitude
region, moisture is supplied mainly from the North Atlantic Ocean
and from inland seas and lakes along the cyclonic storms paths
(Böhner, 2006). During the early Holocene ice sheets in North America and northern Eurasia were still large, as indicated by geological
evidence (Lowe and Walker, 1997) and the sea-level record (Peltier
and Fairbanks, 2006). These remnant ice sheets would have depressed the sea-surface temperatures (SSTs) in the North Atlantic
Ocean (Koç et al., 1993; Kaplan and Wolfe, 2006) and air temperatures as documented in Greenland ice cores (Dahl-Jensen et al.,
1998). Lower temperatures would have reduced evaporation from
the North Atlantic and consequently vapor transport to Eurasian
continent, thus producing relatively dry westerlies. Also, during the
glacial boundary conditions the westerlies were stronger and penetrated further eastward into north-central China as shown by eolian
data and GCM simulations (Vandenberghe et al., 2006). These strong
dry westerlies farther into East Asia in the early Holocene may
have delayed moisture maximum as documented at sites reviewed
above in the mid-latitude transitional zone, as well as in central
Asia (Chen et al., 2008). Also, the strong and dry westerlies would
have blocked the northward movement of rain belts associated with
the subtropical monsoon circulations. The interplay of monsoon and
westerlies may have contributed to the complex patterns at various
sites in the transitional vegetation zone between temperate forest/
steppe and desert in central China.
4.2. Topography-mediated regional climate changes in the northeastern
Tibetan Plateau
The Holocene hydroclimate pattern as shown at Hurleg Lake in
the Qaidam Basin demonstrates the possible orography-mediated regional climate response to large-scale climate controls. During the
Holocene there appears to be opposite changes in moisture conditions at Hurleg Lake (Zhao et al., 2007) and Qinghai Lake (Shen
et al., 2005), only about 250 km to the east but at different elevations
(2800 m in the basin vs. >4000 m asl on the plateau). It is in particular more clear at 6 ka, while maximum aridity occurred in the basin,
but maximum moisture at Qinghai Lake) and over the last 3 ka
(increasing moisture at Hurleg, but decreasing moisture at Qinghai).
The similar out-of-phase patterns at low-elevation sites and surrounding high mountains appear also to occur during the last
1000 years, based on high-resolution multi-proxy records from
Hurleg Lake (Zhao et al., 2009a, 2009b, 2009c) and tree-ring records
from surrounding mountains (Sheppard et al., 2004; Shao et al.,
2006) and during the last 50 years (e.g., Gahai Lake in the Qaidam
Basin vs. Dunde ice core at 5300 m asl; Zhao et al., 2008).
We propose that this difference between low- and high-elevation
sites may be caused by the complex topography around the NE corner
of the Tibetan Plateau (TP) and the resulting convective pattern. The
possible connection between the orographic effect of the TP and dry
climate in central Asia has been well-understood through climate
modeling and observational studies. Broccoli and Manabe (1992) investigated the role of the TP in maintaining mid-latitude dry climates
using GCM simulations and found that the TP has large-scale effects
on atmospheric circulation. The TP caused large-amplitude stationary
waves in the Eurasian continent in the cold season. The dry regions
north of the TP are located upstream of the troughs of these waves,
hence they experience dry subsiding air. The plateau also plays a
major role in responding to and mediating the Asian monsoon circulations through the intense heating of the plateau surface during the
8
Y. Zhao, Z. Yu / Earth-Science Reviews 113 (2012) 1–10
summer. The heating and upward motion of air over the plateau
causes strong air subsidence to the northwest and north of the TP, inducing dry climate in central Asia. Wang et al. (2010) proposed that
Hadley Circulation centered over the Tibetan Plateau during the
early Holocene and resulted in subsidence in the surrounding regions
leading to relatively dry conditions. This large-scale effect of the TP
has also been indicated by observational data (He et al., 1987) and
has been considered as an explanation for spatial pattern of Holocene
wet-dry climate periods in central Asia (Herzschuh, 2006; Chen et al.,
2008). We use the same mechanism to explain the different climate
pattern in the Qaidam Basin and the surrounding mountains and
plateau. This difference in topography not only allows eastward
penetration of the dry westerlies into the Qaidam Basin but also
induces a similar uplifting-subsiding motion regionally, similar to
that described above for the Tibetan Plateau. It appears that such a
mechanism also works at a fine spatial scale as demonstrated in climate simulations. Sato and Kimura (2005) used a regional climate
model of 150-km spatial resolution to simulate the effect of diabatic
heating over the Tibetan Plateau on subsidence in arid climate regions
and found prominent regional-scale subsidence during the summer.
Their high spatial resolution simulations show that the subsidence
extends into the Qaidam Basin, especially in simulations with no condensation, suggesting the dominant role of sensible heat flux induced
by the radiative heating on the plateau surface.
A large topographic feature such as the Tibetan Plateau can modify
the atmospheric circulation. During the instrumental period, Liu and
Yin (2001) show that the plateau may be inducing different regional
precipitation responses in the eastern Tibetan Plateau, because the
flow associated with the North Atlantic Oscillation bifurcates around
the plateau. They found a seesaw pattern in summer precipitation
in northern and southern parts of the eastern Tibetan Plateau. It
would be interesting to test this idea using paleoclimate records during the Holocene or last 2000 years, perhaps by focusing on abundant
lakes and peatlands in the Zoige Basin. Considering that higher elevations have experienced greater warming over the last 50 years as
documented by instrumental records on the Tibetan Plateau (Liu
and Chen, 2000) and by ice core data from different elevations
(Dasuopu at 7200 m, Guliya at 6200 m, and Dunde at 5325 m) on
the Tibetan Plateau during the last 200 years (Thompson et al.,
2003), it is important to understand the elevation-mediated regional
climate responses.
4.3. Possible land-surface and vegetation feedbacks to regional climate
changes
Vegetation and land-surface features have important and significant impacts on regional climate, especially in arid/semi-arid regions.
Vegetation influences regional climate through changes in vegetation
types (vegetation cover, light transmittance) and leaf area index
(canopy thickness, transpiration), both determining water and energy exchanges among soil, vegetation, and the atmosphere. Feedbacks
of vegetation on climate or nonlinear responses of vegetation to
climate have been well documented in climate simulations, e.g., in
desert and steppe regions of northern China (Chen et al, 2004), northern Africa (Claussen, 1997; Ganopolski et al., 1998; Liu et al., 2006);
and humid tropical rain forests (e.g., in Amazonia; Costa and Foley,
2000). Climate-model results show differences between midHolocene and present-day precipitation in the East Asian monsoon
region in summer because during the mid-Holocene, larger areas
were covered by taller vegetation (shrubs, forest instead of grass
and desert). Larger leaf area of this woody vegetation increases
evapotranspiration and, thus, local precipitation (Dallmeyer et al.,
2010).
Some paleoclimate records from the monsoon margin of China
appear to show major vegetation shifts and associated lake level
changes in the mid-Holocene, e.g., at Daihai Lake in the transitional
zone between forest and temperate steppe vegetation (Fig. 2A; Xiao
et al., 2004; Sun et al., 2009). Other sites that show major vegetation
changes during the Holocene are located near the present-day biome
boundaries (Fig. 1B), and the observed vegetation shifts are likely
reflecting the movements of these boundaries. These shifts between
forest steppe and desert steppe and between forest and steppe should
significantly change evapotranspiration and albedo and produce
changes in water and energy exchanges between vegetation/soil
and the atmosphere, thereby affecting regional temperature and
moisture conditions. However, a regional climate model that couples
climate, vegetation, and hydrology at high-spatial resolution will be
needed to simulate the responses of multiple climate fields (including
wind vectors, temperature, water vapor fluxes, humidity) to broadscale and local controls.
5. Summary
We show that Holocene climate histories are complex in the transitional zone between the East Asian monsoon and the westerlydominated central Asia based mostly on pollen records across monsoon margin region. Most sites in the transitional zone between forest
and temperate steppe vegetation in northeastern China show the
wettest climate between 8 and 4 ka. In the transitional vegetation
zone between temperate forest/steppe and desert in central China,
Holocene vegetation and climate pattern is complicated, especially
during the early and middle Holocene. The sites in the transitional
vegetation zone between highland meadow/steppe and semidesert/desert on the Tibetan Plateau reveal relatively uniform change
with wet climate during the early and middle Holocene and dry
climate during the late Holocene.
The synthesis curve of vegetation indices for the monsoon margin
region as a whole shows that the maximum moisture occurred during
the middle Holocene (8–4 ka) and the driest conditions during the
late Holocene (4–0 ka). This general pattern is very different from
either monsoon or westerly regions. In the monsoon region, the
early Holocene was wettest; whereas in central Asia dominated by
the westerlies the early Holocene was driest and the mid-Holocene
wettest. Although the mid-Holocene was wettest in both central
Asia and the monsoon margin region, in the former region the early
Holocene was driest whereas in the latter region the late Holocene
was driest.
We propose that these regional climate changes were controlled
by interactions of large-scale atmospheric circulations, including the
subtropical East Asian monsoon and mid-latitude westerlies, and regional factors, including topography and vegetation. These interacting
and competing factors may have shifted their relative roles during the
Holocene under gradually changing boundary conditions, including
the insolation and waning ice sheets. Understanding the spatial and
temporal patterns and possibly threshold response of regional climates to these large-scale forcing will provide essential information
for revealing the underlying mechanisms of regional climate change.
Acknowledgements
This research was supported by the National Basic Research Program
of China (973 Program, grants #2012CB956102 and 2010CB950202),
the National Natural Science Foundation of China (Grants #41125006
and 41071126), and the US National Science Foundation (EAR
#0518774). We thank Eric Grimm, Paul Wignall and an anonymous reviewer for their helpful comments that improved the manuscript;
the following people for providing the original pollen data: 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), Bo Cheng
(Sanjiaocheng) and Yu Li (Qingtu Lake); and Xiaoli Guo for digitizing
some pollen data.
Y. Zhao, Z. Yu / Earth-Science Reviews 113 (2012) 1–10
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.earscirev.2012.03.001.
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