Vegetation Response to Holocene Climate Change in East Asian Monsoon-Margin Region

Earth-Science Reviews 113 (2012) 1–10

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Vegetation response to Holocene climate change in East Asian monsoon-margin region

Yan Zhao a,⁎, Zicheng Yu b a MOE Key Laboratory of Western China's Environmental System, Research School of Arid Environment and Climate Change, Lanzhou University, Lanzhou 730000, China b Department of Earth and Environmental Sciences, Lehigh University, 1 West Packer Avenue, Bethlehem, PA 18015, USA article info abstract

Article history: Fossil pollen records from 20 sites with reliable chronologies and high-resolution data in the East Asian Received 31 March 2011 monsoon margin region were synthesized to document Holocene vegetation and climate change and to Accepted 1 March 2012 understand the large-scale controls. The vegetation experienced different changes over the Holocene in Available online 9 March 2012 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 Keywords: between steppe/forest and desert, vegetation showed various patterns at different sites. (3) Further west East Asian monsoon Fossil pollen on the Tibetan Plateau near the boundary between highland meadow/steppe and semi-desert/desert, forest Holocene expanded at most sites during the early and middle Holocene. Our synthesis indicates that climate in the Climate change margin region was slightly moist in the early Holocene, wettest in the middle Holocene, and dry in the late Vegetation response Holocene, though there are regional differences as reﬂected by vegetation change. This general pattern is Spatial complexity 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. Introduction ...... 1 2. Study region, data sources and analysis methods ...... 2 3. Temporal and spatial patterns of Holocene vegetation and moisture shifts ...... 2 3.1. Transitional zone between forest and temperate steppe vegetation ...... 2 3.2. Transitional vegetation zone between temperate forest/steppe and desert ...... 5 3.3. Transitional vegetation zone between highland meadow/steppe and semi-desert/desert ...... 5 3.4. General spatial patterns of Holocene vegetation and moisture changes ...... 6 4. Possible mechanisms of complex regional vegetation and climate responses ...... 7 4.1. Interacting controls of low- and high-latitude atmospheric circulations ...... 7 4.2. Topography-mediated regional climate changes in the northeastern Tibetan Plateau ...... 7 4.3. Possible land-surface and vegetation feedbacks to regional climate changes ...... 8 5. Summary ...... 8 Acknowledgements ...... 8 Appendix A. Supplementary data ...... 89 References ...... 9

1. Introduction

The East Asian monsoon is believed to have begun ca. 7 million ⁎ Corresponding author at: Research School of Arid Environment and Climate Change, Lanzhou University, China. Tel.: +86 931 8912329; fax: +86 931 8912330. years ago (An et al., 2000) and has displayed strong variations on E-mail address: [email protected] (Y. Zhao). many time scales (Liu and Ding, 1998; Wang et al., 2001; Yuan

0012-8252/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2012.03.001 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, climate change and ecosystem responses in the East Asian monsoon monsoon intensity follows the precession-driven change in seasonal margin. insolation (Kutzbach, 1981; COHMAP, 1988; Ruddiman, 2008); this insolation-driven monsoon hypothesis has been confirmed by 2. Study region, data sources and analysis methods many paleoclimate proxy records (e.g., Winkler and Wang, 1993; An et al., 2000; Morrill et al., 2003), including the new Chinese cave East Asian monsoonal margin is located at the transition between record spanned over the last 224,000 years (Wang et al., 2008). Dur- the monsoon- and westerly-dominated regions; however, it is not a ing the Holocene, various proxy records show that both Indian Sum- clearly-defined boundary but a broad transitional zone. Due to the mer Monsoon and East Asian monsoon intensities increased sharply large latitudinal and altitudinal extents, this transitional region repre- at the onset of the Holocene, but reached maximum monsoon sents various climatic and vegetation zones. As a result, the vegeta- intensity at 8–9 ka (1 ka=1000 cal yr BP) (Fleitmann et al., 2003; tion across the transitional region from east to west is characterized Dykoski et al., 2005; Wang et al., 2005a, 2005b; Shao et al., 2006). by the ecotones between major biomes, including the ecological This time lag has been attributed to possible response of monsoon in- boundaries between forest and temperate steppe, between temperate tensity to multiple forcing, including changes in precession-driven in- steppe and desert, and between highland meadow/steppe and semi- solation, obliquity-driven insolation and ice volume (Clemens and desert/desert (Fig. 1B). Prell, 2007). Many paleoclimatic records from monsoonal margin have been Further inland on Eurasian continent in the westerly-dominated published. However there is a large disparity in sample resolution central Asia, our recent synthesis indicates that its Holocene moisture and age controls. In this study we used 20 selected pollen records history is very different from the monsoon region (Chen et al., 2008). from lakes or peatlands, with an exception of Dunde ice cap, that A synthesized moisture index curve from 11 lakes in arid central Asia have an analysis resolution of b200 years and multiple age controls shows a consistently dry climate in the early Holocene, an abrupt shift of >4 dating determinations at each site (Table 1). to wet climate at 8 ka, a regional maximum moisture condition at 6 ka Radiocarbon dating was the geochronological technique used to and then decreasing moisture trend for the rest of Holocene (Chen date all the records used in this paper, except for Dunde ice core, et al., 2008). This general pattern is clearly shown in paleoclimate which was dated by ice layer counting and ice flow modeling. Bulk or- records reviewed in the above paper, including Bosten Lake ganic matter, pollen or plant macrofossils were used for 14C dating (Wuennemann et al., 2003; Huang et al., 2009), Issyk-Kul (Ricketts from lacustrine and peat deposits. We have followed the original et al., 2001). The relatively large between-site variability of recon- publications and made old carbon corrections of the measured 14C structed moisture conditions during the mid- and late Holocene sug- dates at Bayanchagan, Sanjiaocheng, Qinghai Lake, Hurleg Lake and gests the various regional or site-specific controls of either regional Zigetang Lake, on the basis of the dating of surface sediments at climate or individual proxy records. Most lakes in northwest China these sites (Table 1). We did not make correction for other sites and elsewhere in central Asia were either not filled up or completely owing to the lack of such information. In this paper, all radiocarbon dried up before 8 ka, but abruptly changed to wet condition after 8 ka dates were calibrated to calendar years before present (BP= across continental interior in central Asia (e.g., Ricketts et al., 2001; 1950 AD) using Calib. 5.0.1 based on IntCal04 calibration dataset Wuennemann et al., 2003; Feng et al., 2005). This extremely dry cli- (Reimer et al., 2004). The chronology at most sites was established mate in central Asia during the early Holocene has been interpreted based on linear interpolation, except at Hurleg Lake (site #18) and as caused by delayed increase of sea-surface temperatures (SSTs) in Tangke (site #12) with 3rd polynomial curve. the North Atlantic Ocean (Chen et al., 2008) and by the enhanced Each of these fossil pollen sequences were resampled into 1000- subsidence of dry air masses induced by maximum summer insola- year bins by averaging tree pollen percentages or Artemisia/Chenopo- tion in the early Holocene (Broccoli and Manabe, 1992; Zhao et al., diaceae (A/C) ratio, Artemisia/Cyperaceae ratio (A/Cy), Ephedra 2007). Cool SSTs due to the presence of waning ice sheets in the distachya/Ephedra fragilis (Ed/Ef) ratio or total pollen concentration early Holocene reduced the moisture transport from the Atlantic to of all samples within that binned time interval. The binned tree pollen Eurasian continent. Also, the maximum insolation in the early Holo- percentages (or other pollen indices) were then standardized to zero cene and heating on the Tibetan Plateau would have induced strong mean and unit standard deviation. All standardized curves were aver- subsidence of dry air masses in surrounding areas north of the aged to generate the composite regional tree pollen record (Z-score), plateau. However, the timing and even directions of moisture changes with standard error of the means as an estimate of variations among during the mid- and late Holocene are highly variable for large vari- sites. We carried out these data synthesis analysis on each sub-region ability of reconstructed moisture conditions in the mid- and late and for all sites. Holocene and appear to lack a coherent spatial pattern across the region (Rhodes et al., 1996; Chen et al., 2008). 3. Temporal and spatial patterns of Holocene vegetation and In semi-arid north and northwest China near the limit of East moisture shifts Asian summer monsoon influences, climate is likely influenced and mediated by the mid-latitude westerlies and regional features, 3.1. Transitional zone between forest and temperate steppe vegetation such as topography and vegetation, in addition to the summer monsoons. As a result, the regional climate changes during the Holocene This zone is located at the boundary between conifer forest and in the monsoonal margin are complex, due to the interactions of temperate steppe in northeastern China (Fig. 1B). We here first these large-scale and regional controlling factors. The general trends describe two pollen records that are representative of vegetation in of climate changes have been broadly documented in East and this region, and then discuss the general pattern from all records. At central Asia over the Holocene, especially at orbital- and multi- Daihai Lake in Inner Mongolia (site #6 in Table 1), pollen record indi- millennial time scales (Herzschuh et al., 2006; Chen et al., 2008). cates a steppe forest (with tree pollen up to ~60%) at 8–3 ka, suggest- However, the major patterns of temporal and spatial variation in ing a wet climate, bracketed by steppe vegetation before 8 ka and past vegetation and climate in the monsoon margin are still poorly after 3 ka (Xiao et al., 2004; Fig. 2A), and lake-level reconstruction documented and understood. Here we review available fossil pollen from the same lake also indicates low and fluctuating lake level at records, in the monsoonal margin from the east in Northeast of 11–8 ka, high and stable lake level at 8–3 ka and decreasing and low China to the west on the northeastern Tibetan Plateau. Our objec- level after 3 ka (Sun et al., 2009; Fig. 2A). The pollen assemblages tives were to document patterns of Holocene vegetation and climate from Bayanchagan Lake (site #3; Fig. 2D) just north of Beijing show change at millennial timescales and to understand the causes of 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 westerly- dominated 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– vegetation, in response to change in precipitation caused by change 6.7 ka, back to steppe after 6.7 ka (Jiang et al., 2006). Jiang et al. in the intensity of the Asian summer monsoon. However, during the (2006) used this pollen sequence to reconstruct climate changes early Holocene, the sites from this region show complex vegetation using modern analogue technique on the basis of 211 surface pollen and moisture patterns. For example, at Bayanchagan, tree pollen samples from northern China. The vegetation sequence and shows gradual increase in the early Holocene and reaches the maxi- paleoclimatic reconstruction suggest that a relatively humid climate mum value at 6 ka before declining in two steps to the lowest values during the early mid-Holocene at 9.2–6.7 ka was favorable for the de- in the last 2 ka (Jiang et al., 2006). At Daihai Lake, tree pollen shows velopment of woodland. slightly increase (Xiao et al., 2004). Haoluku (site #2) shows high All sites in this sub-region, except at Haoluku, show the high tree tree pollen in the early Holocene, but this is a site that has the lowest pollen between 8 and 4 ka with peaks at ~6 ka, which is clearly sampling resolution (Liu et al., 2002; Fig. 2E). At Diaojiao Lake (site reflected in composite tree pollen curve (Fig. 2G). These changes in #4), and Hulun Lake (site #1), tree pollen shows fluctuations (Shi pollen assemblages reflected shifts between forest and steppe 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.

Site no. Site name Latitude, Elevation Precipitation Sample Number Dating material Old carbon Type of Reference longitude (m a.s.l.) (mm/yr) resolution of datesa correction archives (years) (14C yr)*

1 Hulun Lake E112°00′, N48°31′ 545 333 N/A 13 Bulk organic matter – Lake core Wen et al., 2010 2 Haoluku E116°45.42′, 1295 370 200 4 Bulk organic matter – Lake section Liu et al., 2002 N42°57.38′ 3 Bayanchagan E115.21°, N41.65° 1355 400 130 7 Bulk organic matter 570 Lake core Jiang et al., 2006 4 Diaojiao Lake E112°21′, N41°18′ 1800 421 85 4 Bulk organic matter – Lake core Shi and Song, 2003 5 Chasuqi E111°08′, N40°40′ 1000 400 70 4 Bulk organic matter – Peat section Wang and Sun, 1997 6 Daihai Lake E112°33′, N40°29′ 1221 423 b100 8 Bulk organic matter – Lake core Xiao et al., 2004 7 Dadiwan E105°54′, N35°01′ 1400 400 b50 3 Bulk organic matter – Marsh section An et al., 2003 8 Qingtu Lake E103°40′, N39°03′ 1309 115 150 3 Bulk organic matter – Lake section Li et al., 2009 9 Sanjiaocheng E103°20′, N39°00′ 1325 100–500 b100 9 Bulk organic matter 540 Lake section Chen et al., 2006 charcoal 10 Eastern Juyan E101.85°, N41.89° 892 480 140 5 Bulk organic matter – Lake core Herzschuh et al., 2004 11 Zoige core RM E102°21′, N33°57′ 3401 705 150 3 Bulk organic matter – Peat core Shen and Tang, 1996 12 Tangke E103°25′, N32°20′ 3492 70 190 8 Charcoal – Peat core Zhao et al., ZB08-C1 2011 13 Hongyuan E102°31′, N32°47′ 3505 700 175 31 Bulk organic matter – Peat section Zhou et al., 2010 14 Koucha Lake E97.2°, N34.0° 4540 470 200 4 Pollen – Lake core Herzschuh et al., 2009 15 Qinghai lake E99°36′, N36°32′ 3200 350 60 7 Bulk organic matter 1039 Lake core Shen et al., 2005 16 Dalianhai E100°24′, N36°12′ 2850 250 80 10 Plant macrofossil – Lake core Cheng, 2006 17 Dunde E99°36′, N36°32′ 5325 400 1–1000 - Ice layer counting – Ice core Liu et al., 1998 and model 18 Hurleg Lake E96°54′, N37°19′ 2809 100 100 7 Plant macrofossils 2758 Lake core Zhao et al., 2007 19 Zigetang Lake E90.9°′, N32.0° 4560 320 160 5 Bulk organic matter 2010 Lake core Herzschuh et al., 2006 20 Selin Co E88°31′, N31°34′ 4530 290 200 5 Bulk organic matter – Lake core 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 5

3.2. Transitional vegetation zone between temperate forest/steppe and Lake (site #14) on the northeastern Tibetan Plateau and found grad- desert ual increases in total pollen concentration and in tree pollen (especially Betula) since 11 ka, reaching maximum at 7–6 ka (>40% This zone is located at the boundary between temperate steppe tree pollen; Fig. 4F). Tree pollen gradually decreases after 6 ka and and desert, with temperate forest in the highlands, in northwestern reach a minimum after 2 ka. An oxygen isotope record from Qinghai China (Fig. 1B). Dadiwan (site #7) is representative of the pollen Lake shows that a positive water balance occurred earlier than indi- sites in this region with a high sampling resolution of ca. 50 years cated by pollen (Lister et al., 1991), perhaps suggesting different re- (An et al., 2003; Fig. 3D). The pollen data indicate that vegetation sponses of lake hydrology and landscape vegetation (Wei and around Dadiwan changed from a desert steppe at 12–8.5 ka, through Gasse, 1999; Colman et al., 2007). While at Hurleg Lake (site #18), Pinus-dominated steppe woodland (steppe forest) at 8.5–6.5 ka, to the only freshwater lake in the Qaidam Basin on the northeastern desert with sparse steppe after 6.5 ka. This vegetation sequence sug- Tibetan Plateau, both Artemisia-to-Chenopodiaceae (A/C) pollen gests that a relatively humid climate during the mid-Holocene at ratios, a proxy of landscape effective moisture, and sediment carbon- 8.5–6.5 ka was favorable for the development of woodland, while a ate content, an indicator of groundwater/river water inputs and lake relatively dry early and late Holocene allowed arid open vegetation levels, show high moisture conditions at ~8.5–6.5 ka, driest period to develop. at 6.5–3 ka, and increasing moisture after 3 ka (Zhao et al., 2007, Fossil pollen data from this region reveal a complicated pattern, 2009a, 2009b, 2009c; Fig. 4C). This pattern has been conﬁrmed by and the composite curve of tree pollen and other pollen index show other geochemical proxy records (ostracode shell isotopes and trace larges error bars during the early and middle Holocene (Fig. 3E). At elemental ratios) from the same lake (Zhao et al., 2009a, 2009b, Eastern Juyan (site #10), pollen assemblages at 10.7 ka-5.4 ka are 2009c, 2010). characterized by highest values of Chenopodiaceae, Ephedra fragilis- Fossil pollen data from this region reveal uniform pattern, with type and other desert plants, suggesting a dry climate (Herzschuh only a few exceptions, and the composite curve of tree pollen and et al., 2004). Most favorable conditions are reconstructed between other pollen index show small error bars during the Holocene 5.4 ka and 3.9 ka on the basis of relative increase in abundance of (Fig. 4H). Along the south to north regional transect on the eastern Artemisia pollen. The pollen diagram from Sanjiaocheng suggests Tibetan Plateau, tree pollen abundance at all sites increased in the that the region was covered by steppe vegetation at 11.6–7 ka, desert early Holocene and reached the maximum abundance in the mid- or desert steppe at 7–3.8 ka, and desert after 3.8 ka, indicating a wet Holocene, suggesting a wetter climate; however, tree-pollen abun- early Holocene, a dry mid-Holocene and variable late Holocene dance varied from >80% in Hongyuan peatland (site #13), to less (Chen et al., 2006; Fig. 3B). Pollen concentration from Qingtu Lake than 30% at our study site in the central Zoige Basin (Tangke ZB08- shows a maximum during the middle Holocene and rather dry during c1; site #12) and 80% in the northern Zoige Basin (Zoige core RM; the mid-Holocene (Li et al., 2009; Zhao et al., 2008; Fig. 3C). 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; 3.3. Transitional vegetation zone between highland meadow/steppe and Cheng, 2006; Fig. 4). At Koucha Lake (site #14), to the west of these semi-desert/desert sites above-mentioned, pollen assemblages are dominated by Cyper- aceae and Artemisia (Herzschuh et al., 2009), and the vegetation This zone is located at the boundary between highland meadow/ around the lake region changed from steppe before 6.6 ka to high- steppe and semi-desert/desert in the Tibetan Plateau (Fig. 1B). Shen alpine meadow after then, suggesting that climate became cooler et al. (2005) presented a high-resolution pollen record from Qinghai 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 distachya- to-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 (Liuetal.,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 However, it appears that major vegetation shifts occurred at ~8 ka the early and middle Holocene (Liu et al., 1998; Fig. 4C). In central and at ~5 ka at most of these sites in the monsoonal margin region Tibetan Plateau, A/C pollen ratio from Zigetang Lake (site #19) has of China. The changes in effective moisture show different spatial pat- highest value at 6.4–4.2 ka (Herzschuh et al., 2006; Fig. 4B). However, terns in different regions along this geographic transect. Most sites in Selin Lake (site #20), another site from central Tibetan Plateau, shows the transitional zone between forest and temperate steppe vegetation the highest tree pollen abundance during the early Holocene (Sun show the high forest values and the most moist climate between et al., 1993; Fig. 4A). 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 3.4. General spatial patterns of Holocene vegetation and moisture from this region reveal a complicated vegetation pattern, and the changes composite curve of tree pollen and other pollen indices have large error bars during the early and middle Holocene (Fig. 5C). Generally the sites These pollen records along the west–east transect in the semi-arid in the transitional vegetation zone between highland meadow/steppe monsoonal margin region show variable but coherent shifts in and semi-desert/desert reveals relatively uniform change indicated by regional vegetation (Fig. 5), reﬂecting changes in effective moisture. 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 7 and middle Holocene and dry climate during the late Holocene, with probably in response to rapid and intense heating of the Tibetan relatively small error (Fig. 5D). Plateau during summer insolation maximum. The synthesis curve of vegetation indices from all sites shows the At mid-latitude regions, such as near the northern limit of mon- maximum moisture during the middle Holocene at 8–4 ka and the soon influences, however, the mid-latitude westerlies likely also driest period in the last 4 ka (Fig. 5E). This general pattern is very play a major role in mediating the influences of monsoon circulations. different from either the monsoon or westerly regions. In the mon- The intensity and moisture properties of the westerlies depend on soon region, the maximum moisture occurred during the early large-scale boundary conditions. Farther inland in the mid-latitude Holocene (Fig. 5F and G; Wang et al., 2005a, 2005b; Zhao et al., region, moisture is supplied mainly from the North Atlantic Ocean 2009a, 2009b, 2009c), whileas in arid central Asia, which is domi- and from inland seas and lakes along the cyclonic storms paths nated by the westerlies, the early Holocene was driest and the mid- (Böhner, 2006). During the early Holocene ice sheets in North Amer- Holocene was wettest (Fig. 5A; Chen et al., 2008). ica and northern Eurasia were still large, as indicated by geological evidence (Lowe and Walker, 1997) and the sea-level record (Peltier 4. Possible mechanisms of complex regional vegetation and and Fairbanks, 2006). These remnant ice sheets would have de- climate responses pressed the sea-surface temperatures (SSTs) in the North Atlantic Ocean (Koç et al., 1993; Kaplan and Wolfe, 2006) and air tempera- Climate in the transitional zone is influenced by multiple large- tures as documented in Greenland ice cores (Dahl-Jensen et al., scale climate controls and regional land surface factors. These con- 1998). Lower temperatures would have reduced evaporation from trols include subtropical monsoon circulation as a regional expression the North Atlantic and consequently vapor transport to Eurasian of Hadley Cell (Webster, 2005), the mid-latitude westerlies, continent, thus producing relatively dry westerlies. Also, during the topography-induced vertical air motion around the Tibetan Plateau, glacial boundary conditions the westerlies were stronger and pene- and potential regional vegetation feedbacks. As a result of these inter- trated further eastward into north-central China as shown by eolian acting and competing factors, Holocene climate changes of this region data and GCM simulations (Vandenberghe et al., 2006). These strong were complex and regional heterogeneity as shown above. In addi- dry westerlies farther into East Asia in the early Holocene may tion to multiple large-scale climate controls and regional land surface have delayed moisture maximum as documented at sites reviewed factors above-mentioned, human activity might have also influenced above in the mid-latitude transitional zone, as well as in central the Holocene vegetation change in the monsoon margin region. Zhao Asia (Chen et al., 2008). Also, the strong and dry westerlies would et al. (2009a, 2009b, 2009c) reviewed the fossil pollen data in the have blocked the northward movement of rain belts associated with monsoon region and their results showed that human activities the subtropical monsoon circulations. The interplay of monsoon and could be an important factor affecting natural vegetation at a large westerlies may have contributed to the complex patterns at various scale during the last 2 ka. However, we argue that human activity is sites in the transitional vegetation zone between temperate forest/ not a major player in the vegetation and climate change at multi- steppe and desert in central China. millennial scale during most part of the Holocene. 4.2. Topography-mediated regional climate changes in the northeastern 4.1. Interacting controls of low- and high-latitude atmospheric Tibetan Plateau circulations The Holocene hydroclimate pattern as shown at Hurleg Lake in Interacting controls of low- and high-latitude atmospheric circu- the Qaidam Basin demonstrates the possible orography-mediated re- lations might have contributed to the inconsistency of the vegetation gional climate response to large-scale climate controls. During the and moisture change in the transition vegetation zone between Holocene there appears to be opposite changes in moisture condi- temperate forest, steppe and desert. The onset and intensity of mon- tions at Hurleg Lake (Zhao et al., 2007) and Qinghai Lake (Shen soons have generally been attributed to contrasts in the thermal et al., 2005), only about 250 km to the east but at different elevations properties between land surface and oceans, caused by summer in- (2800 m in the basin vs. >4000 m asl on the plateau). It is in particu- solation change and radiative heating (Kutzbach, 1981; Webster lar more clear at 6 ka, while maximum aridity occurred in the basin, and Fasullo, 2003). However, recent studies have reexamined this but maximum moisture at Qinghai Lake) and over the last 3 ka conventional idea and attribute that the onset of monsoon circula- (increasing moisture at Hurleg, but decreasing moisture at Qinghai). tions is in response to large shifts in Intertropical Convergence The similar out-of-phase patterns at low-elevation sites and sur- Zone (ITCZ) (Chao and Chen, 2001) and is caused by interactions rounding high mountains appear also to occur during the last of tropical overturning atmospheric (Hadley) circulation and extra- 1000 years, based on high-resolution multi-proxy records from tropical eddies (Bordoni and Schneider, 2008). This new under- Hurleg Lake (Zhao et al., 2009a, 2009b, 2009c) and tree-ring records standing may shed light on reconciling long-term monsoon history from surrounding mountains (Sheppard et al., 2004; Shao et al., and dynamics during the Holocene. If monsoons are not primarily 2006) and during the last 50 years (e.g., Gahai Lake in the Qaidam controlled by low-latitude insolation-driven heating of the conti- Basin vs. Dunde ice core at 5300 m asl; Zhao et al., 2008). nents, then we may not necessarily expect to see a close match We propose that this difference between low- and high-elevation between the maximum monsoon intensity and summer insolation sites may be caused by the complex topography around the NE corner maximum. This may explain the delay in the timing of monsoon of the Tibetan Plateau (TP) and the resulting convective pattern. The maximum in related to peak summer insolation as documented possible connection between the orographic effect of the TP and dry in many low-latitude paleo-records, such as at Dongge and Sanbao climate in central Asia has been well-understood through climate caves. This delayed response in monsoon precipitation may be modeling and observational studies. Broccoli and Manabe (1992) in- expected farther north, away from the center of monsoon influences, vestigated the role of the TP in maintaining mid-latitude dry climates even just considering the subtropical monsoon circulation. For ex- using GCM simulations and found that the TP has large-scale effects ample, maximum moisture occurred during the mid-Holocene at on atmospheric circulation. The TP caused large-amplitude stationary the most sites in the transition zone between forest and temperate waves in the Eurasian continent in the cold season. The dry regions steppe, and maximum moisture occurred during the mid-Holocene north of the TP are located upstream of the troughs of these waves, or late early Holocene in the transition zone between highland hence they experience dry subsiding air. The plateau also plays a meadow/steppe and semi-desert/desert. The timing difference major role in responding to and mediating the Asian monsoon circu- of maximum moisture conditions occurring in the two zones, is lations 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 zone between forest and temperate steppe vegetation (Fig. 2A; Xiao causes strong air subsidence to the northwest and north of the TP, in- et al., 2004; Sun et al., 2009). Other sites that show major vegetation ducing dry climate in central Asia. Wang et al. (2010) proposed that changes during the Holocene are located near the present-day biome Hadley Circulation centered over the Tibetan Plateau during the boundaries (Fig. 1B), and the observed vegetation shifts are likely early Holocene and resulted in subsidence in the surrounding regions reflecting the movements of these boundaries. These shifts between leading to relatively dry conditions. This large-scale effect of the TP forest steppe and desert steppe and between forest and steppe should has also been indicated by observational data (He et al., 1987) and significantly change evapotranspiration and albedo and produce has been considered as an explanation for spatial pattern of Holocene changes in water and energy exchanges between vegetation/soil wet-dry climate periods in central Asia (Herzschuh, 2006; Chen et al., and the atmosphere, thereby affecting regional temperature and 2008). We use the same mechanism to explain the different climate moisture conditions. However, a regional climate model that couples pattern in the Qaidam Basin and the surrounding mountains and climate, vegetation, and hydrology at high-spatial resolution will be plateau. This difference in topography not only allows eastward needed to simulate the responses of multiple climate fields (including penetration of the dry westerlies into the Qaidam Basin but also wind vectors, temperature, water vapor fluxes, humidity) to broad- induces a similar uplifting-subsiding motion regionally, similar to scale and local controls. that described above for the Tibetan Plateau. It appears that such a mechanism also works at a fine spatial scale as demonstrated in cli- 5. Summary mate simulations. Sato and Kimura (2005) used a regional climate model of 150-km spatial resolution to simulate the effect of diabatic We show that Holocene climate histories are complex in the tran- heating over the Tibetan Plateau on subsidence in arid climate regions sitional zone between the East Asian monsoon and the westerly- and found prominent regional-scale subsidence during the summer. dominated central Asia based mostly on pollen records across mon- Their high spatial resolution simulations show that the subsidence soon margin region. Most sites in the transitional zone between forest extends into the Qaidam Basin, especially in simulations with no con- and temperate steppe vegetation in northeastern China show the densation, suggesting the dominant role of sensible heat flux induced wettest climate between 8 and 4 ka. In the transitional vegetation by the radiative heating on the plateau surface. zone between temperate forest/steppe and desert in central China, A large topographic feature such as the Tibetan Plateau can modify Holocene vegetation and climate pattern is complicated, especially the atmospheric circulation. During the instrumental period, Liu and during the early and middle Holocene. The sites in the transitional Yin (2001) show that the plateau may be inducing different regional vegetation zone between highland meadow/steppe and semi- precipitation responses in the eastern Tibetan Plateau, because the desert/desert on the Tibetan Plateau reveal relatively uniform change flow associated with the North Atlantic Oscillation bifurcates around with wet climate during the early and middle Holocene and dry the plateau. They found a seesaw pattern in summer precipitation climate during the late Holocene. in northern and southern parts of the eastern Tibetan Plateau. It The synthesis curve of vegetation indices for the monsoon margin would be interesting to test this idea using paleoclimate records dur- region as a whole shows that the maximum moisture occurred during ing the Holocene or last 2000 years, perhaps by focusing on abundant the middle Holocene (8–4 ka) and the driest conditions during the lakes and peatlands in the Zoige Basin. Considering that higher eleva- late Holocene (4–0 ka). This general pattern is very different from tions have experienced greater warming over the last 50 years as either monsoon or westerly regions. In the monsoon region, the documented by instrumental records on the Tibetan Plateau (Liu early Holocene was wettest; whereas in central Asia dominated by and Chen, 2000) and by ice core data from different elevations the westerlies the early Holocene was driest and the mid-Holocene (Dasuopu at 7200 m, Guliya at 6200 m, and Dunde at 5325 m) on wettest. Although the mid-Holocene was wettest in both central the Tibetan Plateau during the last 200 years (Thompson et al., Asia and the monsoon margin region, in the former region the early 2003), it is important to understand the elevation-mediated regional Holocene was driest whereas in the latter region the late Holocene climate responses. was driest. We propose that these regional climate changes were controlled 4.3. Possible land-surface and vegetation feedbacks to regional climate by interactions of large-scale atmospheric circulations, including the changes subtropical East Asian monsoon and mid-latitude westerlies, and regional factors, including topography and vegetation. These interacting Vegetation and land-surface features have important and signifi- and competing factors may have shifted their relative roles during the cant impacts on regional climate, especially in arid/semi-arid regions. Holocene under gradually changing boundary conditions, including Vegetation influences regional climate through changes in vegetation the insolation and waning ice sheets. Understanding the spatial and types (vegetation cover, light transmittance) and leaf area index temporal patterns and possibly threshold response of regional cli- (canopy thickness, transpiration), both determining water and ener- mates to these large-scale forcing will provide essential information gy exchanges among soil, vegetation, and the atmosphere. Feedbacks for revealing the underlying mechanisms of regional climate change. of vegetation on climate or nonlinear responses of vegetation to climate have been well documented in climate simulations, e.g., in Acknowledgements desert and steppe regions of northern China (Chen et al, 2004), northern Africa (Claussen, 1997; Ganopolski et al., 1998; Liu et al., 2006); This research was supported by the National Basic Research Program and humid tropical rain forests (e.g., in Amazonia; Costa and Foley, of China (973 Program, grants #2012CB956102 and 2010CB950202), 2000). Climate-model results show differences between mid- the National Natural Science Foundation of China (Grants #41125006 Holocene and present-day precipitation in the East Asian monsoon and 41071126), and the US National Science Foundation (EAR region in summer because during the mid-Holocene, larger areas #0518774). We thank Eric Grimm, Paul Wignall and an anonymous re- were covered by taller vegetation (shrubs, forest instead of grass viewer for their helpful comments that improved the manuscript; and desert). Larger leaf area of this woody vegetation increases the following people for providing the original pollen data: Hongyan Liu evapotranspiration and, thus, local precipitation (Dallmeyer et al., (Haoluku), Ulrike Herzschuh (Zigetang Lake), Qinghai Xu and Jule Xiao 2010). (Daihai Lake), Ji Shen (Qinghai Lake), Wenying Jiang (Bayanchagan Some paleoclimate records from the monsoon margin of China Lake), and Zhaodong Feng and Chengbang An (Dadiwan), Bo Cheng appear to show major vegetation shifts and associated lake level (Sanjiaocheng) and Yu Li (Qingtu Lake); and Xiaoli Guo for digitizing changes in the mid-Holocene, e.g., at Daihai Lake in the transitional some pollen data. Y. Zhao, Z. Yu / Earth-Science Reviews 113 (2012) 1–10 9

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