Quaternary history of the temperate forests of China

Quaternary Science Reviews, Vol. 7, pp. 1-20, 1988. 0277-3791/88 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright © 1988 Pergamon Press plc

QUATERNARY HISTORY OF THE TEMPERATE FORESTS OF CHINA

Kam-biu Liu Department of Geography and Anthropology, Louisiana State University, Baton Rouge, LA 70803-4105, U.S.A.

Pollen data from 80 sites in North China and Northeast China are reviewed to document the Quaternary history of the deciduous forest and the temperate mixed conifer-hardwood forest of China. During the Late Tertiary the forest in North China consisted of an admixture of temperate deciduous hardwoods, subtropical broadleaved evergreen elements, and ancient conifers of tropical and subtropical affinities. Pollen evidence from long boreholes through Quaternary deposits indicates four or five glacial/interglacial climatic cycles. The first glacial episode, known as the Nangou Cold Period, led to the development of a spruce-fir forest of no modern analogue in North China. Deciduous forest was re-established in North China during the intervening interglacials. The subtropical evergreen elements and Tertiary relicts were successively eliminated during the subsequent glacials. In the last glacial, spruce-fir forests which are now confined to mountain slopes above 1500 m descended to the lowlands of North and Northeast China, implying a temperature depression of at least 8-10°C. Cold steppe occurred at least locally near Beijing in the last glacial maximum, but the data base is insufficient to delimit the spatial and temporal extent of this and other paleovegetation types on a regional scale, or to reconstruct the locations of glacial refugia and the dynamics of plant migration. The Holocene pollen stratigraphies suggest a tripartite division, with a period of maximum warmth, the Hypsithermal, in the mid-Holocene. In both North and Northeast China, the forest became more diverse during the mid- Holocene when thermophilous hardwoods expanded at the expense of pine and, in more southerly locations, birch. Unequivocal evidence for intensified summer monsoon during the early Holocene, as predicted by the Kutzbach model based on orbital parameter changes, remains to be found from the pollen records of the temperate forest regions of China.

INTRODUCTION

Among the temperate forests of the world, the ones in East Asia are well-known for their floristic richness and antiquity (C. Wang, 1961; H. Wang, 1979). This is usually attributed to the absence of continental glaciation and, by implication, the smaller magnitude of Pleistocene environmental change in East Asia. Re- cently, Quaternary pollen records from eastern North America and western Europe have revealed a detailed history of plant migration and ecotonal movements in the temperate forests in response to glacial/interglacial climatic oscillations, and have highlighted several important paleoecological questions concerning the stability and diversity of temperate forest communities (e.g. Davis, 1976, 1981; Delcourt and Delcourt, 1979, 1983; van der Hammen et al., 1971; Wright, 1972, 1976, 1977; West, 1970). In East Asia, recent pollen data from Japan have outlined a dynamic history of the temperate forests since the Late Pleistocene (Tsukada, 1982a, 1983, 1985). However, the Quaternary history of the temperate forests of mainland China, the most diverse of the world's temperate forests, remains poorly known to the international scientific community. This information gap is partly due to the paucity of quality palynological and paleoecological data from China per se, and partly due to the language barrier and limited accessibility to Chinese scientific literature from outside.

This paper attempts to bridge this information gap by critically reviewing the pollen records pertaining to the Quaternary history of the temperate forests of China, and highlights some important questions in Chinese Quaternary paleoecology.

MODERN GEOGRAPHICAL SETTING

Climate The line joining Huai He (Huai River) and Qin Ling

(Qin Mountains) is widely taken to be the natural boundary between the temperate and subtropical regions of China (Ren et al., 1979) (Fig. 1). This line roughly parallels the 33°N latitude and corresponds with the 0°C isotherm for January, the 4500°C isoline for accumulated temperature above 10°C, the 900 mm isohyet for mean annual precipitation, and the 1.0 isoline for the aridity index (Ren et al., 1979). In the temperate region north of this line, mean annual temperature decreases poleward from 14°C to about -5°C in the extreme north. Annual precipitation decreases northwestward from nearly 1000 mm along the coast to less than 500 mm along the forest/steppe ecotone, with increasing continentality inland. The moisture gradient, accentuated by NE-SW trending mountain ranges and plateaus acting as rain shadows, is very steep. The climate of temperate China is controlled by the East Asian monsoon, resulting in strong seasonality in both temperature and precipitation. Most of the precipitation falls in the summer when the Summer (Southeast) monsoon prevails. Winter is cold and dry, with frequent outbreaks of cold air from the Siberian high pressure system (Academia Sinica, 1985). Continentality is high owing to the size of the Eurasian landmass.

Vegetation The temperate forests of China as defined in this

paper include two vegetation formations - - the deciduous forest and the mixed conifer-hardwood

2 Kam-biu Liu

MLE, CARTO SECT. LSU

FIG. 1. Physiographic map of China showing major mountain ranges and rivers mentioned in this paper (after Ren et al., 1979)

MLE. CA|TO. $|CT., LSU

FIG. 2. Modern vegetation regions of China (after Wu et al., 1980).

Quaternary History of the Temperate Forests of China 3

forest (Fig. 2). They correspond to the warm temperate and temperate thermal climatic zones, respectively (Ren et al., 1979).

The mixed conifer-hardwood forest. The mixed conifer-hardwood forest occurs mainly in the mountains of Northeast China between 42°N and 50°N, except in the northernmost part of Da Hinggan Ling (Greater Hinggan Mountains) where boreal forest (taiga) prevails. A temperate, monsoon-type continental climate prevails in this region. Mean annual temperature decreases from about 6°C in the south to about -2°C just south of the boreal forest. Pre- cipitation decreases along a southeast-northwest gradient from about 1000 mm to 400 mm annually. Permafrost is widespread in mountain areas north of 47°N. Characteristic dominants in the mixed conifer- hardwood forest include Pinus koraiensis, Picea jezoens&, Abies nephrolopsis, Quercus mongolica, together with various species of Ulmus, Betula, Populus, Tilia, Acer, Juglans, and Fraxinus (Forestry Bureau, 1981). Bogs and fens are widespread, especially on the poorly drained alluvial plains near the confluence of the Heilong, Songhua, and Wusuli rivers (i.e. the Sanjiang Plain) (Ren et al., 1979).

The deciduous forest. The deciduous forest is mainly distributed in North China between 33°N and 42°N latitudes. Mean annual temperature ranges between 14 ° and 10°C. Annual precipitation decreases from about 900 mm in the south to less than 500 mm in the northwest; 60-70% of the precipitation falls in the summer. The aridity index (a ratio between evapor- ation loss and precipitation during the active growing season) is high, between 1.0 and 1.5 (Ren et al., 1979). This region is the most densely populated and most intensively cultivated part of China. Consequently, most of the natural vegetation has been destroyed or seriously altered over several millennia of human activities. Where relatively undisturbed forests occur, such as on mountain slopes, they are dominated by various deciduous species of oak (e.g. Quercus liao- tungensis, Q. dentata, Q. variabilis, Q. mongolica, Q. acutissima, Q. aliena) and several species of pine (e.g. Pinus densiflora, P. tabulaeformis), along with a diverse array of broadleaved hardwoods including Betula, Ulmus, Tilia, Acer, Populus, Celtis, Juglans, Carpinus, Fraxinus, Pistacia (pistache), and many others. Subalpine coniferous forests dominated by Picea meyeri, P. wilsonii, and Larix principis-rup- prechtii occur on mountain slopes between 1600 m and 2200 m (Forestry Bureau, 1981).

QUATERNARY CHRONOLOGY AND PALEOENVIRONMENT

Unlike North America or western Europe, China was not affected by continental glaciation during the Quaternary. The occurrence of the Pliocene Hipparion red clay in as far north as 42°N in northern Liaoning

suggests that a tropical to subtropical humid climate prevailed in much of China during the Tertiary (T. Liu et al., 1986). The dramatic uplift of the Himalayas and the Qinghai-Tibetan Plateau since the Middle Pleisto- cene had major climatic and geomorphic impacts. It significantly altered the general circulation and increased the continentalilty of the Asian landmass, creating conditions favourable to progressive dessi- cation in northwestern China, widerspread loess deposition in central China, and the initiation of alpine glaciers in the mountains of western China and the Qinghai-Tibetan Plateau (T. Liu et al., 1986; Shi et al., 1986; D. Sun and X. Wu, 1986).

Geological and biological data concur to suggest that significant climatic fluctuations occurred in China during the Quaternary, but major controversies exist concerning the nature and chronology of these climatic variations, and the correlation between glacial, loess, and marine stratigraphies. In North China, the most detailed paleoclimatic record comes from the loess stratigraphy in Luochuan, Shaanxi Province, near the forest/steppe ecotone (Site 67, Fig. 3). Distinct alter- nations between loess and paleosol layers indicate frequent fluctuations between cold, dry steppe environments and warm, humid woodlands within the last 2.4 Ma, which can be correlated with the oxygen isotope records of deep-sea cores (T. Liu et al., 1985, 1986; Heller and Liu, 1982). A shorter stratigraphic record spanning the Middle Pleistocene between ca. 800 ka and 230 ka BP is available from cave deposits at Zhoukoudian, the fossil site of the Peking Man (Site 48, Fig. 3) (Yang et al., 1983). Both records seem to indicate more pronounced changes in humi~ty than in temperature during the Quaternary, which are not totally unexpected from their proximity to the modern forest/steppe ecotone.

The climatic record inferred from glacial stratigraphy is much more controversial and contains fewer oscillations. Since J.S. Lee (1947) postulated the occurrence of four Pleistocene glacial stages (Poyang, Dagu, Lushan, Dali) for the Lushan area and Yunnan Province south of Chang Jiang (Yangtze River), attempts have been made to extrapolate this stratigraphic framework to North China and to correlate them with the classical European sequence (Gunz, Mindel, Riss, Wtirm, respectively) (e.g. Mo and Wang, 1984; D. Sun and X. Wu, 1986). Recently, one or two more Early Pleistocene glacial stages have been added to the four 'classical' stages, based on the occurrence of boulder-clay in North China interpreted to be of glacial or glaciofluvial origin (X. Wu, 1983; Cao et al., 1983; M. Zhou, 1985). Since the earliest evidence of glaciation was paleomagnetically dated to the Gauss Epoch or even the Gilbert, glacial geologists tend to favour a longer chronology for the Quaternary, spanning the last 3 to 4 Ma (Cao et al., 1983), a contention supported by many palynologists based on the first occurrence of 'boreal' pollen assemblages in these sediments (K. Zhou, 1984b; K. Zhou et al., 1983; Song et al., 1979). However, the alleged occurrence of till in North China

4 Kam-biu Liu

c c!

0 Single pollen site

l Multiple pollen site

FIG. 3. Location of Quaternary pollen-stratigraphic sites in the deciduous forest and mixed conifer-hardwood forest regions of China. See Table 1 for site descriptions. I = boreal forest; II = mixed conifer-hardwood forest; III = deciduous forest; IV = mixed deciduous-broadleaved evergreen forest; V = subtropical broadleaved evergreen forest; VI = tropical monsoonal rain

forest; VII = steppe; VIII = desert; IX = highland vegetation.

is not unequivocal (Shi et al., 1986) and the basic issue of Quaternary glaciation in Lushan and the whole eastern China championed by J.S. Lee and his students has recently been rigorously challenged (Shi, 1982).

Pollen stratigraphies from long boreholes in the Hebei Plain (part of the North China Plain) have also been used to subdivide the Quaternary based on climatic cycles. Five glacial stages alternating with five interglacials have been delineated over the last 3.06 Ma (Yang et al., 1979) (discussed later). It could be misleading that the terms ‘glacial’ and ‘interglacial’ were used because the pollen data do not imply the occurrence of glaciation in the North China Plain. Tong et al. (1983) and K. Zhou (1984b) postulate a similar scheme consisting of five cold/warm climatic alter- nations but cautiously avoid calling them glacial or interglacial stages.

Another major environmental change affecting North and Northeast China involved sea level changes. Six to nine marine transgressions have been recognized along the Bohai coast during the Quaternary, corresponding to the interglacials or warm episodes (Qian,

1984; M. Zhou, 1983; Yang et al., 1979). There is still much disagreement concerning the chronology and area1 extent of these transgressions.

THE DATA BASE

This paper is the first of a series that document the Quaternary vegetational history of China based on a critical survey of Chinese pollen literature. It results from a long-term project to compile and evaluate Quaternary pollen data from China and to implement a geographic information system to facilitate their stor- age, handling, and analysis. Over the last three decades more than 250 papers or books have been published in China that contain some forms of Quaternary pollen data; much more palynological work has been done but the results remain unpublished or classified. Our data base includes pollen-stratigraphic data (excluding surface samples) from about 180 sites that we have derived from published Chinese literature.

The majority of these pollen study sites are in the temperate mixed forest and deciduous forest regions of


North and Northeast China (Fig. 3). Published data are virtually lacking from the boreal forest. Table 1 lists the locations, site characteristics, time span, and principal references of 80 published pollen records from the temperate forests. Most of the pollen records are of limited stratigraphic and chronological span within the Pleistocene or Holocene. Several long records from boreholes probably span the entire Quaternary, but their dating control (mostly from paleomagnetism) is inadequate and time resolution is poor due to very wide sampling intervals. Most of the pollen records are of Holocene age or at least contain truncated or incom- plete Holocene sections. C-14 dates are available from relatively few of these records. There has not been a 14C dated pollen record that spans continuously the last 30 ka, the period of most dramatic environmental change.

None of the pollen records from temperate China is from lacustrine deposits of small, autonymous lake basins, which are preferred in modern palynological studies because of conditions conducive to continuous organic sedimentation and good pollen preservation (Jacobson and Bradshaw, 1981). Many are from alluvial deposits, geological sections, or lacustrine deposits from large paleolake basins, where the pro- cesses of pollen transport and deposition are poorly known, the conditions of pollen preservation generally poor or non-uniform, and the sedimentation rate probably highly variable. Data on pollen concentration or pollen influx are entirely lacking from this region.

Quality of the published pollen data varies consider- ably. In most cases abbreviated pollen diagrams are presented that contain only a few selected pollen types or broadly lumped groups (e.g. conifers vs broadleaved hardwoods). Pollen records are difficult to compare because different pollen sums are used in different studies and even inconsistently for different ecological groups (e.g. trees, ferns, aquatics) in the same study. Quite often, pollen percentages are given without specifying the pollen sum used in their calculation. For many sites, pollen diagrams are not published; instead, only ranges of pollen percentages for selected taxa in each pollen zone are given. Pollen data from these sites are of limited use except in the most qualitative sense.

Interpretation of the Chinese pollen records is seriously impaired by the lack of systematic studies relating modern pollen rain to regional vegetation and climate. For the whole country relatively few pollen surface samples have been studied or published, and then mostly from very localized environments (e.g.X. Sun and Y. Wu, in press; K. Zhou et al., 1984a) and from offshore marine sediments (Wang et al., 1979, 1980, 1982).

In this paper, I review the principal pollen records and highlight the Quaternary history of the deciduous forest and temperate mixed forest regions in North and Northeast China. Several pollen diagrams are selected for more detailed discussion if they are representative of the vegetational history of the region or if they contain a more detailed record. They are critically

evaluated and re-interpreted if necessary. The ones presented below are redrawn from the original. Some of them are re-zoned if I do not agree with the original zonation, but the latter is also shown for comparison. In a few cases the pollen percentages are recalculated using a different pollen sum as specified.

REGIONAL POLLEN STRATIGRAPHIES AND VEGETATIONAL HISTORY

The Mixed Conifer-Hardwood Forest Region Palynological studies in the mixed conifer-hardwood

forest region of Northeast China were mostly con- ducted since the 1970s. Pollen records from 20 sites have been published (Fig. 3). Most are from peat cores taken from fens and meadows and are of Holocene age. Several are supported by 14C dates.

The Late Pleistocene. Four pollen diagrams from Northeast China contain pollen assemblages attributed to the Late Pleistocene (Sites 9, 10, 12, 13). The assemblages are characterized by maximum pollen percentages of Picea and Abies, along with relatively high percentages of Pinus and Betula. For example, in a pollen diagram from the 853 State Farm (Site 10) in the Sanjiang Plain, Zone 1, estimated to be of Late Pleistocene age, contains up to 30% (of AP sum) Picea and Abies combined, 40% Pinus, 22% Betula, and 25% (of total sum) NAP (mainly Artemisia, Chenopo- diaceae, and Gramineae) (Ye et al., 1983) (Fig. 4). The age of this pollen zone has not been dated at this site, but about a dozen 14C dates ranging from 23,680 _+ 650 to >40 ka BP have been obtained from fossil wood or megafaunal remains associated with this zone at several other localities in Northeast China (Qiu et al., 1981).

The interpretation of this Late Pleistocene pollen assemblage is not straight-forward. Pollen surface samples are lacking from the modern boreal forest (taiga) in Northeast China, but the forest is dominated by larch (Larix gmelinii) and pine (Pinus sylvestris var. mongolica) with little or no spruce and fir (Forestry Bureau, 1981). The absence of Larix pollen in this pollen assemblage therefore does not suggest a southward expansion of taiga to the Sanjiang Plain. The closest analogue probably comes from modern pollen spectra from subalpine spruce-fir forests growing at 1500-1800 m on the north-facing slopes of Changbai Shan (K. Zhou et al., 1984a). Thus this implies a lowering of the subalpine spruce-fir forests, now confined to 1500 m and above, to the Sanjiang Plain (generally at 60-80 m elevations) and other alluvial lowlands. The regional vegetation in much of Northeast China during the Late Pleistocene was probably dominated by spruce and fir, with abundant pine and birch. The high frequencies of Artemisia and Chenopodiaceae pollen probably suggest abundance of these plants in forest openings and floodplains.

The depression of subalpine spruce-fir forests to the lowlands suggests significant temperature lowering during the Late Pleistocene. A much colder climate was

ta Kam-biu Liu

TABLE 1. List of Quaternary Pollen-stratigraphic Sites in the Temperate Forest Regions of China

Site Lat. Long. Time No. Site Name Province (°N) (°E) Site Type Range Reference

1. Jianjihe Heilongjiang 48.7 128.1 peat core LH Yin, 1984 2. Xinqing profile Heilongjiang 48.5 129.6 peat core LH Zhou et al . , 1984c 3. Qiangjin Timber Farm Heilongjiang 48.3 128.3 peat core LH Yin, 1984 4. Tanghongling Heilongjiang 48.3 129.1 sed. core MH-LH Yin, 1984 5. Hongxing profile Heilongjiang 48.2 129.3 peat core MH-LH Zhou et al . , 1984c 6. Qindeli Heilongjiang 48.1 133.2 peat core H Qiu et al . , 1981 7. Cuiluan Heilongjiang 47.7 128.8 peat core LH Yin, 1984 8. Yichun profile Heilongjiang 47.7 128.8 peat core MH-LH Zhou et al . , 1984c 9. 291 State Farm Heilongjiang 46.8 131.5 sed. core LP-MH Ye et al . , 1983

10. 853 State Farm Heilongjiang 46.7 132.8 sed. core LP-H Ye et a l . , 1983 11. Xinjiadian profile Heilongjiang 46.6 130.6 peat core H Zhou et al . , 1984c 12. Bengdebei Heilongjiang 46.6 132.2 sed. core LP-LH Ye et al . , 1981 13. Huangshan Heilongjiang 45.8 126.5 section LP-H Qiu et al . , 1981 14. Yushu Guangming Jilin 44.8 126.6 peat core MH-LH Qiu et al . , 1981 15. Shuangyang Sheling Jilin 43.7 125.5 peat core LH Qiu et al . , 1981 16. Lishu, Xiaoquanyan Jilin 43.3 124.5 peat core H Zhou et al . , 1984c 17. Ying'emen Liaoning 42.1 125.1 peat core H Zhou et al . , 1984c 18. Manbei profile Jilin 42.0 127.5 peat core H Zhou et al . , 1984c 19. Manbei B profile Jilin 42.0 127.5 peat core H Zhou et al . , 1984c 20. Manjiang profile Jilin 41.9 127.6 peat core H Zhou et al . , 1984c 21. Panshan-LP17 Liaoning 41.2 122.0 borehole E P - H Liaoning Geol. Bureau, 1983 22. Dawa-LP24 Liaoning 41.2 122.0 borehole E P - H Liaoning Geol. Bureau, 1983 23. Yingkou-LP26 Liaoning 40.6 122.0 borehole N - H Liaoning Geol. Bureau, 1983 24. Xiaowangzhuang Beijing 40.4 115.9 peat core E H - M H Yan et al . , 1984 25. Bulaodian Beijing 40.3 116.6 peat core MH-LH Zhou 1965 26. Taoshan Beijing 40.2 116.6 peat core MH Yan et al . , 1984 27. Xifu Beijing 40.2 116.7 peat core E H - M H Yan et a l . , 1984 28. Xujiayao Shanxi 40.1 114.0 section LP Zhou et al . , 1983 29. Hutouliang profile Hebeii 40.1 114.5 section EP Liu, 1980 30. Gaolinzhang-319 Beijing 40.1 116.1 peat core EH Zhang et al . , 1981 31. Xinlitun Beijing 40.1 116.2 peat section H Zhou et a l . , 1984c 32. Shizhuang borehole Shanxi 40.0 113.0 borehole N-LP Zhou et al . , !983 33. Datong Basin Shanxi 40.0 113.2 borehole EP Song et a l . , 1979 34. Nangou Hebei 40.0 114.5 section EP Zhou et al . , 1983 35. Yuxian Basin-142 Hebei 40.0 114.7 borehole EP Liu and Tang, 1980 36. Chengqiang Section Hebei 40.0 114.8 section EP Liu and Tang, 1980 37. Xiaojiahe Beijing 40.0 116.2 peat core MH-LH Zhou, 1965 38. Xiaojiahe-07 Beijing 40.0 116.2 peat core M Liu et al . , 1965 39. Lexinju Beijing 40.0 116.4 section LP-MH Zhou et al . , 1978 40. Jishuitan Beijing 40.0 116.5 section LH Zhou et al . , 1978 41. Sanhe-B1 Hebei 40.0 117.1 peat core H Liu et al . , 1965 42. Sanhe-B2 Hebei 40.0 117.1 peat core H Liu et al . , 1965 43. Qianyang Liaoning 40.0 124.1 peat core MH-LH Guiyang Inst. Geochemistry, 1977 44. Dagushan Liaoning 39.9 123.6 peat core MH-LH Guiyang Inst. Geochemistry, 1977 45. Danandao Liaoning 39.8 123.2 peat core LH Guiyang Inst. Geochemistry, 1977 46. Lizhifang Liaoning 39.8 123.5 peat section MH-LH Guiyang Inst. Geochemistry, 1977 47. Zoukoudian Beijing 39.7 115.9 cave section MP Xu, 1965 48. Zoukoudian Beijing 39.7 115.9 cave section EP-MP Yang et al . , 1983 49. Zoukoudian Beijing 39.7 115.9 cave section MP Sun, 1965 50. Fenzhuang Beijing 39.6 115.9 peat section LP Kong and Du, 1980 51. Tangshan-T1 Hebei 39.6 118.2 peat core H Liu et al . , 1965 52. Changxing Dao Liaoning 39.6 121.3 peat section LH Guiyang Inst. Geochemistry, 1977 53. Luanhekou (Le-5) Hebei 39.4 119.2 borehole LP-H Jin, 1984 54. Pulandian Liaoning 39.4 122.0 peat section H Chen et al . , 1965 55. Pulandian Liaoning 39.4 122.0 peat section H Guiyang Inst. Geochemistry, 1977 56. PuI-V borehole Tianjin 39.3 117.7 borehole LP-H Peng and Yan, 1981 57. Bo Hai, 12-1 (marine) 39.0 119.4 core LP-H Jiang et al . , 1981 58. Bo Hai, 12-2 (marine) 39.0 119.4 core LP-H Jin, 1984 59. Cang-9 borehole Hebei 38.7 116.0 borehole N - H Yang et al . , 1979 60. Bo Hai, 25 (marine) 38.6 119.0 core LP-H Jin, 1984 61. Hebei Plain-4 Hebei 38.4 115.7 borehole N-H Tong et al . , 1983 62. Hebei Plain-12 Hebei 38.4 116.9 borehole N-H Tong et al . , 1983 63. Hebei Plain-18 Hebei 38.1 117.6 borehole N-H Tong et aI. , 1983 64. Lishi Sbanxi 37.5 111.1 loess section MP Zhou et al . , 1960 65. Zhangcun Formation Shanxi 37.1 113.0 section EP Song et al . , 1979 66. Liushugou Shanxi 37.1 110.8 loess section EP-LP Zhou et al . , 1984c 67. Heimugou, Luochuan Shaanxi 35.8 109.4 loess section E P - H Liu et al . , 1985 68. Dingcun Shanxi 35.8 111.4 section LP Zhou and Yan, 1984 69. Nianzigou Shaanxi 35.6 109.5 loess section EP-LP Liu et al . , 1985 70. Xuzhou-HX2 Jiangsu 34.9 116.7 borehole LP-H Luo and Liu, 1984 71. Shah Xian Henan 34.8 111.2 section EP Liu, 1965 72. Lianyungang-H70 Jiangsu 34.6 119.2 borehole LP Wang and Gou, 1983 73. Zhangjiapo-1 Shaanxi 34.4 109.5 section EP Inst. Botany, 1966 74. Zhangjiapo-2 Shaanxi 34.4 109.5 section EP Inst. Botany, 1966

Quaternary History of the Temperate Forests of China

TABLE 1. (Continued)

Site Lat. Long. Time No. Site Name Province (°N) (°E) Site Type Range Reference

75. Zhangjiapo-3 Shaanxi 34.4 109.5 section EP Inst. Botany, 1966 76. Beizhuangcun Shaanxi 34.3 109.5 section LP Inst. Botany, 1966 77. Guanyun-GK6 Jiangsu 34.3 119.2 borehole LP-H Wang and Gou, 1983 78. Laochihe, Lantian Shaanxi 34.2 109.4 section LP Inst. Botany, 1966 79. Guannan-GK8 Jiangsu 34.1 119.3 borehole LP-H Wang and Gou, 1983 80. TQ-1 borehole Anhui 33.3 115.6 borehole EP Jin et al., 1986

N = Neogene; EP = Early Pleistocene; MP = Middle Pleistocene; LP = Late Pleistocene; H = Holocene; EH = Early Holocene; MH = Mid-Holocene; LH = Late Holocene.

also implied by the widespread occurrence of the Late Pleistocene Mammuthus-Coelodonta fauna in North- east China. Fossils of Mammuthus primigenius and Coelodonta antiquitatis ranging in age from 12 ka BP to >40 ka BP have been found from over 150 localities in the region (Qiu et al., 1981). At several localities sediments bearing the Picea-Abies pollen assemblage are convoluted by periglacial activities, and are in turn overlain by loess (Qiu et al., 1981). The biotic and physical evidence seems to suggest a climate 7-12°C colder than that of today.

The Holocene. Seven pollen records from North- east China probably span the entire Holocene or at least much of it (Sites 6, 11, 16, 17, 18, 19, 20). A tripartite division of the Holocene is justified based on the pollen evidence. In the pollen diagram from the 853 State Farm (Fig. 4), three pollen zones occur above the basal Picea-Abies pollen zone (Ye et al., 1983). Zone II, presumably of early Holocene age, contains high percentages of Betula pollen, whereas the abundance of conifer pollen has declined. In Zone III, the pollen of broadleaved trees increases to maximum frequencies. Besides Betula, these broadleaved deciduous trees include Alnus, Ulmus, and Quercus. The pollen of Myriophyllum (not shown in Fig. 4) is also abundant in this zone. Zone IV is characterized by an increase in Pinus pollen and diminishing frequencies of Betula and other broadleaved deciduous taxa.

Parallel succession of pollen assemblage zones occurs in other Holocene pollen sequences from Northeast China, although there are regional variations in species compositions. In some pollen diagrams, such as the one from Manbei B (Site 19), the early Holocene pollen zone (Zone 1) is dominated by Pinus instead of Betula, while the late Holocene pollen zone (Zone 3) is characterized by a rise in the Betula pollen curve (K. Zhou et al., 1984c) (Fig. 5).

The mid-Holocene pollen zone (Zone 2), characterized by maximum pollen frequencies of broadleaved hardwood trees, is the most consistent feature in pollen diagrams across Northeast China. Besides Betula pollen, Quercus and Ulmus pollen is the predominant components among the broadleaved hardwood tree pollen, along with Tilia, Juglans, Alnus, Corylus, Carpinus, and Acer in minor amounts. The upper

boundary of the mid-Holocene hardwood pollen zone is directly dated at 5100 + 85 BP at the 853 State Farm (Ye et al., 1983) and interpolated at about 4500 BP at Tanghongling (Site 4; Yin, 1984), but the lower boundary has not been directly dated in a published pollen sequence. An inferred age of 8000-2500 BP for this pollen zone, largely based on data from the Liaodong Peninsula south of this region (Qiu et al., 1981), remains to be substantiated.

The pollen data thus suggest that a mixed conifer- hardwood forest dominated by pine, birch, as well as oak and elm has prevailed in Northeast China through- out the Holocene. During the mid-Holocene, the forest became more diverse as the thermophilous hardwood trees such as oak, elm, walnut, lime, and maple increased in abundance at the expense of pine, and in some cases birch. These vegetational changes are consistent with a warmer, and probably more humid, climate during the mid-Holocene. An intensified summer monsoon could have caused these climatic and vegetational changes (discussed below).

A relatively warm and humid climate during the mid- Holocene was favourable for the initiation of peat bogs in Northeast China. Recent studies on peatland stratigraphies (K. Zhou et al., 1984c; Yin, 1984) suggest that many peat bogs in Northeast China began to form during the mid- to late Holocene. The age of peat bogs seems to be older in the southern part of this region and becomes progressively younger towards the north (K. Zhou et al., 1984c). In the late Holocene, progressive paludification under a cooler climate resulting in reduced evapotranspiration favours expansion of peatland in the northern part of the region. A transition from herbaceous peat to Sphagnum peat is characteristic of many peat stratigraphies spanning the last several millennia (e.g. Yin, 1984).

The Deciduous Forest Region A dense network consisting of at least 60 palyno-

logical study sites exists in the deciduous forest region, especially in the vicinity of Beijing and the Liaodong Peninsula.

Pollen diagrams from seven long boreholes (Sites 21, 22, 23, 59, 61, 62, 63) in the Lower Liao He (Liao River) alluvial plain and the Hebei plain span the entire Quaternary and probably include the Pliocene (Yang et al., 1979; Liaoning Geological Survey, 1983; Tong et

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al., 1983). The pollen diagram from the Cang-9 borehole will be discussed below to illustrate the general trends of vegetational changes during the Quaternary (Yang et al., 1979).

The Cang-9 borehole. The Cang-9 borehole (Site 59) is located in the Hebei Plain near Runqiu. The core is 556 m long. It contains the 'red clay stratum' at 456-483.5 m, which is a regional stratigraphic marker widely recognized to represent the lower boundary of the Quaternary System for the North China Plain and has been dated by paleomagnetism to contain the Mammoth Event (3.06 Ma) (Yang et al., 1979).

The pollen diagram is divided into ten zones (Fig. 6). Pollen Zone 1, beneath the 'red clay stratum', consists of a diverse mixture of pollen from temperate deciduous hardwoods (Ulmus, Betula, Quercus, Jug- lans), subtropical broadleaved taxa (Liquidambar, Myrica), and some Tertiary relicts (Podocarpus, Ginkgo), while Picea, Abies, Pinus, and Larix pollen is either infrequent or absent. In other pollen diagrams from this region, the pollen of Symplocos (sweetleaf) Carya, or Liquidambar is present at 10-30% of total pollen in the corresponding zones, along with the pollen of Hammamelis (witch-hazel), Magnolia, Myrtaceae, Sapindus (soapberry), Melia (chinaberry), Lauraceae, Rhus, Taxodium, Tsuga, Keteleeria, and Cedrus in minor amounts (Yang et al., 1979; Liaoning Geological Survey, 1983). The pollen assemblage suggests a predominantly deciduous forest dominated by temperate hardwoods such as elm, birch, oak, and walnut with an admixture of tropical or subtropical broadleaved evergreen elements and Tertiary relicts. The modern ranges of the latter two groups of plants occur mostly south of the study region. This Pliocene vegetation community thus lacks any modern analogue.

Zones 2, 4, 6, and 8 generally contain abundant pollen of Picea (>30%), Abies (>10%), and Pinus (>35%) but relatively low percentages of temperate hardwoods; the pollen of Salix, Artemisia, Gramineae and Cyperaceae also seems to be more frequent. On the other hand, Zones 3, 5, 7, and 9 are generally characterized by higher pollen percentages of Quercus, along with Ulmus, Tilia, Celtis, Corylus, and Typha.

The pollen assemblages in Zones 2, 4, 6, and 8 indicate a coniferous forest or woodland dominated by spruce, fir, and pine. They were interpreted to correspond to four Pleistocene glacial stages in North China (Yang et al., 1979). Evidence of a fifth glacial stage has been reported from several other pollen diagrams in North China (Yang et al., 1979; Tang, 1981), although it is missing in the Cang-9 borehole. Based on litho- stratigraphic correlation between the Cang-9 borehole and four neighboring boreholes on the North China Plain which have been dated paleomagnetically, the following bracketing dates were tentatively assigned to the four earlier glacial stages (Yang et al., 1979): first glacial, 3.06-2.70 Ma; second glacial, 2.20-1.79 Ma; third glacial, 1.00-0.70 Ma; fourth glacial, 0.3-0.2 Ma. The fifth glacial was estimated to span between 100 ka

and 10 ka BP based on a 14C date of 22,900 + 1100 BP obtained from an interstadial pollen assemblage in a neighboring borehole (Yang et al., 1979).

The pollen assemblages in Zones 3, 5, 7, 9 were interpreted to represent four interglacial stages, including the Holocene. The vegetation was probably a predominantly deciduous forest dominated by temperate hardwoods, particularly oak, together with pine. The pollen of other deciduous hardwoods, such as Ulmus, Juglans, Celtis, Tilia, Corylus, and Carpinus, is not present consistently in all interglacials. It is possible that the absence of their pollen in some interglacials is due to poor pollen representation and inadequate sampling efforts, rather than the failure of these species to reach the site owing to unfavourable climate or migration barriers. The generally higher pollen frequencies of aquatic plants (e.g. Typha, Myrio- phyllum) during the interglacials probably imply a more humid climate, although the presence of Ephedra pollen might suggest the otherwise.

The first glacial. The vegetation and climate of North China during the Pleistocene glacials is a subject of great paleoecological interest. Among the five glacials the first and the last have been the subject of several studies. Pollen evidence for the first glacial is documented from the sediments of the Nihewan Formation, which had previously been recognized as a stratotype of the Early Pleistocene in North China. Pollen stratigraphy from the Hutouliang section representing the lowest member of the Nihewan Formation in the Yangyuan basin, Hebei Province (Site 29), consists of three zones (J. Liu, 1980). Zone 1, the basal zone, is dominated by the pollen of conifers (>90% of AP sum), especially Pinus, Picea, and Abies. Zone 2 is an herb zone dominated by the pollen of Artemisia, Chenopodiaceae, and Cruciferae (NAP up to 70% of total pollen sum), with minor amounts of broadleaved tree pollen (up to 10%) such as Betula, Quercus, Ulmus, and Tilia. Zone 3 is similar to Zone 1 in being dominated by the pollen of Pinus, Picea, and Abies, but also contains abundant pollen of Cupressaceae and herbs. Unfortunately the pollen diagram for the Hutouliang section was not published, the above descriptions were obtained from the original publication (J. Liu, 1980). The pollen diagram from the nearby Nangou section of the same formation (Site 34) is shown in Fig. 7 to show the tripartite oscillations representative of the Hutouliang section (K. Zhou et al., 1983).

Similar oscillations in the pollen stratigraphy have also been found in the Chengqiang section (Site 36) of the Nihewan formation (Liu and Tang, 1980), and in pollen Zones IV, V, and VI of the Yuxian borehole-142 (Site 35) (Liu and Tang, 1980). In addition, pollen assemblages containing abundant Picea and Abies pollen (collectively to at least 20% of AP sum, but mostly over 40%) have been reported from sediments attributed to Early Pleistocene age at several other localities in North China. These include the Zhangjiapo

12 Kam-biu Liu

o~ " O c

o J

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o w

@

:,c . o ~, oq / / /

!~.P .P / /

c

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25 Grains

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Trees ~.~ Herbs ~ '

Z?.o Xoo °

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3

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--= - - 3 w

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% ~ ' g 0 ' g 0 ' ~ ' 2 0 ' 8'0~ ' ,0 20 J 'f AP sum

Clayey sand and gravel, containing caliche

~ Cross-bedded gravel

Gray clay with sandy layers, containing leaf impressions and fossil snails

i I r~ l f-'-I r'--I r~-i 20 40 80 I080 20 20 20 20

; : .~ NAP sum J-J

brown sand and Light gravel

Dark sandy clay, containing fossil snails and shell fragments

Red clay

FIG. 7. Pollen diagram for the Early Pleistocene Nangou section (after K. Z h o u et al. , 1983). Pollen percentages for the tree taxa are based on an A P sum. Herb pollen percentages are based on a sum of all N A P taxa. Note variable percentage scales at

bot tom.

sections near Lantian (Sites 73, 74, 75); Shan Xian near Sanmenxia (Site 71); Zhangcun Formation, Yushe Basin (Site 65); and the Shizhuang borehole, Datong Basin (Site 32) (Institute of Botany, 1966; M. Liu, 1965; Song, 1958; Song et al., 1979; K. Zhou et al., 1983).

These Early Pleistocene Picea-Abies pollen assemblages suggest a coniferous forest dominated by spruce and fir. However, in most profiles these pollen assemblages contain trace amounts of Tertiary relicts of tropical or subtropical affinities, such as Cedrus, Tsuga, Keteleeria, Dacrydium (rimu), Podocarpus, Pseudo- larix, Taxodiaceae, and Ginkgoaceae (e.g.K. Zhou et al., 1983; Institute of Botany, 1966). These ancient conifers were well-represented in the Tertiary flora of North China but have disappeared from the deciduous forest region since the Early Pleistocene; today they are largely confined to the tropical and subtropical regions south of the Yangtze River, especially in cool, moist habitats in mountain ranges (Institute of Botany, 1966). The Early Pleistocene coniferous forest does not have a modern analogue. Simplistic reconstruction of paleo- temperature based on the abundance of Picea and

Abies pollen alone, often attempted in Chinese literature, is therefore unsafe. However, it would be reasonable to infer a cooler and more humid, less continental climate for the Early Pleistocene glacial episodes than that prevailing in North China today.

Many authors (e.g. Song et al., 1979; K. Zhou et al., 1983; Cao et al., 1983; Liu and Tang, 1980) argue that the lower boundary of the Quaternary System in North China should be defined palyno-stratigraphically by the first evidence of significant climatic cooling, hence the bottom of the Picea-Abies pollen zone. K. Zhou et al. (1983) have proposed to call this early Pleistocene cold episode the 'Nangou Cold Period' after the pollen stratigraphy of the Nangou type section in the Yang- yuan Basin, Hebei Province (Fig. 7). But there is disagreement as to whether the Nangou section and the Hutouliang section, both containing pollen evidence for a cold phase at the base of the Nihewan Formation, are contemporaneous (K. Zhou et al., 1983; Song et al., 1979; Xia and Liu, 1984).

Opinions diverge concerning the interpretation of Zone 2, the herb pollen zone, in the Hutouliang section. Cao et al. (1983) and K. Zhou et al. (1983)


interpreted this pollen assemblage as analogous to alpine tundra that exists above the subalpine spruce-fir forest in the mountain ranges of North China today. Thus the oscillations in the pollen stratigraphy from Zone 1 to Zone 3 represent a glacial climatic cycle, with the coldest phase occurring in Zone 2 when spruce-fir forest was replaced by periglacial tundra. However, Liu and Tang (1980) and Xia and Liu (1984) interpreted the high percentages of Artemisia, Chenopodiaceae, and Cruciferae as representing steppe vegetation, thus implying a relatively dry and warm climate. The location of these sites near the modern ecotone between forest and steppe would certainly make their pollen records sensitive to past fluctuations in effective precipitation. Unfortunately the lack of surface pollen spectra from modern alpine tundra and steppes in North China, and the lack of pollen concentration data from these fossil pollen samples, preclude more objec- tive evaluation of these contrasting interpretations.

The last glacial. Vegetational changes during the later phases of the last glacial episode (corresponding to the Wisconsin-W0rm glaciation) are documented in two fairly detailed pollen stratigraphies from North China; one located near the forest/steppe ecotone in the Weihe Basin, the other near Beijing.

The Beizhuangcun Section: A section of gray-brown clay exposed at 490 m elevation along the You He (You River) at Beizhuangcun (Site 76) contained abundant pollen and plant macrofossils of Late Pleisto- cene age (Institute of Botany, 1966). At the base of the exposure is a gravel unit which had been interpreted by some as till, a claim yet to be substantiated by sedimentological evidence. The pollen diagram can be divided into five zones (Fig. 8). Both Zone 1 and Zone 2 are dominated by the pollen of Picea and Abies, indicating a spruce-fir forest. Within Zone 2, the lower part of the gray-brown lacustrine clay unit contained abundant megafossils of spruce, including well- preserved cones, wood, seeds, and leaves. The cones and wood were positively identified to those of Picea wilsonii. Zone 2 contains increasing frequencies of Pinus and Betula pollen; Picea and Abies pollen declines slightly but remains abundant. The vegetation was probably still a coniferous forest dominated by spruce and fir, but with a mixture of birch and pine. Zone 3 is characterized by the successive peaks of broadleaved deciduous pollen types such as Quercus, Carpinus, Corylus, Ulmaceae, and Fraxinus. Picea and Abies pollen decreases rapidly, followed by a dramatic increase in Compositae pollen. This zone records a transient phase of mixed conifer-hardwood forest

4 5 5 -

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140

120

100

80

60

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loess (yellow & red) clay

~ ' ~ grovel

FIG. 8. Pollen diagram from Beizhuangcun (after Institute of Botany, 1966). Pollen percentages are based on a sum of all taxa.

14 Kam-biu Liu

during a period of rapid vegetational changes. Zone 4 is dominated by herb pollen, especially Compositae, as well as Artemisia, undoubtedly indicating a steppe vegetation. Zone 5 comes from the red and yellow loessic deposits that overlie unconformably the gray- brown clay of the preceding zones. It contains fewer pollen but the assemblage is still dominated by Com- positae and Artemisia, thus suggesting a steppe. The increasing percentages of Pinus and Betula pollen could have been due to long distance transport; alternatively, the forest margin might have been closer to the site.

A X4C date of 23,100 + 850 BP had been obtained from spruce wood in the lower part of pollen Zone 2 above the basal gravel layer (Xu et al., 1980), but two more dates of 30,900 BP and 26 ka BP obtained recently are thought to be more reliable (M. Zhou et al., 1987). The upper part of the stratigraphy, corresponding to pollen Zone 4, is dated to about 9 ka BP (An Zhisheng, pets. cornrnun., 1987). Thus the pollen stratigraphy spans the Late Wisconsin-Wiirm glacial maximum to the early Holocene. It shows rapid vegetational changes from a spruce-fir forest, through a brief transition to mixed conifer-hardwood forest, to steppe. It is not sure to what extent are the successive pollen peaks of broadleaved hardwood trees in Zone 3 controlled by climate or a function of species immi- gration from neighboring areas (Davis, 1976), but the presence of their pollen in Zone 1 seems to suggest that their populations were not far away from the area.

The occurrence of full-glacial spruce-fir forests at Beizhuangcun at an elevation of 490 m is of great paleoclimatic significance. The area has a mean annual temperature of about 14°C and precipitation of 580 mm per year. In North China, modern populations of Picea wilsonii are confined to the north-facing slopes of mountains between 1600 m and 2100 m; in Taibai Shan

(part of Qin Ling), south of the study region, they occur above an elevation of 2000 m. For Abies (.4. fargesis, A. chensiensis), their elevational limits in Qin Ling are even higher, between 2300 and 3700 m (Institute of Botany, 1966). The descent of spruce-fir forests to Beizhuangcun, some 1500 m below their modern limits, therefore implies substantial cooling during the Late Pleistocene. The mean annual temperature was probably 8-10°C lower than that of today.

The Fenzhuang Borehole: A complex history of Late Pleistocene vegetational changes for the northern part of the deciduous forest is revealed in an exceptionally well-dated pollen diagram from Fenzhuang near Beijing (Site 50) (Kong and Du, 1980). The pollen sequence mainly comes from the middle section (4.64-8.50 m) of a 20 m borehole located in the piedmont lowlands of Xi Shan (Xi Mountain). The lithology of the polleniferous section is largely peat and black muck. Both the underlying red clay and the overlying yellow clayey sand are devoid of pollen. The pollen diagram was divided into four zones (with Roman numerals) and two subzones by Kong and Du (1980); but I reinterpreted the data and re-zoned the pollen stratigraphy (Fig. 9). The ensuing discussion is based on my own zonation scheme and interpretation. The base of the pollen diagram, at 14 m depth, has an inferred age of about 30 ka BP. Zone 1 contains few pollen and is dominated by Artemisia pollen (to 90% of total sum) with few other taxa present. The bulk of the samples are from gray clayey sand in the upper part of the zone, which spans between ca. 17 ka BP and 13 ka BP. In Zone 2 (ca. 13 ka- l l ,400 BP), conifer pollen, particularly Larix, Abies, and Picea, increases. Artemisia declines dramatically, but Typha pollen increases sharply. The pollen of broadleaved hard-

F E N Z H U A N G B O R E H O L E , B E I J I N G

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FIG. 9. Pollen diagram from Fenzhuang (redrawn and recalculated from Kong and Du, 1980). Pollen percentages are based on a sum of all taxa.


woods such as Betula, Ulmus, Tilia, and Quercus are present in low frequencies. Zone 3 (ca. 11,400-11 ka BP) is characterized by a resurgence of Artemisia pollen, and prominent peaks in Tilia pollen and the spores of Selaginella sinensis. The pollen of Typha, Larix, Abies, Picea, Ulmus, Quercus, among others, declines to minimum values or disappears. Artemisia pollen decreases sharply again in Zone 4 (ca. 11 ka- 10 ka BP), while Typha, and Umbelliferae increase. Except at two levels, Tilia pollen generally occurs at low percentages in this zone.

Prior to 13 ka BP (Zone 1) the vegetation near Beijing was probably a cold steppe or steppe-tundra. The climate was probably too cold and dry to support any forest. A coniferous forest dominated by spruce, fir, and larch invaded the area after 13 ka BP (Zone 2). The vegetation was probably analogous to the subalpine spruce-fir-larch forest that remains locally at elevations above 1800 m in mountains west and northwest of Beijing today (Forestry Bureau, 1981). This could have been brought about by a moister and warmer climate, which also led to an increase in aquatic plants and the beginning of peat accumulation. The climate at that time, nevertheless, was probably still colder than the present by some 6-9°C.

The interpretation of Zone 3 is most intriguing. The dramatic increase in Artemisia pollen and Selaginella sinensis (a fern of dry habitats that is widely distributed in North and Northeast China) spores suggests re- vertence to a drier climate between 11,400 and 11 ka BP. The disappearance of Larix and Picea pollen and the abrupt decline in the pollen of aquatic plants also indicate increased aridity. The pollen peak of Tilia, however, seems to suggest otherwise. Tilia grows on north-facing slopes between 600 and 1300 m elevations in the mountains near Beijing today. It is a late- successional tree that is often associated with oak, birch, poplar, and other thermophilous hardwoods in the deciduous forest, and prefers mesic sites with deep, moist soils (Kong and Du, 1980). The abundance of Tilia pollen, implying a deciduous forest, thus suggests a warmer, and perhaps moister, climate than that prevailing in the preceding zone. One possible ex- planation is that as the climate continued to warm, aridity increased because of higher evapotranspiration, causing coniferous forest to be replaced by dry steppe in the lowlands, while lime-dominated forest persisted locally in moist habitats at higher elevations and in piedmont areas. After 11 ka BP a mixed forest consisting of pine, lime, birch, and elm was established in response to a return to a moister climate. Wetland expanded in the lowlands, and peat accumulation was resumed.

The Holocene. Despite the large number of Late Quaternary pollen study sites in North China (e .g .J . Liu et al., 1965; Chen et al., 1965; K. Zhou, 1965, 1985; K. Zhou et al., 1978, 1984c; Yan et al., 1984; Zhang et al., 1981; Tong et al., 1983), few have yielded a 14C dated pollen diagram that spans the entire Holocene.

The pollen records from Xinlitun and Pulandian will be discussed below to highlight the Holocene history of the deciduous forest in North China (Guiyang Institute of Geochemistry, 1977; K. Zhou et al., 1984c).

The Xinlitun Peat Stratigraphy: A Holocene pollen diagram from a peatland at Xinlitun near Beijing (Site 31; K. Zhou et al., 1984c) supplements the truncated sequence from Fenzhuang discussed above. It can be divided into three zones: a Pinus zone at the bottom, a Quercus-herbs zone in the middle, and a Pinus zone at the top (Fig. 10). The increase in the pollen of Quercus, as well as Carpinus, Acer, Betula, and Juglans, in Zone 2 suggests expansion of deciduous hardwoods at the expense of pine, probably indicating a warmer climate during the mid-Holocene, the Hypsithermal (Deevey and Flint, 1957; Wright, 1976). The climate during the early part of the mid-Holocene might have been more moist, as suggested by the peak in Typha pollen. The progressive rise in the Gramineae and Artemisia pollen curves towards the upper part of the zone probably suggests increasing aridity. It is also notable that a Tilia pollen peak occurs in the upper part of Zone 1. It may be correlated with a similar peak in the pollen diagram from Fenzhuang dated at ca. 11 ka BP, indicating a mixed forest locally dominated by lime.

The Pulandian Peat Stratigraphy: A tripartite division is also justified for the Holocene pollen diagram from a peatland at Pulandian in the Liaodong Peninsula (Site 55; Guiyang Institute of Geochemistry, 1977) (Fig. 11). But here Zone 1, spanning the early Holocene to about 8 ka BP, is dominated by Betula pollen with some Ulmus but little Pinus. A Quercus zone (Zone 2) and a Pinus zone (Zone 3) follow, very much like the sequence from Xinlitun. The Quercus zone is bracketed by 14C dates of 8070 + 190 BP and 4310 + 125 BP, and dates of 1040 _+ 210 BP and 700 + 90 BP have been obtained from Lotus seeds near the base of Zone 3 (Guiyang Institute of Geochemistry, 1977; Libby, 1951). The pollen diagram thus suggests that the early Holocene vegetation in the Liaodong Peninsula was a deciduous forest dominated by birch, along with some elm, perhaps similar to pure birch forests that are common on south-facing, drier slopes in the mixed forest of Northeast China today (Wang, 1961). During the mid-Holocene, birch retreated northward and was replaced by oak, probably in response to Hypsithermal warmth. Based on this and other pollen diagrams from the Liaodong Peninsula (not shown in Fig. 11), the deciduous forest during ca. 8 ka-4 ka BP also contained a high diversity of other thermophilous broadleaved hardwoods such as Tilia, Juglans, Carpinus, Ulmus, Alnus, and Pterocarya (winged nut) (Chen et al., 1965; Guiyang Institute of Geochemistry, 1977). The modern pine-oak forest that prevails in the Liaodong Peninsula was established when pine increased in abundance during the late Holocene, probably due to climatic cooling. The pollen data are not detailed enough to permit an evaluation of human

16 Kam-biu Liu

XinLitun Peat Land

/j/,.,

1750 ± 150-- I ~

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Ctayey Organic Sedge Herba- Sand sand mud peat ceous

peat Mte, Car'co. Sect,, LSU

FIG. 10. Pollen diagram from Xinlitun (after K. Zhou et al., 1984c). Pollen percentages are based on an AP sum.

0-

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Herbs & Aquatics r

0 100 % Fine Sand Grey

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POLLEN I ZONES

Fossil Black Peat Silt Lotus Organic Seeds Clay

SEDIMENT SYMBOLS MLE. CARTO. SECT, L~U

FIG. 11. Pollen diagram from the Pulandian peatland (after Guiyang Institute of Geochemistry, 1977). Percentages for arboreal pollen taxa are based on an AP sum. The pollen curve for broadleaved trees is the sum of Quercus, Ulmus, Alnus, and all other

broadleaved hardwood taxa except Betula. Percentages for non-arboreal pollen (NAP) taxa are based on a NAP sum.

impact on the vegetation over the last several millennia.

DISCUSSION AND SYNTHESIS

Unlike their North American or European counter- parts, the temperate forests of China had not been affected by continental glaciations during the Pleisto-

cene. Despite the absence of continental ice sheets, evidence from loess deposition (T. Liu et al . , 1985), landform development (Nanjing University, 1974) and megafaunal assemblages (B. Zhou, 1978) from the temperate forest regions of China indicates drastic environmental changes during the Pleistocene, corresponding to glacial/interglacial climatic cycles. The pollen evidence reviewed in this paper is compatible with this scenario of drastic environmental changes.


The Late Tertiary forest in North and Northeast China contained an admixture of temperate deciduous hardwoods, subtropical broadleaved evergreen trees, and some ancient conifers of tropical and subtropical affinities. Climatic changes of the Pleistocene caused the progressive retreat of the tropical and subtropical elements to the south of the region, leading to the establishment of the extant temperate forests which are dominated by deciduous hardwoods and northern pines. Podocarpus, Keteleeria, Cedrus, Carya, Ptero- carya, Myrica, Melia, Symplocos, and Liquidambar, for example, the pollen of which is present in Pliocene sediments of the North China plain (Tong et al., 1983; Yang et al., 1979), are now exotic to the temperate forests of China (Academia Sinica, 1983).

Five glacial/interglacial cycles are generally recognized for the temperate regions of China based on pollen-stratigraphic data (Tang, 1981; Tong et al., 1983). The first episode of major glacial cooling, called the Nangou Cold Period, brought a coniferous forest of boreal character to North China. But this coniferous forest, though dominated by spruce and fir, was still mixed with minor amounts of Tertiary relicts and subtropical evergreen hardwoods, thus lacking any modern analogue. Each subsequent glacial saw the return of spruce-fir forests to the temperate region of China, but the subtropical elements and Tertiary relicts were progressively eliminated in the later glacials. In the Late Pleistocene and Holocene, the flora of temperate China was essentially modern in character.

It is most likely that the glacial coniferous forest that prevailed in North China was not derived from a southward expansion of the taiga, but instead from a descent of the subalpine spruce-fir forests to the lowland. The modern taiga is dominated by larch and pine with little spruce and fir, and is confined to the extreme northeast, whereas spruce and fir are domi- nant in the subalpine forest between ca. 1500-2200 m. A cooling of some 8-10°C could have permitted the subalpine spruce-fir forest to descend to the North China Plain and the Wei River lowlands. The paucity of pollen study sites and inadequate dating control does not permit reconstruction of glacial refuges for the thermophilous hardwood species. But based on the presence of their pollen in the glacial Picea-Abies pollen assemblages, one may speculate that local populations of these thermophilous hardwood species could have survived the glacial climate in suitable microhabitats amidst the coniferous forest.

During the interglacials the spruce-fir forest retreated to the mountains and broadleaved deciduous forest became re-established in the lowlands. The forest was dominated by Quercus, Ulmus, Betula, Celtis, Juglans, and other temperate hardwood trees which are also characteristic of the deciduous forest today. Tertiary relicts like Podocarpus, Cedrus, and Pseudolarix were still present during the Early Pleisto- cene interglacials, but had disappeared since the Middle Pleistocene. It has been inferred from geological evidence that the climate of the Middle Pleisto-

cene interglacial was much warmer than that of today, based on the occurrence of lateritic paleosols north of Huai He (34°N), some 5-10 ° north of the northern limit of this soil today (Yang and Xu, 1980). The sparse pollen data from North China provide no strong indication for substantial northward expansion of subtropical forest elements, although the sporadic occurrence of the pollen of Symplocos, Pistacia, and Ostrya in the interglacial deposits near Beijing probably imply the greater abundance of these thermophilous trees during the Middle Pleistocene (Sun, 1965; Xu, 1965; Yang etal., 1983).

A pollen stratigraphy from temperate China that continuously spans the last 30 ka is still lacking, but the truncated pollen diagrams from Beizhuangcun and Fenzhuang contain a fairly detailed history of the Late Pleistocene/Holocene transition. Both show that a spruce-fir forest prevailed during the Late Pleistocene, rapidly changing to a mixed pine-hardwood forest. But the chronology and complexity of vegetational changes between the two areas are quite different. In Beizhuan- gcun, a more inland location near the forest/steppe ecotone, the spruce-fir forest had existed by 23 ka BP and changed unidirectionally to a steppe, suggesting increasing aridity as the climate warmed. In Fenz- huang, on the other hand, a cold steppe, represented by a basal Artemisia pollen zone, prevailed for a long time before it was replaced by spruce-fir forest ca. 13 ka. The climate of the Beijing area during the Wisconsin-Wiirm full-glacial times was probably very cold and dry; frequent outbreaks of arctic air masses from Siberia could have caused an expansion of cold steppe southeastward to replace forest. The Fenzhuang pollen diagram also shows a possible climatic oscillation between ca. 11,400 and 11 ka BP, as reflected by abrupt increases in Artemisia and Selaginella sinensis, indicating a return to drier conditions. It is tempting to relate these changes to the Younger Dryas oscillation of 11 ka-10 ka BP that has been documented from both sides of the North Atlantic Ocean (Rind et al., 1986). However, the paleoecological significance of this pollen zone, which contains maxima in both Tilia and Artemisia pollen, remains unclear. Any such attempt in paleoclimatic reconstruction and inter-continental correlation is premature.

A tripartite division of the Holocene is justified for North and Northeast China based on the pollen records. The climate seemed to warm to a maximum during the mid-Holocene, followed by Neoglacial cooling. Thermophilous hardwoods such as oak, elm, lime, and ironwood increased in frequency at the expense of pine during the mid-Holocene, indicating a warmer and more humid climate. This climatic change can be explained by a strengthened summer monsoon, bringing increased moisture to more northern and inland locations. Results from climate modeling experiments predict intensified monsoon circulations during the early Holocene due to orbital parameter changes, resulting in higher summer temperature and precipitation in northern Eurasia, including northern

18 Kam-biu Liu

China (Kutzbach, 1980; Kutzbach and Guet ter , 1984, 1986; Rind et al. , 1986). Unequivocal evidence for an early Holocene thermal max imum has not been found in the pollen data f rom North and Northeas t China. The only hint might be found in the pollen diagram from Beizhuangcun, which shows replacement of forest by steppe near the top of the truncated sequence, suggesting an early Holocene climate drier than the present. However , a s trengthened summer monsoon is expected to bring a moister rather than a drier climate to this ecotonal area between forest and steppe, just contrary to what the data suggest.

Based on archaeological and phenological evidence Chu (1973) concluded that a warmer and moister climate prevailed in Nor th China prior to ca. 3 ka BP. It was followed by a fluctuating trend of climatic cooling interrupted by warmer episodes during ca. 2700-2000 BP, 1350-950 BP, and 800-700 BP. The pollen data do not show clear evidence of the short- te rm climatic fluctuations during the last three millennia, but this may simply be a function of the poor resolution and sensitivity of the pollen data.

Unlike that of North America , the vegetat ion of China has been extensively disturbed and modified by human activities over a period of at least four or five millennia. Thus it is hard to isolate the role of climate f rom that of human disturbance in the vegetat ional changes registered in the pollen records. In Europe , for example , the elm decline at the Atlantic/Sub-boreal transition (ca. 5 ka BP) was attr ibuted to climatic change but has been reinterpreted in terms of forest clearance by prehistoric people (Garbet t , 1981). Even in North America , the arrival of European settlers two or three centuries ago significantly altered the vegetat ion, leaving a distinctive imprint in the pollen records across the t empera te region, the A m b r o s i a rise (McAndrews, 1988). It has become increasingly re- alized that other non-climatic factors, such as disease, have played an important role in vegetat ional changes (e.g. Davis, 1978). In the t empera te forests of North and Northeast China, both climatic cooling and human disturbance could have contributed to the late Holo- cene Pinus rise detected in the pollen records, an event poorly dated but often arbitrarily assigned an age of 2500 BP based on a presumed correlation with the Bly t t -Sernander system from Europe (e.g. Guiyang Institute of Geochemis t ry , 1977). More data and dates will be needed to evaluate the relative contributions between climatic and anthropogenic factors in vegetat ional changes during the historical period. Research has not begun to document the human impact on the vegetat ion of China by means of palynological methods, but it could be a very productive line of investigation given the wealth of supporting information f rom historical records (e.g. Ho, 1969; Chu, 1973).

ACKNOWLEDGEMENTS

I thank Hong-lie Qiu for assistance in computer data management, M.L. Eggart for cartographic assistance, and M. Monceaux for typing

the manuscript. This research was supported by grants from the National Geographic Society and the Association of American Geographers, and a Summer Faculty Research Grant from the LSU Council on Research. Travel funds from the LSU College of Arts and Sciences are also acknowledged.

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Quaternary history of the temperate forests of China

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