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A preliminary mapped summary of Holocene pollen data for Northeast

Article in Quaternary Science Reviews · April 1998 DOI: 10.1016/S0277-3791(98)00017-1

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The user has requested enhancement of the downloaded file. Quaternary Science Reviews, Vol. 17, pp. 669—688, 1998 ( 1998 Elsevier Science Ltd. Printed in Great Britain. All rights reserved. PII: S0277-3791(98)00017–1 0277—3791/98, $19.00

A PRELIMINARY MAPPED SUMMARY OF HOLOCENE POLLEN DATA FOR

GUOYU REN* and LANSHENG ZHANG- *National Climate Center, No. 46 Baishiqiao Road, Beijing 100081, Peoples+ Republic of China -Beijing Normal University, Xinjiekouwai Street, Beijing, 100871, Peoples+ Republic of China

Abstract—Maps of pollen data have long been unavailable for continental despite their importance for paleoecological and paleoclimatic studies. Pollen data from 65 Holocene sites have allowed the mapping of eight pollen taxa and seven time slices for Northeast China. These pollen maps show significant vegetation changes during the last 10,000 years in the current forest of Northeast China, with the early Holocene characterized by widely distributed Betula trees, the mid-Holocene by the development of temperate deciduous forest mainly consisting of Quercus and ºlmus trees, and the late Holocene by the marked increase in the abundance of Pinus trees and the development of temperate mixed conifer and deciduous forest. An unexpected finding from the pollen mapping research is the south-to-north propagation of the mid-to-late-Holocene forest decline, which may have been caused by the persistent human activities and the gradually northward expansion of agriculture during the last 5000 years. It is thus evident that caution has to be taken when reconstructing the mid-to-late-Holocene climates from pollen records in some regions of Northeast China. The classic paleoclimate reconstruction for the southern part of the study area and possibly for North and needs to be re-evaluated. ( Elsevier Science Ltd. All rights reserved

INTRODUCTION Here we present the preliminary research results for the Holocene pollen maps from northeast China and Isopoll and isochrone maps have been widely used in describe the methods used to produce contours the studies of late Quaternary paleoecology and whether isopolls or isochrones. The maps are then used paleoclimatology (Bernabo and Webb, 1977; Huntley to interpret the past vegetation. The underlying causes and Birks, 1983; Huntley, 1990; Webb, 1985; Anderson for the vegetation changes are discussed and a few et al., 1994; Ren, 1994). These maps illustrate the conclusions are drawn. changing patterns of past vegetation (Bernabo and Webb, 1977), show how major ecotones have moved (Webb, 1983; Bernabo and Webb, 1977), and aid inter- MODERN ENVIRONMENT OF THE STUDY pretations of past climates (Bartlein et al., 1984; Hun- AREA tley, 1990; COHMAP Members, 1988). Furthermore, this acquired paleoecological and paleoclimatological knowledge aids researchers in testing and calibrating Physiography ecological and climatic models (Huntley, 1990; CO- The study area extends from 37° to 59°N and 110 to HMAP Members, 1988; IGBP, 1994), in estimating 135°E and includes: , Jilin prov- past variations in the terrestrial carbon reserves (Pren- inces, the Northern part of Hebei Province (including tice and Sarnthein, 1993), and in assessing effects of Beijing Municipality) and Auton- human activities on Holocene vegetation and environ- omous east of 110°E (Figs 1 and 2). Major ment (Ren, 1994). topographical features of the area are the extensive Time series of maps summarize late Quaternary Northeast Plain surrounded by a series of mountains pollen data for and (Webb, and hills. The consists of the 1983; Anderson and Brubaker, 1994; Huntley and , and , each Birks, 1983; Huntley, 1990). In the continental of which is no more than 400 m above sea level. Two East Asia, however, pollen-mapping has not been done series of the northeast-southwest-trending mountains, before 1994, largely due to the low density of sites the Daxingan Mts. and the Changbai Mts., run parallel with well-dated records. Ren (1994) made a first along the western and eastern sides of the Songnen attempt to summarize the Holocene vegetation and Plain and Liaohe Plain, while the Xiaoxingan Range climatic changes of Northeast China through mapping with a northwest—southeast orientation lies to the of the available fossil pollen data. The maps revealed north of the Songnen Plain. some interesting patterns in vegetation change that Both the Liaohe River and the Songhuajiang are hard to identify from analysis of pollen data at River originate in the Daxingan Mts., but the former single sites. flows eastward and then southward into the ,

669 670 Quaternary Science Reviews: Volume 17 and the latter runs up to the northeastern corner of the ized by widely scattered bogs and swamps, which can study area, where it combines with the other two large also be found in the other regions, though to a less rivers, the Heilongjing River and the Wusulijiang extent. River, forming a low-lying Sanjiang Plain character- Climate Nearly every part of Northeast China is strongly influenced by temperate east Asian monsoon. Mean January temperatures range from !6°C in the south to !30°C in northern Daxingan Mts. The mean July temperatures have a much more homogeneous distri- bution, with the highest temperatures occurring in the south and the inland plains (Fig. 2). Annual precipita- tion decreases from more than 1000 mm in northern Hebei Province and southeastern Liaoning Province to less than 300 mm in some stations west of Daxingan Mts. (Fig. 3). Eighty to ninety percent of the total annual precipitation falls in the three summer months (June—August). Little snow or rain falls in winter and spring, and most regions are normally dry during the beginning stage of plant growth, a unique climatic phenomenon not characteristic of the same latitude areas of Europe and North America. The climates are most like those in Kansas, Minnesota, and Alberta in North America.

Vegetation Modern vegetation cover consists of the temperate types, including temperate moist forest, temperate forest steppe and temperate steppe (Fig. 4). Forest is found in mountain and hill regions, and steppe dominates the major fluvial plains and the Inner Mongolia Plateau west of the Daxingan Mts. A mixed plant belt called forest steppe between the forest of the Changbai Mts. and the steppe in the Songnen and FIG. 1. Physiographic regions of Northeast China. Liaohe Plains.

FIG. 2. Mean January and July temperature (°C) in northeast China. G. Ren and L. Zhang: Summary of Holocene Pollen Data 671

Warm temperate deciduous forest grows in the south of the study area, where varied species of Quercus are common, and species of ºlmus, Juglans, ¹ilia, Carpinus, Corylus, Pinus and Betula are also important components of the forest. Between the cold- temperate conifer forest and the warm-temperate de- ciduous forest is temperate mixed conifer and decidu- ous forest, in which Pinus koraiensis is the dominant species. Other important species include Abies holo- phylla, Carpinus cordota, Acer mono, Quercus mon- golica, ºlmus propinqua, Fraxinus mand shurica, Jug- lans mand shurica, ¹ilia amurensis and Betula platy- phylla. As a representative type for Northeast China, the mixed forest covers the Changbai Mts., Xiaoxingan Range and the western pediment belts facing the Song- nen Plain (, 1980). Temperate steppe mainly develops on the central plains and the Inner Mongolia Plateau. Its floristic components mainly include the families of Gramineae, Chenopodiaceae, Compositae and Cyperaceae. One genus most commonly found in the steppe is Artemisia (Wu, 1980; Academic Sinica, 1985). The best developed forest steppe appears on the eastern fringe of the Song- nen and Liaohe Plains, where trees and herbs grow alternately in the different topographical units. Of the FIG. 3. Annual precipitation (mm) in northeast China. trees, those corresponding to the deciduous compo- nents of the mixed forest to the east are usually the most common. At present, however, nearly the whole forest steppe belt and part of the steppe close to it are farmed (Fig. 4).

DATA AND METHODS

Data Pollen data are mainly taken from the published research reports for single sites, with the exception of a few sites that came from unpublished dissertations. Considerable variation exists among the data in such aspects as the topographic position of the sampling sites, sediment type of the samples, sampling interval, radiocarbon-dating control, laboratory analysis pro- cedure, and percentage calculation method. These problems have to be taken into consideration before confidence is given to the data. Topographic position: Pollen studies have been done in varied topographic positions. The sites for pollen mapping were selected for the regions to minim- ize differences in elevation. When the data in moun- tainous regions were used, only those from the basal belt of the vertical floristic distributions were chosen. The upper limit of the basal belt is approximately FIG. 4. Modern vegetation types in Northeast China. Revised from 900 m in the Changbai Mts., and it declines to about Wu 1980. 600 m in the Xiaoxingan Range. Few sites were used from the other mountainous regions. In the northern end of the Daxingan Mts., cold Sediment type: Continuous lacustrine and peat sedi- temperate forest appears, with ¸arix dahurica as its ments are most favorable for paleoecological studies dominant species. This forest is a southwards extension that use fossil pollen data (Birks and Birks, 1980). of the Siberian conifer forest along the mountains. Often the pollen deposition in peat can be more 672 Quaternary Science Reviews: Volume 17

TABLE 1. Fossil pollen sites selected from the published research reports for Northeast China

No. Site Location Time range Radiocarbon Authors used (ka BP) Dates

1 Gushantun Huinan, 0—10 4 Liu (1989) Jilin 2 Dadianzi Huinan, 0—10 4 Sun and Yuan Jilin (1990) 3 Sandaomiao Changbai 0—10 3 Yuan and Sun Mts., Jilin (1990) 4 Harbaling Dunhua, 0.8—2 2 Peat Group Jilin (1983) 5 Liangsuixiang Luohe, Jilin 0, 8 2 Wang and Xia (1990) 6 Chuangye Fuyuan, 0—10 3 Xia (1988) Heilongjiang 7 Qindeli Sanjiang, 0—10 5 Xia (1988) Heilongjiang 8 Bielahonghe Sanjiang, Heilongjiang 0.8—6 3 Xia (1988) 9 Yangmuxiang Mishan, 0—2 1 Xia (1988) Heilongjiang 10 Qinghe Baoqin, 0—2 1 Xia (1988) Heilongjiang 11 Shenjiadian Huachuan, 0—4 3 Xia (1988) Heilongjiang 12 Jiajihe Xiaoxingan, 0—4 2 Yin (1984) Heilongjiang 13 Tanghongling Xiaoxingan, 0.8—6 3 Yin (1984) Heilongjiang 14 Qianjing Xiaoxingan, Heilongjiang 0.8—4 1 Yin (1984) 15 Heilongjiang 2 2 Liu (1989) 16 853farmland Baoqin, 4—61 Yeet al. (1983) Heilongjiang 17 Laodaomiao Hailin, 2—4 1 Xiao and Sun Heilongjiang (1987) 18 Hailangxiang Ningan, Heilongjiang 0—4 1 Xiao and Sun (1987) 19 Gonghelai Houqi, 0.8—6 3 Xia et al. (1993) Inner Mongolia 20 Bolahu Qianqi, Jilin 2—6 5 Xia (1993) 21 Maili Houqi, 0—4 7 Ren (1994) Inner Mongolia 22 Seling Suangyang, 0—0.8 1 Qiu et al. (1981) Jilin 23 Zhoujiayoufang2 Yushu, Jilin 4—10 1 Wang and Xia (1988) 24 Daluoshu Naiman, 0, 2, 10 6 Han (1992) Inner Mongolia 25 Zhoujiayoufang1 Yushu, Jilin 6 4 Sun et al. (1985) 26 Shaolanghe Wudan, 21 Wuet al. (1992) Inner Mongolia 27 Dongwenggen Tailai, 0, 4 4 Ye et al. (1989) Heilongjiang 28 Resuitang Chifeng, 6 2 Jiang (1992) Inner Mongolia 29 Wudan Chifeng, 2 1 Jiang (1992) Inner Mongolia 30 Wulanaodu Chifeng, 82 Wuet al. (1992) Inner Mongolia 31 Dadianzi Aohan, 4 2 Kong et al. (1991) Inner Mongolia 32 Hongshengxiang Tailai, 0, 4, 6 2 Qiu et al. (1992) Heilongjiang 33 Kaitong Tongyu, 0, 8 1 Qiu et al. (1992) Heilongjiang 34 Xiaonan Changcun, 6 1 Wang and Xia, Jilin (1988) G. Ren and L. Zhang: Summary of Holocene Pollen Data 673

TABLE 1. (Continued)

No. Site Location Time range Radiocarbon Authors used (ka BP) Dates

35 Hetun Naiman, 8—10 1 Wang (1991) Inner Mongolia 36 Mangshigou Naiman, 10 1 Qui (1991) Inner Mongolia 37 Xingbarhudongqi Inner Mongolia 2—4 2 Xia (1991) 38 Dalainuor Inner Mongolia 2—10 4 Li et al. (1990) 39 Pulandian Dalian, 4—10 4 Chen et al. (1977) Liaoning 40 Dagushan Donggou, 4—8 3 Chen et al. (1977) Liaoning 41 Qianyang Donggou, 0.8—8 2 Chen et al. (1977) Liaoning 42 Qianwatun Zhuanghe, 0.8—6 2 Chen et al. (1977) Liaoning 43 Paoya Fuxian, 2—10 11 Zhao (1989) Liaoning 44 Lianhuashan Fuxian, 6—10 9 Zhao (1989) Liaoning 45 Bachagou Changxing, 2 1 Chen et al. (1977) Liaoning 46 Ewenkeqi Inner Mongolia 0.8—2 1 Xia (1993) 47 Sanxianpu Changling, 0.8—4 2 Wang and Xia Jilin (1988) 48 Hailarbeishan Hailar, 4 3 Xia (1993) Inner Mongolia 49 Yangerzhuang Huanghua, 2—10 3 Xu et al. (1993) Hebei 50 Maohebei Hebei 4—10 4 Li and Liang, (1985) 51 Baiyangdian Hebei 2—10 3 Xu et al. (1988) 52 Fenzhuang Fangshan, 10 5 Zhang et al. Beijing (1981) 53 Xinlitun Haidian, 2 1 Zhang et al. Beijing (1981) 54 Xinlitun2 Haidian, 0.8—2 1 Zhou (1984) Beijing 55 Gaolizhang Haidian, 8—10 2 Zhang et al. Beijing (1981) 56 Taoshan Huairou, 4—6 2 Zhang et al. Beijing (1981) 57 Dawuangzhuang Yanqing, Beijing 6—8 3 Kong (1982) 58 Wuliying Yanqing, Beijing 2—6 3 Kong (1982) 59 Daziying Hebei 8—10 2 Li and Liang, (1985) 60 Nanzhuangtou Xusui, 10 7 Yuan et al. (1989) Hebei 61 Summer Palace Beijing 2 4 Zhou et al. (1993) 62 Mengchun Changzhou, 6—82 Xuet al. (1993) Hebei 63 Donggaochun Pinggu, 2 1 Wang (1991) Beijing 64 Huangshan Harbin, 10 3 Wang and Xia, Heilongjiang (1988) 65 Jiangnancun Yongji, Jilin 8 1 Sun et al. (1980)

heterogeneous and indicate the past local and extra- etration from plants growing on the mire surface can local plant changes better than these data from lake sometimes make peat dates too young. sediments, because the data from peat are generally Most pollen samples in the study area were prepared free of the effects of pollen focusing and pollen re- from peat, in both the developing bogs and swamps deposition inherent in the lake basins (Jacobos and and in the buried peat. Some come from other sedi- Bradshaw, 1981). The high carbon content of peat can ment layers, and precaution was taken when the data also make radiocarbon dating in peat more reliable from the non-peat layers were used. Generally, only than dating in lacustrine sediments, though root pen- data obtained from the lake sediment samples are 674 Quaternary Science Reviews: Volume 17 chosen, and the data values occurring in the fluvial Xiaoxingan Range, Songnen Plain, upper Liaohe Plain sediment, loess and paleosols were compiled merely as and the northern part of Hebei Province. The Daxin- reference data rather than being used as the basis for gan Mts. and Inner Mongolia Plateau are poorly rep- preparing pollen maps. resented. Sampling interval: Data reliability depends to a large extent on the time resolution of pollen time Methods series, which in turn is determined by the sampling intervals within a given core or section. Wide sampling The procedure for preparing pollen maps approxim- intervals are common for the fossil pollen studies made ately follows that of Webb et al. (1983) and Webb (1985), before 1985, and it naturally weakens our confidence in though some revisions have been made that are better them. In spite of this, certain constraints can be set in suited to the specific conditions of the study area. selecting data in order to ensure high precision. Only Pollen taxa and time level: A few of the most com- those pollen sites with the sampling intervals finer than mon families or genera in Northeast China were se- 1 sample per 600 yr were put into the data set. In a few lected for mapping. They include Picea/Abies, Pinus, areas in which the data are very rare, sites were in- Quercus, ºlmus, Betula, Artemisia, and Cheno- cluded with a time resolution that was more coarse podiaceae pollen. Total arboreal pollen was also map- than 1 sample per 600 yr. ped. The percentages of Picea and Abies are combined Dating control: Before 1982, many pollen studies because they are usually not given separately in the were published with no radiocarbon dates. All of them published reports. are omitted, and no dating procedure based on pollen- Six time slices in 2000-yr intervals were chosen from stratigraphic correlation was permitted. When a core 10,000 yr B.P. (10 ka) to 0 yr B.P. (0 ka), and 800 yr or section did not have enough radiocarbon dates (e.g. B.P. (0.8 ka) was also chosen as a mapping time slice in fewer than three in the whole Holocene period), only order to investigate the vegetation change in the Medi- the pollen percentage values at the fixed mapping levels eval Warm Period. The number of sites with data differ most close to the carbon dates were selected. If more among the time slices with 21 at 10 ka, 24 at 8 ka, 30 at than three radiocarbon dates exist in a pollen time 6 ka, 34 at 4 ka, 37 at 2 ka, 22 at 0.8 ka, and 27 at 0 ka. series, age assignments for the fixed mapping levels Coverage is best after 6 ka because peat accumulated were performed based on a graph of the age—depth mostly during the last 6000 years in Northeast China relationship. Such a method was not applied, however, (Ma et al., 1989; Ren, 1994). For modern percentage if the sediment from which the samples were taken was values, ten extra sites with surface pollen data were also heterogeneous, and a simple linear interpolation be- included. tween the carbon dates was used instead. All of the Data extraction: Original percentage values for all of dates used in the present paper are in uncalibrated the fossil pollen sites were read from the pollen dia- radiocarbon time. grams given in the references quoted in Table 1. A total of 65 fossil pollen sites were used after the Smoothing for the percentage curves was performed by above considerations were applied (Table 1 and Fig. 5). hand before digitizing in order to buffer against the The regions with good data coverage include the influences of extreme values. Liaodong Peninsula, Changbai Mts., Sanjiang Plain, Standardization: Because Chinese palynologists used different sums for calculating pollen percentages, a standardization was needed to make the data compa- rable. This required recalculation of the percentage values using a sum of total pollen grains from terres- trial plants excluding Cyperaceae. The most frequently published sums include: (1) the total pollen and spore grains counted; (2) the total pollen and spore grains from terrestrial plants; and (3) the total pollen grains from trees. In a few cases, two sums, the total pollen grains from trees and the total pollen and spore grains from other plants, were used. Map construction: Isopolls were drawn by hand for the selected taxa and time slices. Base maps with a scale of 1 : 9 million were used. The interval between the contours of pollen percentages is prescribed as 5% except for that of Arboreal pollen and Artemisia, for which the interval is 10%. Specific isopolls were chosen before isochrones were drawn by overlapping on to one base map the isopolls of the same taxon from different time slices. The isochrones represent 30% for Artemisia, 20% for Pinus, 10% for Betual, and 5% FIG. 5. Distribution of the fossil pollen sites used in this study. for Picea/Abies, ºlmus, Quercus and Chenopodiaceae G. Ren and L. Zhang: Summary of Holocene Pollen Data 675 pollen. The 40% isopoll for Arboreal pollen today pollen appear in the Changbai Mts., and Quercus pol- corresponds well to the central axis of the forest steppe len percentages were even higher than 30% at a few in the east of the Songnen Plain, and this isopoll was places on the Liaodong Peninsula. In the eastern part therefore chosen for mapping. of the Songnen and Liaohe Plains, increased percent- ages of the deciduous tree pollen were noticeable as well. The trends in pollen abundance changes that ANALYSIS OF POLLEN MAPS started at the beginning of Holocene seem to have persisted until about 6 ka. The isopolls for given time slices are shown in From 6 to 4 ka, tree pollen percentages continued to Figs 6—12, and the isochrones for different taxa are rise slightly in the study area except at sites on the presented in Fig. 13. Several major features of the Liaodong Peninsula and in Hebei Province, where tree spatial distribution and temporal variation of pollen pollen percentages declined markedly (Fig. 9 and taxa are evident on these maps. Fig. 13). Betula percentages decreased on the Liaodong Peninsula, but Betula percentages rose slightly at sites Early Holocene (10— 7 ka) on the Sanjiang Plain and in the Xiaoxingan Range. The most remarkable event at this time was the rapid At 10 ka, the beginning time of Holocene, tree pollen increase of Pinus pollen percentages in almost every was rare everywhere compared to later dates (Fig. 6). It part of the study area. The appearance of the 20% was about 70% in the Changbai Mts., 40—60% on the isopoll for Pinus pollen records this event on the map Liaodong Peninsula, and 10—40% in the Songnen and for 4 ka. This isopoll surrounds an extensive area from Liaohe Plains. The most abundant arboreal taxon was the Liaodong Peninsula to the Xiaoxingan Range and Betula pollen, which reached 25—30% in the Changbai Sanjiang Plain. In the meantime, Picea/Abies percent- Mts. and the Sanjiang Plain and 20—25% in the ages rose in the central and northern forest regions, Liaodong Peninsula. Picea/Abies percentages are gen- though not so strongly as Pinus pollen percentages. erally 5—10% in the Changbai Mts., 1—5% on the Pollen percentages from deciduous trees, ºlmus in Sanjiang Plain, and around 1% on the Liaodong Pen- particular, generally decreased from 6 to 4 ka, which, at insula. Pinus pollen abundance was at its lowest for the least in the central and northern regions, may be result- Holocene. Quercus and Artemisia pollen occurred in ed from the big increase in Pinus pollen percentages. relatively high abundance where Picea/Abies pollen was low on the Liadong Peninsula. Otherwise the Late Holocene (3—0 ka) pollen for deciduous trees such as ºlmus and Quercus was in low abundance and was not usually higher than The pollen change trends starting from 4 ka BP have 10% in the eastern forest regions. lasted throughout the late Holocene. Tree percentages Tree pollen percentages increased in the east at 8 ka continue to decline in Liaodong Peninsula and Hebei (Fig. 7 and Fig. 13), with ºlmus and Quercus pollen Province, as seen from the isopolls for 2 ka BP (Fig. 10) accounting for the largest portion of the increase. ºl- and isochrones (Fig. 13). Moreover, they begin to de- mus percentages increased up to 15 to 20% on the crease as well in the southeastern part of Horqin Sand- Sanjiang Plain and in the Changbai Mts., while Quer- land, to the far north of Liaodong Peninsula. In the cus pollen percentages exceeded 20% in the Changbai central and northern regions, however, tree percent- Mts. and on the Liaodong Peninsula. The abundance ages remain unchanged or even a continuous rising in of Betula pollen had declined by this time and was some sites, which still may be induced by the steady below 10% on the Sanjiang Plain, between 15% and increase of Pinus pollen during the last 3000 years. By 20% in the Changbai Mts., and 10—15% on the 2 ka BP, Pinus percentages reaches more than 30% in Liaodong Peninsula. Little change is recorded for the a few sites of Changbai Mts. and Xiaoxingan Range, other tree taxa. though they start to decrease in Liaodong Peninsula. Further declination of deciduous tree pollen occurs in almost every forest region. Mid-Holocene (7 to 3 ka) This period witnessed some remarkable changes in the fossil pollen record of Northeast China. Most no- VEGETATION RECONSTRUCTION ticeable were the rise of Pinus percentages almost The pollen maps and their interpretation can be everywhere and the decline of tree pollen percentages used to reconstruct vegetation change through differ- in the southern regions. ent stages of the Holocene. For this discussion, the Tree percentages continued to increase in the first whole Holocene is divided into three periods, and the half of the mid-Holocene. They rose to 90% or higher vegetation condition during each of them is described. at some sites in the Changbai Mts. and on the Liaodong Peninsula by 6 ka (Fig. 8 and Fig. 13). The Early Holocene birch period increase in tree pollen percentages mainly resulted from the further increase in Quercus and ºlmus pollen Represented by the time slice of 10 ka, this stage may percentages. Isopolls of 20% Quercus and 20% ºlmus last from as early as 12 to 8 ka, if the records for 12 ka 676 Quaternary Science Reviews: Volume 17

FIG. 6. Isopolls for 10,000 yr B.P. in Northeast China. The values shown on the map represent the pollen percentages. Regions with sparse data are indicated by dash lines. G. Ren and L. Zhang: Summary of Holocene Pollen Data 677

FIG. 7. Isopolls for 8000 yr B.P. The values shown on the map represent the pollen percentages. Regions with sparse data are indicated by dash lines. 678 Quaternary Science Reviews: Volume 17

FIG. 8. Isopolls for 6000 yr B.P. The values shown on the map represent the pollen percentages. Regions with sparse data are indicated by dash lines. G. Ren and L. Zhang: Summary of Holocene Pollen Data 679

FIG. 9. Isopolls for 4000 yr B.P. The values shown on the map represent the pollen percentages. Regions with sparse data are indicated by dash lines. 680 Quaternary Science Reviews: Volume 17

FIG. 10. Isopolls for 2000 yr B.P. The values shown on the map represent the pollen percentages. Regions with sparse data are indicated by dash lines. G. Ren and L. Zhang: Summary of Holocene Pollen Data 681

FIG. 11. Isopolls for 800 yr B.P. The values shown on the map represent the pollen percentages. Regions with sparse data are indicated by dash lines. 682 Quaternary Science Reviews: Volume 17

FIG. 12. Isopolls for 0 yr B.P. The values shown on the map represent the pollen percentages. Regions with sparse data are indicated by dash lines. G. Ren and L. Zhang: Summary of Holocene Pollen Data 683

FIG. 13. Isochrones for the selected pollen taxa in Northeast China. The values shown on the map represent thousands years before present. Specific isopolls are set at 40% for Arboreal pollen, 30% for Artemisia, 20% for Pinus, 10% for Betula, and 5% for the other mapped taxa. 684 Quaternary Science Reviews: Volume 17 at a few sites are representative. The forest extent was koraiensis probably increased. But in the south, the small compared to any subsequent time, and steppe main portion of the change may come from the in- attained its maximum development in the plains and creases of Pinus densiflora and Pinus tabuleaformis. The pediments. Forests were restricted to the eastern other big change was the decline of forests on the mountains and hills, in which Betula was the dominant Liaodong Peninsula and in Hebei Province. Popula- component, followed in abundance by Picea, Abies, tions of major tree taxa started to decrease, though ºlmus and Quercus trees. Picea and Abies trees were the most significant decline occurred for the deciduous more common in the north, while ºlmus and Quercus tree populations including Quercus and ºlmus. Pollen trees occurred more frequently in the south. Pinus trees data from representative sites indicate that the above probably grew in scattered patches, as suggested by two changes actually started around 5 ka, a little Pinus pollen percentages that did not exceed 15% at earlier than 4 ka, and they continued during the late any site. Holocene. In the forests, trees were relatively sparse because the pollen accumulation rate remained low (Sun and Late Holocene mixed forest period Yuan, 1990; Yaun and Sun, 1990; Liu, 1989). Shrub Betula populations might have been large at least in The decline in the southern forest that began be- the north (Xia, 1988; Xia, pers. comm.). At the begin- tween 6 and 4 ka persisted throughout the late Holo- ning of Holocene, the southern hill regions and the cene. As a result, the abundance of major tree taxa lower Changbai Mts. were probably covered by an decreased on the Liaodong Peninsula and in the north- open forest, and an even more open forest environment ern part of Hebei Province, and the area of the forest prevailed on the Sanjiang Plain. decline expanded to the central regions, for instance, to At the end of this time period, ºlmus and Quercus the southeastern Horqin Sandland by 3 ka, and to populations were spreading at expense of Betula and a few sites in the Changbai Mts. by 2 to 1 ka. Picea/Abies. Quercus mongolica was probably the In the northern regions, including the Xiaoxingan dominant species of Quercus in the north, but it was Range, Sanjiang Plain, and some sites in the Changbai probably less abundant than Quercus liaotongica in the Mts., no long term forest decline ever occurred. The south. The extensive central plains were still covered by forests of these regions became denser than before, steppe that mainly consisted of Artemisia, Che- mainly because the population of Pinus koraiensis in- nopodiceae and Gramineae as prolific pollen pro- creased in abundance. The major deciduous trees may ducers. not have decreased as much as shown by the pollen percentages that, to a large extent, were affected by the increased pollen deposition from Pinus populations. Mid-Holocene deciduous tree period The tree taxa that declined in the south included not The deciduous forest developed in the eastern wet merely deciduous taxa like Quercus and ºlmus, that regions around 6 ka. Some deciduous trees that are had declined at 4 ka, but also Pinus and Betula. Herbs, common today dominated the forests. Quercus and especially Artemisia and Chenopodiceae, increased ºlmus, of course, were the most important tree taxa. their abundance and coverage during the time of the They grew in large populations in the eastern regions forest decline and at some sites even dominated the and northern Hebei Province. Quercus populations landscapes that were originally forested. were probably much more abundant than ºlmus populations in the south, especially on the Liaodong DISCUSSION Peninsula, though the opposite was true on the San- jiang Plain in the north. The other deciduous trees taxa The isopolls and isochrones prepared from the avail- also became more common. able pollen data summarize and organize many of the On the other hand, abundances of Betula, Picea and spatial and temporal variations evident in the inter- Abies populations decreased across much of the east- pretations of the data from single sites. The pollen ern forests. Pinus populations remained sparse, though maps have an exceptional advantage over the site ana- some evidence that their abundance began to rise on lyses. The standardized data and maps allow correla- the Liaodong Peninsula. In fact, Pinus populations tion among different sites and investigation of the were probably least common between 9 and 6 ka. macro-spatial patterns in vegetational distribution and In the east of the Songnen and Liaohe Plains, the change. The mapped patterns are key to interpreting ecotone between forest and steppe shift westwards the potential causes of the past vegetation changes in (Fig. 13), and the extent of the central steppe shrank. Northeast China. However, much of the plains were still occupied by various herbs, not by an open forest or woodland. Forest decline since 5 ka By 4 ka, two of the big changes had started. One was the large expansion of Pinus (Sun and Weng, 1992), and Interpretation of the fossil pollen maps produced an to a smaller degree, Picea/Abies, populations within unexpected discovery that the forest declined and that the forest regions between 6 and 4 ka. In the Changbai the belt of decline shifted northwards during the last Mts. and the regions to the north, populations of Pinus 5000 years. Around 5 ka, the forests on the Liaodong G. Ren and L. Zhang: Summary of Holocene Pollen Data 685

Peninsula and in Hebei Province began to decline. crease (Ting, 1989, Zhao and Xie, 1988), leading to After that, the forest decline in the southern regions a further deforestation. At the same time, the area of continued, and the decline extended to many sites of settlement and agriculture moved slowly from south to the central regions during the last 3000 years. These north, causing the belt in which forests declined for first include the southeast of the Horqin Sandland at about time to shift northwards as well. In the northern re- 3 ka and the western Changbai Mts. about 1 ka. In the gions such as the Xiaoxingan Range and Sanjiang northern regions, including the Xiaoxingan Range and Plain, the arrival of agriculture was so late that the Sanjiang Plain, no long-term trend in forest decline forest decline has not yet been identified in the coarsely ever appeared. sampled pollen records and resultant pollen maps. The spatial and temporal patterns of the forest change strongly resist the explanation of climate cha- Climatic implications nges. Within such an area like Northeast China, no reliable evidence has ever been found to show that the Fossil pollen indicate the natural changes in vegeta- climate change on the century to millennium time scale tion as well as climate, and the Holocene pollen re- could propagate in any horizontal direction. Although cords from the Liaodong Peninsula have been widely the decrease in tree abundance mainly resulted from held as the classical ones for reconstructing the climate such deciduous trees as Quercus, ºlmus and Juglans on for the last 10,000 years (Chen et al., 1977; Zhou et al., the Liaodong Peninsula around 4 ka, Pinus accounted 1984; Kong et al., 1982; Du et al., 1989; Li and Liang, for an increasing percentage of the decline soon after 1985; Xia, 1988; Shi and Kong, 1992). According to the that. In the Changbai Mts., pollen accumulation rate prevailing reconstruction, the Holocene climate of data from a few sites indicate an almost simultaneous and Northeast China was broadly decrease in the abundance of the main trees during the divided into three stages, which were designated as last 3000 years. The unselectiveness of the decline of a cold-dry period (10—7.5 ka), a warm-wet period trees in the closed forests is rather difficult to explain (7.5—2.5 ka) and cool-dry period (2.5—0 ka). These peri- by climate changes. ods were correlated with the international classical According to the historic records and the archae- Nordic climate periods (Birks and Birks, 1980; Bradley, ological evidence, on the other hand, human popula- 1985). tion density and agricultural activity intensity were During the last decade, another group of scientists larger in the southern regions than in the central and argued, on the basis of some pollen and the other proxy northern ones of the last 5000 years (Tan, 1982; Zhu, data, that Holocene could be divided into warm-wet 1991; Tong, 1989; Tian, 1993; Ren, 1994). In fact, when early stage and cool-dry late stage, with 5 ka as a divid- agriculture was widely practiced by the early settlers on ing point (An and Liu, 1987; An et al., 1990, 1993; the Liaodong Peninsula and in Hebei Province at 5 ka, Wang and Feng, 1991). Although this suggestion is in only a few groups of nomads and hunters cruised on agreement with the model simulations and the changes the vast central and northern regions (Zhu, 1991; He, of earth’s orbital parameters (COHMAP Members, 1991; Jilin Archaeological Institute, 1991; Heilongjiang 1988; Kutzbach and Guetter, 1984; Kutzbach 1989), it Archaeological Institute, 1991; Zhao and Xie, 1988). By is not supported by the lack of forest in the early 3 ka, however, the agriculture may have shifted north Holocene in Northeast China, which may indicate to the southeastern Horqin Sandland and the neigh- drier conditions then than in the mid-to-late Holocene. boring regions, where the earliest cultivation relics, It is also not supported by many of the mid-to-late named as ‘‘Gaotaishan Culture’’, were frequently dis- Holocene pollen time series that show some asyn- covered in the last decade (Xin, 1988; Tian, 1988; chronous variations with each other and with those Zhang and Xu, 1991; Liaoning Archaeological Insti- used as evidence by this group of Chinese scientists. tute 1989). Afterwards, the ancient farmers might One of the largest differences between the above two settled in the central plains and in some valleys of the sets of climate reconstructions is for the beginning ages western Changbai Mts. between 2 and 1 ka BP (Jilin for the ‘cool-dry’ late Holocene, defined mainly on the Archaeological Institute, 1991; Heilongjiang Archae- basis of the decline of tree pollen. However, as men- ological Institute, 1991). In the high north, including tioned above, because human activities may have been the Xiaoxingan Range and Sanjiang Plain, significant the main cause of the decline in tree abundance, the exploitation and settlement began only after the found- pollen data and the reconstructed vegetation are prob- ing of the People’s Republic of China. lematic for use in indicating climatic change after, e.g. Human activities instead of the climate changes may 5 ka on the Liaodong Peninsula and in Hebei Prov- therefore have led to the late Holocene forest decline ince. The sharp divergence of the interpretations may that the patterns on the pollen maps show. Early be- be no more than an indirect reflection of the differences tween 6 and 4 ka, the Liaodong Peninsula and Hebei in the beginning dates for human modification of the Province were more densely populated, and agricul- natural environment. ture there was well developed. Human deforestation, Some pollen records, however, were free of human therefore, led to the forest decline in these regions disturbance and can be used to reconstruct the climate (Goudie, 1981; Whitlow, 1987). After 4 ka, human history for Northeast China. In the northern most part population in the southern regions continued to in- of the study area, a vast territory remained unsettled 686 Quaternary Science Reviews: Volume 17 before 1950. The fossil pollen data from the Xiaoxin- Caixing, Qin Yanfei, Bi Jun and Wang Jue. Special gan Range and the Shanjiang Plain, and also some thanks are due to T. Webb for his invaluable comments sites in the western Changbai Mts., can be used to and editing of the manuscript. This work is supported indicate climate changes not only for the early and by the National Key Basic Research Programme of middle Holocene, but also for the last few millennia. China, No. 27. 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