The Surface Pollen of the Woodland–Steppe Ecotone in Southeastern

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ELSEVIER Review of Palaeobotany and Palynology 105 (1999) 237±250

The surface pollen of the woodland±steppe ecotone in southeastern Inner Mongolia, China

Hongyan Liu a,b,Ł, Haiting Cui a, Richard Pott b, Martin Speier b a Department of Geography, Peking University, Beijing 100871, PR China b Institut fuÈr Geobotanik der UniversitaÈt Hannover, Nienburger Strasse 17, D-30167 Hannover, Germany Received 28 July 1998; revised version received 4 November 1998; accepted 9 November 1998

Abstract

Surface pollen deposition in a modern woodland±steppe ecotone in the southeastern edge of the Inner Mongolia Plateau, situated between the deciduous broadleaved forest zone and the steppe zone in Asia, has been investigated. The following pollen and spore groups can be classi®ed according to the relationship between surface pollen and plant community: (1) pollen group representing the whole study region: Artemisia, Betula, Chenopodiaceae, Polygonaceae and Plantago; (2) pollen group representing a vegetation zone: Pinus; (3) pollen and spore group representing a certain community type: Quercus, Picea, Ostryopsis, Corylus and spores; and (4) pollen group having a low representation: Ulmus, Gramineae, Leguminosae and Compositae. According to the pollen composition in each vegetation zone, four vegetation zones of the investigated area could be characterized by the following combinations of pollen types: (1) woodland zone: Betula±Pinus±Artemisia; (2) woodland±grassland zone: Betula±Artemisia; (3) woodland±steppe zone: Artemisia±Betula± Chenopodiaceae; and (4) steppe zone: Artemisia±Chenopodiaceae. Some ratios of various pollen types have been proved to be better at indicating the vegetation gradient than single pollen types. For example, C=A (Chenopodiaceae=Artemisia), A=B(Artemisia=Betula)andAP=NAP (Arboreal pollen=Non-Arboreal Pollen) show distinctly the gradient from the woodland zone to the steppe zone. This investigation has a future application for the reconstruction of Holocene vegetation and climate history in the investigated area. Various pollen combinations can be used as `indicator units' for the diagnosis of vegetation types from fossil pollen analysis. Some problems of the interpolation of vegetation from fossil pollen data in the semi-arid area have also been discussed. High Artemisia pollen value do not indicate an Artemisia-dominated steppe vegetation due to the under-representation of Gramineae pollen. Plantago pollen in semi-arid and arid area can hardly be regarded as an indicator of human interference.  1999 Elsevier Science B.V. All rights reserved.

Keywords: woodland±steppe; ecotone; Inner Mongolia China; recent pollen rain; semi-arid area

1. Introduction teristics uniquely de®ned by space and time scales and by the strength of interactions between adjacent An ecotone is ªa zone of transition between ad- ecological systemsº (Holland, 1988). An ecotone is jacent ecological systems, having a set of charac- generally regarded as a sensitive region for climate and vegetation change (Di Castri, 1988). The analy- Ł Corresponding author. Fax: C86-10-62751187; E-mail: sis of the vegetation history of the woodland±steppe [email protected] ecotone is important for regional and global scale cli-

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matic change. The examination of the present surface in the valleys and lower on the high peaks of the pollen composition is an important basis for the in- Jibei Mountains. There is, however, no obvious trend terpretation of vegetation history. Some researchers inside the Plateau. have considered the distribution of the surface pollen The soil types of the investigated area vary owing in different vegetation zones of different areas, for to the great climatic gradients and different geomor- example, Central Europe (Schneider, 1984), the Mid- phologic types. Brown soil, grey forest soil, light dle East (El-Moslimany, 1990), the Near East (Van chernozem and chestnut soil dominate in a sequence Zeist and Bottema, 1991), North America (Liu and from southeast to northwest. Sandy soil prevails Lam, 1985), East Europe (Monoszon, see Xia, 1990) in the Hunshandak sandy land. Other azonal soil and North China (Wang, 1995). But very little has types, for example subalpine meadow soil, occur been done on the distribution of surface pollen within locally (Editorial Committee of serial maps of re- an ecotone between two vegetation zones. sources in Inner Mongolia, 1988; Soil Survey Of®ce The woodland±steppe ecotone in the southeast- of Chifeng, 1989). ern edge of the Inner Mongolia Plateau has been selected as the area for this research. The rela- 2.2. Vegetation of the investigated area tionship between pollen types and plant community types is analyzed in detail. Surface pollen combi- The vegetation of the investigated area is dis- nation for various vegetation zones are presented. cussed by Liu (1998) in detail. Nomenclature follows For the purpose of interpolating vegetation from fos- Editorial Committee of ªFlora of Inner Mongoliaº sil pollen assemblage, some problems of vegetation (1994). reconstruction in semi-arid area are also discussed. From the southeast to the northwest, four vege- This investigation can provide a basis for the recon- tation zones can be determined: the woodland zone, struction of Holocene vegetation and climate history the woodland±grassland zone, the woodland±steppe in the investigated area as well as those with similar zone and the steppe zone (see Fig. 1). conditions. In the woodland zone (Fig. 2), the most frequent community types are Quercus mongolica-woodland, Betula dahurica-woodland and Pinus tabulae- 2. Investigated area and methods formis-woodland. The woodland patches are usually small owing to their cultivation in the past 100 years. 2.1. General features of the investigated area The woodland±grassland zone is connected with the woodland zone and occurs at an altitude of more The geographical coordinates of the investigated than 1450 m. Its vegetation is a mosaic of Valeriana area are 42±43.75ëN, 115.75±117.75ëE (Fig. 1). The of®cinalis±Betula platyphylla-woodland, Picea mey- main part of it is situated in the southeastern edge of eri-woodland and Polygonum viviparum-grassland the Inner Mongolia Plateau at an altitude of 1100± (Fig. 3). Fens appear in the lowlands. 1400 m. The Xilinguole volcanic platform is situated The woodland±steppe zone occurs at an alti- in the north and the Hunshandak sandy land occurs in tude of below 1450 m and is connected with the the west of the investigated area. In the investigated steppe zone. In this zone, the steppe communities area, the Inner Mongolia Plateau is bordered by the form a background, while the woodlands occur in Greater Khingan Mountains in the northeast and the the form of island (Fig. 4). Ostryopsis davidiana± Jibei Mountains in the south and southeast. The Betula platyphylla-woodland and Populus davidi- altitude of the valleys reaches 500±700 m there, ana-woodland both occur in this zone. The steppe while the highest mountain exceeds 2000 m. communities are different in various topological po- The annual precipitation decreases sharply from sitions. The footslopes are occupied by Leymus chi- the southeast to the northwest. In the valleys of the nensis-steppe. The lower parts of the steep slopes are Jibei Mountains, the annual precipitation reaches 450 usually occupied by Stipa baicalensis-steppe, while mm, but in the Xilinguole platform, it decreases to their upper parts are occupied by woodland commu- 320±350 mm. The temperatures are relatively higher nities. ______中国科技论文在线 http:\\www.paper.edu.cn

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Fig. 1. Map of the investigated area. (A) Geomorphologic features and vegetation zones (1 D woodland zone; 2 D woodland±grassland zone; 3 D woodland±steppe zone; 4 D steppe zone. (B) Location of the investigated area in the vegetation map of China (after Wu, 1983). ______中国科技论文在线 http:\\www.paper.edu.cn

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Fig. 2. The woodland in the Dajuzi region (Jibei Mountains) represents a view of the woodland area in the Inner Mongolia Plateau characterized by Betula platyphylla, Picea meyeri and Larix principis±ruprechtii.

In the steppe zone, Leymus chinensis-steppe oc- and acetolysis after cooking and sieving (Moore et curs at an altitude of less than 1300 m, other- al., 1991). Four hundred to six hundred pollen grains wise Stipa baicalensis-steppe prevails. The dunes are and spores were counted for each sample under dominated by Ulmus pumila-open woodland (Fig. 5). microscope. The percentage of each type of total pollen and spores in a sample has been calculated. 2.3. Study method Samples having scarce pollen were omitted from the calculation. Seventy one sites were selected for ®eld survey. The typical woodland, shrub, grassland and steppe communities of each site have been examined by 3. The surface pollen composition of different the phytosociological method comprehensively de- community types scribed by Mueller-Dombois and Ellenberg (1974). One hundred and forty eight plots have been made 3.1. Woodland and shrub communities totally. The releveÂs are described by Liu (1998) in detail. Fig. 6 shows the comparison of the main sur- Surface pollen were sampled for each vegeta- face pollen types from different sites in woodlands tional plot. Mosses and the upper part of the soil and shrubs. The relationship between surface pollen were collected regularly. In this text the surface shrublands and vegetation communities can be sum- pollen samples have the same serial numbers as the marized as follows: vegetation samples from which they were taken. (1) Picea: Picea pollen mainly occurs in the All pollen samples were treated with heavy liquid Picea meyeri-woodland with values of up to 90%. ______中国科技论文在线 http:\\www.paper.edu.cn

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Fig. 3. A view of the landscape in the woodland±grassland zone near Baiyinaobao (Greater Khingan Mountains). In the foreground small patches of Picea meyeri-woodlands can be seen. In background the chains of mountains of the Greater Khingan Mountain range can be recognized.

A few plots of Picea meyeri-woodland have a very below 30%. The pine pollen values are below 10% at low spruce pollen values, which is possibly due to the edge of the steppe zone (Darhan region). In gen- the dif®cult regeneration of spruce because Picea eral, pine pollen can be dispersed widely but shows meyeri-woodland is a relict vegetation type in the a different representation pattern in the investigated investigated area. Some plots with Betula platyphylla area. in the tree layer have relatively low spruce pollen (3) Quercus: This pollen type mainly occurs in the values due to the high birch pollen production. In Quercus mongolica±woodland with values of less other vegetation types, the spruce pollen values are than 10%. Some Quercus mongolica±woodland plots extremely low. Therefore, we can infer that spruce (e.g. No. 61) have no oak pollen, which is possibly woodland exist locally when spruce pollen values due to strong human interference. Quercus pollen exceed 10%. also occur in some plots of Populus davidiana± (2) Pinus:HighvaluesofPinus pollen are found woodland and Betula platyphylla-woodland, for ex- in the Pinus tabulaeformis-woodland as well as in the ample, No. 113 and No. 119, which is ascribed to the Quercus mongolica-woodland. It has values of below local occurrence of Quercus mongolica trees at these 10% in Betula dahurica-woodland and Betula platy- plots. All this evidence shows that the oak pollen is phylla-woodland and rarely occurs in the Populus da- poorly dispersed and can indicate the local existence vidiana-woodland and Picea meyeri-woodland. Pine of oak trees. pollen values vary widely in the Pinus tabulae- (4) Betula: Birch pollen is dominant in the formis-woodland. In the woodland zone, the values total investigated area. Its values can exceed of pine pollen exceed 70%. In the ecotone they are 40% in Betula platyphylla-woodland and Betula ______中国科技论文在线 http:\\www.paper.edu.cn

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Fig. 4. A view of the landscape in the woodland±steppe zone in the Haoluku region. In the front: Juniperus rigida.

dahurica-woodland. In some plots of Picea mey- seen in Quercus mongolica-woodland and Betula eri-woodland, the values of birch pollen are rela- platyphylla-woodland, which has a relation with tively higher because of the occurrence of Betula the higher Ostryopsis and Corylus coverage in platyphylla in the tree layer. In Populus davidi- these plots or the Ostryopsis davidiana-shrubland ana-woodland, the values differ. When Betula platy- nearby. phylla occurs in the tree layer, the value of birch (7) Artemisia: The following Artemisia species pollen can reach 70%. If no birch tree occurs, the grow in the woodland and shrub communities of the values of birch pollen range between 10 and 50%. investigated area: Artemisia gmelinii, A. mongolica, Two factors can explain this phenomenon. First, A. mandshurica, A. laciniata, A. capillaris, A. laven- the poplar pollen production is low and rarely well dulaefolia, A. eriopoda, A. palustris, A. integrifolia preserved, and second, the birch pollen is widely dis- and A. argyi. Some plots near the steppe zone also persed. From the results of the investigated area we contain A. halodendron, A. scorparlia, A. sieversiana can conclude that birch woodland may exist locally and A. frigida. The frequencies of Artemisia in most only when the Betula pollen values are over 30%. plots are less than 5%. As for the pollen values, (5) Ulmus: This pollen type only occurs in a few Artemisia is a prevailing pollen type and occurs in plots with values of below 2%. It does not show a all vegetation types in the investigated area. How- clear indication of a vegetation type. ever, the pollen values of Artemisia have no correla- (6) Ostryopsis and Corylus: Pollen grains of Os- tion with their vegetation coverages. It is relatively tryopsis and Corylus are dif®cult to distinguish. more frequent in the Populus davidiana-woodland This pollen type mainly occurs in the Ostryop- because of the low dispersion of poplar pollen. In sis davidiana-shrubland. A few peaks can also be the birch woodland it differs from 5 to 40%. In ______中国科技论文在线 http:\\www.paper.edu.cn

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Fig. 5. Steppe community dominated by Stipa krylovii (Darhan region). Ulmus pumila-woodlands characterize natural dune formations.

Pinus tabulaeformis-woodland and Ostryopsis da- and the pollen values of Gramineae. It could be vidiana-shrubland it exceeds 50%. In conclusion, due to the high production of Betula and Artemisia Artemisia pollen has a high over-representation in pollen that in¯uences the total composition of the the woodland and shrub communities of the whole pollen samples. investigated area. (10) Cyperaceae: The coverage values of Cyper- (8) Chenopodiaceae: The Chenopodiaceae species aceae in the woodland and shrub communities are which occur in the woodland and shrub communi- relatively high, sometimes they can reach 20%. Some ties of the investigated area include Chenopodium species, for example Carex lanceolata, dominate in album, C. urbicum subsp. sinicum, C. glaucum, C. the herb layer of most woodland communities. How- aristatum, C. acuminatum and Salsola collina. The ever, Cyperaceae pollen occur only in a few plots occurrence of these species is in¯uenced by human with values usually below 2%. Thus, Cyperaceae has disturbance. They have a coverage of less than 1% a low pollen representation, which is probably due to in the vegetation and occur only in one third of the their lower pollen production. woodland plots. However, most plots contain pollen (11) Leguminosae: The coverage of Leguminosae grains of Chenopodiaceae, the maximum value is may reach 10%, but its pollen values are usually approximately 30%, indicating that Chenopodiaceae below 5%. No obvious correlation has been found pollen also has a high over-representation in the between them. whole investigated area. (12) Polygonaceae: Only one third of the plots (9) Gramineae: The coverage of Gramineae in contain Polygonaceae species with a coverage of some vegetation plots can reach 10%. But the values below 1%. But most plots contain pollen grains of of Gramineae pollen in most plots are below 5%. No Polygonaceae, the maximum exceeds 5%. Therefore obvious correlation is found between the coverage Polygonaceae pollen is over-represented. ______中国科技论文在线 http:\\www.paper.edu.cn

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Fig. 6. Surface pollen and spore of woodland and shrub communities. ______中国科技论文在线 http:\\www.paper.edu.cn

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(13) Compositae: Except for Artemisia species, zone with values of 30±50%. Its values can reach Compositae species occurring in woodland and 20% in the woodland±steppe zone and steppe zone. shrub communities with a high constancy include The high values and broad distribution also indicate Dendranthema chanetii, D. zawadskii, Taraxacum its over-representation. mongolicum and Saussurea spp. Saussurea ussurien- (5) Ulmus: This pollen type only occurs in the sis is the dominant species of some plots of the Ulmus pumila-open woodland or the adjacent region birch woodland in the investigated area. The veg- of the Hunshandak sandy land with a value of below etation coverage of Compositae species (excluding 2%. But some plots of Ulmus pumila-open woodland Artemisia) sometimes reach 10%. But the pollen val- do not show any elm pollen, which may be a result ues are usually less than 2%. Compositae pollen is of the low pollen production of elm. therefore under-represented. (6) Corylus and Ostryopsis: Their pollen values in (14) Plantago: Plantago species and their pollen some plots near the Ostryopsis davidiana±shrubland grains occur only in a few plots. are 15±25%, in other plots they are usually below 5%. (15) Spores: Only 1=5 of the plots contain ferns (7) Artemisia: Artemisia is the dominant pollen and usually with a vegetation coverage of less than type of all the plots. Its values are usually over 1%. The spores rarely occur in the woodland and 40%. Artemisia species in the community is mainly shrub communities of the investigated area with a Artemisia frigida, although some plots also contain values of less than 1%. A. capillaris and A. halodendron. The coverage of Artemisia in communities varies from 5 to 25%. In 3.2. Grassland, steppe and open woodland relation to the results from the woodland and shrub communities communities, Artemisia pollen is over-represented in this area. Fig. 7 shows the surface pollen composition of (8) Chenopodiaceae: This pollen type occurs in grassland, steppe and open woodland communities. all the plots and has higher values in the steppe The relationships between the surface pollen and the zone. Chenopodiaceae species in the communities communities are summarized as follows: are similar to those in the woodland and shrub com- (1) Picea: The values of spruce pollen in plots munities. Only half of the vegetation plots contain from grassland, steppe and open woodland commu- Chenopodiaceae species with a coverage of below nities are usually below 5%. The plots with high 5%, but almost all the plots have Chenopodiaceae values of spruce pollen are usually near the Picea pollen, indicating that Chenopodiaceae pollen has an meyeri-woodland. This result is similar to that of the over-representation. woodland and shrub communities. (9) Gramineae: The coverage of Gramineae (2) Pinus: The plots inside or near the woodland species is commonly between 5 and 25%. Gramineae zone have high pine pollen values, the maximum species are dominant in the Stipa baicalensis steppe reaches 50%. In other plots they are usually below and are an important part of the Leymus chinensis 10%. This ®nding also shows the spatial tendency of steppe, but their pollen values are usually below 5% pine pollen from woodland zone to steppe zone in in almost all the plots. Therefore it is concluded that the investigated area. the pollen of Gramineae has a low representation. (3) Quercus: This pollen type usually occurs in (10) Cyperaceae: This pollen type occurs in about the grassland with a value of less than 2%. In steppe 40% of the plots. But Cyperaceae species occur in and open woodland it rarely occurs. The plot No. 120 almost all the plots of grassland, steppe and open has a maximum value of 5% because it is located woodland communities with a coverage of 1±5%, near the Populus davidiana woodland with Quercus which indicates the low representation of Cyperaceae mongolica in the tree layer. The phenomenon also pollen. supports the result from the woodland and shrub (11) Leguminosae: Leguminosae pollen have high communities. values between 5 and 15% in the grassland com- (4) Betula: The birch pollen occurs in all the communities. However, in steppe and open woodland munity types, especially in the woodland±grassland communities they occur only sporadically. Legumi- ______中国科技论文在线 http:\\www.paper.edu.cn

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Fig. 7. Surface pollen and spore of grassland, steppe and open woodland communities. ______中国科技论文在线 http:\\www.paper.edu.cn

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nosae are pollinated by insects and have a low pollen 4. The correlation of the surface pollen with the production. The relatively high percentage of Legu- vegetation zones minosae pollen in the grassland communities may be caused by the relatively low percentage of Artemisia 4.1. The distribution of major pollen types in pollen in them. different vegetation zones (12) Polygonaceae: Polygonaceae pollen occur in most plots. The relatively high pollen values of Fig. 8 shows the average values of major surface Polygonaceae in the woodland±grassland zone may pollen types in the different vegetation zones. The be due to the same reason as for the Leguminosae. surface pollen composition of each vegetation zone (13) Compositae (except Artemisia): Compositae can be summarized as follows: species which occur in grassland, steppe and open (1) Woodland zone: Pinus pollen accounts for woodland include e.g. Heteropappus altaicus var. 25% of the total pollen sum. The average pollen millefolius, Leontopodium longifolium, Saussurea value of Betula is 36% and Artemisia 21%. The amara, Senecio campester. They are distributed in other pollen types are relatively rare. So the pollen almost all the plots, but their pollen occurs in only composition of the woodland zone can thus be sum- half of the plots, indicating a low representation. marized as a Betula±Pinus±Artemisia combination. (14) Plantago: Only eight plots of grassland, (2) Woodland±grassland zone: Betula has the steppe and open woodland in the investigated area largest portion with 40%. Artemisia takes the sec- contain Plantago species. Plantago asiatica and ond position with a portion of 36%. The values of Plantago depressa which are introduced by human Pinus pollen is only 5%. The pollen composition interference occur in the Ulmus pumila open wood- of this zone can be regarded as a Betula±Artemisia land. Plantago major occurs in grassland and fen. combination. In contrast, half of the plots contain Plantago pollen (3) Woodland±steppe zone: The average pollen grains, indicating its over-representation. value for Artemisia reaches 50%. The value of Be- (15) Spores: Almost no fern occur in the grassland tula is only 16%. In comparison with the woodland± and steppe communities. The spores only occur in a grassland zone, the pollen value of Chenopodiaceae few plots with values of less than 1%. is higher with a frequency from 6 to 15%. An

Fig. 8. Surface pollen composition in different vegetation zones. ______中国科技论文在线 http:\\www.paper.edu.cn

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Fig. 9. Some ratios of surface pollen in different vegetational zones.

Artemisia±Betula±Chenopodiaceae pollen combina- 5. Conclusions and discussion tion is typical in this zone. (4) Steppe zone: The Artemisia pollen accounts 5.1. The relationship between the surface pollen and for 47% of the total pollen sum. Chenopodiaceae vegetation have a value of 25%. The pollen values of other types are low. So an Artemisia±Chenopodiaceae pollen The relationship between surface pollen and vege- combination is evident. tation can be affected by various factors, for example pollen production, the process of blooming and the 4.2. The change of the ratios of the surface pollen means of dispersion (insect or wind). These param- with the vegetation zones eters make it almost impossible to reconstruct the community composition according to pollen (e.g. Fig. 9 shows the change of the ratio A=B(Artemi- Li, 1993; Speier, 1994). In spite of these dif®cul- sia=Betula), C=A (Chenopodiaceae=Artemisia), and ties, pollen analysis is an important method for re- AP=NAP (Arboreal Pollen=Non-Arboreal Pollen) constructing large vegetational units, for example, with a change in the vegetation zone. These three woodland, grassland, meadow and so on, by us- ratios shows an obvious spatial gradient. From the ing `indicator units' or `pollen representation types' woodland zone to the steppe zone, the A=BandC=A (Behre, 1981; Pott, 1985, 1986). ratios increase markedly. This gradient has a relation The important pollen types in the investigated with the spatial distribution of the surface pollen area include: Picea, Pinus, Quercus, Betula, Ul- of Artemisia, Chenopodiaceae and Betula. Although mus, Ostryopsis, Corylus, Artemisia, Chenopodi- the maximum value of Artemisia pollen occurs in aceae, Gramineae, Cyperaceae, Leguminosae, Polyg- the woodland±steppe zone and that of Betula pollen onaceae, Compositae and Plantago. The following occurs in the woodland±grassland zone, the A=Band pollen and spore groups can be classi®ed accord- C=A ratio both have a maximum in the steppe zone ing to the relationship between surface pollen and and a minimum in the woodland zone. From the community: (1) pollen group representing the whole woodland zone to the steppe zone, the AP=NAP ratio region: Artemisia, Betula, Chenopodiaceae, Polyg- decreases, which corresponds with the change of the onaceae and Plantago; (2) pollen group represent- vegetation. ing a vegetation zone: Pinus; (3) pollen and spore group representing a certain community type: Quer- cus, Picea, Ostryopsis and Corylus, spores; and (4) pollen group having a low representation: Ulmus, Gramineae, Leguminosae and Compositae. ______中国科技论文在线 http:\\www.paper.edu.cn

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Researches in the Middle East and the Near East the investigated area. So a higher Artemisia pollen (El-Moslimany, 1990; Van Zeist and Bottema, 1991) value can indicate the existence of steppe, but the have shown that pollen of Chenopodiaceae and Plan- dominant species is not certain. tago dominate in the arid zone and the pollen of Some Plantago species are commonly regarded as Artemisia dominates in the semi-arid zone. Research indicators for human disturbance. But in the inves- in the summer-green broadleaved forest zone demon- tigated area, Plantago species are scarce in commu- strates the over-representation of Betula pollen (Yao, nities under different human disturbance. El-Mosli- 1989; Li et al., 1993; Yu and Liu, 1997). In the inves- many (1990) has found that Plantago is a dominant tigated area, Artemisia, Betula and Chenopodiaceae pollen type in the arid zone of the Middle East. The are the dominant pollen types, which coincides with over-representation of Plantago pollen in the study the result of the above researchers. The transition of region may be caused by Plantago pollen transported the ecotone determines that dominant pollen types of from the arid zone. In the study region, higher values different vegetation zones can coexist here. Climate of Chenopodiaceae pollen can indicate strong human and vegetation are both crucial factors for surface disturbance. pollen. In spite of the high coverage of Gramineae in part of the steppe communities, Artemisia pollen dominates the woodland±steppe zone and the steppe 5.2. Spatial distribution of surface pollen zone. Pine pollen can be well dispersed (e.g. Andersen, According to the pollen composition in each veg- 1970; Li, 1993; Li et al., 1993). In the investigated etation zone, four zones of the investigated area area, pine pollen has only a higher value in the can be characterized as: (1) woodland zone: Betula± woodland zone. In the ecotone and the steppe zone, Pinus±Artemisia pollen combination; (2) woodland± its values are low. Two reasons can be given to grassland zone: Betula±Artemisia pollen combina- explain this phenomenon. One reason is that the tion; (3) woodland±steppe zone: Artemisia±Betula± production of pine pollen is relatively low because Chenopodiaceae pollen combination; and (4) steppe the investigated area is at the northern limit of Pinus zone: Artemisia±Chenopodiaceae pollen combina- tabulaeformis-woodland. The second reason is that tion. These pollen compositions can be used as `in- the prevailing wind direction in the investigated area dicators' for the diagnosis of vegetation types from is from northwest to southeast, which is favourable fossil pollen assemblages. for the dispersal of Chenopodiaceae, Artemisia and Some ratios of different pollen types have been Plantago pollen from the steppe zone to the ecotone proved to be better at indicating the vegetation gra- and the woodland zone, but unfavorable for the dient than single pollen types. For example, C=A dispersal of pine pollen from the woodland zone to (Chenopodiaceae=Artemisia) is used as a mois- the ecotone and the steppe zone. ture indicator in the Middle East (El-Moslimany, Different opinions are given for Quercus pollen. 1990). In the investigated area, the highest Betula Van Zeist and Bottema (1991) found that the Quer- pollen percentage is in the woodland±grassland zone cus pollen is over-represented in Iran. Some Chinese rather than in the woodland zone, while the high- researchers (e.g. Li et al., 1993) found that Quercus est Artemisia pollen percentage is in the woodland± pollen probably represent its situation correctly in steppe zone rather than in the steppe zone. But C=A the community. In this research the results corre- increases from the woodland zone to the steppe zone, spond with the latter. This result may be caused by which coincides with the vegetation gradient. A=B different Quercus species in different regions of the (Artemisia=Betula)andAP=NAP (Arboreal Pollen= world and also the values of the other taxa. Non-Arboreal Pollen) also have a good gradient in Some researchers deduced that the vegetation accordance to the vegetation. These three ratios can was an Artemisia-dominated steppe when Artemisia also be used for the reconstruction of the past vege- pollen values are high. Due to the lower representa- tation of the investigated area. tion of the Gramineae pollen, Artemisia also has a higher value in the Gramineae-dominated steppe in ______中国科技论文在线 http:\\www.paper.edu.cn

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