www.elsevier.com/locate/revpalbo

Review of Palaeobotany and Pal

Temporal and spatial changes of vegetation cover on the

Chinese Loess Plateau through the last glacial cycle:

evidence from spore-pollen records

Hanchao Jiang*, Zhongli Ding

Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China

Received 5 November 2003; accepted 26 August 2004

Abstract

To reconstruct the palaeovegetation cover of the Chinese Loess Plateau over different time intervals of the last glacial–

interglacial cycle, the spore-pollen representation of loess-soil samples in six loess sections along a north–south transect is

analyzed and previously published pollen data reviewed. Results show that the temporal changes in the vegetation cover of the

Plateau are controlled essentially by the history of the East-Asian summer monsoon; there is a general pattern that the pollen

components indicating relatively humid climatic conditions are better represented during interglacial and interstadial periods

than during glacial and stadial periods. The herbaceous pollen concentrations show a general increase from south to north with

the decrease of monsoonal precipitation over the Loess Plateau. Through the last 130 ka or so, the main body of the Loess

Plateau has been covered by an Artemisia-dominated grassland vegetation, suggesting that future rehabilitation of the landscape

that aims for soil–water preservation on the Loess Plateau should focus on the planting of herbs.

D 2004 Published by Elsevier B.V.

Keywords: Loess deposits; pollen record; grassland; last glacial cycle; Chinese Loess Plateau

1. Introduction

On the Chinese Loess Plateau, continuous soil

erosion has been imposing a tremendous challenge to

the social and economic development of this relatively

impoverished region (Liu, 1985). Severe soil erosion

results mainly from the destruction of the vegetation

0034-6667/$ - see front matter D 2004 Published by Elsevier B.V.

doi:10.1016/j.revpalbo.2004.08.003

* Corresponding author. Tel.: +86 10 62007542; fax: +86 10

62032495.

E-mail address: hanchaojiang@mail.igcas.ac.cn (H. Jiang).

cover due to a long period of improper land use. To

mitigate this environmental issue, the Chinese govern-

ment currently strives to reverse this trend, by

reducing agricultural use of the land and by planting

trees and grasses. However, there is controversy over

the nature of the ecosystem that should be recon-

structed in this region. Based on the interpretations of

historical documents, one group of authors (Shi, 1981,

1991; Zhu, 1983, 1994) concluded that relatively

dense forests were present over a vast area of the

Loess Plateau before devastation by human activities

ynology 133 (2005) 23–37

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–3724

and accordingly recommended the planting of trees on

the Loess Plateau. However, another group of authors

(Liu, 1985; Liu et al., 1996; Sun et al., 1997; Xie et

al., 2002) insists that climatic conditions are only

suitable for grass growth in the Loess Plateau.

Consequently, systematic palaeovegetation studies

are urgently needed before a practical strategy of

vegetation reconstruction is implemented.

At present, the climate of the Loess Plateau is

essentially controlled by the East-Asian monsoon,

which leads to a clear northward decrease in annual

precipitation over the Plateau (Fig. 1). Because the

Loess Plateau is located relatively far from the Pacific

Ocean, an overall dry climatic condition and a steep

rainfall gradient are present. The Plateau straddles the

semi-humid, semi-arid and arid climatic zones. It is

Fig. 1. Location map of the Chinese Loess Plateau showing the study sectio

of the Plateau (mm). The inset map shows the locality of the Loess Plate

around and within the Loess Plateau are shown.

therefore likely that a zonation in the native vegetation

cover should have been present and changed over

different time intervals in response to monsoon

changes. Previous vegetation studies (Sun, 1989;

Sun and Zhao, 1991; Lu et al., 1995, 1996, 1999;

Sun et al., 1997) have mainly focused on the southern

and middle parts of the Loess Plateau, inhibiting a full

picture of native vegetation and its changes to be

obtained over the entire Plateau. In this study, we

carried out spore-pollen analyses of six loess sections

along a north–south transect of the Plateau and

combined the results with a compilation of the

previously published pollen data. The spatial and

temporal vegetation changes since the last interglacial

period provide a preliminarily reconstruction for the

whole Loess Plateau.

ns mentioned in the text and the annual average precipitation pattern

au in continental China. The deserts (dotted) and mountains (black)

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–37 25

2. Materials and methods

Six naturally exposed loess sections, located at

Jiyuan, Mubo, Qingyang, Ningxian, Binxian and

Lantian along a north–south transect, were selected

for study (Fig. 1). All of the sections are situated on

the dLoess YuanT—a broad, flat, high tableland. Table

1 provides some information on the environment of

individual sites. At present, both temperature and

precipitation show a southward increase; the mean

annual temperature increases from 7.9 to 13.0 8C and

the mean annual precipitation from 316.9 to 720 mm

at Jiyuan and Lantian, respectively (Table 1). Under

the influence of the East Asia summer monsoon,

climate seasonality (warm, humid summer and cold,

dry winter) is very evident on the Loess Plateau and

the rainfall of the three summer months (JJA) usually

accounts for 50–60% of the total annual precipitation,

leaving winter and spring extremely dry. Furthermore,

in the rainy summer, much of the rain falls in a few

rainstorms, leading to a relatively low effectiveness of

the rainwater for plant growth.

As the sections lie in the flat tablelands where dense

crops are planted, the modern natural vegetation is

hard to determine. Nevertheless, a general picture of

natural vegetation distributions over the Loess Plateau

has been proposed. According to data in Sun et al.

(1997), the transect straddles three natural vegetation

zones from south to north. Lantian and Binxian (Fig.

1) are located in the zone of warm-temperate forest,

which is dominated by broadleaved deciduous oaks

(mainly Quercus variabilis, Q. alina and Q. liaotun-

gensis) with some other deciduous taxa such as Betula,

Populus, Acer, Tilia and Ulmus. Ningxian is situated

in the zone of warm-temperate forest-steppe, which is

mainly composed of Bothriochloa ischaemum, Stipa

bungeana and Lespideza dahurica, with the forest taxa

Table 1

Geographical and stratigraphic information of the studied loess sections

Section Latitude

(8N)Longitude

(8E)Elevation

(m)

MAT

(8C)

Jiyuan 37.14 107.39 1730 7.9

Mubo 36.43 107.46 1430 8.5

Qingyang 36.1 107.83 1290 9.4

Ningxian 35.44 107.94 1200 8.8

Binxian 34.96 108.05 1225 11

Lantian 34.15 109.27 720 13

of Pinus tabulaeformis, Quercus and Betula occurring

only in shady valleys or on high mountains. Qingyang,

Mubo and Jiyuan occur in warm-temperate steppe,

which is characterized by Stipa bungeana, S. brevi-

flora, Artemisia girald and Caragana spp.

All the six sections have an S1–L1–S0 stratigraph-

ical sequence (Table 1) and are readily correlated in the

field, suggesting that the deposition of eolian materials

has never been interrupted by other natural processes

such as fluvial and ice erosion or deposition at the sites.

Soil S1, formed in the last interglacial period, has a

brownish or reddish color and an A–Bw–C or A–Bt–C

horizon sequence. It has a polygenetical characteristic,

showing a general decrease in thickness and pedogenic

development from north to south. The last glacial loess

unit, L1, has a yellowish color and a massive structure,

ranging in thickness from 5 m in the south of the

transect to 29.32 m in the north. In general, the L1 loess

unit can be subdivided into five horizons, named L1–1,

L1–2, L1–3, L1–4 and L1–5. L1–2 and L1–4 are weakly

developed soils, while the other three horizons are

typical loess (Ding et al., 1999, 2001). The Holocene

soil, S0, shows a dark color due to relatively high

organic matter accumulation during its formation.

In the field, we took samples at 2–5-cm intervals for

soils and at 5–10-cm intervals for loess beds. Grain size

has been analyzed for all the samples (Ding et al., 1998;

Yang, 2001; Yang and Ding, 2004). Results show that

grain size in the loess horizons is much coarser than in

soils for each section of the transect (Fig. 2).

Correlation of the grain size curves also indicates the

continuity of the loess records. Based on the grain-size

records, we selected three samples from S0, L1–1, L1–4,

L1–5 and S1 in each section. The samples from L1–1 and

L1–5 are located in the areas of largest grain size, while

the S0, L1–4 and S1 samples are from the areas of

smallest grain size (Fig. 2). In the transect, a total of 90

MAP

(mm)

Thickness

of S0 (m)

Thickness

of L1 (m)

Thickness

of S1 (m)

316.9 1.18 29.32 7.3

470 1.4 19.45 3.65

537 1.65 9.85 4

562 1.7 8.7 2.3

579 1.75 7.5 2.85

720 1.4 5 2.3

Fig. 2. Stratigraphic division and grain size records of the six loess sections along the transect. Squares indicate the sampling positions for pollen

analyses.

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–3726

samples were selected for pollen analysis. Although the

chronology of the loess sections has not been accurately

established, this sampling strategy should still assure

the use of roughly synchronously depositedmaterial for

each representative time interval along the transect.

A total of 80–90 g of sediment from each sample

were subjected to concentrated hydrofluoric acid

treatment for 24 h in order to dissolve silicate minerals

and hence reduce sample volume. After subsequent

treatment with hydrochloric acid and 5% caustic soda,

pollen grains were extracted by two changes of heavy

liquid (chromium iodide).

3. Temporal vegetation changes along the transect

Examination under the microscope revealed rela-

tively abundant pollen and spores in nearly half of the

samples but the other samples failed to provide a

satisfactory pollen sum. Of the total of 90 samples

analyzed, 46 containing 72–412 pollen and spore grains

are illustrated in the pollen diagram (Fig. 3). Previous

studies have shown that a paleoenvironmental infer-

ence based on a single sample from which only 50–100

grains are counted would be quite dubious. However, if

a sequence of some 5–10 samples in which such

numbers are counted, bears similar characteristics, the

conclusions are much more sound and could be relied

upon (Horowitz, 1992). Accordingly, more than 70

grains is regarded as an acceptable pollen count in this

study and the palynological sum is composed of all taxa

from regional dominants to local aquatics. In this study,

a total of 47 pollen and spore types were identified, with

the herbaceous pollen taxa being dominant in most of

the samples. Comparatively, the diversity and abun-

dance of arboreal pollen taxa are less important.

Although different floral elements have different

moisture demands, trees, shrubs, mesic herbs, ferns

and algae demand much more moisture than other

herbs. Accordingly, we here choose to use the

percentage of AP (arboreal pollen)+MH (mesic herba-

ceous pollen)+SP (spores) as an indicator of relatively

humid environments. In this study, we found 10 taxa

of mesic herbs, including Liliaceae, Gramineae,

Cruciferae, Ranunculaceae, Cyperaceae, Polygona-

ceae, Polygonum, Pteroxygonum, Gentianaceae and

Umbelliferae. In general, the ecological scope of

Gramineae is extensive, but most components of it

grow in relatively humid environments (e.g. Horo-

witz, 1992; Schabitz, 1994; Campo et al., 1996). Ke

(1995) found that Algae are relatively widely present

in loess records, particularly in the southern and

middle parts of the Plateau, implying the existence of

scattered water ponds within it. Previous studies

(Wang and Sun, 1994; Sun et al., 1997) showed that

fern spores can be also used to indicate relatively

humid environments. Here, we describe the pollen

assemblages of the sections from south to north.

Fig. 3. Pollen percentage diagram for the six sections from south to north on the Chinese Loess Plateau.

H.Jia

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ofPalaeobotanyandPalyn

ology133(2005)23–37

27

Fig.3(continued).

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–3728

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–37 29

3.1. Lantian section

The pollen diagram of the Lantian section is shown

in Fig. 3a. Of the 15 samples analyzed, only 7 had

enough pollen and spore grains (N70) for reasonable

statistical analysis. A most striking feature of the

pollen record is that herbaceous pollen taxa dominate

(65.9–100%) in all the loess-soil horizons, except for

the S1 soil in which the alga Concentricystis is

predominant (97.6%).

The pollen assemblages of the S0, L1–1, L1–4 and

L1–5 units are all dominated by Artemisia (53.7–

88.9%), with low contents of Compositae (0.4–

17.7%), Chenopodiaceae (0–7.2%), Gramineae (0–

4.3%), Gentianaceae (0–1.4%), Cruciferae (0–1.5%),

Polygonum (0–4.2%) and Polygonaceae (0–7.7%). In

these units, the mesic herbaceous elements, including

Gramineae, Gentianaceae, Cruciferae, Polygonum and

Polygonaceae, range from 1.6% to 10.8%, with the

highest content occurring in S0.

Arboreal pollen types, with contents of no more

than 18.3%, are also identified in S0, L1–1, L1–4 and

L1–5. They are mostly temperate deciduous trees such

as Quercus (0–6.4%) and Ulmus (0–10.4%). Trace

amounts of other arboreal pollen grains are scattered

in the loess-soil horizons. These are Pinus, Abies,

Podocarpus, Taxodiaceae, Tsuga, Cedrus, Juglans,

Betula, Ephedra, Tamaricaceae and Rhamnaceae.

Ferns, mostly Polypodiaceae (0–4.1%), Selaginella

sinensis (0–1.5%), Araiostegia (0–0.6%), Cyathea-

ceae (0–0.8%) and unidentified species (0–6.2%), are

only found in the S0 and L1–4 horizons with a content

ranging from 4.7% to 9.2%. The alga Concentricystis

reaches 20.5% in L1–5 and 4.1% in L1–4, respectively.

As shown in Fig. 3a, the value of AP+MH+SP at

Lantian reaches up to 97.6% in the last interglacial

soil of S1, whereas it is below 36% in other horizons.

3.2. Binxian section

In the Binxian section, 11 samples satisfy the

pollen statistical requirement (Fig. 3b). The pollen

assemblages of the two samples of the S1 horizon are

dominated by either the xerophilous shrub Zygophyl-

laceae (96.7%) or the mesic shrub Rosaceae (95.5%),

and the uppermost sample of S0 has relatively high

fern spores (37%). The other eight pollen samples are

mostly composed of the herbaceous taxa (78.3–

94.4%), in which Artemisia is predominant (68.6–

93%). Other herbaceous pollen types such as Com-

positae (0.6–13.7%) and Chenopodiaceae (0–2.2%)

are also identified in these samples. The values of

mesic herbaceous pollen taxa, which consist of

Gramineae (0–1.5%), Ranunculaceae (0–0.4%), Cru-

ciferae (0–21.7%), Cyperaceae (0–1.1%) and Poly-

gonum (0–5.5%), are 5.5–23.6% in S0 and 3.7–4.3%

in L1–1, respectively.

Throughout the section, tree pollen is no more than

5% except for the uppermost sample of the S0 horizon,

in which it reaches 21.4% and is composed predom-

inantly of Pinus (14%), Picea (4.3%) and Ulmus

(2.3%). Trace amounts of Abies, Carya, Tamarica-

ceae, Ephedra and Quercus pollen are also present. A

relatively high pollen content of shrub Zygophylla-

ceae species is found in one sample of L1–1 (15.9%)

and Elaeagnaceae in one sample of L1–4 (11.4%).

Fern spores are detected in all horizons but S1. The

average content of fern spores is 18.1% in S0, and

decreases markedly to 6.3% in L1–1, 3.4% in L1–4 and

6.5% in L1–5. The major fern types are Polypodiaceae

(0–11.3%), Cyatheaceae (0–4.3%), Selaginella sinen-

sis (0–1.6%), Ophioglossaceae (0–1.6%), Athyriaceae

(0–1.2%) and unidentified others (0–18.7%).

As shown in Fig. 3b, the average value of

AP+MH+SP is as high as 65.8% in S0 and 99.7% in

S1, whereas it is below 26% in other horizons.

3.3. Ningxian section

In the Ningxian section, significant amounts of

pollen grains were extracted from only eight samples

(Fig. 3c). The pollen assemblages of the samples are all

dominated by herbaceous taxa, with the highest content

at 98.1% in L1–1 and the lowest content of 55.6% in S0.

Again, Artemisia is the highest pollen element,

which ranges from 45.7% to 91.3% in the samples.

Compositae pollen is also detected in all the samples,

with up to 35.6% in L1–5 and below 13% in other

horizons. Gramineae has a value of about 15% in the

horizons of L1–4 and S1. The percentages of other

herbaceous pollen taxa such as Cruciferae, Cyper-

aceae, Pteroxygonum, Polygonum and Chenopodia-

ceae are generally no more than 3% and they are

scattered through the horizons.

The eight samples contain tree pollen of 2–12.8%,

mainly derived from the temperate deciduous species

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–3730

of Quercus and Betula. Other tree pollen components,

such as Juglans, Carya, Alnus, Ulmus, Pinus, Picea

and Abies , are only present in trace amounts.

Relatively high percentages of shrub pollen types

are found in three samples; they are Corylus (38.1%)

in S0, Tamaricaceae (30.3%) in L1–5 and Zygophylla-

ceae (8.4%) in S1.

Fern spores are detected in seven samples and

mainly composed of the monolete spores Polypodia-

ceae and unidentified taxa. The percentage of fern

spores is 8.5–21% in L1–4 and mostly below 5% in

other samples.

The AP+MH+SP value reaches the highest in S0(46.7%) and the lowest in L1–1 (4.9%), and varies

between 8.6% and 41% in other horizons (Fig. 3c).

3.4. Qingyang section

Eight samples in the S0, L1–4, L1–5 and S1 horizons

at Qingyang contained suitable amounts of pollen

grains (Fig. 3d); these are predominantly composed of

herbaceous components (82.7–98%). In the last

glacial maximum loess deposit of L1–1, 68 and 51

pollen grains were also extracted from two samples,

which respectively contain as high as 84.3% and

95.5% of Artemisia. Although insufficient for formal

statistical analysis, the dominance of pollen grains of

Artemisia in the two samples still gives a good

indication that the Qingyang area was covered by

steppe vegetation during the last glacial maximum.

Artemisia percentages are about 55% in two

samples of L1–4 and 77% in one sample of S1, while

other samples have Artemisia values of about 90%.

Compositae persistently occurs in the samples, with

contents of 0.6–9.3%. The abundance of Chenopo-

diaceae reaches 27.1% in one sample of L1–4 and is

less than 3% in other samples. The mesic herbs,

mainly Gramineae and Polygonum, are found in

nearly all the samples but with values mostly below

7%; the highest content of the mesic herbs (23.5%),

mainly Gramineae, occurs in one sample of L1–4.

Other herbaceous pollen types, such as Umbelliferae,

Cyperaceae, Pteroxygonum and Caryophyllaceae, are

only sporadically present in the horizons.

In the Qingyang section, the arboreal pollen

content, although present in seven samples, is never

more than 4.6%, and is made up of Quercus, Ulmus,

Pinus, Picea, Abies, Tamaricaceae, Ephedra, Rosa-

ceae and Corylus. Ferns, including Polypodiaceae (0–

6.7%), Cyatheaceae (0–0.8%) and other unidentified

taxa (0–9.7%), are present only in the L1–4 and S1horizons, with percentages of 3.8–12.8%.

In the entire section, the highest AP+MH+SP value

(40.8%) occurs in one sample of L1–4, whereas it is

below 12% in other samples (Fig. 3d).

3.5. Mubo section

Compared to the pollen records in the above

sections, the diversity and abundance of palyno-

morphs of the pollen assemblages at Mubo are both

significantly reduced (Fig. 3e). Unfortunately, only

five samples in the S0, L1–1 and L1–4 horizons

contained sufficient pollen grains for statistical anal-

ysis, although close to 60 grains were recorded in the

two horizons of L1–5 and S1. Herbaceous pollen

ranges from 83.1% to 100%, with Artemisia being

dominant (56.2–99.3%). Two mesic herbaceous pol-

len taxa are detected in S0 and L1–1, Gentianaceae

with 25.8% in one sample of L1–1, and Polygonum

with between 0.8% and 2.6% in S0. Other herbaceous

pollen types are Compositae and Chenopodiaceae, but

their contents are below 4% throughout the section.

Less than 1.3% arboreal pollen is found in three

samples from the S0, L1–1 and L1–4 horizons, and they

are composed of Pinus, Picea and Abies species. Fern

spores, mainly Polypodiaceae and other unidentified

taxa, are also present in three samples with percen-

tages of 10–16.9%.

The highest AP+MH+SP value (42.7%) occurs in

one sample of L1–1, while this value is below 15% in

other samples (Fig. 3e).

3.6. Jiyuan section

Of the 15 samples analyzed at Jiyuan, only 7 (1 in

S0, 1 in L1–4, 3 in L1–5 and 2 in S1) satisfy the statistical

requirement (Fig. 3f). The pollen assemblages in the

Jiyuan section appear to be much more variable

between samples than in the other sections mentioned

above. The pollen assemblage of the S0 sample is

mainly composed of Quercus (81.3%) and fern spores

(13.2%), while tree pollen, including Pinus, Picea and

Abies, dominates one sample of L1–5 (96.1%). In the

other five samples, herbaceous pollen types have

comparatively high contents, ranging from 54.2% to

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–37 31

72.9%. Of the herbaceous pollen taxon group, Arte-

misia (20.8–53.4%) and Compositae (1–33.3%) com-

monly occur, whereas other types, mainly mesic herbs,

are found only in a few samples. For example,

Gramineae occurs only in L1–5 (1.1–15.9%) and one

sample of S1 (3.3%), Liliaceae occurs in only one

sample of L1–5, and Polygonum and Pteroxygonum

only in L1–4 (14.4% and 1.3%, respectively).

Five samples of Jiyuan section contain arboreal

components (9.2–42.4%). In the two samples of L1–5,

the tree elements are mainly Pinus (16.7–17.7%),

Picea (0.8–2.5%), Abies (5.8–13.3%) and Tsuga (1–

3.5%), while Ulmus is the major tree taxon found in

L1–4 (7.8%) and the two samples of S1 (32.9–39.6%).

Other tree pollen taxa, such as Podocarpus, Taxodia-

ceae and Cedrus, are sporadically distributed in the

five samples.

Fig. 4. Spatial vegetation distributions of the Loess P

The fern spores Polypodiaceae, Selaginella sinen-

sis, Araiostegia and other unidentified taxa, are found

in the horizons of L1–4, L1–5 and S1, with values

below 6.5%, and the alga content reaches 19.6% in

one sample of L1–4.

The AP+MH+SP value is above 95% in S0 and one

sample of L1–5, whereas it varies between 41.2% and

57.2% in other samples (Fig. 3f).

4. Spatial vegetation distribution over different

time intervals

In previous studies, pollen records of seven sites

over the Loess Plateau have been analyzed by different

groups of authors. They are located respectively at

Fuxian (Ke et al., 1993), Luochuan (Sun et al., 1998),

lateau during the last interglacial period (S1).

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–3732

Yaoxian (Li et al., 2003), Fuping (Sun and Zhao,

1991), Qishan (Zhao and Huang, 1999), Yangguo (Sun

et al., 1997) and Xian (Sun and Zhao, 1991) (Fig. 1).

Some of the pollen records cover the full S1–L1–S0stratigraphical sequence and others are reconstructed

only for the Holocene. The published pollen records

plus these reported here allow us to tentatively map the

spatial vegetation distributions of the Plateau over

different time intervals. In this study, we selected the

pollen data of three adjacent samples that correspond to

the climate patterns associated with S0, L1–1, L1–4, L1–5

and S1 depositional environments from the sections.

The averaged percentages of each pollen type were

then calculated for each horizon. We use histograms to

depict the pollen distributions for each site. In these

Fig. 5. Spatial vegetation distributions of the Loess Plate

histograms, the content of herbs is designated as A,

shrubs as B, trees as C, ferns and algae as D, mesic

herbs as E, and the total content of shrubs, trees, ferns

and algae and mesic herbs as F (see Figs. 4–8).

The spatial distributions of pollen over different

time intervals of the last glacial cycle are described

below for S1, L1–5, L1–4, L1–1 and S0.

4.1. S1

As shown in Fig. 4, significant amounts of pollen

grains have been obtained from eight sites of the last

interglacial soil (S1). Previous studies have shown that

during the last interglacial period, the Asian summer

monsoon was significantly enhanced, leading to an

au during the deposition of the L1–5 loess horizon.

Fig. 6. Spatial vegetation distributions of the Loess Plateau during the L1–4 horizon deposition.

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–37 33

increase of rainfall over the Loess Plateau (An et al.,

1991; Heller et al., 1993; Liu et al., 1995). This

relatively wet condition is registered in the Lantian

pollen record, which is dominated by Concentricystis,

a fresh, shallow water-living alga (Ke, 1995), suggest-

ing the presence of a wetland at the sampling site. But

this type of environment may have been scattered. At

a nearby site, Yangguo, Sun et al. (1997) were not

able to extract enough pollen grains from the

corresponding horizon in S1 unit, which was inter-

preted as resulting from a strong oxidation of organic

matter and pollen.

North to the Weihe river, the main body of the

Loess Plateau appears to be covered by a grassland

vegetation during S1; herbaceous taxa reach a content

of up to about 90% of the pollen and spores at most

sites. One exception is the Binxian section in which

the dominant pollen is xerophilous Zygophyllaceae

and mesic Rosaceae, suggesting the growth of shrubs

at this site. Jiyuan is situated in the northernmost part

of the Loess Plateau with a relatively high elevation

and low annual temperature (Table 1). The geo-

morphology of this area is characterized by small, flat

tablelands surrounded by gullies up to 300 m in depth.

The pollen diagram at Jiyuan shows that although

herbaceous pollen has a percentage of 58.1%, Ulmus

is as high as 36.2%. This pollen assemblage may

suggest that the tablelands at Jiyuan were covered by

a grassland vegetation, whereas elm trees were

growing in the gullies.

Fig. 7. Spatial vegetation distributions of the Loess Plateau during the last glacial maximum (L1–1).

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–3734

4.2. L1–5

During the deposition of the L1–5 loess horizon, the

main body of the Loess Plateau experienced dry

climatic conditions, as indicated by the dominance of

the herbaceous pollen taxa and very low pollen

contents of trees and mesic herbs (Fig. 5). But the

relatively high F (B+C+D+E) values at Yangguo,

Lantian and Jiyuan suggest that the climate at the

southernmost and northernmost parts of the Plateau

may have been considerably wetter than in the main

part of the Plateau. At Lantian, the content of

Concentricystis reaches 20.5%, implying at least the

occasional presence of wetlands. The pollen percent-

age of the mesic herbs is about 38% at Yangguo,

being the highest among all the studied localities. The

total pollen concentration of Pinus, Picea and Abies at

Jiyuan, the northernmost site, is over 50% (Fig. 5),

indicating the existence of coniferous forest under the

cold-humid climatic conditions.

4.3. L1–4

Previous studies have shown that the climate of the

L1–4 stage was comparatively humid within the last

glacial period, as caused by an enhanced summer

monsoon (An et al., 1991; Heller et al., 1993; Liu et

al., 1995). However, the Loess Plateau was still

covered mainly by a grassland vegetation, dominated

by Artemisia (Fig. 6). Relatively wetter conditions

Fig. 8. Spatial vegetation distributions of the Loess Plateau during the S0 soil development.

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–37 35

were localized at the southernmost and northernmost

parts, as for stage L1–5. The content of Concentricystis

and mesic herbs at Yangguo reaches about 20% and

40%, and at Jiyuan, alga has a content of 19.6%

accompanied by 7.8% of Ulmus and 14.4% of the

mesic herb Polygonum. These suggest that the south-

ernmost and northernmost parts of the Plateau may

have a more humid condition than the main body of

the Plateau during L1–4 period of soil development.

4.4. L1–1

The pollen representation of the Loess Plateau

during the Last Glacial Maximum is registered in

eight sites (Fig. 7). Values for herbaceous pollen,

mainly Artemisia, are mostly over 90%, indicating

much more arid conditions relative to the earlier time

intervals. Comparatively, the composite F value is

considerably higher in the southern part than in the

northern part. For example, at Lantian, Yangguo and

Binxian, the F values range from 16% to 23%,

whereas this value is below 5% at Fuxian, Ningxian

and Luochuan.

4.5. S0

The pollen diagrams of 12 sites (Fig. 8) reveal

that during the period of S0 soil development, the

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–3736

diversity and abundance of vegetation significantly

increased in the Loess Plateau, particularly in the

southern part. For instance, we detected at Lantian,

Yangguo, Xi’an, Fuping and Binxian considerable

tree pollen including Pinus, Picea, Ulmus, Corylus

and Quercus, and high values of Polypodiaceae,

Selaginella sinensis and Concentricystis. At Jiyuan,

the northernmost site, a Quercus woodland was

present during the Holocene optimum, as indicated

by the 81.3% value for Quercus pollen. Even if a

relatively humid condition can be inferred, the main

area of the Plateau was still covered by a grassland

vegetation during this time interval, since 9 of the 12

sites, have herbaceous pollen contents of over 70%

(Fig. 8).

5. Discussion and conclusions

The most striking feature that our records reveal is

an Artemisia-dominated grassland on the Loess

Plateau both in glacial and interglacial periods. This

means that even in the relatively humid interglacial

periods caused by the enhanced summer monsoon

climate, a moisture limitation was still present that

hindered the development of a forest cover over the

main body of the Loess Plateau. Previous studies have

shown the widespread occurrence of Artemisia-

dominated vegetation in the semi-arid temperate zone

in the late Pleistocene (El-Moslimany, 1990), similar

to the situation of the Loess Plateau.

The comparison of vegetation patterns between

the glacial and interglacial periods shows that the

Loess Plateau generally had a more abundant and

diversified vegetation cover in the interglacial

periods than in the glacial periods. The pollen

concentrations of trees, ferns and algae, and mesic

herbs are significantly higher in the interglacials and

interstadials. These features imply a much higher

precipitation probably caused by an enhanced mon-

soon circulation in the interglacial periods if a higher

evaporation rate under a higher average temperature

is taken into consideration. This is consistent with

the hypotheses derived from other proxy studies (Liu

and Ding, 1998).

Throughout the Loess Plateau, there is a general

characteristic that the southern part of the Plateau has

had a more diversified vegetation cover than the

northern part over most of the studied time intervals,

and the pollen values of trees, ferns and algae, and

mesic herbs substantially decrease from south to

north. This may be closely associated with the

relatively steep gradients of temperature and precip-

itation of the Asian monsoon climate, as mentioned

above. Observations have shown that there is a clear

zonation of pedogenic characteristics (Liu, 1985) over

the Loess Plateau. In the southern part, the paleosols

had undergone relatively strong leaching and differ-

entiation, forming an A–Bt–C horizon sequence. In

the middle Plateau region, a typical A–Bw–Cca

horizon sequence formed, while an A–C horizon

sequence occurred in the northern part. Obviously, the

spatial distributions of both paleosols and paleovege-

tation support the monsoon control of the climate in

the Loess Plateau.

The Loess Plateau has various different geomor-

phological units, which can exert different controls on

moisture conditions and, consequently, on vegetation

patterns. The pollen records so far obtained from the

Loess Plateau are mostly from the loess dYuanT—the

flat, broad tablelands. Compared to the loess gullies,

loess dYuansT have poorer moisture-retention capa-

bility. But if forests had widely occupied the gullies

around the dYuansT, high tree pollen concentrations

would have been extracted from our sections, as is in

the Jiyuan section. Obviously, our results show that it

is not in the case.

Our major conclusion is that in the last 130 ka or

so, the main body of the Loess Plateau has been

covered by an Artemisia-dominated herbaceous veg-

etation, which should be taken into thorough consid-

eration in future rehabilitation of native ecosystems on

the Loess Plateau.

Acknowledgements

Sampling assistance in the field from Yang Shiling

and Chen Zhuo is acknowledged. We thank Professors

John Dodson, A. Peter Kershaw and Dr. Li Xiaoqiang

for their critical comments on an earlier version of the

paper. This study was financially supported by the

Innovation Project of the Chinese Academy of

Sciences (KZCX2-SW-118), the NSF of China (grant

90202020, 40021202) and the National Key Project

for Basic Research (G1999043402).

H. Jiang, Z. Ding / Review of Palaeobotany and Palynology 133 (2005) 23–37 37

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