Modern pollen samples from alpine vegetation on the Tibetan Plateau

RESEARCH ARTICLE

© 2001 Blackwell Science Ltd. http://www.blackwell-science.com/geb

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Global Ecology & Biogeography

(2001)

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Blackwell Science, Ltd

Modern pollen samples from alpine vegetation on the Tibetan Plateau

GE YU*, LINGYU TANG†, XIANGDONG YANG*, XIANKUN KE‡ and SANDY P. HARRISON§ *

Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China,

†

Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China,

‡

Department of Urban and Resource Sciences, Nanjing University,

Nanjing 210093, China, and

§

Max Planck Institute for Biogeochemistry, 07743 Jena, Germany

ABSTRACT

1

A set of 316 modern surface pollen samples,sampling all the alpine vegetation types thatoccur on the Tibetan Plateau, has been compiledand analysed. Between 82 and 92% of the pollenpresent in these samples is derived from only 28major taxa. These 28 taxa include examples ofboth tree (AP) and herb (NAP) pollen types.

2

Most of the modern surface pollen samplesaccurately reflect the composition of the modernvegetation in the sampling region. However, air-borne dust-trap pollen samples do not provide areliable assessment of the modern vegetation.Dust-trap samples contain much higher percent-ages of tree pollen than non-dust-trap samples,and many of the taxa present are exotic. In theextremely windy environments of the TibetanPlateau, contamination of dust-trap samples bylong-distance transport of exotic pollen is aserious problem.

3

The most characteristic vegetation types pre-sent on the Tibetan Plateau are alpine meadows,steppe and desert. Non-arboreal pollen (NAP)therefore dominates the pollen samples in mostregions. Percentages of arboreal pollen (AP) arehigh in samples from the southern and easternTibetan Plateau, where alpine forests are animportant component of the vegetation. Therelative importance of forest and non-forestvegetation across the Plateau clearly follows

climatic gradients: forests occur on the southernand eastern margins of the Plateau, supported bythe penetration of moisture-bearing airmassesassociated with the Indian and Pacific summermonsoons; open, treeless vegetation is dominantin the interior and northern margins of thePlateau, far from these moisture sources.

4

The different types of non-forest vegetation arecharacterized by different modern pollen assem-blages. Thus, alpine deserts are characterized byhigh percentages of Chenopodiaceae and

Artemisia

,with

Ephedra

and

Nitraria

. Alpine meadows arecharacterized by high percentages of Cyperaceaeand

Artemisia

, with Ranunculaceae and Polygo-naceae. Alpine steppe is characterized by highabundances of

Artemisia

, with Compositae, Cru-ciferae and Chenopodiaceae. Although

Artemisia

is a common component of all non-forest vegeta-tion types on the Tibetan Plateau, the presenceof other taxa makes it possible to discriminatebetween the different vegetation types.

5

The good agreement between modern vegeta-tion and modern surface pollen samples acrossthe Tibetan Plateau provides a measure of thereliability of using pollen data to reconstruct pastvegetation patterns in non-forested areas.

Key words

Alpine vegetation, dust trap, isopolmaps, modern pollen, non-arboreal pollen, pollen–vegetation relationships, surface samples, TibetanPlateau.

Corresponding author: [email protected] Nomenclature follows Institute of Biology CAS (1994).

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INTRODUCTION

Surface pollen records have been widely used totest the methods used to interpret fossil pollenrecords, to reconstruct past vegetation patterns,or to make vegetation-based reconstructions ofpast climates (e.g. Wright, 1967; Huntley &Birks, 1983; El-Moslimany, 1990; Guiot

et al.

,1993; Hjelmroos & Franzen, 1994; Prentice

et al.

,1996; Yu

et al.

, 1998, 2000). This approach isbased on the assumption that surface pollenrecords reflect modern regional vegetation pat-terns. Most of the studies that have establishedthe relationship between modern vegetation andsurface pollen samples have focused on lowlandforest vegetation types. Here we evaluate howwell surface pollen samples reflect the regionalpatterns of nonforest vegetation types in alpineenvironments on the Tibetan Plateau.

The Tibetan Plateau, with an area of

≈

2,000,000 km

2

and elevations up to 4000 m a.s.l.(Ren

et al.

, 1979) is characterized by differenttypes of alpine vegetation (Chinese Academy ofSciences, 1988). There are no large-scale vegeta-tion maps that cover the whole area. Surfacepollen samples have been collected as part ofpalaeo-environmental studies of the TibetanPlateau (e.g. Du & Kong, 1983; Huang

et al.

, 1983;Li & Liu, 1988; Jarvis, 1993; Shan

et al.

, 1996;Van Campo

et al.

, 1996; Wang

et al.

, 1996; Wu& Xiao, 1996; Tang

et al.

, 1998). Although thereare now samples from all of the alpine vegeta-tion zones on the Tibetan Plateau (Fig. 1), therehas been no attempt to synthesize the avail-able modern surface sample data for the wholeof the Tibetan Plateau or to examine the rela-tionship between surface pollen samples and theregional vegetation patterns. The vegetation onthe Tibetan Plateau, unlike that of the lowlandsof eastern China, is undisturbed by humanactivities and therefore provides an excellent testof the assumptions that modern surface pollensamples reflect the natural vegetation of theregion.

We have compiled the modern surface pollendata available from the Tibetan Plateau and weuse these data: (1) to map the spatial patterns inthe abundance of specific pollen taxa, and (2) toanalyse the relationship between the modernpollen assemblages and regional vegetationpatterns.

GEOGRAPHICAL SETTING

The Tibetan Plateau is located between 75 and105

°

E and 25–40

°

N and has an average elevationof

c.

4000 m a.s.l. As an extensive, high-elevationland-mass, the Plateau affects both regional andglobal climates. Insolation-induced warming ofthis large surface produces a regional heat sourcewhich strengthens the land–sea contrast betweenthe Indian Ocean and the Asian continent andthus plays a key role in the generation of thePacific summer monsoon. In addition, the moun-tainous mass of the Plateau blocks the moisture-bearing air-masses of the Indian Monsoon frompenetrating into inland Asia during the summerseason. The Plateau also affects winter rainfallpatterns by splitting the upper-level westerly airflow into a northern and a southern branch (Ren

et al.

, 1979). As a result of these seasonal atmo-spheric circulation patterns, north-western Chinaand the interior of the Tibetan Plateau areextremely cold and arid throughout the year,while the southern and eastern margins of theplateau are seasonally wet (China Academy ofSciences, 1988; Zhang

et al.

, 1991). These climatepatterns are reflected in the regional vegetation.Forest occurs in the south-east of the Plateau,alpine shrubland, meadowland and steppe occurin the interior, and sparsely vegetated desertoccurs in the north-west (Tibetan InvestigationGroup, 1988).

Six major vegetation regions are recognized onthe Plateau. The approximate extent of eachregion has been mapped (Fig. 1) based ondescriptions of their regional distribution in theliterature.

I.

Montane broad-leaved and conifer forest.

This vegetation type is found along the south-eastern margin of the plateau, which is charac-terized by high mountains and deeply incisedvalleys with elevational differences that canexceed 3000 m. The climate is subtropical andmonsoonal, with a mean annual temperature of16.0

°

C, coldest month temperatures of 7.7

°

Cand warmest month temperatures of 21.5

°

C. Theannual precipitation ranges from 850 to 1500 mmand is concentrated in the monsoon seasonbetween May and October (Editorial Committeeof Chinese Vegetation, 1980). At lower elevations(2000 and 3000 m a.s.l.) there are evergreenneedle-leaved and deciduous broad-leaved mixed


Modern pollen sam

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Fig. 1 Pollen sites and sample types on the Tibetan Plateau. Dashed lines indicate the boundaries of six vegetation regions (I, II, III, IV, V and VI) describedin the text.

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forests, dominated by

Pinus yunanensis

,

Tsugachinensis

,

T. dumosa

,

Quercus

spp. and

Q. acutis-sima

and commonly with

Acer

,

Betula

,

Corylus

,

Carpinus

,

Lonicera

,

Rhododendron

and

Cinarun-diaria

present. At higher elevations (3000 and4000 m a.s.l.) there are evergreen conifer forestsdominated by

Abies fabri

with a

Rhododendron

shrub understorey. Above 4000 m a.s.l. can befound alpine shrubland and steppe vegetation,with sparse ground cover, including taxa such as

Sabina

, Gramineae and Compositae (EditorialBoard of Sichuan Vegetation, 1980).

II.

Alpine meadows and steppe.

These vegeta-tion types occur in the eastern part of thePlateau at elevations between 3400 and 4500 ma.s.l. The climate is cold and relatively wet,with a mean annual temperature of 0.9

°

C, coldestmonth temperatures of –4.7

°

C and warmestmonth temperatures of 10.3

°

C. Annual precip-itation is between 560 and 860 mm. Evaporationis low and relative humidity is between 64 and73%. Alpine meadows occur in flat-lying areasand alpine steppe on the mountain slopes (Chai

et al.

, 1965). These vegetation assemblages aredominated by non-arboreal species, includingCyperaceae, Gramineae,

Artemisia

, Rosaceae,Compositae, Caryophyllaceae, Saxifragaceae,Ericaceae and

Polygonum

. However, pure standsof evergreen needle-leaved trees (

Picea purpurrea

and

Abies likiangensis

) grow on the wetter slopesof the mountains (Chai

et al.

, 1965; EditorialBoard of Sichuan Vegetation, 1980).

III.

Alpine meadows and shrubland

. Thesevegetation types are characteristic of the south-ern Plateau at elevations over 4000 m a.s.l. Theclimate is cold, with a mean annual temperatureof 1.5

°

C, coldest month temperatures of –8.0

°

Cand warmest month temperatures of 9.5

°

C(Tibetan Investigation Group, 1988). The annualprecipitation is between 400 and 550 mm, evapor-ation is low and there is a positive water budget(Zhou

et al.

, 1976; Wang, 1987). The meadow vegeta-tion, which occurs at elevations below 4400 ma.s.l., is dominated by Gramineae, Compositae,Cyperaceae, Ranunculaceae, Leguminosae,Liliaceae, Primulaceae, Caryophyllaceae, Labiatae,Saxifragaceae and Gentianaceae.

Kobresia

and

Stipa

are important components of the meadowvegetation, with herbaceaous forms of

Arte-mesia

, including

A. wellbyi

and

A. stracheyi

.Shrublands occur between 4400 and 4700 m

a.s.l., dominated by

Sabina pingii

,

Caraganaversicolor

and

Potentilla

spp. Above 4700 m a.s.l.,there is alpine steppe with

K. pygmaea

. At higherelevations (above 5200 m a.s.l.) there is only asparse vegetation cover (Editorial Board ofSichuan Vegetation, 1980).

IV.

Alpine steppe

. This vegetation type occursin the interior of the Plateau, where the climateis characterized by cold, dry conditions and ashort growing season. In the Bange Co-Selin Cobasin (4530 m a.s.l.), for example, the annualmean temperature is –3

°

C, the total annualprecipitation is 290 mm and the total annualevaporation is 2176 mm (Zheng

et al.

, 1989). Thevegetation is dominated by Gramineae (

Stipasubsessiliflora

and

S. purpurea

),

Artemisia

forbs(

Kobresia littledalei

) and Cyperaceae (

Carexmoorcroftii

) (Tibetan Investigation Group, 1988;Huang

et al.

, 1993). However, stands of

Pinus

occuron mountain slopes at elevations over 4500 ma.s.l. (Huang

et al.

, 1996; Wu & Xiao, 1996).V.

Alpine steppe–desert and montane forest

.These vegetation types are found in the northernpart of the Plateau where there are large basins(2500–3500 m a.s.l.) surrounded by high moun-tains (up to 4000 m a.s.l.). The climate is verycold and dry, with coldest month temperatures of–12.7

°

C and warmest month temperatures of12.4

°

C. The annual precipitation in the QinghaiLake basin, for example, is between 377 and395 mm, but the potential evaporation over thebasin is

≈

3.8 times higher than the rainfall(Lanzhou Glaciology Institute, 1979). The vegeta-tion is dominated by Chenopodiaceae,

Artemisia

,

Ephedra

,

Nitraria

,

Tamarix

,

Zygophyllum

,Polygonaceae, Compositae and Gramineae.Conifer forests, with

Picea crassifolia

,

P. wilsonii

and

Sabina przowalskii

, are found between 2500and 3000 m a.s.l. on the northern, rainshadowslopes of the mountains. Conifer and broad-leaved deciduous forests, with

Picea crassifolia

,

P. wilsoni

,

Quercus

,

Pinus

and

Ostryopsis davidiana

,occur on the wetter south-eastern slopes of themountains. Open woodlands and shrubs, charac-terized by

Populus suaveolensi P. davidiana

,

Sabina chinensis

and

Rhododendron

, with

Sorbus

,

Ulmus

,

Salix

and

Betula

, grow at elevationsabove 3000 m a.s.l. (Lanzhou GlaciologyInstitute, 1979).

VI.

Alpine desert.

This vegetation type is foundin the western and the north-western parts of


Modern pollen samples from the Tibetan Plateau 507

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the Plateau at elevations above 4500 m a.s.l. Theclimate is extremely arid, with annual precipita-tion of ≈ 60 mm and a relative humidity of only31–33%. The daily temperature is below freezingfor seven months of the year (Editorial Com-mittee of Chinese Vegetation, 1980). The sparsevegetation is characteristic of alpine desert,and is characterized by Chenopodiaceae, Zygo-phyllum, Artemisia, Gramineae, Ephedra andNitraria, and small amounts of Leguminosae,Cruciferae and Rosaceae (Tibetan InvestigationGroup, 1988).

DATA AND METHODS

Pollen data

We have compiled a total of 316 modern surfacepollen samples from the Tibetan Plateau. Thisdataset includes 126 unpublished samples obtainedby the authors, 172 samples from the Chinesepollen dataset put together for the BIOME 6000vegetation mapping project (Yu et al., 2000), andan additional 18 samples obtained from theliterature (Table 1). The samples are mainly fullpollen counts (307 samples) but nine sampleswere digitized from published pollen diagrams.The non-digitized samples have pollen countsbetween 93 and 955 grains. Only 10% of thesamples had fewer than 150 grains. While werecognize that samples with low pollen countsmay yield statistically less reliable results, all ofthe samples containing fewer than 150 grainscome from the desert regions of central andnorthern Tibet. Exclusion of these samples wouldtherefore mean that one of the characteristicvegetation types of the Tibetan Plateau was notsampled. Credibility measures (see below) havebeen used to assess the reliability of these samples.Most of the surface samples (273) were collectedfrom surface deposits (soil, moss, wetlands, lakesediments, river sediments), but some (42) arefrom seasonally deployed dust traps and one wastaken from the Dunde icecap (Table 1).

The pollen sites are located between 27 and40°N and 75–105°E (Fig. 1) and cover an eleva-tion range from 1370 m to 5325 m a.s.l. Overhalf (52.5%) of the sites were located at eleva-tions > 4000 m a.s.l., corresponding to the meanelevation of the Tibetan Plateau. A few sites(4%) sample the high elevation (> 5000 m a.s.l.)

vegetation characteristic of the mountain rangesin the interior of the Tibetan Plateau. The remain-ing sites come from interior basins (< 4000 ma.s.l.) or the lower-elevations (< 4000 m a.s.l.) ofthe mountain ranges marginal to the TibetanPlateau.

Pollen nomenclature

A total of 190 pollen taxa were represented in theoriginal data set. We standardized the nomencla-ture used by different authors according to thenomenclature system of the Institute of Botanyof the Chinese Academy of Science (Institute ofBotany of Chinese Academy of Sciences, 1994).Genus-level identifications were amalgamatedinto the appropriate families, except in the casewhere specific families are not routinely used inpalynological analyses. For example, Chinesepalynologists do not routinely group the generain the Tamaricaceae (e.g. Tamarix, Myricariaand Reaumuria) because they have distinctivedistribution patterns. However, genera in otherfamilies (and particularly families with onlyherbaceous species) are often grouped togetherby at least some palynologists. To standardizeour taxon list for each pollen sample, weamalgamated genera from the Caryophyllaceae,Cyperaceae, Chenopodiaceae, Cupressaceae, Eri-caceae, Labiatae, Leguminosae, Liliaceae, Polygo-niaceae, Primulaceae, Rosaceae, Rubiaceae andScrophulariaceae families. We also amalgamatedgenera from the Compositae, with the exceptionof the taxon Artemisia. Artemisia is an importantindicator in steppe and desert vegetation, andChinese palynologists routinely list it separatelyfrom other genera in the Compositae.

As a result we recognize 28 pollen taxa: Abies,Artemisia, Betula, Caryophyllaceae (includingArenaria), Chenopodiaceae, Compositae (inc. Aster,Centaurea and Liguliflorae), Cruciferae, Cupres-saceae (inc. Juniperus and Sabina), Cyperaceae(inc. Carex), Ephedra, Ericaceae (inc. Monotropa,Pyrola and Rhododendron), Gramineae, Labiatae(inc. Lamium, Origanum, Phlomis), Leguminosae(inc. Astragalus, Caragana, Lotus and Papilion-aceae), Liliaceae (inc. Allium and Lilium), Nitraria,Pinus, Picea, Polygonaceae (inc. Calligonum,Polygonum, Rheum and Rumex), Primulaceae(inc. Androsace, Anagallis and Primula), Quercus,Ranunculaceae (inc. Aconitum, Caltha, Clematis


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Table 1 Site information for the modern pollen surface samples from the Tibetan Plateau

Site name Lat. (N) Long. (E) Elev. (m) Sample type

Number of samples

Vegetation type References/note

Baxi 33.55–32.67 103.02–102.5 3440–3600 soil 16 meadow forest Wang et al., 1996Kunlun Mt 35.60–39.60 74.90–79.40 2200–4700 dust flux 10 steppe–desert Weng et al., 1993Lanzhou 35.91 104.10 1800–2300 soil 3 woodland, desert–shrub Wang et al., 1991Qinghai 33.30–36.38 94.33–101.33 4192 moss 94 meadow–steppe Liu et al., 1995Southern Tibet 29.26–31.59 90.61–101.86 3710–4590 soil 17 forest–steppe, meadow Tang et al., 1998 Western Tibet 32.50–39.50 76.90–81.10 2400–4500 dust flux 32 desert–steppe Huang et al., 1993Angren 29.2 87.4 4300 soil 1 steppe this studyBangda 30.30 97.30 4140 moss 1 scrub this studyBayi 29.70 94.40 3485 moss 1 forest this studyBayixi 29.70 94.50 3375 moss 1 forest this studyBodongquan 35.52 93.87 4662–4672 soil 2 steppe this studyBudla 29.67 91.25 3000 moss 1 shrub this studyCaidamo 36.71–36.73 99.18–98.89 3100–3120 soil 3 desert this studyCuomorong 31.62 92.07 4410 mud 1 steppe this studyCuona 32.07–32.15 91.40–91.45 4588–4740 soil 2 steppe this studyDalijia 35.56–35.57 102.74–102.84 2760–3612 moss 3 steppe–shrub this studyDangxiong 30.30 90.70 4570 moss 1 steppe this studyDingri 28.30–28.50 86.40–86.8 4500–5000 soil 2 meadow–steppe this studyDulan 36.06–36.19 98.06–98.17 3290–3360 soil 2 desert this studyGahai 34.24–34.35 102.30–102.31 3440–3610 moss 3 steppe this studyGeermo 36.08–36.35 94.81–95.09 2817–3184 soil 2 desert this studyHezuo 34.62–34.94 102.46–102.87 2996–3147 soil 4 steppe–shrub this studyHuangshui 34.22–36.59 101.23 2732 soil 1 desert–steppe this studyJiafan 30.6–80 92.20–90.90 4450–4830 moss 2 meadow this studyJiaxidong 29.80 92.40 4880 moss 1 scrub this studyKangding 31.00 101.90 3030 moss 1 forest this studyKekexili 34.22–34.58 92.41–92.72 4532–4680 soil 7 steppe–desert this studyKeligao 36 97.56 3110 soil 1 desert–shrub this studyKunlun 35.68–35.77 94.05–94.32 4090–4690 soil 2 steppe–desert this studyLasha 30.42 90.99 4219 soil 1 steppe this studyLuoqui 34.6 102.38–102.404 3400–3500 moss 2 forest this studyMinshan 35.38 102.89 1964 moss 1 shrub–forest this studyNaqu 30.62–31.40 91.40–92.25 4510–4700 soil 5 steppe–meadow this study

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Table 1 continued.


Number of samples


Nayini 30.80 92.70 4860 moss 1 woodland this studyNianqingtanggula 30.54–30.68 91.10–91.40 4440–5152 moss 7 steppe this studyQinghai 36.59–36.63 100.10–100.74 3000–3250 soil 3 steppe this studyReyueshan 36.41–36.43 101.03–101.11 3361–3440 soil 2 steppe this studyRuomohong 36.37–36.38 96.18–96.43 2763–2940 organics 2 desert this studyTanggula 33.21–32.91 91.86–91.95 5010–5121 soil 4 steppe this studyWudaoliang 34.62–35.26 92.80–93.15 4676–4758 soil 2 steppe this studyXiangpishan 36.75 99.61 3817 soil 1 steppe this studyYangbajing 30.00 90.70 4100 moss 1 steppe this studyYangbajing 30.08 90.56 4323 moss 1 steppe this studyYanshiping 33.58 92.07 4700 soil 1 steppe this studyYidun 29.7–30.30 98.00–99.50 4390–4630 moss 2 schrub meadow this studyZhilin 29.80 94.40 3625 moss 1 forest this studyZhuchang 29.6 98.30 3360 moss 1 steppe this studyZigetangcuo 32.01–32.03 90.93–90.95 4560–4600 soil 7 steppe–swamp this studyZoige 33.71–34.02 102.48–1.2.74 3438–3600 moss 3 forest–steppe this studyZuogongshan 29.70–29.90 97.60–98.00 3960–4630 moss 2 shrub meadow this studyDunde 38.10 96.40 5325 glacial 1 no vegetation Liu et al., 1998Damagou 37.00 80.70 1370 fluvial 1 desert–woodland XJIETRE, 1994Hetian 37.50 79.80 1500 fluvial 1 desert–woodland XJIETRE, 1994Unnamed lake at Ali 31.95–31.98 90.46–90.73 4450–4585 lacustrine 5 steppe this studyAngren Co 29.3 87.18 4300 lacustrine 1 steppe Huang et al., 1983Bangong Co 33.7 79 4241 lacustrine 1 desert–steppe Huang et al., 1996 (digitized data)Beilike Lake 36.67 89 4680 lacustrine 1 desert–steppe Huang et al., 1996 (digitized data)Bunan Lake 35.98 90.11 4800 lacustrine 1 desert–steppe Shan et al., 1996 (digitized data)Chaerhan Lake 36.92 94.98 2677 lacustrine 1 desert Du & Kong, 1983 Chen Co 28.85 90.5 4429 lacustrine 2 meadow this studyCuoer 31.47–31.57 91.50–91.52 4515–4545 lacustrine 2 steppe this studyCuona 32.07–32.15 91.40–91.45 4588–4740 lacustrine 2 steppe this studyDahaizi Lake 27.5 102.4 3660 lacustrine 1 alpine conifer Li & Liu, 1988Erlu Co 31.55 91.7 4530 lacustrine 1 steppe this studyGeren Co 34.59–34.63 92.39–92.45 4650 lacustrine 3 steppe–desert this study

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Table 1 continued.


Number of samples


Gounong Co 34.35 92.2 4670 lacustrine 1 desert Shan et al., 1996 (digitized data)Hang Co 31.32 91.79 4370 lacustrine 1 steppe this studyHidden Lake 29.7 98 4980 lacustrine 1 meadow this studyNamo Co 30.68–30.71 90.85–90.87 4710–4718 lacustrine 4 steppe this studyNariping Co 31.29 91.47 4520 lacustrine 1 steppe this studyNariyong Co 28.3 91.95 4750 lacustrine 1 meadow–woodland Huang et al., 1983Peiku Co 28.83 85.33 4590 lacustrine 1 meadow–woodland Huang, 1996Peng Co 31.52–31.54 91.03–91.62 4522–4545 lacustrine 2 steppe this studyQinghai Lake 36.88 100.1 3194 lacustrine 2 steppe–alpine conifer this studyRencuo 30.7 96.70 4450 lacustrine 1 meadow this studySelin Co 31.57 89.1 4530 lacustrine 1 steppe Sun et al., 1993 (digitized data)Sumxi Co 34.63 80.25 5058 lacustrine 1 desert van Campo & Gasse, 1993

(digitized data)Wulanwula Lake 34.8 90.5 4854 lacustrine 1 desert–steppe Shan et al., 1996 (digitized data)Xiaoshazi Lake 36.97 90.73 4106 lacustrine 1 desert–steppe Huang et al., 1996 (digitized data)Ximen Co 33.38 101.1 4020 lacustrine 1 meadow–woodland this studyXiongou Co 31.04 91.63 4637 lacustrine 1 steppe this studyXuge Co 31.97 90.33 4540 lacustrine 1 steppe this studyZabuye Lake 34.3 84.1 4421 lacustrine 1 steppe–desert Wu & Xiao, 1996 (digitized data)Zarinanmu Co 30.95 85.5 4653 lacustrine 1 meadow–woodland Huang et al., 1983Zigetang Co 32.01–32.03 90.93–90.95 4560–4600 lacustrine 1 steppe–swamp this study

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and Thalictrum), Rosaceae (inc. Potentilla, Rosaand Spiraea), Salix, Scrophulariaceae (inc.Verbascum and Veronica), Tamarix, Tsuga andUmbelliferae. We recalculated the abundance ofeach of the 28 major taxa as a percentage of thetotal pollen sum. These 28 taxa represent nearlyall of the pollen present in many samples. In theworst case, these taxa represent > 82% of thepollen present.

Pollen percentage diagrams and isopollen maps

The abundance of the 28 major taxa at each siteis shown as standard pollen percentage diagrams(see Appendix). We also calculated the totalpercentage of arboreal pollen (AP), non-arborealpollen (NAP) and ferns (FS) at each site. Therelative proportions of AP, NAP and FS ateach site, grouped by vegetation region, are shownin Fig. 2a. The spatial patterns in the relativeabundance of AP, NAP and FS are shown inFig. 2b. These histograms were obtained byaveraging the results from all of the sites presentin a series of 2° latitude × 2° longitude grid cells(15 columns × 7 rows) across the area between 75and 105°E and 27–40°N.

Spatial patterns in the abundance of each majortaxon (Fig. 3) are shown using isopol maps(Huntley & Birks, 1983). For contouring pur-poses, the pollen percentages at individual siteswere gridded using linear interpolation betweensites. Different contour intervals were used foreach of the maps. Thus, we used a 20% contourinterval from 10% for abundant taxa (Artemisia,Chenopodiaceae, Cyperaceae, Pinus), a 5% con-tour interval from 5% for moderately abundanttaxa (Compositae, Gramineae, Ephedra, Quercus,Betula, Picea, Ranunculaceae), a 2% contourinterval from 2% for less abundant taxa (Tsuga,Abies, Salix, Tamarix, Nitraria, Cupressaceae,Cruciferae, Rosaceae, Polygonaceae, Caryophyl-laceae, Ericaceae, Leguminosae, Umbelliferae,Labiatae) and a 1% contour interval from 1% forrare taxa (Primulaceae, Liliaceae, Scrophulari-aceae). An additional contour line was added toeach map (except those showing rare taxa) to dis-tinguish areas with abnormally low percentagesof each taxon. The percentages used were 5%,2.5% and 1% for each of the other three classes,respectively.

Evaluation of data quality and credibility

The uneven distribution of the modern pollen sitescould result in artefacts when the pollen percent-ages are interpolated for contouring purposes.We developed a three-category scheme to assessthe credibility of the interpolation. We dividedthe area between 75 and 105°E and 27–40°N intoa 2° latitude × 2° longitude grid (15 columns ×7 rows). The number of pollen sites in each gridcell was summed. Grid cells with > 5 pollen siteswere assigned to the highest credibility category,grid cells with 1–5 pollen sites were assigned tothe intermediate credibility category and gridcells with no sites are considered unreliable(Fig. 3d). About 17% of the grid cells fall withinthe highest credibility category and 25% of the cellsfall within the intermediate credibility category.

RESULTS

Relative abundance of AP/NAP/FS

Herbaceaous pollen taxa (NAP) are abundant atall pollen sites across the Tibetan Plateau(Fig. 2a). NAP is the dominant component ofpollen assemblages from vegetation regions II, Vand VI and at most sites in Regions III and IV.NAP percentages comprise 60–80% of the totalpollen count in the northern and the westernparts of the Plateau, 40–60% in the centralPlateau, and decline to between 20 and 40% inthe southern and eastern parts of the Plateau(Fig. 2b). Tree pollen (AP) is only abundant inthe southern and eastern parts of the TibetanPlateau (i.e. Region I and the southern parts ofRegions III and IV). In Region I, AP comprisesbetween 30 and 95% of the total pollen, depend-ing on the site. In the driest and coldest region(VI) in the north-western part of the Plateau, APpercentages never rise above 20%. Ferns are notcommon on the Tibetan Plateau, and the per-centages are never more than 6.5% in any region.

The relative abundance of AP and NAP inthe modern surface samples is consistent withthe broadscale vegetation patterns shown by themodern vegetation. Most areas of the TibetanPlateau are characterized by non-forest vegeta-tion types (alpine meadows, steppe, desert andshrublands). Extensive forests are confined to thesouthern and eastern margins of the Plateau. The


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Fig. 2 Relative proportions of arboreal pollen (AP), non-arboreal pollen (NAP) and ferns (FS) at eachmodern pollen site (a). The sites are grouped according to the six vegetation regions that have been identifiedon the Tibetan Plateau. Spatial patterns in the relative proportions of AP, NAP and FS are illustrated withhistograms (b) of the average percentages of each type obtained from a given 2° latitude × 2° longitude gridcell. NAP is dominant in Regions II, V and VI and most of Regions III and IV, reflecting the treelessvegetation across most of the Plateau. AP is abundant in Region I and the southern part of Regions III andIV, reflecting the alpine forest vegetation found in the southern and eastern parts of the Tibetan Plateau.




presence of AP in low abundance, even in low-elevation sites in Regions II, V and VI, probablyrepresents the local tree populations at higherelevations or low-distance transport of pollenfrom forested areas outside the Plateau region.

Pollen percentage diagrams

The good correspondence between modernvegetation patterns and modern pollen samples

is verified by consideration of the relative abund-ance of the 28 main taxa (Appendix) in samplesfrom each vegetation region. Pinus, Picea andQuercus are the most important taxa in lower-elevation sites, while Artemisia and Cyperaceaebecome increasingly important in higher-elevationsites in Region I. The abundance of these key taxareflects the characteristic montane broad-leavedand conifer forests of this region, and the pro-gressive increase in the importance of Artemisia

Fig. 3 Isopol maps showing the interpolated percentage of each major taxon based on surface samples fromthe Tibetan Plateau. The site data from which the isopol maps are reconstructed are shown in Fig. 1. Thecredibility of the isopol reconstructions (bottom left panel, d) has been assessed using a three-categoryscheme. Grid cells falling in the highest credibility category are marked by a thick cross. Grid cells falling inthe intermediate credibility category are marked by a thin cross. Cells falling in the lowest credibility category(i.e. with no samples in the grid cell) are marked by a white cross.


514 G. Yu et al.


and Cyperaceae with elevation reflects the presenceof shrublands at the highest elevations withinthe region. Cyperaceae and Artemisia are thedominant taxa in Regions II and III, regionsgenerally characterized by alpine meadow and/orsteppe-meadow vegetation. However, somesamples (samples 61–75) from the southern partof Region III have rather high percentages of Tsuga(< 15%), Quercus (< 15%) and Betula (< 20%).The abundance of trees in these sites is consistentwith the presence of alpine forest vegetation onthe southern margin of the region. Region IV ischaracterized by high percentages of Artemisia,Chenopodiaceae and Cyperaceae pollen, reflect-ing the alpine steppe vegetation. Relatively high

percentages of Tsuga occur in samples (samples20 and 33) from the southern margins of thisregion, where there are montane conifer forests.Samples from the alpine desert vegetation ofRegion V have extremely high percentages (up to85%) of Chenopodiaceae and Artemisia, andmoderate representation of Ephedra. Somesamples also contain typical desert taxa such asTamarix and Nitraria. Tree pollen (Pinus, Piceaand Cupressaceae) occurs in moderate abundanceat some higher-elevation sites from this region,close to the areas where montane forests arefound. Samples from low-lying sites with thealpine desert in Region VI are again dominatedby Artemisia and Chenopodiaceae, with Ephedra

Fig. 3 continued.




and Nitraria. Most of the dust trap samples inthe data set come from Region VI (samples 40–60). These samples show a systematic bias, relat-ive to other samples from this region, with muchhigher percentages of Pinus (2–8%) pollen thanwould be considered normal given the vegetation.This illustrates the well-known tendency of Pinuspollen, in particular, to be transported long dis-tances from the vegetation source by the wind.

Isopollen maps

Isopol maps of individual taxa show that the mod-ern surface pollen samples accurately reflect thedifferent vegetation types on the Tibetan Plateau.

The abundance of Artemisia (Fig. 3a) ishighest (30–50%) in regions where the vegeta-tion is alpine steppe or alpine desert in thecentral, western and north-eastern parts of theTibetan Plateau. This shows that Artemisia is amajor component of both types of vegetation.Artemisia also occurs in low to moderate abund-ance in other non-forest vegetation types, includ-ing alpine shrubland and alpine meadows.

Chenopodiaceae percentages are very high(50–70%) in the alpine desert region of northernTibet (Fig. 3a). The alpine deserts are alsocharacterized by rather high percentages (upto 10%) of Ephedra, Nitraria, Tamarix andScrophulariaceae.

Fig. 3 continued.


516 G. Yu et al.


The highest percentages of Compositae (< 30%)(Fig. 3a) occur in the alpine steppe vegetationregions of the central and southern TibetanPlateau. The abundance of Cruciferae (> 5%) isalso high in the alpine steppe of the centralTibetan Plateau. Although they never occur inhigh abundances (< 10%), Caryophyllaceae,Labiatae, Liliaceae, Primulaceae and Umbelliferaeare found in alpine steppe in the central part ofthe Tibetan Plateau (Fig. 3b).

The highest percentages of Cyperaceae (> 50%)are found in areas characterized by alpinemeadows in eastern-central Tibet (Fig. 3b). The dis-tribution of Ranunculaceae and Polygonaceae(Fig. 3b), with highest abundances in the south-eastern and north-western parts of the Plateau,corresponds with the distribution of alpinemeadows and wetlands.

If dust-trap samples are not considered, highabundances of tree taxa are confined to thesouthern and eastern margins of the Plateau.Relatively high percentages of Pinus (10–30%)(Fig. 3c) occur in the conifer forest belt of the

south-eastern and south-western Tibetan Plateau.In the south-east, Pinus occurs together withPicea and Abies. In the south-west, Pinus occursin conjunction with Tsuga. Cupressaceae (mainlywith Juniperus and Sabina) shows a differentdistribution from the other conifers, and isonly present in high abundance (> 2%) in themountains of the north-western Tibetan Plateau.

The samples with abundant Salix, Quercus andBetula (Fig. 3c) accurately reflect the distributionof broad-leaved forest along the south-easternmargin of the Tibetan Plateau. The close spacingof the isopols indicates that the pollen of thesebroad-leaved tree taxa are not transported very farfrom the forest source areas. The patterns of abund-ance (Fig. 3d) evident in tree-shrub taxa such asRosaceae and Ericaceae (including Rhododendron)closely mirror the distribution of broad-leavedforests, as might be expected given that these taxaare a major component of the forest understorey.

Moderate abundances of Gramineae (5–10%)are found in all types of vegetation from theTibetan Plateau (Fig. 3d). Given the low pollen

Fig. 3 continued.




productivity of this taxon, this implies thatGramineae are a fundamental component ofTibetan alpine vegetation. Leguminosae alsooccur in relatively high abundance over much ofthe Plateau (Fig. 3d). This is not a reflection ofthe fundamental importance of the Leguminosaeto all types of alpine vegetation, but is due to thefact that this taxon encompasses very diverse lifeforms (from forbs, through shrubs to trees) whichcannot be distinguished on the basis of theirpollen. Samples from south-eastern and south-western Tibet most likely contain primarilyarboreal species of the Leguminosae, while samplesfrom the steppe region in the north-western partof the Plateau are presumably composed offorb and shrub species of the Leguminosae.

DISCUSSION AND CONCLUSIONS

Sampling

The pollen data used in this study came fromsamples of several different types, including soil,moss polsters, lacustrine or fluvial sediments, andairborne dust traps. Pollen from most of thesetypes of samples appear to reflect closely regionalvegetation types on the Tibetan Plateau.Arboreal and shrub pollen (AP), including suchspecies as Quercus, Castanea, Juglans, Alnus andeven Lithocarpus, appears to be over-representedin dust trap samples. In Region VI where thevegetation is primarily alpine desert, for example,up to 20% AP is found in dust-trap samples. Suchhigh abundances are not found in other types ofsample from this region. We conclude that dust-trap samples do not provide a good representa-tion of either the local or regional vegetation onthe Tibetan Plateau. This may reflect the windi-ness of this region. Mean annual wind speeds of4.5 m/s are common over the north-western partof the Tibetan Plateau (i.e. Region VI) and thereare between 80 and 100 days with strong winds(> 12 m/s) per year (Li et al., 1991). Such strongwinds can transport pollen from forested regionsto the south and west of the Plateau over verylong distances into the Plateau interior.

Pollen patterns and the vegetation

With the exception of airborne dust-trapsamples, modern surface pollen samples closely

reflect the modern vegetation on the TibetanPlateau. Analysis of isopol maps shows thatspecific suites of taxa are characteristic of specificvegetation types. Thus, high abundances ofCyperaceae, Rosaceae, Ranunculaceae and Poly-gonaceae, with lower abundances of Artemisia,identify alpine meadow vegetation. Similarly, pro-viding airborne dust trap samples are excludedfrom consideration, high percentages of Pinus, Abiesand Picea identify regions of alpine conifer forests.Abundant Quercus, Betula and Salix pollen onlyoccurs in relatively moist areas characterized bybroad-leaved forest. Long-distance transport ofbroad-leaved tree pollen is clearly not as common asthe transport of conifer pollen, since there is an evenbetter agreement between the modern surface pollensamples and the distribution of broad-leaved foreststhan of conifer forests.

Artemisia and Chenopodiaceae occur inabundance in both the steppe and desert vegeta-tion types. Previous studies have indicated thatthe ratio of the two taxa can be used to distin-guish between steppe and desert vegetation (e.g.El-Moslimany, 1990; Yu et al., 1998). Yu et al.(1998) suggested that Artemisia pollen is moreabundant in steppe, while Chenopodiaceae pollenis most abundant in desert vegetation in China.The present study provides quantitative confirma-tion of this idea. Alpine desert vegetation in thenorthern part of the Tibetan Plateau is charac-terized by modern surface pollen samples withmore than 50% Chenopodiaceae and less than20% Artemisia (along with a number of othertaxa including Nitraria and Ephedra). Conversely,the steppe vegetation in the northern part of theTibetan Plateau is characterized by surfacepollen samples with more than 30% Artemisiaand less than 30% Chenopodiaceae. Theseconclusions may not apply to lowland desertand steppe vegetation. However, our results suggestthat attempts to discriminate non-forest vegeta-tion types objectively from pollen data (e.g.Prentice & Webb, 1998; Yu et al., 1998, 2000;Prentice et al., 2000) may be quite sensitive to thetreatment of these two taxa.

There is considerable ecological diversitywithin some of the taxa considered in this ana-lysis. The Leguminosae, for example, include indi-vidual species with very different life forms andcharacteristic of different environments. Astra-galus, for example, is a forb found in alpine


518 G. Yu et al.


steppe, while Caragana grows under colderconditions in the alpine desert and Lotus isan obligate aquatic. Since it is not possible todistinguish different taxa within the Legumi-nosae on the basis of their pollen, there willalways be some uncertainties attached to the eco-logical interpretation of assemblages dominatedby this taxon. A similar situation exists with theRosaceae, which includes typical alpine meadowshrubs (e.g. Potentilla) and temperate trees (Rosaand Spirea). A further important example, in thecontext of China, is Rhododendron, which is amember of the Ericaceae but occupies a very dif-ferent climatic niche from other taxa in this family.

Despite various uncertainties, our study showsthat modern surface pollen samples provide areasonably accurate representation of the modernvegetation. Surface pollen samples thereforeprovide an excellent way of testing the variousmethods for reconstructing environmental orclimatic parameters during the recent geologicalpast from palaeovegetation data. Increased densityof modern pollen sampling across the TibetanPlateau could considerably improve our abilityto discriminate different vegetation types. Indeed,there are very few modern pollen samples frommany parts of Asia (including much of India andSouth-east Asia) and systematic sampling ofthese regions would be useful in order to extendthe range of potential modern analogues for pastvegetation types.

ACKNOWLEDGMENTS

We thank Han Huiyou (Baxi region), Huang C-Xuan (West Kunlun Mt), Liu Guangxiu (NuergaiPlateau), Zheng Zhuo (Kunlun Mt), Huang Fei(Peiku Co), Liu Jinlin (Dahaizi Lake), SunXiangjun (Selin Co) and Kam-biu Liu (DundeIcecap) for providing pollen data. Pollen percent-age diagrams were plotted using the Psimpollprogram, developed by Keith Bennett. Finan-cial support was provided by the National(G1998040800) and Chinese Academy of Science’sKey Project for Basic Research on the TibetanPlateau (KZ951-A1–204 and KZ95T-06), theHundred Talents Project of the Chinese Academyof Science, the Chinese National Science Foun-dation (no. 49971075 and no. 49976026). Wethank two unnamed referees for their helpfulreviews of an earlier version of the manuscript.

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APPENDIX

The relative proportion of the 28 major taxa inmodern surface pollen samples from the TibetanPlateau. The 316 samples have been allocated, onthe basis of their geographical location alone, toone of the six vegetation regions identified fromthe literature (Regions I, II, III, IV, V and VI).

In each histogram, the values of the x-axis are inunits of pollen taxon percentage, and the valuesof the y-axis are ordered by sample numberswithin each vegetation region.

The appendix of six pollen percentage dia-grams can be obtained from the website ofGlobal Ecology and Biogeography: http://www.blackwell-science.com/geb


Documents

Modern pollen samples from alpine vegetation on the Tibetan Plateau