Vegetation and fire history of a Chinese site in southern tropical Xishuangbanna derived from...

ORIGINALARTICLE

Vegetation and fire history of a Chinesesite in southern tropical Xishuangbannaderived from phytolith and charcoalrecords from Holocene sediments

Yansheng Gu1,2*, Deborah M. Pearsall3, Shucheng Xie4 and Jianxin Yu1

1Key Laboratory of Biogeology and

Environmental Geology (China University of

Geosciences), Ministry of Education, Wuhan

430074, China, 2Institute of Ecology and

Environment, China University of Geosciences,

Wuhan 430074, China, 3Department of

Anthropology, University of Missouri,

Columbia, MO 65211, USA and 4State Key

Laboratory of Geological Processes and Mineral

Resources, China University of Geosciences,

Wuhan 430074, China

*Correspondence: Yansheng Gu, Key Laboratory

of Biogeology and Environmental Geology

(China University of Geosciences), Ministry of

Education, Wuhan 430074, China.

E-mail: [email protected]

ABSTRACT

Aim The aims of this paper are to reconstruct the vegetation and fire history over

the past 2000 years in a well-preserved rain-forest area, to understand

interactions between climate, fire, and vegetation, and to predict how rain

forest responds to global warming and increased intensity of human activity.

Location Xishuangbanna, south-west China, 21–22� N, 101–102� E.

Methods Phytolith (plant opal silica bodies) morphotypes, assemblages, and

indices were used to reconstruct palaeovegetation and palaeoclimate changes in

detail. Micro-charcoal particles found in phytolith slides, together with burnt

phytoliths and highly weathered bulliform cells, were employed to reconstruct a

record of past fire occurrence. A survey of field sediments, lithology, and 14C

dating were also employed.

Results Phytoliths were divided into 11 groups and classified into 33 well-

described morphotypes according to their shape under light microscopy and their

presumed anatomical origins and ecological significance. The phytolith

assemblages were divided into six significant zones that reveal a complete

history of vegetation changes corresponding to climate variation and fire

occurrence. Phytolith assemblages and indices show that the palaeoclimate in the

study area is characterized by the alternation of warm–wet and cool–dry

conditions. Phytolith and charcoal records reveal that 12 fire episodes occurred.

Comparison of burnt phytoliths with an aridity index (Iph) shows that fire

episodes have a strong relationship with drought events.

Main conclusions Our results indicate that fire occurrence in the tropical rain

forest of Xishuangbanna is predominantly under the control of natural climate

variability (drought events). Nearly every fire episode is coupled with a climatic

event and has triggered vegetation composition changes marked by a pronounced

expansion of grasses. This indicates that drought interacts with fire to exert a

strong influence on the ecological dynamics of the rain forest. However, the

impact of human activity in recent centuries is also significant. Our results are

important for understanding the interactions between climate, fire, and

vegetation, and for predicting how rain forest responds to global warming and

increased human activity.

Keywords

Charcoal, China, climate change, fire history, Holocene sediments, phytoliths,

tropical rain forest, Xishuangbanna.

Journal of Biogeography (J. Biogeogr.) (2008) 35, 325–341

ª 2007 The Authors www.blackwellpublishing.com/jbi 325Journal compilation ª 2007 Blackwell Publishing Ltd doi:10.1111/j.1365-2699.2007.01763.x

INTRODUCTION

A given rain forest will experience variations in rainfall pattern

and in the frequency and intensity of droughts. The increase in

research into droughts in tropical forests has resulted in

increased attention being paid to forest–climate relationships

and rain-forest deforestation and fragmentation (Walsh, 1996;

Walsh & Newbery, 1999; Potts, 2003). These issues are

especially relevant in tropical monsoon areas, where climate

is characterized by a distinctive distribution of precipitation.

Severe droughts can lead to the destruction of habitat and a

decrease in biodiversity in the Tropics, especially if accom-

panied by fires, which accelerate these changes (Woods, 1989;

Kellman & Meave, 1997; Harrison, 2000, 2001; Laurance et al.,

2000; Williamson et al., 2000; Potts, 2003; van Nieuwstadt &

Sheil, 2005). Global warming could also exacerbate habitat

destruction if it promotes a drier climate or strong El Nino-

Southern Oscillation (ENSO) events in the Tropics (Laurance

& Williamson, 2001). An understanding of drought and fire

history is thus important in evaluating interactions among

natural climate variability (drought events), vegetation, and

human disturbance, at least during the last several millennia,

and may help to predict how tropical ecosystems will respond

to future climate change (Marchant, 2005).

Primary rain forest is well preserved in tropical Xis-

huangbanna (Fig. 1). The climate history of the region,

including drought episodes, has been reconstructed in part

using pollen and documentary records from surrounding areas

(Liu et al., 1986; Sun et al., 1986; Walker, 1986; Zhu, 1997;

Zhu & Cai, 2005). The lack of stratigraphic palaeoecological

data from Xishuangbanna has prevented us from determining

the extent to which the rain forest of this region is sensitive to

climate change. Little is known about the relationship between

climate, vegetation and fire events in the tropical rain forests of

Xishuangbanna.

The aims of this paper are to reconstruct the local

palaeovegetation, palaeoclimate, and fire history of Xis-

huangbanna, and to understand their interrelationships using

phytolith and charcoal records from fluvial sediments in the

well-preserved rain forest area. Phytoliths are an important

and reliable tool for reconstructing local vegetation and

climate. Grass phytoliths, in particular, offer a promising

means to differentiate between grasses at the subfamily level

and to infer subtle changes in palaeoenvironmental conditions

(Twiss, 1987, 1992; Fredlund & Tieszen, 1994, 1997; Alexandre

et al., 1997; Lu & Liu, 2003b). Grass phytolith indices have

been successfully used to reconstruct humidity and aridity in

marine sediments and grasslands (Diester-Haass et al., 1973;

Twiss, 1987; Fredlund & Tieszen, 1994, 1997; Alexandre et al.,

1997; Barboni et al., 1999; Parker et al., 2004), although they

have not been widely used in tropical rain-forest areas. Burnt

phytoliths and highly weathered bulliform cells, together with

Figure 1 Map showing the distribution of

modern tropical rain forest in southern Xis-

huangbanna, the sampling site, and places

referred to in the text (Rain forest types are

after Zhu, 1997).

Y. Gu et al.

326 Journal of Biogeography 35, 325–341ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd

charcoal particles, are used here as indicators of fire episodes in

association with changes in vegetation and climate.

MATERIALS AND METHODS

Sampling

The samples studied here were taken from a site in tropical

Xishuangbanna in south-west China (Fig. 1). The study area is

located on the southern margin (21–22� N, 101–102� E) of

Xishuangbannan Dai Autonomous Prefecture in Yunnan

Province. Altitude ranges from 477 to 2007 m. The tropical

rain forest, which is part of the tropical Asian flora, occurs

mostly in the lower hills, mountain valleys, and the basin

below 900 m altitude (Zhu, 1997). It is mainly composed of

mixed rain forest and dipterocarp rain forest, with the former

widely distributed in the study area (Fig. 1). The tropical rain

forest in Xishuangbanna has been conserved since 1958, and is

now part of a State Nature Reserve in which there is little

human activity (Zhu, 1997; Zhu et al., 1998). The rain forest

has a high biodiversity: over 3500 species of higher plants, 700

species of higher animals, and 1500 species of insects have been

recorded (Cao & Zhang, 1997; Pu et al., 2001). The regional

climate is dominated by the south-west monsoon (Indian

Ocean monsoon), but the south-east monsoon also affects the

region (Zhang, 1988). The migrating rain-belt of the south-

west monsoon has a strong influence on the rain-forest

ecosystem, resulting in a marked dry season (November to

April) and a wet season (May to October). At Menglun station,

a mixed rain-forest area, the annual mean precipitation is

1463.3 mm, of which 85% falls during the wet season. In

contrast to the precipitation pattern, the monthly mean

temperature remains stable (21.7�C), although it rises slightly

in the wet season (25.7�C) (Liu & Li, 1996).

Samples were collected from a 1.5-m-deep profile of well-

developed fluvial sediments (clay and silt) in a seasonal pond

inside a 3-km-long gully (Fig. 2). The stratigraphy is described

in Table 1. Phytolith samples were taken at 2-cm intervals

from the top to a depth of 50 cm, and at 5-cm intervals from

50 cm to the bottom of the profile. The macro-charcoal

remains (> 1 mm) were identified by their shape and size and

described in situ.

Dating

A sample for radiocarbon dating was collected at a depth of

95–100 cm from an area where macro-charcoals were abun-

dant (layer 4 in Fig. 2). This was wet-sieved to remove

particles larger than 0.3 mm. 14C dating was performed at the

State Key Laboratory of Earthquake Dynamics of the China

Earthquake Administration, by liquid scintillation counting of

Figure 2 Sampling profile and lithological

features of sediments.

Table 1 Stratigraphic description of sediments from the tropical

rain forest in Xishuangbanna.

Depth

(cm)

Layer

no. Lithological description

0–15 (1) Predominantly black clay with roots and organics;

locally with abundant macro-charcoal particles;

macro-charcoal size up to 0.8 cm in diameter with

mostly angular shapes

15–65 (2) Black silt clay with locally abundant macro-charcoal

particles; macro-charcoal size up to 0.5 cm in

diameter with mostly angular shapes; locally with

circular quartz particles of size up to 0.2 cm in

diameter

65–85 (3) Light yellow silt locally with elliptic pebbles of size

up to 4 cm in diameter; circular quartz particles of

size up to 0.5 cm in diameter; abundant macro-

charcoal particles up to 0.3 cm in diameter with

mostly angular shapes

85–100 (4) Black silt clay with abundant macro-charcoal

particles of size up to 0.8 cm in diameter; locally

with elliptic pebbles up to 2 cm in diameter

Vegetation and fire history from phytolith and charcoal records

Journal of Biogeography 35, 325–341 327ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd

benzene samples after washing with 5% NaOH and 10% HCl

solutions. Age was calculated using software from the LKB

Company according to the Chinese sugar standard; calibra-

tion was performed using calib v4.1.2 and a conventional

half-life of 5568 years (Stuiver & Reimer, 1993). Dating

showed that the sediments in the profile we studied were

deposited from before 1780 ± 110 yr bp until the present.

Tang (1992) reported a 14C date of 1660 ± 150 yr bp at a

depth of 1 m in fluvial sediments deposited near the study

area, which indicates a stable sedimentation rate in the area

(Tang, 1992).

Phytolith extraction, identification and counting

Phytolith extraction was carried out at the Key Laboratory of

Biogeology and Environmental Geology at the Chinese

University of Geosciences. The extraction method followed

Wang & Lu (1993). Air-dried samples of 10 g were

deflocculated in NaHCO3, and this was followed by carbon-

ate removal using 15% HCl and oxidation of organic matter

using a 30% solution of H2O2. After removal of sand grains

and clay minerals by sieving and gravity sedimentation,

respectively, phytoliths were transferred into a ZnI2 solution

(density 2.30 g cm)3), centrifuged at 2000 rpm for 10 min,

and washed with 1% acetic acid. After drying, phytoliths

were embedded in Canada balsam to make slides for

microscopic observation. Identification and counting for 35

phytolith samples was carried out at the Phytolith Laboratory

of the Department of Anthropology, University of Missouri-

Columbia. Extracts were scanned at 400 · magnification

using a Zeiss light microscope. For each sample (each slide),

phytoliths were identified and counted in 10 fields; counts

ranged from 126 to 720 phytoliths (with the majority around

400).

The identification of phytoliths in geological sediments is

based on morphological comparison with modern specimens,

as geological reference specimens are not available. In this

study, phytoliths were described and compared with the

phytolith reference collections at the University of Missouri

(see MU phytolith website: http://www.missouri.edu/~phyto)

and with published sources (Twiss et al., 1969; Brown, 1984;

Mulholland, 1989; Twiss, 1992; Bozarth, 1993; Wang & Lu,

1993; Carnelli et al., 2004; Stromberg, 2004; Lu et al., 2006;

Piperno, 2006). All phytolith morphotypes identified here

were described using International Code for Phytolith

Nomenclature (ICPN) 1.0 protocols (Madella et al., 2005)

whenever possible.

An aridity index (Iph) and a climate index (Ic) were

calculated from phytolith count data using the approaches of

Diester-Haass (Diester-Haass et al., 1973; Alexandre et al.,

1997) and Twiss (1987, 1992). The aridity index, Iph, is the

ratio of Chloridoideae to total Chloridoideae and Panicoideae

phytoliths. The climate index, Ic, is the ratio of Pooideae to

total Pooideae, Panicoideae, and Chloridoideae phytoliths,

with high values corresponding to cool temperatures (Twiss,

1992).

Phytolith morphotypes and classification

Phytolith morphotypes are described in detail (see Table 2)

and illustrated in Fig. 3. The major plant groups documented

in the profile samples are Poaceae (Gramineae), Cyperaceae,

Asteraceae (Compositae), ferns, gymnosperms, and broad-

leaved trees (Fig. 3). Poaceae phytoliths include short cells,

long cells, bulliform cells, and hair cells (grass-type). Short cells

can be classified as Pooideae (rondel and trapeziform),

Panicoideae (bilobate, cross, and polylobate), Chloridoideae

(square saddle), Bambusoideae (oblong concave saddle or

collapsed saddle) or Arundinoideae (trapeziform saddle or

plateaued saddle), based on their micro-morphological char-

acteristics and typical descriptions (Twiss et al., 1969; Brown,

1984; Twiss, 1987, 1992; Mulholland, 1989; Wang & Lu, 1993;

Fredlund & Tieszen, 1994; Piperno & Pearsall, 1998) (Fig. 3).

Sponge spicules and diatoms were also present in samples in

association with phytoliths.

It is important to mention that classification based on short

cells does not correspond completely to the grass subfamilies

designated above. For example, rondel short cells can occur in

Oryzoideae and Panicoideae; crosses can occur in Bambuso-

ideae, Oryzoideae and Arundinoideae; and bilobates can occur

in Bambusoideae, Oryzoideae and Arundinoideae (Twiss et al.,

1969; Twiss, 1992; Barboni et al., 1999). It should also be noted

that a small number of Pooideae phytoliths are produced in

Chloridoieae and Panicoideae (Brown, 1984; Mulholland,

1989; Barboni et al., 1999). These overlaps should be taken

into account during phytolith interpretation.

The correct classification of saddles is of great significance

for interpreting a climate signal on the basis of the climate (Ic)

and aridity (Iph) indices. Bambusoideae contribute large

numbers of oblong concave saddles (collapsed saddles), and

Arundinoideae produce many distinctive phytoliths called

trapeziform saddles (plateaued saddles). Compared with

saddles from the above two grass subfamilies, saddles from

Chloridoideae are either shorter and thicker or have different

three-dimensional structures. In particular, square saddles are

mostly ‘squat’, meaning that the axis of the phytolith

exhibiting the double-edge, saddle-like outline is wider than

the other axes, or the two axes are of equal dimensions

(Piperno, 2006). These distinctive morphological characteris-

tics of saddles permit reliable subfamily-level classification,

enabling Bambusoideae to be distinguished from Chlorido-

ideae and Arundinoideae (Wang & Lu, 1993; Piperno &

Pearsall, 1998; Lu et al., 2006; Piperno, 2006).

Opaque platelets with systematic perforations are produced

in the inflorescences of selected species of Asteraceae (Piperno,

1988; Bozarth, 1992; Mercader et al., 2000). The Cyperaceae

are characterized by polyhedrons with conical projections

(Piperno, 1988, 1989, 2006; Bozarth, 1992; Ollendorf, 1992;

Wang & Lu, 1993; Lu et al., 2006; Pearsall et al., 2006) (Fig. 3).

Ferns are rich in elongates with two undulating ridges, prisms

that are sinuate or cavate, and oblong lacunose phytoliths

(Piperno, 1988; Wang & Lu, 1993; Carter, 2002). Gymnosperm

trees produce predominantly parallelepipedal contorteds and

Y. Gu et al.


Tab

le2

Des

crip

tio

no

fp

hyt

oli

thm

orp

ho

typ

es,

anat

om

ical

ori

gin

,p

alae

oec

olo

gica

lsi

gnifi

can

cean

dav

aila

ble

lite

ratu

re.

Maj

or

pla

nt

gro

up

Nam

eO

ther

Nam

esF

ig.

3M

orp

ho

typ

ed

escr

ipti

on

Size

ran

ge

(lm

)

An

ato

mic

al

Ori

gin

Pal

aeo

eco

logi

cal

sign

ifica

nce

Ref

eren

ces

Pan

ico

idea

e

gras

ses

Bil

ob

ate*

Du

mb

bel

l(a

)–(d

)B

ilo

bat

e,h

avin

gtw

o

lob

es

<25

Ep

ider

mal

sho

rtce

ll

Mo

sto

fth

eP

anic

oid

eae

are

C4

pla

nts

flo

uri

shin

g

inw

arm

,tr

op

ical

to

sub

tro

pic

alre

gio

ns

wit

h

am

od

erat

eam

ou

nt

of

avai

lab

leso

ilm

ois

ture

.

Tw

iss

etal

.(1

969)

,B

row

n

(198

4),

Pip

ern

o(1

988)

,T

wis

s

(199

2),

Mu

lho

llan

d&

Rap

p

(199

2),

Wan

g&

Lu

(199

3),

Pea

rsal

l(2

000)

,M

adel

laet

al.

(200

5)

Cro

ss*

(e),

(g)–

(h)

Qu

adra

-lo

bat

e,h

avin

g

fou

rlo

bes

<15

Tw

iss

etal

.(1

969)

,B

row

n

(198

4),

Pip

ern

o(1

988)

,T

wis

s

(199

2),

Mu

lho

llan

d&

Rap

p

(199

2),

Pea

rsal

l(2

000)

,M

adel

la

etal

.(2

005)

Cyl

ind

rica

l

po

lylo

bat

e*

Po

lylo

bat

eo

r

com

ple

x

du

mb

bel

l

(am

),(a

p)–

(aq

)P

oly

lob

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hav

ing

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re

than

two

lob

esli

nea

rly

arra

nge

d

<35

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iss

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.(1

969)

,T

wis

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992)

,

Mu

lho

llan

d&

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p(1

992)

,

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rsal

l(2

000)

,M

adel

laet

al.

(200

5)

Bam

bu

soid

eae

gras

ses

Ob

lon

g

con

cave

sad

dle

*

Lo

ng

sad

dle

or

coll

apse

d

sad

dle

(f),

(i)–

(k)

Co

llap

sed

sad

dle

or

Ch

usq

uea

-typ

esa

dd

le,

usu

ally

wid

th<

hei

ght

<20

Bam

bu

soid

eae

gras

ses

are

C3

pla

nts

flo

uri

shin

gin

war

mo

rh

ot,

hu

mid

tro

pic

alto

sub

tro

pic

al

regi

on

s.

Pip

ern

o(1

988)

,T

wis

s(1

992)

,

Wan

g&

Lu

(199

3),

Pip

ern

o&

Pea

rsal

l(1

998)

,Sa

se&

Ho

son

o

(200

1),

Li

etal

.(2

005)

,

Mad

ella

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.(2

005)

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al.

(200

6)

Aru

nd

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idea

e

gras

ses

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pez

ifo

rm

sad

dle

*

(m)–

(n)

Pla

teau

edsa

dd

leo

r

Ph

ragm

ites

-typ

eo

r

Ari

stid

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pe

<15

Mo

sto

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e

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nd

ino

idea

ear

eC

3

pla

nts

flo

uri

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gin

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m,

sem

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dto

mo

ist

regi

on

s.

Wan

g&

Lu

(199

3),

Pip

ern

o&

Pea

rsal

l(1

998)

,L

u&

Liu

(200

3a),

Pip

ern

o(2

006)

Od

dSi

de

view

like

acu

pw

ith

a

rou

nd

bas

ean

dca

vate

sid

e

<10

Ch

lori

do

idea

e

gras

ses

Squ

are

sad

dle

*

Sho

rtsa

dd

le(o

)–(s

)C

hlo

rid

oid

eae

sad

dle

,

usu

ally

wid

th‡

hei

ght

<15

Ch

lori

do

idea

egr

asse

sar

e

typ

ical

C4

gras

ses

gro

win

gin

war

m,

arid

to

sem

iari

dre

gio

ns

wh

ere

soil

mo

istu

reis

very

low

.

Tw

iss

etal

.(1

969)

,B

row

n

(198

4),

Mu

lho

llan

d(1

989)

,

Tw

iss

(199

2),

Mu

lho

llan

d&

Rap

p(1

992)

,W

ang

&L

u

(199

3),

Pip

ern

o&

Pea

rsal

l

(199

8),

Bar

bo

ni

etal

.(1

999)

,G

ob

etz

&B

oza

rth

(200

1),

Lu

&L

iu

(200

3b),

Mad

ella

etal

.(2

005)

Po

oid

eae

gras

ses

Ro

nd

el*

Co

nic

al,

hat

-sh

aped

,

circ

ula

r,o

val

(t)–

(u)

Co

nic

al<

15P

oo

idea

egr

asse

sar

ety

pic

al

C3

pla

nts

gro

win

gin

coo

l

and

wet

regi

on

sw

ith

hig

h

lati

tud

eso

rel

evat

ion

s.

Mu

lho

llan

d(1

989)

,M

ulh

oll

and

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app

(199

2),

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g&

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(199

3),

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dlu

nd

&T

iesz

en

(199

4),

Pea

rsal

l(2

000)

,M

adel

la

etal

.(2

005)



Tab

le2

con

tin

ued

.

Maj

or

pla

nt

gro

up

Nam

eO

ther

Nam

esF

ig.

3M

orp

ho

typ

ed

escr

ipti

on

Size

ran

ge

(lm

)

An

ato

mic

al

Ori

gin

Pal

aeo

eco

logi

cal

sign

ifica

nce

Ref

eren

ces

Tra

pez

ifo

rm

po

lylo

bat

e*

Tra

pez

oid

or

po

lylo

bat

e,

cren

ate,

ob

lon

g

(af)

–(a

g)T

rap

ezif

orm

po

lylo

bat

e<

30T

wis

set

al.

(196

9),

Bro

wn

(198

4),

Kap

lan

etal

.(1

992)

,

Mu

lho

llan

d&

Rap

p(1

992)

,

Tw

iss

(199

2),

Wan

g&

Lu

(199

3),

Fre

dlu

nd

&T

iesz

en(1

994)

Tra

pez

ifo

rm

sin

uat

e*

(ah

)T

rap

ezif

orm

sin

uat

e<

35P

ears

all

(200

0),

Lu

&L

iu

(200

3b),

Mad

ella

etal

.(2

005)

,

Lu

etal

.(2

006)

No

taxa

tom

ical

sign

ifica

nce

Elo

nga

te

smo

oth

*

Elo

nga

teo

r

qu

adri

late

ral,

rect

angl

e,

trap

ezo

id,

lon

gce

llw

all

(aj)

Elo

nga

tew

ith

smo

oth

edge

<45

Ep

ider

mal

lon

gce

ll

Elo

nga

tep

hyt

oli

ths

are

mo

stly

abu

nd

ant

in

tem

per

ate

tosu

btr

op

ical

regi

on

s.

Tw

iss

etal

.(1

969)

,K

apla

net

al.

(199

2),

Mu

lho

llan

d&

Rap

p(1

992)

,P

ears

all

&

Eli

zab

eth

(199

2),

Tw

iss

(199

2),

Pip

ern

o&

Pea

rsal

l(1

998)

,

Mad

ella

etal

.(2

005)

,

Wan

g&

Lu

(199

3),

Lu

etal

.(2

006)

Elo

nga

te

ech

inat

e*

(al)

Elo

nga

tew

ith

ech

inat

e

edge

<10

0

Cu

nei

form

bu

llif

orm

cell

*

Fan

-sh

aped

,

bu

llif

orm

(y)–

(aa)

Cu

nei

form

<70

Lea

fb

ull

ifo

rm

cell

Bu

llif

orm

cell

sar

em

ost

ly

abu

nd

ant

insu

btr

op

ical

to

tro

pic

alre

gio

ns.

Tw

iss

etal

.(1

969)

,T

wis

s(1

992)

,

Mu

lho

llan

d&

Rap

p(1

992)

,

Pea

rsal

l&

Eli

zab

eth

(199

2),

Wan

g&

Lu

(199

3),

Bre

mo

nd

etal

.

(200

5),

Lu

&L

iu(2

005)

,

Mad

ella

etal

.(2

005)

,L

uet

al.

(200

6)

Par

alle

lep

iped

al

bu

llif

orm

cell

*

Squ

are,

rect

angu

lar

(x),

(ab

)P

aral

lele

pip

edal

<40

Un

cifo

rmh

air

cell

*

Po

int-

shap

ed,

hai

rce

ll,

lon

g

cell

app

end

age,

tric

ho

me

(ai)

,(a

k)Sc

uti

form

or

lan

ceo

late

<50

Hai

rce

ll

mes

op

hyl

lin

gras

ses

Un

cifo

rmh

air

cell

sar

e

mo

stly

abu

nd

ant

in

tem

per

ate

tosu

btr

op

ical

regi

on

s.

Tw

iss

etal

.(1

969)

,B

row

n

(198

4),

Pip

ern

o(1

988)

,K

apla

net

al.

(199

2),

Mu

lho

llan

d&

Rap

p(1

992)

,

Pea

rsal

l&

Eli

zab

eth

(199

2),

Tw

iss

(199

2),

Wan

g&

Lu

(199

3),

Lu

&L

iu

(200

3b),

Mad

ella

etal

.(2

005)

,

Lu

etal

.(2

006)

Cyp

erac

eae

Po

lyh

edro

nw

ith

con

ical

pro

ject

ion

Co

ne-

shap

ed,

hat

-sh

aped

,

con

es,

ron

del

con

cave

(am

)-(a

n)

To

pvi

ew,

po

lyh

edro

n

wit

hco

nic

alp

roje

ctio

ns,

usu

ally

wit

hp

itte

d

surf

ace

<40

Lea

fo

rse

ed

epid

erm

alce

ll

Mo

stly

dis

trib

ute

din

wet

hab

itat

.

Pip

ern

o(1

988,

1989

,20

06),

Bo

zart

h

(199

2),

Oll

end

orf

(199

2),

Wan

g&

Lu

(199

3),

Pea

rsal

let

al.

(200

6)

Y. Gu et al.


Tab

le2

con

tin

ued

.

Maj

or

pla

nt

gro

up

Nam

eO

ther

Nam

esF

ig.

3M

orp

ho

typ

ed

escr

ipti

on

Size

ran

ge

(lm

)

An

ato

mic

al

Ori

gin

Pal

aeo

eco

logi

cal

sign

ifica

nce

Ref

eren

ces

Ast

erac

eae

Op

aqu

e

per

fora

ted

pla

tele

t

(ac)

Op

aqu

ep

erfo

rate

d

pla

tele

t

<80

Ep

ider

mal

cell

Mo

stly

dis

trib

ute

din

coo

l,

sem

iari

dre

gio

ns.

Pip

ern

o(1

988)

,B

oza

rth

(199

2),

Mer

cad

eret

al.

(200

0)

Fer

nE

lon

gate

un

du

lati

ng

(ao

)E

lon

gate

wit

htw

o

par

alle

led

un

du

lati

ng

rid

ges

Mo

stly

dis

trib

ute

din

war

m

and

wet

hab

itat

s.

Pip

ern

o(1

988)

Pri

smsi

nu

ate

(ap

)P

rism

-sh

aped

wit

h

sin

uat

eed

ges

<60

Wan

g&

Lu

(199

3)

Ob

lon

g

lacu

no

se*

Tab

ula

rw

ith

cava

teed

ges

and

surf

aces

<70

Gym

no

sper

mP

aral

lele

pip

edal

con

tort

ed*

Po

lyh

edra

l,

Po

lyh

edro

n-

shap

ed

blo

cky

or

elo

nga

te

(aq

)–(a

r)P

oly

hed

ron

,u

sual

ly

blo

cky

or

elo

nga

tew

ith

smo

oth

or

pit

ted

surf

ace

<80

Ep

ider

mal

or

sub

epid

erm

al,

end

od

erm

alan

d

trac

hei

d

Mo

stly

dis

trib

ute

din

coo

l

toco

ldan

dse

mia

rid

regi

on

sw

ith

hig

hla

titu

des

or

elev

atio

ns.

Bo

zart

h(1

993)

,W

ang

&L

u

(199

3),

Stro

mb

erg

(200

4),

Car

nel

liet

al.

(200

4)

Cla

vate

con

tort

ed*

Bo

ot-

shap

ed(b

c)B

ent

like

ab

oo

t,w

ith

gran

ula

tesu

rfac

e

<40

Wan

g&

Lu

(199

3)

Bro

ad-l

eave

dA

bb

revi

ated

stel

late

*

An

ticl

inca

l

epid

erm

is,

jigs

aw-s

hap

ed

(av)

–(a

w)

Has

sin

uo

us

shap

e,n

o

do

ub

leo

utl

ines

<50

Ep

ider

mal

cell

Ab

un

dan

tin

dec

idu

ou

s

tree

s.

Pip

ern

o(1

988)

,B

oza

rth

(199

2),

Wan

g&

Lu

(199

3),

Sase

&

Ho

son

o(2

001)

,L

u&

Liu

(200

5),

Pea

rsal

let

al.

(200

6)

Aci

cula

rh

air

cell

*

Hai

rce

ll(a

s)–(

at)

Has

ab

igb

ase

<60

Hai

rce

lls

mes

op

hyl

lin

sid

e

tree

s

Ab

un

dan

tin

tro

pic

al

regi

on

s.

Cyl

ind

ric

scle

reid

*

Scle

reid

(bk)

–(b

l)A

vari

ably

shap

ed

scle

ren

chym

ato

us

cell

wit

ha

cen

tral

spin

eo

r

rid

ge,

usu

ally

iso

dia

met

ric

<10

0P

aren

chym

ao

f

leaf

Ab

un

dan

tin

sub

tro

pic

al,

tro

pic

alre

gio

ns.

Pip

ern

o(1

988,

2006

),P

ears

all

&

Eli

zab

eth

(199

2),

Kea

lho

fer

&

Pip

ern

o(1

998)

,St

rom

ber

g

(200

4),

Pea

rsal

let

al.

(200

6)

Glo

bu

lar

face

tate

*

Hem

isp

her

e(b

d)–

(be)

Fac

eted

,to

pvi

ew,

usu

ally

sph

eric

alb

od

y,ed

ges

are

scal

lop

ed

<20

Ep

ider

mal

cell

Glo

bu

lar

face

tate

ph

yto

lith

s

are

abu

nd

ant

in

sub

tro

pic

al,

tro

pic

al

regi

on

s.

Kea

lho

fer

&P

iper

no

(199

8),

Ru

nge

(199

9),

Pea

rsal

let

al.

(200

6)

Glo

bu

lar

gran

ula

te*

Sph

eric

al

rugo

se,

sph

ero

id

(ax)

–(b

a)G

lob

ula

rsp

her

ical

wit

h

gran

ula

tesu

rfac

e

<30

Ab

un

dan

tin

sub

tro

pic

al

and

tro

pic

alre

gio

ns.

Pip

ern

o(1

988,

2006

),W

ang

&

Lu

(199

3),

Kea

lho

fer

&P

iper

no

(199

8),

Th

orn

(200

4)



Tab

le2

con

tin

ued

.

Maj

or

pla

nt

gro

up

Nam

eO

ther

Nam

esF

ig.

3M

orp

ho

typ

ed

escr

ipti

on

Size

ran

ge

(lm

)

An

ato

mic

al

Ori

gin

Pal

aeo

eco

logi

cal

sign

ifica

nce

Ref

eren

ces

Glo

bu

lar

psi

late

*

(bb

)G

lob

ula

rp

sila

te<

20E

pid

erm

alce

llA

bu

nd

ant

insu

btr

op

ical

and

tro

pic

alre

gio

ns.

Mu

ltif

acet

ed

blo

cky

Po

lyh

edro

n

aggr

egat

e

(bh

)–(b

i)M

ult

ifac

eted

,b

lock

yo

r

qu

adri

late

ral

or

irre

gula

r

<70

Cel

lw

all

Pip

ern

o(1

988)

,W

ang

&L

u

(199

3),

Kea

lho

fer

&P

iper

no

(199

8),

Go

bet

z&

Bo

zart

h(2

001)

Sase

&H

oso

no

(200

1),

Pla

tele

tp

oly

gon

Po

lyh

edra

l

pla

te-l

ike

(au

)A

po

lygo

nh

avin

gm

ore

than

five

sid

esw

ith

smo

oth

surf

ace

<50

Ep

ider

mal

cell

Pip

ern

o(1

988)

,B

oza

rth

(199

2),

Eli

zab

eth

&R

ow

lett

(199

3),

Wan

g&

Lu

(199

3)

Seed

ph

yto

lith

Po

lygo

n,

hav

ing

mo

re

than

six

sid

esw

ith

pit

ted

surf

ace

<25

Seed

epid

erm

al

cell

Ab

un

dan

tin

tro

pic

al

regi

on

s.

Pip

ern

o(1

988)

Tri

-ste

llat

e

tru

nca

te

Bea

k-sh

aped

(bj)

Ap

oly

gon

hav

ing

thre

e

sid

esco

mp

are

sph

eric

al

tria

ngl

e

<50

Ep

ider

mal

cell

Ab

un

dan

tin

sub

tro

pic

al,

tro

pic

alre

gio

ns.

Wan

g&

Lu

(199

3),

Pea

rsal

let

al.

(200

6)

Vas

cula

rti

ssu

e*V

ascu

lar

cell

s,

trac

hei

d,

pip

e-sh

aped

(bf)

–(b

g)V

ascu

lar

cell

or

tiss

ue,

usu

ally

spin

dle

-sh

aped

lam

inat

e

<15

0V

ascu

lar

tiss

ues

Pip

ern

o(1

988)

,W

ang

&L

u

(199

3),

Ru

nge

(199

9)

*An

ICP

N.

Y. Gu et al.


clavate contorted phytoliths (Bozarth, 1993; Wang & Lu, 1993;

Sase & Hosono, 2001; Carnelli et al., 2004; Stromberg, 2004).

Broad-leaved trees produce many kinds of phytoliths, inclu-

ding the following types: multifaceted blocky, platelet polygon,

abbreviated stellate, globular psilate, globular granulate, vas-

cular tissue (spindle laminate), globular facetate, tri-stellate

truncate, cylindrical sclereid, acicular hair cell (tree-type), and

seed phytoliths. The first three are abundant in deciduous trees

(Pearsall et al., 2006; Piperno, 1988; Bozarth, 1992; Wang &

Lu, 1993; Kealhofer & Piperno, 1998; Sase & Hosono, 2001;

Gobetz & Bozarth, 2001; Lu & Liu, 2005); the rest are

abundant in subtropical and tropical regions, and originate

mostly from ligneous dicots (Piperno, 1988; Barboni et al.,

1999).

Analysis of fire evidence

Direct evidence for fire was established by counting burnt

phytoliths and charcoal particles (see Appendix S1 in the

supplementary material). Burning is associated with surface

vegetation clearance and increased slope erosion on hills.

Increased erosion leads to increased and immediate transport

of phytoliths, which reduces the time for which phytoliths are

exposed in the source areas. The degree of weathering on large

phytoliths, such as bulliform cells, is therefore an indirect

indicator of fire events. Compared with fresh bulliform cells,

highly weathered bulliform cells have rebuilt edges and

surfaces marked with a number of cavities (see Appendix

S2). The frequency of occurrence of highly weathered bulli-

form cells should decrease in sediments formed during fire

episodes. As an indirect indicator, highly weathered bulliform

cells have a positive relationship with wet and warm condi-

tions, which suggests that they indicate the recovery of surface

vegetation and stable weathering processes.

Micro-charcoal particles in slides can be classified into two

size groups, namely 100–25 and < 25 lm. Micro-charcoals of

size 100–25 lm are abundant in fire episodes, whereas micro-

charcoals < 25 lm are abundant in pre-fire periods (see

Appendix S3). Micro-charcoal particles found in phytolith

slides, together with burnt phytoliths and highly weathered

bulliform cells, were counted for 20 fields at 250 · magni-

fication. Tallies of all phytoliths were made for the same

fields. Abundances of burnt phytoliths, highly weathered

bulliform cells, and micro-charcoal particles relative to the

total number of phytoliths counted for each sample were

calculated, providing a relative measure of the severity of the

fire episode.

RESULTS

Phytolith assemblages and indices

On the basis of phytolith identifications and phytolith indices,

six zones of phytolith assemblages can be discriminated and

interpreted in terms of climate and major vegetation constit-

uents (Figs 4 & 5).

Zone 1 (100–87 cm):

The Ic (climatic index) and Iph (aridity index) ratios vary

from 29% to 64%, and from 64% to 83%, respectively.

Poaceae phytoliths make up the greatest part of the

assemblage, varying from 67% to 76%, with bulliform cells

and long cells making the greatest contributions. The

proportion of fern and Asteraceae phytoliths increase within

Zone 1 together with Poaceae. Broad-leaved tree types occur

in reverse proportion to Poaceae phytoliths, decreasing from

16% to 10%. Globular granulate, acicular hair cells, vascular

tissues and sclereids are common broad-leaved trees types.

Gymnosperm phytoliths remain stable. It should be noted

that the short cells in this zone are present in relatively low

quantities, which might influence the high Iph value of 83%

(see Appendix S4).

Zone 2 (87–72 cm):

Poaceae phytoliths still make up the greatest part of the

assemblage. Bulliform cells, long cells, and Bambusoideae,

Panicoideae, and Pooideae short cells make the greatest

contributions. Broad-leaved tree phytoliths are present in

inverse proportion to the Poaceae phytoliths; globular granu-

late and sclereid forms are dominant, and multi-faceted

blocky, platelet polygon, and vascular tissues are common.

Ferns and Asteraceae decline slightly, together with Poaceae.

Gymnosperm phytoliths increase slightly.

Zone 3 (72–58 cm):

As in Zone 1, Poaceae phytoliths are dominant in assemblage

zone 3. Bulliform cells, long cells, and Bambusoideae, Arun-

dinoideae, and Pooideae short cells are abundant. Arundino-

ideae phytoliths increase, while Bambusoideae phytoliths

decrease. Broad-leaved tree phytoliths still occur in inverse

proportion to Poaceae phytoliths. Globular granulate, globular

psilate, and sclereids are dominant; multifaceted blocky,

globular facetate, acicular hair cell, and vascular tissues are

common. Fern and gymnosperm phytoliths decrease gradually.

Zone 4 (58–44 cm):

This zone is similar to Zone 2. Poaceae phytoliths are still

dominant. The assemblage of broad-leaved tree phytoliths is

dominated by globular granulate, globular psilate, multifaceted

blocky, and sclereids. Globular facetate, acicular hair cell, and

vascular tissues also occur.

Zone 5 (44–9 cm):

The Ic and Iph ratios are variable within this zone. The

majority of the Ic values are lower than 50%, and fluctuate

around 30%. The pattern is similar for the Iph ratio. Poaceae

phytoliths are dominant in this zone. Bulliform cells, long cells,

and Bambusoideae, Pooideae and Panicoideae short cells are



abundant; Chloridoideae and Arundinoideae short cells and

hair cell (grass-type) phytoliths are common. The percentage

of Pooideae and Asteraceae phytoliths changes in inverse

proportion to Panicoideae phytoliths. Chloridoideae and

Arundinoideae phytoliths occur in inverse proportion to

Bambusoideae phytoliths. Broad-leaved tree phytoliths fluctu-

ate in inverse proportion to Poaceae phytoliths. The broad-

leaved tree assemblage is dominated by globular granulate,

(a)

(o)

(v) (w) (x) (y) (z) (aa) (ab) (ac)

(ad) (ae) (af) (ag) (ah) (ai)

(aj) (ak) (al) (am) (an)

(ao) (ap) (aq) (ar) (as) (at)

(au) (av) (aw) (ax) (ay) (az) (ba) (bb)

(bc) (bd) (be) (bf) (bg) (bh)

(bl)(bk)(bj)(bi)

(p) (q) (r) (s) (t) (u)

(h)(g) (i) (j) (k) (l) (m) (n)

(b) (c) (d) (e) (f)

Y. Gu et al.


globular psilate, acicular hair cell, and sclereids; and platelet

polygon, globular facetate, and vascular tissues are also present.

The percentage of sponge spicules is low, but diatoms occur

frequently in this zone, and approach the highest percentage

(5%) at 16 cm.

Zone 6 (9–0 cm):

The Ic and Iph ratios decrease gradually. Poaceae phytoliths

make up the greatest part of this assemblage zone; hair cells

and Chloridoideae and Arundinoideae short cells are com-

mon. Broad-leaved tree phytoliths change in inverse pro-

portion to Poaceae phytoliths, and globular granulate;

acicular hair cells, vascular tissues, and sclereids are com-

mon.

Fire episodes based on phytolith and charcoal

evidence

Macro-charcoal and micro-charcoal particles in the sediments

vary in shape from angular to irregular (see Appendix S1).

Most burnt phytoliths are from Poaceae, ferns, and broad-

leaved trees. These phytoliths contain black occluded carbon

(Piperno, 1988; Kealhofer & Penny, 1998), and reveal burning

directly. They are also occasionally slightly distorted in shape

(see Appendix S2). On the basis of charcoal and burnt

Figure 4 Percentage of main phytolith morphotypes and assemblages in sediments (count based on 10 fields at 400 ·).

Figure 3 Major phytolith morphotypes from geological sediments from the tropical rain forest in Xishuangbanna. Poaceae phytoliths (a)–

(al) and non-Poaceae phytoliths (am)–(bl) (the bar is 10 lm): (a)–(d) bilobates (Panicoideae type); (e), (g)–(h) crosses (Panicoideae type);

(f), (i)–(k) oblong concave saddles (Bambusoideae type); (l) Bambusoideae variant 3 bilobate; (m)–(n) trapeziform saddles (Arundinoideae

type); (o)–(s) square saddles (Chloridoideae type); (t)–(u) rondels (Pooideae type); (v)–(w) complex rondels (unknown origin); (x), (ab)

parallelepipedal bulliform cells; (y)–(aa) cuneiform bulliform cells; (ac) opaque perforated platelets (Asteraceae); (ad)–(ae) cylindrical

polylobates (Panicoideae type); (af)–(ag) trapeziform polylobates (Pooideae type); (ah) trapeziform sinuates (Pooideae type); (ai), (ak)

unciform hair cell (grass type); (aj) elongate smooth; (al) elongate echinate; (am)–(an) polyhedrons with conical projection (Cyperaceae,

Cyperus sp.) (Piperno, 1989); (ao) elongate undulating (fern type); (ap) prism sinuate (fern type); (aq)–(ar) parallelepipedal contorted

(gymnosperm type); (as)–(at) acicular hair cell (tree type); (au) platelets polygon (broad-leaved tree type); (av)–(aw) abbreviated stellate

(broad-leaved tree type); (ax)–(ba) globular granulate (broad-leaved tree type); (bb) globular psilate (broad-leaved tree type); (bc) clavate

contorted phytoliths (gymnosperm type); (bd)–(be) globular facetates (broad-leaved tree type); (bf)–(bg) vascular tissues (broad-leaved tree

type); (bh)–(bi) multifaceted blocky (broad-leaved tree type); (bj) tri-stellate truncate (broad-leaved tree type); (bk)–(bl) cylindrical sclereid

(broad-leaved tree type).



phytolith occurrences, 12 episodes and six zones of fire events

can be identified in the profile (Fig. 6).

Episode 1:

The percentage of burnt phytoliths decreases gradually from 90%

to 70%, from the start to the end of the episode, corresponding to

Zone 1. The number of micro-charcoal particles decreases

gradually from 130 to 65 particles, corresponding to the decrease

in burnt phytoliths. The percentage of highly weathered bulli-

form cells changes in inverse proportion to the percentage of

burnt phytoliths, fluctuating from 5% to 10%.

Episode 2:

Corresponding to Zone 2, the percentage of burnt phytoliths

decreases rapidly within this zone; there is a peak at 80 cm,

corresponding to fire Episode 2. Micro-charcoal particles show

a similar pattern. The percentage of highly weathered bulliform

cells is again inversely proportional to the percentage of burnt

phytoliths.

Episodes 3–4:

Corresponding to Zone 3, the percentage of burnt phytoliths

and the sum of micro-charcoal particles increase rapidly in this

zone. The percentage of highly weathered bulliform cells

decreases in inverse proportion to the percentage of burnt

phytoliths.

Episode 5:

As in Episode 2, the percentages of burnt phytoliths and micro-

charcoal particles decrease rapidly in Zone 4. There is a peak at

Figure 6 Evidence of fire occurrence and

positive feedbacks between drought, fire and

vegetation composition.

Figure 5 The abundance of grass sub-

families and results for phytolith indices.

Y. Gu et al.


50 cm, which corresponds to fire episode 5. The percentage of

highly weathered bulliform cells increases in inverse propor-

tion to the percentage of burnt phytoliths.

Episodes 6–11:

The percentages of burnt phytoliths, charcoal, and highly

weathered bulliform cells show frequent and rapid variability.

Burnt phytoliths have six evident peaks varying from 50% to

96%, corresponding to fire episodes 6–11. The sum of

micro-charcoal particles changes in concert with the burnt

phytoliths. The percentage of highly weathered bulliform

cells is inversely proportional to the percentage of burnt

phytoliths.

Episode 12:

The percentage of burnt phytoliths and charcoal declines

rapidly; there is a single peak at 6 cm, corresponding to fire

episode 12. The percentage of highly weathered bulliform cells

again demonstrates an inverse relationship with the percentage

of burnt phytoliths (Fig. 7a).

Nearly every fire episode involved in the burning of grasses

and trees, and burnt phytoliths occur mostly in plant taxa from

the Poaceae, ferns, and broad-leaved trees. This is critical in

determining the intensity and range of fire episodes in the

study area.

DISCUSSION

Climate change and palaeoenvironmental analysis

Previous studies have demonstrated that the aridity (Iph) and

climate (Ic) indices have significant potential as climatic

indicators for sedimentary palaeoenvironmental interpretation

(Twiss, 1987, 1992; Barboni et al., 1999; Parker et al., 2004).

Mean annual precipitation, together with mean annual tem-

perature, has a decisive influence on the spatial distributions of

phytolith types in the study area (Lu et al., 2006). Our results

are consistent with modern soil phytolith assemblages in

southern tropical China (Lu et al., 2006): bulliform cells

(except for fan-reed), long saddles (oblong concave saddle),

Palmae (Arecaceae, palm family), and broad-leaf types are very

abundant; dumbbell (bilobate), elongate, point-shaped (hair

cell), pteridophyte (fern) and gymnosperm types are moder-

ately abundant; and short saddles (square saddle) and

Pooideae are rare (Fig. 4). An Iph value of 30% was selected

here as the boundary for humidity–aridity, consistent with the

approach used in intertropical Africa (Alexandre et al., 1997).

On this basis, the humidity history of the study area is

characterized by alternating wet and dry intervals over the past

2000 years. In particular, 11 drought events have been

identified (a1 to a11 in Fig. 5), which might be related to

decreased precipitation from the Indian summer monsoon

(Denniston et al., 2000). Comparable with the Iph index, the

Ic index also indicates a trend of fluctuating warmth-coolness,

with 11 cold events (temperature below average) observed in

the study region over the past 2000 years (Fig. 5). The majority

of the Ic values are < 50%, and the relatively high Ic value

(> 50%) could be explained by the presence of Sporobolus,

which produces Pooideae-type phytoliths even though it

belongs to the Chloridoideae group (Barboni et al., 1999).

Comparable fluctuations in regional climate have been docu-

mented during the late Pleistocene–early Holocene in

Xishuangbanna (Liu et al., 1986; Walker, 1986). Modern

meteorological data from Mengla County also show that,

during a 40-year interval, 25% of years experienced low

temperatures (Tan et al., 2002). Moreover, historical docu-

ments and lake sediments in the neighbouring areas (e.g.

Kunming and Erhai) produce markedly fluctuating tempera-

ture records for the last 2000 years (Sun et al., 1986; Zhang

et al., 2001), which is consistent with the phytolith record

reported herein. Our results demonstrate the significant

potential of phytoliths for identifying subtle climate changes

on a short time-scale.

To summarize, our phytolith records document six climate

periods marked by occurrence of drought events and cold

events. Zone 1 is marked by cool and dry conditions, with the

presence of Pooideae, Chloridoideae, Bambusoideae, and

Asteraceae. Zone 2 is characterized by warm and wet condi-

tions, with the presence of Panicoideae, Pooideae, and

Bambusoideae. In addition, a brief cold and dry interval

occurs at 80 cm. Zone 3 is marked by cool–dry conditions,

with the presence of Pooideae, Chloridoideae, Bambusoideae,

(a) (b)

Figure 7 Correlation analysis among burnt

phytoliths, highly weathered bulliform cells

and Iph (aridity index): (a) burnt phytoliths

are negatively correlated with highly weath-

ered bulliform cells; (b) burnt phytoliths are

positively correlated with Iph.



Panidoideae, Arundinoideae, ferns, and Asteraceae. There is a

relatively wet interval at 65 cm, which is the lithological

boundary between silt and clay in the profile. Zone 4 is

dominated by warm and wet conditions, with the presence of

Panicoideae, Pooideae, Bambusoideae and Arundinoideae,

corresponding to the Medieval Warm Period in the Erhai

area (Zhang et al., 2001). Within this zone, there are brief cold

and dry intervals at about 50 cm. Zone 5 is dominated by

alternating cold–dry and warm–wet intervals, corresponding to

the Little Ice Age in the neighbouring areas (Sun et al., 1986;

Zhang et al., 2001). Zone 6 is characterized by rapid warming

and wet conditions, with the presence of Panicoideae, Pooideae,

Bambusoideae, and Arundinoideae. There is a cold event at

6 cm. The rapid warming and wet conditions of the most recent

zone suggest that rain forest is sensitive to global warming. It is

evident that Panicoideae, Bambusoideae, Cyperaceae, sponge

spicules, and diatoms occur in warm and wet conditions.

However, Pooideae and Asteraceae exist in cool–dry or

cool–wet circumstances (Figs 4 & 5).

Drought, fire and vegetation succession

Modern species composition, physiognomy and plant diversity

surveys show that trees dominate rain forest and have a higher

abundance and diversity than shrubs and herbs (Zhu et al.,

1999; Zhu & Cai, 2005; Li et al., 2004). Rain-forest compo-

sition and structure might change and grasses increase rapidly

in an open habitat after a fire occurrence and disturbance.

Owing to recurring fire episodes, grasses (Poaceae) and ferns

increase as a result of recurring episodes of fire caused by

drought and other factors at the expense of broad-leaved and

gymnosperm trees. In contrast, broad-leaved and gymnosperm

trees increase during pre-fire and wet conditions at the expense

of grasses and ferns. The vegetation structure of the forest is

thus disturbed again and again by the fire regime. It is

therefore likely that both drought and fire, the latter in

particular, have a significant influence on vegetation compo-

sition and structure in the study area. A similar kind of

relationship is observed today in the Amazonian rain forest,

where modern drought and fire have damaged the forest,

causing fragmentation, forest loss, and climate change (Will-

iamson et al., 2000; Laurance & Williamson, 2001).

It is noteworthy that nearly all fire episodes identified in the

profile are associated with evidence for droughts (Fig. 6). The

significant positive correlation between the Iph index and

burnt phytoliths (Fig. 7b) sheds further light on the relation-

ship between fire and drought. More significantly, the relative

abundances of vegetation elements such as grasses, gymno-

sperms, and broad-leaved trees show positive feedbacks to pre-

fire and fire episodes over the past 2000 years.

It should be noted that a fire episode occurs at 30 cm in

association with a relatively low Iph value of 25%. This implies

that fire can occur when the Iph index approaches 25%, and that

this value may be a cut-off value for this region. An exceptional

fire episode, Episode 12, occurs at 6 cm, in association with

decreasing abundances of both grasses and trees. This might

record human interference in the rain forest, corresponding to

the rapid growth of local populations during the 19th and 20th

centuries (Lee, 1982; Hansen, 1999; Fukao, 2004).

CONCLUSIONS

This first phytolith and charcoal record from tropical rain-

forest sediments in south-western China provides an excellent

framework for analysing the interaction between climate, fire,

and vegetation history over the last 2000 years. Our main

findings are as follows.

1. Phytolith assemblages and indices indicate that the region

has experienced six periods of climate change marked by

alternating warm–wet and cool–dry conditions. Eleven

drought events and cold events occurred. Recent rapid

warming and wet conditions indicate that rain forest is

sensitive to global warming.

2. Burnt phytoliths, highly weathered bulliform cells, and

charcoal particles reveal the occurrence of 12 fire episodes.

3. Fire apparently influences drought occurrence and exerts a

strong influence on vegetation structure. Nearly every fire

episode, coupled with a climatic shift, triggered rapid changes

in the composition of the flora. Grasses expanded in response

to an increased occurrence of fire, which confirms the

expectation that drought interacts with fire to affect rain-

forest ecosystems. In the last fire episode, grasses and trees

both declined. This suggests that human activity in recent

centuries has affected fire occurrence and vegetation.

ACKNOWLEDGEMENTS

This project was funded by the National Natural Science

Foundation of China (grants 40502015, 40232025 and

40525008). The authors are grateful for the assistance of the

Xushuangbanna Tropical Botanical Garden of the Chinese

Academy of Sciences, and for helpful information and sugges-

tions provided by Professor Zhu Hua and Mr Wang Hong. We

thank to Professor Yang Fengqing and Dr Wang Hongmei for

help with the fieldwork. We are grateful to Professor Zhou

Xiugao and master candidate Qin Yangmin, who helped with

sediment laboratory processing. Many thanks should be given

to the referees for their valuable comments and constructive

suggestions on an earlier version of the manuscript.

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SUPPLEMENTARY MATERIAL

The following supplementary material is available for this

article online:

Appendix S1 Macro-charcoal, micro-charcoal and burnt

phytoliths in sediments.

Appendix S2 Burnt phytoliths, highly weathered bulliform

cells and other silicon remains.

Appendix S3 Statistics of burnt phytoliths, highly weathered

bulliform cells and micro-charcoals in 20 fields at 250 · mag-

nification expressed as percentages.

Appendix S4 Statistics of phytolith types expressed as

percentages.

This material is available as part of the online article from:

http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2699.

2007.01763.x

Please note: Blackwell Publishing are not responsible for the

content or functionality of any supplementary materials sup-

plied by the authors. Any queries (other than missing material)

should be directed to the corresponding author for the article.

BIOSKETCHES

Yansheng Gu has a PhD from Wuhan University. Currently

he is an associate professor at the Institute of Ecology and

Environment, China University of Geosciences. He is interes-

ted in Quaternary geology and environmental changes in

southern China.

Deborah Pearsall has a PhD from the University of Illinois.

Currently she is a professor in the Department of Anthropol-

ogy, director of the Palaeoethnobotany Laboratory, University

of Missouri-Columbia, and a palaeoethnobotanist specializing

in macro-remain analysis, phytolith analysis, and starch-grain

analysis.

Shucheng Xie has a PhD from the China University of

Geosciences. Currently he is a professor at the Key Laboratory

of Biogeology and Environmental Geology, China University

of Geosciences. He is interested in Quaternary vegetation and

climate change.

Editor: Peter Linder



Vegetation and fire history of a Chinese site in southern tropical Xishuangbanna derived from...

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Transcript of Vegetation and fire history of a Chinese site in southern tropical Xishuangbanna derived from...