Botanical Studies (2008) 49: 373-383.

*

Corresponding author: E-mail: wuning@cib.ac.cn;

zhangyc@cib.ac.cn; Tel: +86-28-85213782; Fax:

+86-28-85222753.

INTRODUCTION

The resources for photosynthetic productivity such as

light, soil water and mineral nutrients are often heteroge-

neously distributed in natural ecosystems (Frankland et

al., 1963; Chazdon and Pearcy, 1991; Kelly and Canham,

1992; Ackerly and Bazzaz, 1995; Muraoka et al., 1997;

Oshima et al., 1997). Resource heterogeneity usually oc-

curs at different spatial and temporal scales and affects

many ecologically important processes and phenomena,

which can range from responses of populations (Wiens,

1976; Fowler, 1988; Levin, 1992), to individuals or parts

of individuals (Shorrocks and Swingland, 1990; Caldwell

and Pearcy, 1994).

Contrast is defined as the degree of difference between

patches or between a patch and its surrounding matrix

(Kotliar and Wiens, 1990). The sink-source hypothesis

states that photosynthetic rates may be regulated, at least

in part, by the balance between source tissues (net export-

ers of photosynthates) and sink tissues (net consumers

of carbohydrates) (King et al., 1967; Neales and Incoll,

1968; Sweet and Wareing, 1966; Wareing et al., 1968).

For clonal plants, the contrast of patchiness, which is re-

sponsible for the establishment of source-sink relations be-

tween adult interconnected ramets (Marshall, 1990), plays

a prominent role as the main external driver of integration

effects (Stuefer, 1996). Many studies have reported that

clonal plants in heterogeneous environments show a higher

capacity for integration (Chen et al., 2004) and division of

labor (Alpert and Stuefer, 1997; Roiloa et al., 2007).

Many species of plants are capable of clonal growth in

natural communities (Oborny and Bartha, 1995; Klime.

et al., 1997). Clonal growth allows plants to form large

systems consisting of a variable number of ramets located

Clonal integration of Fragaria orientalis driven by

contrasting water availability between adjacent patches

Yun-Chun ZHANG

1, 2, 3

, Qiao-Ying ZHANG

4

, Eshetu YIRDAW

5

, Peng LUO

1

, and Ning WU

1,

*

1

Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu, Sichuan Province, 610041, P. R. China

2

Shandong Institute of Light Industry, Jinan, Shandong Province, 250353, P. R. China

3

Graduate School of the Chinese Academy of Sciences, Beijing, 10039, P. R. China

4

State Key Laboratory of Biocontrol, Sun Yat-Sen University, Guangzhou, Guangdong Province, 510275, P. R. China

5

Viikki Tropical Resources Institute (VITRI), P.O. Box 27, FIN-00014 University of Helsinki, Finland

(Received August 27, 2007; Accepted May 1, 2008)

ABSTRACT.

Experimental studies have shown that clonal plants can reciprocally translocate resources be-

tween interconnected ramets in heterogeneous environments. Resource contrast between patches in heteroge-

neous environments is the main external driver of integration effects. It was hypothesized that translocation

of water between interconnected ramets was enhanced under higher levels of contrasts in water availability.

A pot experiment with clonal fragments consisting of two interconnected ramets of F. orientalis¡Xa stolonif-

erous herb widely distributed in China¡Xwas conducted. In the experiment, each of the ramets of the clonal

fragments was allocated to either a high or low water treatment. The ramets in the high water treatment were

maintained at 90% field capacity while those in the low water treatment were maintained at 90, 60 and 30%

field capacity, respectively. The ramet developmental stage (proximal or distal from mother rosette) was

accounted for by allocating either proximal or distal ramets in the contrasting patches. In order to assess the

role of physiological integration among ramets, the stolon was severed for half of the clonal fragments. Stolon

severing and drought stress had significant effects on plant performance, which increased with the increase of

contrast. However, the directionality of stressed ramets had no influence on most treatments. Based on per-

formance measures, a cost-benefit analysis showed that the dry ramets benefited from clonal integration at the

cost of the connected wet ramets. The results indicated that this clonal species is able to withstand soil-water

heterogeneity through physiological integration, which is promoted under increasing levels of water contrast.

Our results suggest that clonal plants might be suitable for vegetation restoration in dry areas due to superior

survival strategies.

Keywords: Clonal plant; Cost-benefit analysis; Drought stress; Fragaria orientalis; Heterogeneous environ-

ment; Physiological integration; Water transport.

ECOLOGY

374

Botanical Studies, Vol. 49, 2008

at some distance from each other, which remain connected

by stolons or rhizomes for a variable period of time. This

kind of reproduction implies that a clonal system in which

ramets are connected by stolons or rhizomes is more likely

to experience spatial heterogeneity than non-clonal plants

(Roiloa and Retuerto, 2006b).

One of the most advantageous features of clonal plants

is their capacity to exchange resources like water, photo-

assimilates, and nutrients (Lau and Young, 1988; Alpert,

1991; D¡¦Hertefeldt and Jonsdottir, 1994; Lin et al., 2006;

Wu et al., 2007; Hu et al., 2008) with non-resource agents-

like defense compounds, signaling molecules, or patho-

gens (Gomez and Stuefer, 2006) between interconnected

ramets that are potentially independent (physiologically

integrated). The advantages of physiological integration

have been shown in a number of studies in which the

connections among ramets were artificially severed and

the impact on ramet performance measured (Mattheis et

al., 1976; Hartnett and Bazzaz, 1983; Jonsdottir and Cal-

laghan, 1988; Schmid and Bazzaz, 1987; Schmid et al.,

1988; Marshall and Anderson-Taylor, 1992; Bullock et al.,

1994; Yu et al., 2004). Physiological integration is espe-

cially advantageous when transport of resources is directed

from old to young ramets (Hartnett and Bazzaz, 1983;

B.ezina et al., 2006), from ramets in favorable microsites

to ramets in adverse microsites (Alpert, 1991; Pennings

and Callaway, 2000; Saitoh et al., 2002; Chen et al., 2004),

or when reciprocal resources are shared between con-

nected ramets of a clone in an environment where the two

resources tend not to occur together (division of labor)

(Roiloa et al., 2007). However, integration also has its

disadvantages, such as the energy cost of maintenance for

inter-ramet connections (Pitelka and Ashmun, 1985; Jons-

dottir and Watson, 1997; Kelly, 1995) and the rapid spread

of pathogens throughout the system of interconnected ra-

mets (Stuefer et al., 2004).

Most previous experiments investigating integration in

clonal plants in response to environmental heterogeneity

have used only simple contrast models, such as contrast

and no contrast, severed vs. unsevered stolon, and under

shade or in the open (Salzman and Parker, 1985; De

Kroon et al., 1996; Chen et al., 2004; Roiloa and Retuerto,

2006a). In natural environments, ramets of clonal plants

often experience more complex habitats. However, little

research has been done under these more realistic scenarios

of habitat heterogeneity. Here, we add novel information

by using a more complex system of heterogeneity with

different levels of water contrasts.

In the transitional zone from the Qinghai-Tibetan Pla-

teau to the Sichuan Basin, there is an ecotone between arid

valley and montane ecosystems. The landscape of this kind

of semi-arid area is characterized by vegetation consisting

of scrubland, shrubland, and grassland (Liu, 1994), which

provide suitable habitats for various clonal plants (Yu et

al., 2002). Drought stress is an important environmental

factor inhibiting plant growth and productivity (Li et al.,

2000; Li and Wang, 2003; Yin et al., 2005). Clonal plants

play important roles in the ecotone (Chen, 2004), and over

many centuries, they have developed various mechanisms

to enhance their drought adaptation (He et al., 2007). For

example, the dry ramets of the Carex species get benefits

from wet ramets through physical integration (De Kroon et

al., 1996). Variation in mountainous terrain and nonunifor-

mity of the vegetation cause heterogeneous distribution of

water.

A pot experiment was used to test how the contrast be-

tween adjacent patches affected the strength of clonal in-

tegration in the stoloniferous herb Fragaria orientalis. In

our study, the stolon between adjacent and connected sib-

ling ramets was either severed or kept intact under differ-

ent degrees of heterogeneous water supply treatment. We

predicted that 1) drought would affect the growth and sur-

vival of severed ramets of F. orientalis; 2) the integration

between ramets of F. orientalis would be enhanced under

higher levels of water contrast, and 3) under drought-stress

the performance of ramets is different in proximal or distal

ramets.

MATERIALS AND METHODS

Study site and plants

Our study was conducted in the Maoxian Ecological

Station (31¢X41¡¦07" N, 103¢X53¡¦58" E; 1,816 m a.s.l.) of the

Chinese Academy of Sciences in western Sichuan Prov-

ince, P.R. China. The station is in an ecologically fragile

region of the eastern Qinghai-Tibetan Plateau, located at

the upper reaches of the Yangtze River. It has an annual

mean temperature of 8.6¢XC, a mean annual precipitation

of 919.5 mm, and an annual potential evaporation of 795.8

mm. The yearly sunshine duration is ca. 1139.8 h.

Fragaria orientalis Losinsk. (Rosaceae) is a stolonif-

erous, perennial herb that is widely distributed in Korea,

Mongolia, eastern Russia, and China. In China, it is com-

mon in the north and in the eastern Qinghai-Tibet Plateau,

inhabiting forests and meadows on mountain slopes (Yu et

al., 1985; Guan et al., 2004). Stems and petioles are pilose

(more densely in upper parts) or glabrescent. Leaves are

composed of three leaflets with a slender petiole borne on

the vertical stems with compressed nodes. The axillary

buds on the vertical stems may grow out and form stolons.

The stolons usually produce roots when reaching a moist

substratum and often form a sympodial network of stolons

above the ground. In the arid valley, variation in mountain-

ous terrain and nonuniformity of vegetation cause hetero-

geneous distribution of water. This spatial heterogeneity

of water is characterized as patchiness, even on a small

scale relevant to individual plants (Cook, 1983; Alpert and

Mooney, 1986; Jackson and Caldwell, 1993). In this habi-

tat, ramet systems of stoloniferous herb F. orientalis often

experience patchiness of soil moisture. As growing in

different patches, its interconnected ramets can transport

and share water acquired by ramets in different patches

due to clonal integration. So, F. orientalis provided proper

materials for the experiment.

ZHANG et al. ¡X High water contrast enhances clonal integration

375

The experiment

On 10 June 2006, fifteen plants of F. orientalis, each

consisting of more than twenty newly produced ramets,

were excavated around the Maoxian Ecological Station.

The plants were at least 1,000 m away from one another,

and thus could be considered to represent fifteen distinct

genotypes (Yu et al., 2002). All the second and third

youngest ramets with connected stolon of these original

plants were dissected into clonal fragments. One ramet in

each fragment was designated the initial proximal part, in-

dicating its relative proximity to the mother rosette, while

the other was designated the initial distal part. With the

stolon still intact between two ramets, these clonal frag-

ments were planted in trays of sand for about three weeks.

Once well established (rooted), they were size-standard-

ized (Yu et al., 2002) and transplanted into plastic pots

(20 cm in diameter and 15 cm in height). Each fragment

was grown in a pair of plastic pots allowing for unsevered

fragments to be connected by a stolon. Each pot was filled

with homogenized soil to a depth of 14 cm. The proximal

and the distal ramets of each clonal fragment were planted

separately in adjacent pots, and they were connected by an

undamaged stolon. Pots of severed clonal fragments were

set up in the same procedure. The subjects were grown in

a glasshouse under a semi-controlled environment, with a

day temperature range of 12-31¢XC and a night temperature

range of 9-15¢XC, and a relative humidity range of 35-85%.

After one week, all ramets were size-standardized again

so that ramets of a similar area remained. At the beginning

of the experiment, all ramets were about 2 cm tall. The

unsevered clonal fragments were divided into two groups:

for one group, all the proximal ramets (wet ramets) were

well-watered [up to 90% of field capacity (FC)], and the

distal ramets (dry ramets) were subject to a well-watered

treatment [90% of FC] and to two drought-stressed treat -

ments (60% and 30% of FC). The other group was just the

reverse, i.e., the distal ramets (wet ramets) were treated

with 90% of FC while the proximal ramets (dry ramets)

were treated with 90%, 60% and 30% of FC, respectively.

Severed clonal fragments were treated in the same way.

The experiment used a factorial design with different water

level treatments (90%, 60% or 30% of FC), position of the

stressed ramets (proximal or distal), and stolon severing

treatment (severed or not) as main effects (Figure 1 and

Table 1). In each treatment, there were fifteen replicates,

and each of them was derived from one of the fifteen origi-

nal rosettes. That is to say, each of the fifteen replicates

came from a different one of the fifteen genotypes. A

total of 150 pairs of ramets (10 levels of treatments ¡Ñ 15

replicates) were used in this experiment.

In the well-watered treatment, the pots were re-watered

to 90% of FC by replacing the amount of water transpired

every second day. In the drought-stressed treatments, the

pots were watered to 60% and 30% of FC every second

day to keep different drought levels in the soil. Evapora-

tion from the soil surface was prevented by enclosing each

pot with a plastic bag, which was sealed at the base of the

stem of each ramet. A total of 8 g of slow-release fertil-

izer (13% N, 10% P and 14% K-Xinjin, Xinjin Compound

Fertilizer Factory, Sichuan, China) was added to each pot

before planting. An empirical relationship between plant

fresh weight (Y, g) and plant leaf area (X, cm

2

): Y = 0.096

X - 0.158 (R

2

= 0.923, P<0.001) was used to correct pot

water for changes in plant biomass. In addition, 15 ad-

ditional control pots were equipped with dry grass stems

to model the evaporation situation with living F. orientalis

and were enclosed in plastic bags in the same way as the

ramets. These pots were also weighed every second day

in order to estimate evaporation from the soil surface. At

the end of the experiment, all parts of the plant in each pot

were marked and harvested on 25 September.

Measurements and analysis

Total number of ramets and leaves per pot were count-

ed. Total leaf area per pot was measured using a CI-203

Laser Area Meter (CID Inc.). The length of root, petiole

and stolon of each ramet was measured. Every ramet

was then dissected into roots, leaf laminaes, petioles, and

stolons, and the biomass of each part was determined after

drying at 70¢XC for 48 h.

A three-way ANOVA, with the main effects mentioned

above, i.e., effect of severing, effect of water supply, and

effect of directionality, was applied to analyze the respons-

es of traits at the clonal fragment level and ramet level.

Traits included biomass, number of ramets, height, root

length, stolon length, leaf area, number of leaves, petiole

Figure 1. Schematic diagram of the experiment, showing different levels of water supply given to pairs of connected or severed ramets

and proximal or distal ramets.

376

Botanical Studies, Vol. 49, 2008

length, and root-shoot ratio. Duncan¡¦s multiple range test

was employed to compare the means in all measured char-

acters.

Costs and benefits of clonal integration were calculated

separately for the dry ramets and the wet ramets in terms

of biomass and number of ramets. Costs and benefits (Sal-

zman and Parker, 1985; Slade and Hutchings, 1987a; Ev-

ans, 1991; van Kleunen and Stuefer, 1999; Yu et al., 2002)

were defined as differences in biomass and number of

ramets between interconnected ramets and the correspond-

ing ramets in the severing treatment.

RESULTS

Effect of severing

At the end of the experiment, we found some of the

severed dry ramets in the high contrast treatment had died,

i.e., 53% of the distal ramets and 47% of the proximal

ramets. All others survived (Table 1).

Stolon severing did not affect root-shoot ratio under

homogeneity. The root-shoot ratio of dry ramets was sig-

nificantly higher than that of wet ramets when they were

connected, especially in high contrast. However, dry

ramets and wet ramets showed no difference when they

were severed. Under a heterogeneous water supply, the

root-shoot ratio of connected wet ramets was higher than

that of severed ones (Figure 2).

Stolon severing increased significantly the biomass,

number of ramets and petiole length of wet ramets and

those of dry ramets under a homogeneous water supply.

When stolons were severed, the biomass, height, and

leaf area of dry ramets under a heterogeneous water

supply decreased significantly, and the number of ramets,

number of leaves, and stolon length of dry ramets under

Figure 2. Root-shoot ratio. Ho, Homogeneity (no contrast); LP,

proximal ramets under low contrast; LD, distal ramets under low

contrast; HP, proximal ramets under high contrast; HD, distal

ramets under high contrast. Filled bars show severed ramets.

Empty bars show connected ramets. Error bars represent SE of

the mean. Vertical bars sharing the same lowercase letter are not

different at p=0.05.

Table 1. Experimental design and survival of ramets.

Treatment

Wet ramets

Survival Stolon connection

Dry ramets

Survival

Homogeneity

90% FC, proximal ramets 100%

Connected

90% FC, distal ramets

100%

(No contrast)

90% FC, proximal ramets 100%

Severed

90% FC, distal ramets

100%

Heterogeneity

Low contrast 90% FC, proximal ramets 100%

Connected

60% FC, distal ramets

100%

90% FC, proximal ramets 100%

Severed

60% FC, distal ramets

100%

90% FC, distal ramets

100%

Connected

60% FC, proximal ramets 100%

90% FC, distal ramets

100%

Severed

60% FC, proximal ramets 100%

High contrast 90% FC, proximal ramets 100%

Connected

30% FC, distal ramets

100%

90% FC, proximal ramets 100%

Severed

30% FC, distal ramets

53%

90% FC, distal ramets

100%

Connected

30% FC, proximal ramets 100%

90% FC, distal ramets

100%

Severed

30% FC, proximal ramets 47%

Clonal fragment consisting of two successive ramets of F. orientalis were treated as two interconnected parts growing in two adja-

cent pots. The wet ramets were offered 90% FC, and the dry ramets were offered 90% FC, 60% FC and 30% FC. Half of the wet

ramets and the dry ramets came from proximal ramets and the other half came from distal ramets. The stolon connection between

the wet and the dry ramets of the same clonal fragment was either severed or not.

ZHANG et al. ¡X High water contrast enhances clonal integration

377

high contrast also decreased significantly. When both ra-

mets were in moist pots, severed fragments presented a

higher biomass, more ramets and leaves, and greater root

and stolon length, but lower height and petiole length in

comparison with connected fragments. Severed fragments

showed no difference in biomass, height, root length,

stolon length, leaf area, or number of leaves in a heteroge-

neous water available environment (Figure 3).

Effect of water supply

No significant influence on root-shoot ratio was found

among severed wet ramets. While for other ramets, root-

shoot ratios under heterogeneous water supply were all

bigger than those under homogeneous water supply.

No significant differences were found either under low

contrast or high contrast when wet ramets and dry ramets

were connected, but root-shoot ratios of severed dry

ramets increased with the increase of contrast (Figure 2).

Both biomass, number of ramets, leaf area, petiole

length of dry ramets and height, number of leaves,

stolon length of severed dry ramets decreased under

heterogeneous water supply. The number of leaves

and petiole length of connected dry ramets showed

no significant difference between low contrast and

homogeneous water supply, but a significant decrease was

found under high contrast compared with homogeneous

water supply. The biomass of dry ramets and number of

ramets, leaf area, number of leaves, and stolon length

of severed dry ramets decreased significantly with the

increase of contrast. The biomass and leaf area of clonal

fragments decreased under heterogeneous water supply

(Figure 3).

Effect of directionality

Change of directionality showed no significant

influences on connected ramets in the same water supply

treatment (Figure 2). When stolons were connected, the

biomass of dry proximal ramets was higher than that

of dry distal ramets under low contrast, but there was

no difference under high contrast. The parameters of

connected ramets showed no differences under change of

directionality except in the number of ramets and in the

leaf area of wet ramets under a similar water supply. For

the whole clonal fragment, significant differences were

only found in the number of leaves of connected ramets

under high contrast, and no differences were found on the

other parameters of intact ramets (Figure 3).

Cost-benefit analysis

The cost-benefit analysis of biomass exhibited consider-

able benefits of clonal integration for dry ramets at the cost

of wet ones, and such benefits increased with the increas-

ing contrast between patches (Figure 3A).

The cost-benefit analysis in terms of the number of ra-

mets also showed significant benefits of clonal integration

for dry ramets at the cost of wet ones in high contrast, but

the beneficial effect was less significant in conditions with

lower water availability contrast (Figure 3B). The cost-

benefit analysis of ramet livability also proved the benefits

of clonal integration for dry ramets under high contrast

(Table 1).

DISCUSSION

Thirty percent of FC showed markedly negative ef-

fects on the survival and growth of the severed ramets of

F. orientalis. During the experiments, 53% of the distal

ramets and 47% of the proximal ones among severed dry

ramets in high contrast treatments died, indicating that

drought was a major stress factor in the model environ-

ment, and that ramets in favorable habitats can provide

support to drought-stressed ramets. Similar results were

obtained for root-shoot ratio, biomass, number of ramets,

height, leaf area, number of leaves, and petiole length.

Thus, physiological integration between ramets of F. ori-

entalis was beneficial for ramet establishment in patches

with heterogeneous moisture. The effects of drought on

F. orientalis was consistent with studies on other clonal

species such as Psammochloa villosa (Dong and Alaten,

1999), Carex hirta and C. flacca (De Kroon et al., 1996),

Lycopodium flabelliforme (Lau and Young, 1988), Primula

sieboldii (Noda et al., 2004), Potentilla anserina (van

Kleunen and Stuefer, 1999), C. flacca (de Kroon et al.,

1998), Distichlis spicata (Alpert, 1990), Pennisetum

centrasiaticum and Leymus secalinus (Ren et al., 1999),

and Hydrocotyle bonariensis (Evans, 1991), where it was

found that drought had negative effects on severed ramets,

and the negative effects were ameliorated in connected

ramets using a simple one contrast model. In our ex-

periment, homogenous (no contrast) and low contrast

treatment did not affect the survival of severed dry ramets,

which showed that the benefits of clonal integration were

different among various contrasts.

Under a heterogeneous water available environment,

severed clones showed a higher root-shoot ratio for dry

ramets but a lower one for wet ramets. However, the root-

shoot ratios of connected clones showed no difference

between dry and wet ramets. When the stolon between

ramets was severed, each ramet was an independent plant,

and its performance conformed to the classical theory of

biomass allocation, which predicts an increase of biomass

allocated to organs uptaking the limited resource. The dis-

similar performance of connected clones was the result of

a division of labor, at least in part, between dry and wet

ramets (Alpert and Stuefer, 1997; Hutchings and Wijesing-

he, 1997). i.e. in order to easily uptake locally abundant

water, more biomass was allocated to roots of wet ramets

connected to dry ones under heterogeneous water supply,

which made the root-shoot ratio of connected wet ramets

higher than that of severed ones. Water of dry ramets was

partly supplied from the root system of wet ones. This

was inconsistent with the finding regarding Cinnamomum

tamala and Calamagrostis epigeios (Zhang, 2002). Stolon

severing had no appreciable influence on root-shoot ratio

in a homogenous water available environment, which sug-

378

Botanical Studies, Vol. 49, 2008

Figure 3. (A) Biomass, (B) Number of ramets, (C) height, (D) root length, (E) stolon length, (F) leaf area, (G) number of leaves, and

(H) petiole length of the wet and the dry ramets. The letters on the bar: P, proximal ramets; D, distal ramets; --, connected stolon; //,

severed stolon. Error bars represent SE of the means. The values for entire clonal fragments are the sum of those for the wet and the

dry ramets. For the wet and the dry ramets, horizontal bars sharing the same lowercase letter are not different at p=0.05. Characters of

the clonal fragments marked by the same capital letter are not different at p=0.05.

ZHANG et al. ¡X High water contrast enhances clonal integration

379

gested that there was equity in water exchange between

ramets or that physiological integration did not occur when

ramets were in the same water availability environment.

Severed fragments with both ramets in moist pots,

severed dry ramets under homogeneous water supply

and all wet ramets, had higher biomass, more ramets and

leaves, and greater root and stolon length, but lower height

and petiole length. This was inconsistent with a previous

study of Alternanthera philoxeroides (Liu, 2005). Severed

stolons eliminated the acropetal constraints on resource

transport (Liu, 2005), which enhanced the growth of ra-

mets as new individuals. In addition, severing gets rid of

the energy cost of maintenance for inter-ramet connec-

tions (Caraco and Kelly, 1991). Severed ramets invested

that energy in their vegetative growth. As for severed dry

ramets under heterogeneity, their growth was inhibited by

drought because there was no water supply from connected

ramets.

As for contrasts of water availability, root-shoot ratios

of severed dry ramets growing under high contrast were

higher than those under low contrast, but no difference

was found in contrast with connected dry ramets. A

plausible explanation is that wet ramets provided more

water to high contrast-treated ramets when they were

connected. In other words, dry ramets gained more ben-

efit from connected untreated ramets in high contrast,

which slowed the increase of biomass allocated to organs

uptaking water. In our experiment, the biomass and leaf

area of clonal fragments, biomass, number of ramets, leaf

area, petiole length of dry ramets and height, number of

leaves, and stolon length of severed dry ramets decreased

under heterogeneous water supply. This suggested that a

decreased water supply affected growth of whole clonal

fragments and dry ramets, especially severed dry ramets.

The biomass and morphological responses of stressed ra-

mets to drought were similar to those under other stresses

such as shading (Slade and Hutchings, 1987a, b), nutrient

depletion (Slade and Hutchings, 1987a, b; Evans, 1991),

salinity (Evans and Whitney, 1992), or pathogen exposure

(D'Hertefeldt and van der Putten, 1998). Neither the num-

ber of leaves nor the petiole length of connected dry ramets

showed a significant difference between low contrast and

homogeneous water supply, but a significant decrease was

found under high contrast compared with homogeneous

water supply. The biomass of dry ramets and number of

ramets, leaf area, number of leaves, and stolon length

of severed dry ramets decreased significantly with the

increase of contrast. Those results indicate that integration

between adjacent ramets is enhanced under higher water

contrast. According to the sink-source hypothesis, water

transport may be regulated by the balance between source

tissues (water uptake tissues, such as roots) and sink tis-

sues (transpiration tissues, such as leaves). In a clonal sys-

tem, drought stressed ramets may behave like strong sinks

and wet ramets like sources. For clonal plants, the contrast

of patchiness played a prominent role as the main external

driver of integration effects (Stuefer, 1996). Higher water

contrast increased the gap of source and sink. Thus, higher

water contrast enhanced the integration between ramets of

clonal plant.

Water transport was less influenced by directional con-

straints in plants because it was a passive process driven

by water potential gradients, which arose from intra-clonal

differences in water loss and water uptake (Marshall,

1990; Stuefer, 1996). Previous studies on C. arenaria,

C. flacca and C. hirta also found that water translocation

was equally effective in the acropetal (movement from

the older to the younger part of the clone) and basipetal

(movement from the younger to the older part of the

clone) directions (De Kroon et al., 1996; D¡¦Hertefeldt

and Jonsdottir, 1999). Tracer studies in C. arenaria and

Clintonia borealis have shown that several clonal spe-

cies assimilates are predominantly transported acropetally

(towards developmentally younger clone parts), and to

a much lesser extent in a basipetal direction (Pitelka and

Ashmun, 1985; Marshall, 1990; D¡¦Hertefeldt and Jonsdot-

tir, 1999). However, in F. chiloensis assimilates move in

acropetal and basipetal directions (Alpert, 1996). In our

study, transport of assimilates may be affected by both

basipetal direction and water contrast. When stolons were

connected, the biomass of dry proximal ramets was higher

than that of dry distal ramets under low contrast, but it

showed no difference under high contrast. This suggested

that both basipetal direction and water contrast worked

on assimilate transport in low contrast but water contrast

predominantly worked on assimilate transport. Changing

the direction of clonal fragments had no significant influ-

ence on most of the parameters for either whole fragments

or ramets under the same water contrast treatments. A

plausible explanation was that both of the ramets in the

pair have become functionally independent, and therefore,

transport that was driven by source-sink relations linked to

development decreased. Transport was primarily driven by

water contrast.

Translocation of substances to resource-deficient ramets

potentially reduced the performance of the supplying part

of a clonal fragment (Pitelka and Ashmun, 1985; Caraco

and Kelly, 1991). Although integration for the supporting

part of the clones has a cost, in general the benefits at a

whole clone level outweigh it. When clonal fragments of

F. orientalis were partially dry, and stolon connections

between the dry and wet ramets remained intact, clonal in-

tegration conferred great benefits (in terms of biomass pro -

duction and number of ramets) on the dry ramets (which

presumably imported resources) and extracted significant

cost from the connected wet ramets (which presumably

exported resources). This contrasts with the results in Pon-

teilla anserina (Stuefer, 1995; van Kleunen and Stuefer,

1999) and Hydrocotyle bonariensis (Evans, 1991), which

showed that clonal integration confers benefits on stressed

ramets but no costs on the connected favored ramets. The

reason wet ramets incurred no costs in the present experi-

ment may be because ramets in a locally inferior environ-

ment can be helped by their neighbors but at some cost to

380

Botanical Studies, Vol. 49, 2008

the contributing ramets (Salzman and Parker, 1985). De

Kroon et al. (1996) showed that when a ramet pair of C.

flacca or C. hirta was exposed to a heterogeneous water

supply, water translocation strongly increased to a level at

which 30-60% of the water acquired by the wet ramet was

exported towards the dry ramet. Too much water export

may affect the performance of donor ramets. An alterna-

tive explanation for the costs to exporting ramets can be

that maintenance of tissues connecting ramets imposes

metabolic costs (Pitelka and Ashmun, 1985) and vascular

translocation may require an expenditure of energy (Ep-

stein, 1972) affecting the growth of wet ramets. As a result

of this positive cost-benefit balance, the ability of whole

clonal fragments of F. orientalis to better cope with partial

drought was greatly enhanced by the increase of dry ramet

survival, especially in higher water availability contrast.

Clonal integration can therefore be understood as part of

a tolerance strategy that enhances the survival and growth

of clonal plants growing in patchy environments (Grime,

1979; Pitelka and Ashmun, 1985). The clonal integration

of F. orientalis can be appreciated as an adaptive strategy

for the species.

It can be concluded from our findings that F. orientalis

is able to withstand a heterogeneous distribution of avail-

able water, depending on physiological integration which

would be enhanced under higher levels of water contrast.

Furthermore, our results suggest clonal plants may be suit-

able for vegetation restoration in drought areas as they

have superior survival strategies.

Acknowledgements. We are very grateful to two anony-

mous reviewers, Dr. Yi Shaoliang and Dr. Chen Jinsong

for their valuable suggestions on manuscript improvement.

This work was financially supported by the Important

Directional Project of the Chinese Academy of Sciences

(KSCX2-YW-418), the key project of the Chinese Acade-

my of Sciences Knowledge Innovation Program (KZCX2-

XB2-02), the National Natural Science Foundation of

China (40671181), and the key project of the National

Natural Science Foundation of China (90511008).

LITERATURE CITED

Ackerly, D.D. and F.A. Bazzaz. 1995. Seedling crown orienta-

tion and interception of diffuse-radiation in tropical forest

gaps. Ecology 76: 1134-1146.

Alpert, P. 1990. Water sharing among ramets in a desert popu-

l ation of Distichlis spicata (Poaceae ). Am. J . Bot. 77:

1648-1651.

Alpert, P. 1991 Nitrogen sharing among ramets increases clonal

growth in Fragaria chiloensis. Ecology 72: 69-80.

Alpert, P. 1996 Nutrient sharing in natural clonal fragments of

Fragaria chiloensis. J. Ecol. 84: 395-406.

Alpert, P. and H.A. Mooney. 1986. Resource sharing among ra-

mets in the clonal herb, Fragaria chiloensis. Oecologia 70:

227-233.

Alpert, P. and J. Stuefer. 1997. Division of Labour in Clonal

Plants. Backhuys, Leiden, The Netherlands.

B.ezina, S., T. Koubek, Z. Munzbergova, and T. Herben. 2006.

Ecological benefits of integration of Calamagrostis epigejos

ramets under field conditions. Flora 201: 461-467.

Bullock, J.M., A.M. Mortimer, and M. Begon. 1994. Physi-

ological integration among tillers of Holcus lanatus: Age-

dependence and respons es to clipping and competition.

New Phytol. 128: 737-747.

Caldwell, M. and R. Pearcy. 1994. Exploitation of

Environmental Heterogeneity by Plants. Academic Press,

London.

Caraco, T. and C.K. Kelly. 1991. On the adaptive value of physi-

ological integration in clonal plants. Ecology 72: 81-93.

Chazdon, R.L. and R.W. Pearcy. 1991. The importance of sun-

flecks for forest understory plants-photosynthetic machinery

appears adapted to brief, unpredictable periods of radiation.

Bioscience 41: 760-766.

Chen, J. 2004. Study on the Adaptive Strategies of Clonal Plants

in Heterogeneous Environments. Wuhan University, Wu-

han.

Chen, J.S., D. Yu, Q. Liu, and M. Dong. 2004. Clonal integra-

tion of the stoloniferous herb Fragaria vesca from different

altitudes in southwest china. Flora 199: 342-350.

Cook, R.E. 1983. Clonal plant populations. Am. Sci. 71:

244-253.

D¡¦Hertefeldt, T. and I.S. Jonsdottir. 1994. Effects of re-

source a vailability on integration and clonal growth in

Maianthemum bifolium. Folia Geobotanica & P hytotaxo-

nomica 29: 167-179.

D¡¦Hertefeldt, T. and I.S. Jonsdottir. 1999. Extensive physiologi-

cal integration in intact clonal systems of Carex arenaria. J.

Ecol. 87: 258-264.

D¡¦Hertefeldt, T. and W.H. Van Der Putten. 1998. Physiological

integration of the clonal plant Carex arenaria and its re-

sponse to soil-borne pathogens. Oikos 81: 229-237.

De Kroon, H., B. Fransen, J.W.A. Van Rheenen, A. Van Dijk,

and R. Kreulen. 1996. High levels of inter-ramet water

translocation in two rhizomatous Carex species, as quanti-

fied by deuterium labelling. Oecologia 106:73-84.

De Kroon, H., E. Van Der Zalm, J.W.A. Van Rheenen, A. Van

Dijk, and R. Kreulen. 1998. The interaction between water

and nitrogen translocation in a rhizomatous sedge (Carex

flacca). Oecologia 116: 38-49.

Dong, M. and B. Alaten. 1999. Clonal plasticity in response to

rhizome severing and heterogeneous resource supply in

the rhizomatous grass Psammochloa villosa in an inner

Mongolian dune, China. Plant Ecol. 141: 53-58.

Eps tein, E. 1972. Mineral Nutrition of Plants : Principles and

Perspectives. John Wiey and Sons Inc., New York.

Evans, J.P. 1991. The effect of resource integration on fitness

related traits in a clonal dune perennial, Hydrocotyle bonar-

iensis. Oecologia 86: 268-275.

Evans, J.P. and S. Whitney. 1992. Clonal integration across a

ZHANG et al. ¡X High water contrast enhances clonal integration

381

salt gradient by a nonhalophyte, Hydrocotyle bonariensis

(Apiaceae). Am. J. Bot. 79: 1344-1347.

Fowler, N. 1988. The Effects of Environmental Heterogeneity in

Space and Time on the Regulation of Populations and Com-

munities. Blackwell Scientific Publications, Oxford.

F rankland, J.C., J .D. Ovington, and C. Macrae. 1963. Spatial

and seasonal variations in soil, litter and ground vegetation

in some lake district woodlands. J. Ecol. 51: 97-112.

Gomez, S. and J.F. Stuefer. 2006. Members only: Induced sys-

temic resis tance to herbivory in a clonal plant network.

Oecologia 147: 461-468.

Grime, J.P. 1979. Plant Strategies and Vegetation Processes.

John Wiley & Sons New York.

Guan, W.B., J.A. Wu, G.L. Liang, B. Wang, K.M. Ma, G.H.

Liu, and X.L. Wang. 2004. Vegetation classification and

the main types in the headwater area of the Minjiang river.

China Meteorological Press, Beijing.

Hartnett, D.C. and F.A. Bazzaz. 1983. Physiological integration

among intraclonal ramets in Solidago canadensis. Ecology

64: 779-788.

He, B., G.-Q. Li, H.-Y. Gao, W.-H. Chen, G. Li, W.-L. Qiao, and

J.-B. Ni. 2007. A comparison study on the clonal growth of

Hippophae rhamniodes l. Subsp. Sinensis at different soil

moisture condition. J. Yunnan Univ. (Natural Sciences Edi-

tion) 29: 101-107.

Hu, J.B., X.X. Gao, J. Liu, C.H. XIE, and J.W. Li. 2008. Plant

regeneration from petiole callus of Amorphophallus albus

and analysis of somaclonal variation of regenerated plants

by RAPD and ISSR markers. Bot. Stud. 49: 189-197.

Hutchings, M.J. and D.K. Wijesinghe. 1997. Patchy habitats,

division of labour and growth dividends in clonal plants.

Trends Ecol. Evolution. 12: 390-394.

Jackson, R.B. and M.M. Caldwell. 1993. Geostatistical patterns

of soil heterogeneity around individual perennial plants. J.

Ecol. 81: 683-692.

Jonsdottir, I.S. and M.A. Watson. 1997. Extensive physiologi-

cal integration: An adaptive trait in resource-poor environ-

ments. In H. de Kroon and J. van Groenendael (eds.), The

Ecology and Evolution of Clonal Plants. Backhuys Publish-

ers, Leiden, pp. 109-136.

J onsdottir, I.S . and T.V. Callaghan. 1988. Interrelationships

between different generations of interconnected tillers of

Carex bigelowii. Oikos 52: 120-128.

Kelly, C.K. 1995. Thoughts on clonal integration: Facing the

evolutionary context. Evol. Ecol. 9: 575-585.

Kelly, V.R. and C.D. Canham. 1992. Resource heterogeneity in

oldfields. J. Veget. Sci. 3: 545-552.

King, R.W., I.F. Wardlaw, and L.T. Evans. 1967. Effect of as-

similate utilization on photosynthetic rate in wheat. Planta

77: 261-276.

Klime., L., J. Klimesova, R. Hendriks, and J. Van Groenendael.

1997. Clonal plant architecture: A comparative analysis of

form and function. In H. de Kroon and J. van Groenendael

(eds.), The Ecology and Evolution of Clonal Plants. Back-

huys Publishers, Leiden, pp. 1-29.

Kotliar, N.B. and J.A. Wiens. 1990. Multiple scales of patchiness

and patch structure: A hierarchical framework for the study

of heterogeneity. Oikos 59: 253-260.

Lau, R.R. and D.R. Young. 1988. Influence of physiological

integration on survivorship and water relations in a clonal

herb. Ecology 69: 215-219.

Levin, S.A. 1992. The problem of pattern and scale in ecology:

The Robert H. MacArthur Award Lecture. Ecology 73:

1943-1967.

Li, C., F. Berninger, J. Koskela, and E. Sonninen. 2000. Drought

responses of Eucalyptus microtheca provenances depend on

seasonality of rainfall in their place of origin. Aust. J. Plant

Physiol. 27: 231-238.

Li, C.Y. and K.Y. Wang. 2003. Differences in drought responses

of three contrasting Eucalyptus microtheca f. Muell. Popu-

lations. Forest Ecol. Manag. 179: 377-385.

Lin, K.H. P.Y. Chao, C.M. Yang, W.C. Cheng, H.F. Lo, and T.R.

Chang. 2006. The effects of flooding and drought stresses

on the antioxidant constituents in sweet potato leaves. Bot.

Stud. 47: 417-426.

Liu, J. 2005. The Distribution Pattern and Characteristics of the

Invasive Plant Species In China. Shan Dong University, Ji-

nan.

Liu, W.B. 1994. Mani types of the semi-arid valley scrubs in the

upper reaches of the Minjiang river. Mt Res. 12: 27-31.

Marshall, C. 1990. S ource-Sink Relations of Interconnected

Ramets. SPB Academic Publishing, The Hague.

Marshall, C. and G. Anderson-Taylor. 1992. Mineral nutritional

inter-relations amongst stolons and tiller ramets in Agrostis

stolonifera l. New Phytol. 122: 339-347.

Mattheis, P.J., L.L. Tieszen, and M.C. Lewis. 1976. Responses

of Dupontia fischeri to simulated lemming grazing in an

alaskan arctic tundra. Ann. Bot. 40: 179-197.

Muraoka, H., Y. Tang, H. Koizumi, and I. Was hitani. 1997.

Combined effects of light and water availability on pho-

tosynthesis and growth of arisaema heterophyllum in the

forest understory and an open site. Oecologia (Berlin) 112:

26-34.

Neales, T.F. and L.D. Incoll. 1968. The control of leaf photo-

synthesis rate by the level of assimilate concentration in the

leaf: A review of the hypothesis. Bot. Rev. 34: 107-125.

Noda, H., H. Muraoka, and I. Washitani. 2004. Morphologi-

cal and physiological acclimation responses to contrasting

light and water regimes in Primula sieboldii. Ecol Res. 19:

331-340.

Oborny, B. and S. Bartha. 1995. Clonality in plant communi-

ties¡Xan overview. Abst. Bot. 19: 115-127.

Oshima, K., Y. Tang, and I. Washitani. 1997. Spatial and sea-

sonal patterns of microsite light availability in a remnant

fragment of deciduous riparian forest and their implication

in the conservation of Arisaema heterophyllum, a threatened

plant species. J. Plant Res. 110: 321-327.

Pennings , S .C. and R.M. Callaway. 2000. The advantages of

382

Botanical Studies, Vol. 49, 2008

clonal integration under different ecological conditions: A

community-wide test. Ecology 81: 709-716.

Pitelka, L.F. and J.W. Ashmun. 1985. Physiology and integra-

tion of ramets in clonal plants. In J.B.C. Jackson, L.W. Buss

and R.E. Cook (eds.), Population Biology and Evolution of

Clonal Organisms. Yale University Press, New Haven, pp.

399-437.

Ren, A., Y. Gao, Y. Liang, S. Chen, S. Liu, and N. Liu. 1999. Ef-

fect of drought stress on clonal growth of pennisetum cen-

trasiaticum and Leymus secalinus. J. Desert Res. 19: 30-34.

Roiloa, S.R. and R. Retuerto. 2006a. Physiological integration

ameliorates effects of serpentine soils in the clonal herb

Fragaria vesca. Physiol. Plant. 128: 662-676.

Roiloa, S.R. and R. Retuerto. 2006b. Small-scale heterogene-

ity in soil quality influences photosynthetic efficiency and

habitat selection in a clonal plant. Ann. Bot. (London) 98:

1043-1052.

Roiloa, S.R., P. Alpert, N. Tharayil, G. Hancock, and P.C. Bhow-

mik. 2007. Greater capacity for division of labour in clones

of Fragaria chiloensis from patchier habitats. J. Ecol. 95:

397-405.

Saitoh, T., K. Seiwa, and A. Nishiwaki. 2002. Importance of

physiological integration of dwarf bamboo to persistence in

forest understorey: A field experiment. J. Ecol. 90: 78-85.

Salzman, A.G. and M.A. Parker. 1985. Neighbors ameliorate lo-

cal salinity stress for a rhizomatous plant in a heterogeneous

environment. Oecologia 65: 273-277.

Schmid, B. and F.A. Bazzaz. 1987. Clonal integration and popu-

lation structure in perennials: Effects of severing rhizome

connections. Ecology 68: 2016-2022.

Schmid, B., G.M. Puttick, K.H. Burgess, and F.A. Bazzaz. 1988.

Clonal integration and effects of simulated herbivory in old-

field perennials. Oecologia 75: 465-471.

Shorrocks, B. and R. Swingland. 1990. Living in a Patchy Envi-

ronment. Oxford University Press, Oxford.

Slade, A.J. and M.J. Hutchings. 1987a. An analysis of the costs

and benefits of physiological integration between ramets in

the clonal perennial herb Glechoma hederacea. Oecologia

73: 425-431.

Slade, A.J. and M.J. Hutchings. 1987b. Clonal integration and

plasticity in foraging behaviour in Glechoma hederacea. J.

Ecol. 75: 1023-1036.

Stuefer, J.F. 1995. Separating the effects of assimilate and water

integration in clonal fragments by the use of steam-girdling.

Abstr. Bot. 19: 75-81.

Stuefer, J.F. 1996. Potential and limitations of current concepts

regarding the response of clonal plants to environmental

heterogeneity. Vegetatio 127: 55-70.

Stuefer, J.F., H. Dekroon, and H.J. During. 1996. Exploitation of

environmental heterogeneity by spatial division of labour in

a clonal plant. Funct. Ecol. 10: 328-334.

Stuefer, J.F., S. Gomez, and T. Van Molken. 2004. Clonal inte-

gration beyond resource sharing: Implications for defence

signalling and disease transmission in clonal plant networks.

Evol. Ecol. 18: 647-667.

Sweet, G.B. and P.F. Wareing. 1966. Role of plant growth in

regulating photosynthesis. Nature 210: 77-79.

Van Kleunen, M. and J.F. Stuefer. 1999. Quantifying the effects

of reciprocal assimilate and water translocation in a clonal

plant by the use of steam-girdling. Oikos 85: 135-145.

Wareing, P.F., M.M. Khalifa, and K.J . Treharne. 1968. Rate-

limiting processes in photosynthesis at saturating light in-

tensities. Nature 220: 453-457.

Wiens, J.A. 1976. Population responses to patchy environments.

Annu. Rev. Ecol. Syst. 7: 81-120.

Wu, Q.S., Y.N. Zou, R.X. Xia, and M.Y. Wang. 2007. Five Glo-

mus species affect water relations of Citrus tangerine dur-

ing drought stress. Bot. Stud. 48: 147-154.

Yin, C., X. Wang, B. Duan, J. Luo, and C. Li. 2005. Early

growth, dry matter allocation and water use efficiency of

two sympatric Populus species as affected by water stress.

Environ. Exp. Bot. 53: 315-322.

Yu, D., L. Lu, C. Gu, K. Guan, and C. Li. 1985. Flora of china,

Science Press, Beijing, pp. 353-354.

Yu, F., M. Dong, and B. Krusi. 2004. Clonal integration helps

psammochloa villosa survive sand burial in an inland dune.

New Phytol. 162: 697-704.

Yu, F., Y. Chen and M. Dong. 2002. Clonal integration enhances

survival and performance of Potentilla anserina, suffering

from partial sand burial on ordos plateau, China. Evol. Ecol.

15: 303-318.

Zhang, C. 2002. Ecological Adaptation of the Clonal Growth in

two Rhizonmatous Plants in Mu-Us Sandland. The Chinese

Academy of Sciences, Beijing.

ZHANG et al. ¡X High water contrast enhances clonal integration

383