Response of Acroptilon repens to simulated herbivory and soil disturbance

RESEARCH ARTICLE

Response of Acroptilon repens to simulatedherbivory and soil disturbanceO. Koloren1, S. Uygur1, F.N. Uygur1 & U. Schaffner2

1 Department of Plant Protection, Agricultural Faculty, Cxukurova University, Adana, TR 01330, Turkey

2 CABI Bioscience Switzerland Centre, CH-2800 Delemont, Switzerland

Keywords

Acroptilon repens; seed output; simulated

herbivory; soil disturbance; weed control.

Correspondence

U. Schaffner, CABI Bioscience Switzerland Cen-

tre, chemin des Grillons 1, CH-2800 Delemont,

Switzerland. Email: [email protected]

Received: 15 April 2004; revised version

accepted: 3 July 2005.

doi:10.1111/j.1744-7348.2005.00014.x

Abstract

Acroptilon repens is an invasive weed in North America but also causes prob-

lems in disturbed habitats in its native range in Asia. In order to test the effect

of simulated biological control and soil disturbance on established A. repens

patches and the competing vegetation, two levels of shoot clipping as well as

soil tillage were imposed on A. repens patches in an undisturbed meadow and

at two fallowland sites in the native range of the weed. At the meadow site,

2 years of partial clipping of shoots and of soil tillage had no influence on

A. repens performance, while soil tillage significantly reduced the above-

ground biomass of the competing vegetation. At the fallowland sites, which

had been continuously cultivated for several years prior to the experiment,

A. repens shoot density, biomass and number of seed heads were significantly

higher in the undisturbed control than in the tillage plots. The total number

of seed heads per unit area increased with shoot density up to 200 shoots

m22. These results indicate that A. repens has considerable regrowth capacities

that allow established patches to tolerate substantial losses of above-ground

biomass and that the competitive ability of A. repens is favoured both when soil

disturbance is imposed on previously undisturbed sites, as well as when

repeated soil disturbance is abandoned. The only promising nonchemical

herbicide-based approach to reduce the competitive ability and seed output of

A. repens appears to be a long-term management that enhances the inter-

specific plant competition by reducing soil disturbance and selectively damaging

A. repens.

Introduction

An effective weed management requires detailed knowl-

edge of the effect of potential control measures on the

population dynamics of the weed in the presence of com-

peting vegetation (Briese, 1993; Shea, 1998; Edwards

et al., 2000). The assessment of the impact of control

measures should therefore preferentially be carried out

under field conditions. This is particularly the case in

clonal weeds because the competitive ability and com-

pensatory growth of shoots may depend on their clonal

integration (Schmid et al., 1988).

Field studies with clonal plants revealed that herbivory

andmechanical damage in combination with interspecific

competition can reduce growth and reproductive output

of shoots (Root, 1996; Piqueras, 1999), shoot density

(Lowday & Marrs, 1992) as well as clonal growth

(Meyer & Schmid, 1999). However, because of the con-

siderable regrowth capacities of clonal plants, short-term

studies measuring the effects of mechanical or biological

control on clonal weeds may be unable to reveal sig-

nificant effects of the imposed treatments, although

the same treatments, repeatedly applied in a long-

term experiment, may have considerable impact on the

vigour of the weed (Lowday & Marrs, 1992). As an

alternative to such long-term studies, one may meas-

ure the change in performance of a clonal plant when

it is released from long-term damage or disturbance.

Annals of Applied Biology ISSN 0003-4746

Ann Appl Biol 147 (2005) 101–107 ª 2005 The AuthorsJournal compilation ª 2005 Association of Applied Biologists

101

Assessing demographic parameters of the weed and the

competing vegetation after the release from long-term

disturbance such as mowing or soil tillage cannot only

provide evidence that the weed indeed had been suffer-

ing from the disturbance imposed but may also illumi-

nate whether it is the weed or the competing vegetation

that can more quickly recover from a specific distur-

bance regime.

Russian knapweed, Acroptilon repens (L.) CD, a clonal

plant native to Asia, was accidentally introduced into

North America in the late 19th century and has

invaded all the western states of USA and the western

provinces of Canada since (Watson, 1980). This plant

is also known to display a weedy character in its

native range. In Turkey, for example, A. repens is a

major weed in orchards, where the soil is regularly til-

led in order to reduce competition for water between

the fruit trees and herbaceous vegetation (Mordovets

& Golovin, 1983; Sozeri & Maden, 1994). Both in its

native and exotic range, initial colonisation of a site

by A. repens involves establishment of genets from

seeds or from small root fragments, but subsequent

population development seems to occur almost ex-

clusively by the production of shoots via clonal

growth (Bottoms et al., 2001; O. Koloren, S. Uygur,

F.N. Uygur & U. Schaffner, personal observation). In

Wyoming, USA, A. repens has primarily invaded large

areas of low-value land (J.L. Baker, Fremont County

Weed and Pest, Lander, Wyoming, USA, personal com-

munication), rendering chemical herbicide-based con-

trol methods both economically and environmentally

inadequate.

The objective of the present study was to assess how

established A. repens patches and the competing vegeta-

tion respond to simulated above-ground herbivory on

A. repens and to soil disturbance. The treatments were

chosen to simulate either feeding damage by the special-

ist herbivore Cochylimopha nomanada Esch (Lepidoptera,

Cochylidae), one of the candidate species for biological

control of A. repens in North America (clipping of shoots

in spring; Schaffner et al., 2001), or the typical distur-

bance regime in those habitats in which A. repens tends

to become an aggressive weed even in its native range

(soil tillage). Specifically, the following questions were

asked. (a) How does simulated herbivory or soil distur-

bance affect the performance of A. repens and its compet-

ing vegetation in an undisturbed meadow? (b) How does

simulated herbivory or cessation of soil disturbance

affect the performance of A. repens and its competing

vegetation in fallowland that had been continuously dis-

turbed for several years prior to the experiment? (c)

How does a change in shoot density affect reproductive

output in A. repens?

Material and methods

Sites

Field sites were set up in central Turkey in an undisturbed

meadow (one site near Goreme; 38�389N, 34�439E) and in

1-year fallowland (two sites near Bor; 37�449N, 34�339E).Central Turkey is characterised by a continental climate

with high summer temperatures (mean maximum

temperature for the period 1990–99 was 41.8�C) and

low winter temperatures (mean minimum temperature,

236.2�C) and low precipitation (mean annual precipita-

tion, 392.0 mm). The site in the undisturbed meadow,

which had not been cultivated for at least 10 years, was

monitored for 2 consecutive years (2000–01), while the

two sites on the fallowland were studied only during one

season each (2000 and 2001).

Experimental design

On each site, four blocks were set up consisting of four

2- � 2-m plots, each (total of 16 plots site21) arranged in

a randomised block design. The four plots within a block

were randomly subjected to one of the following four

treatments: (a) soil tillage (approximately 20 cm deep)

using a hand plough, (b) removal of one thirds of all

shoots, (c) removal of two thirds of all shoots and (d)

undisturbed control. Removal of shoots was effected

by clipping shoots at ground level and simulated herbiv-

ory by the biological control candidate Cochylimorpha

nomadana. The larvae of thismoth complete their develop-

ment in late spring in the upper parts of the roots and in

the shoot base; infested A. repens shoots die back before

seed-set (Schaffner et al., 2001). At the meadow site, the

same treatments were applied in May of the first and the

second year. At the beginning of the experiment,

A. repens shoot density and mean height were assessed

on all plots. Immediately after the treatments were

imposed, shoot density was recorded again, except for

those plots where the soil was tilled and then in all plots

at monthly intervals until autumn. On freshly tilled soil,

no shoot density was recorded because it was difficult to

unambiguously assess the number of shoots that were

still attached to roots. In September, the number of

A. repens seed heads was recorded, above-ground bio-

mass harvested, separated into A. repens and ‘other vege-

tation’ and the dry weight determined. The number of

seed heads was determined as a surrogate for reproduc-

tive output; prestudies have shown that the number of

seed heads per shoot is significantly correlated with the

total number of seeds per shoot (Pearson’s correlation

coefficient r = 0.665, P < 0.0001, n = 60). At the meadow

site, the number of seed heads and above-ground biomass

Response of A. repens to simulated herbivory and disturbance O. Koloren et al.

102 Ann Appl Biol 147 (2005) 101–107 ª 2005 The Authors

Journal compilation ª 2005 Association of Applied Biologists

of A. repens as well as above-ground biomass of the com-

peting vegetation were only assessed at the end of the

second year.

In order to determine the relationship between shoot

density and reproductive output over a broad range of

shoot densities, twenty-four 1- � 1-m plots were set up

in a completely randomised design on a fallowland in

central Turkey. On six plots each, the number of shoots

per m2 was maintained at 1, 4, 8 or 16 by clipping all

surplus shoots. In addition, four plots were laid out at

one side of the field, which had a very high shoot den-

sity (mean of 208 shoots m22). Shoot density on these

four plots was not altered. At the end of the season,

the total number of seed heads was recorded on all 28

plots.

Statistics

Means ± SE for initial shoot density and shoot height

between the meadow and the fallowland habitats, and

between the treatments within the different habitats,

were compared using one-way analysis of variance

(ANOVA). To test for effects of simulated herbivory and

soil disturbance on A. repens and its competing vegeta-

tion, analysis of covariance (ANCOVA) was performed

on biomass (A. repens and other vegetation), shoot den-

sity and number of seed heads (A. repens only), using

shoot density of A. repens at the beginning of the experi-

ment as covariate. Comparisons of the adjusted means of

the treatments were made using the Tukey–Kramer test

(Quinn & Keough 2002).

Results

Initial mean shoot density of A. repens at the meadow and

the fallowland sites were 77.0 ± 5.1 and 100.7 ± 11.2

plot21, respectively, and did not differ significantly

(ANOVA; P > 0.15). However, mean shoot height at the

onset of the experiment was significantly greater at the

meadow (22.3 ± 1.5 cm) than at the two fallowland

sites (18.2 ± 2.3 cm) (ANOVA; d.f. = 1, 46; F = 7.463;

P < 0.01). At all sites, initial shoot density and shoot

length did not differ significantly among the groups as-

signed to the different treatments (ANOVA; all P > 0.5).

Undisturbed meadow

At the undisturbed meadow site, shoot density, above-

ground biomass and number of seed heads of A. repens at

the end of the second season did not differ between the

treatments (Table 1). In both years, shoot density in the

simulated herbivory plots recovered within 1 month

after the application of the treatments (Fig. 1). In the soil

tillage treatment, shoot density reached the level of that

in the control plots 2 months after treatment.

Biomass of the other vegetation at the end of the second

season was significantly reduced by soil disturbance, com-

pared to the two simulated herbivory treatments and the

control (Table 1; Fig. 2). In the soil disturbance plots,

other vegetation made up 15% of the total above-

ground biomass compared to approximately 50% in the

control plots. No difference was found between the con-

trol and the simulated herbivory treatments (Table 1;

Figs 1 and 2).

Table 1 Results of analyses of covariance for shoot density, biomass and number of seed heads of Acroptilon repens and biomass of other

vegetation at the undisturbed meadow site (at the end of the second season) and the two fallowland sites (after one season each). Number of

A. repens shoots at the beginning of the experiment was included as covariate in the model. Numbers in parentheses give degrees of freedom

Undisturbed Meadow Fallowland, Site 1 Fallowland, Site 2

MS F MS F MS F

A. repens shoot density

Treatment (3, 8) 37.2 0.138 1738.4 7.637** 3218.9 4.421*

Initial shoot density (1, 8) 5035.2 18.663** 5722.4 25.139*** 11 960.0 16.426**

A. repens biomass

Treatment (3, 8) 5414.3 2.586 1425.8 4.324* 25 272.6 10.533**

Initial shoot density (1, 8) 49 483.1 23.639*** 1638.1 4.968 27 901.8 11.629**

A. repens seed heads

Treatment (3, 8) 11 317.7 1.027 16.9 1.954 54 365.1 5.473*

Initial shoot density (1, 8) 237 331.1 21.543** 4.5 0.520 15 823.2 1.593

Other vegetation biomass

Treatment (3, 8) 13 196.5 5.412* — — 3244.2 6.380*

Initial shoot density (1, 8) 2000.2 0.820 — — 5555.4 10.926***

*Significant at P < 0.05; **significant at P < 0.01; and ***significant at P < 0.001.

O. Koloren et al. Response of A. repens to simulated herbivory and disturbance


103

Fallowland

At the 1-year fallowland sites, the simulated herbivory

and the control plots did not differ in any of the A. repens

parameters measured at the end of the season. As in the

undisturbed meadow, shoot density recovered in the

fallowland within 1 month after the simulated herbivory

treatments had been applied (Fig. 3). In contrast, shoot

density, above-ground biomass and number of seed

heads were significantly reduced in the soil disturbance

treatment in at least one of the two experiments com-

pared to the control plots (Table 1; Figs 3 and 4). The

number of seed heads produced on the fallowland site

studied in 2000 was very low, which may explain the

lack of significant treatment effects.

Biomass of the other vegetation was lowest on the

soil disturbance plots; it differed significantly between

the soil disturbance and the simulated herbivory plots

but not between the soil disturbance and the control

plots (Fig. 4).

Shoot density and seed output

The mean number of seed heads per shoot increased with

shoot densities, peaked at approximately 10 shoots m22

and declined at higher shoot densities (Fig. 5). The total

number of seed heads per area increased with shoot

density throughout.

Discussion

In order to assess the reponse of A. repens to simulated

herbivory and soil disturbance under as natural con-

ditions as possible, rhizome connections at the border of

the experimental plots were not severed. Clonal identi-

ties within established A. repens patches are difficult to

establish unequivocally, which makes this study open to

the criticism that plot responses to treatments may not

be independent. One of the suggested benefits of clonal

integration is that shoots suffering damage may reduce

negative effects by receiving support from undamaged

0

10

20

30

40

May AugJun Jul May Jun Jul Aug

2000 2001

Num

ber

of s

hoot

s /

m2

Figure 1 Shoot density (mean ± SE) of an Acroptilon repens patch in undisturbed meadow monitored during 2 years of consecutive treatment. d, con-

trol; :, one third of the shoots removed; n, two thirds of the shoots removed; ¤, soil tillage. Arrows indicate timing of treatments. SE, standard error.

0

25

50

75

C 1/3 2/3 T

Bio

mas

s / m

2

(g d

ry w

eigh

t)

0

50

100

150

C 1/3 2/3 T

Seed

hea

ds /

m2

aa

b

a

Figure 2 Biomass of Acroptilon repens (black bars) and other vegetation (open bars) per plot (LHS), and number of A. repens seed heads per m2 (RHS)

in undisturbed meadow after 2 years of experimental treatment. Treatments: C, control; 1/3; one third of shoots removed; 2/3, two third of the shoots

removed; T, soil tillage. Given are means ± SE; different letters indicate significant differences at P < 0.05 between individual treatments. SE, standard

error, LHS, left-hand side; RHS, right-hand side.




neighbours (Schmid et al., 1988). If the same mechanism

would operate in A. repens, then the results of this study

may be biased against detecting differences in perfor-

mance between different treatments.

Response to soil disturbance

Soil disturbance on the undisturbedmeadow led to a strik-

ing increase in the proportion of total above-ground bio-

mass comprising A. repens compared to the control plots.

While the competing vegetation was largely destroyed

by soil disturbance, A. repens was able to fully compen-

sate for the damage inflicted for at least two consecutive

seasons. On the other hand, abandonment of long-term

soil disturbance led to a substantial increase in above-

ground biomass and seed production of A. repens com-

pared to the continuous soil disturbance treatment. This

suggests that, despite the absence of significant effects of

soil disturbance on previously undisturbed A. repens

patches in the short-term experiment at the meadow

0

10

20

30

40

May AugJun Jul

site a (2000)

Num

ber

of s

hoot

s / m

2

Num

ber

of s

hoot

s / m

2

0

25

50

75

May AugJun Jul

site b (2001)

Figure 3 Shoot density (mean ± SE) of two different Acroptilon repens patches in 1-year fallowland. One site each was studied in 2000 (a) and 2001

(b), respectively. d, control; :, one third of the shoots removed; n, two thirds of the shoots removed; ¤, soil tillage. Arrows indicate timing of treat-

ments. SE, standard error.

0

20

40

60

80

100

120

C 1/3 2/3 T

a

a

a

b

0

10

20

30

40

50

60

70

80

C 1/3 2/3 T

Bio

mas

s / m

2

(g d

ry w

eigh

t)

a

a

a

ab

b

b

c

ac

0

1

2

3

4

5

C 1/3 2/3 T

Seed

hea

ds /

m2

Seed

hea

ds /

m2

0

5

10

15

20

25

30

C 1/3 2/3 T

Bio

mas

s / m

2

(g d

ry w

eigh

t) ab

a

b

ab

a)

b)

Figure 4 Biomass of Acroptilon repens (black bars) and other vegetation (white bars) per plot (LHS), and number of A. repens seed heads per m2

(RHS) in 1-year fallowland. One site each was studied in 2000 (a) and 2001 (b), respectively. Treatments: C, control; 1/3, one third of shoots removed;

2/3, two thirds of the shoots removed; T, soil tillage. Given are means ± SE; different letters indicate significant differences at P < 0.05 between indi-

vidual treatments. SE, standard error, LHS, left-hand side; RHS, right-hand side.



105

site, long-term disturbance can significantly affect the

performance of established A. repens patches. However,

A. repens appeared to recover more quickly from long-

term soil disturbance than the competing vegetation. As

in other clonal weeds, the increase in dominance by

A. repens during and after intensive soil cultivation may

be at least partly because of the creation and dispersal of

small root fragments, from which shoot recruitment can

rapidly occur (Edwards et al., 2000).

Repeated soil disturbance has been suggested as a

way of controlling clonal weeds, although single events

may dramatically increase the abundance of the weed

(Donald, 1990). The fact that repeated soil disturbance as

it is being practised in orchards or along roadsides in the

native range helps A. repens to establish almost complete

monocultures (U. Schaffner, personal observations) sug-

gests that repeated soil disturbance is not a feasible con-

trol method against this clonal weed.

Response to simulated herbivory

In our short-term experiments, the simulated herbivory

treatment appeared to have negligible impact onA. repens,

both on the meadow as well as on the fallowland sites.

The target weed was able to compensate for the sub-

stantial losses of above-ground biomass within a few

weeks. Such a high regrowth capacity after herbivory or

mechanical damage has been found also in other clonal

plant species (Schmid et al., 1988; Lowday & Marrs,

1992). In Solidago altissima L., for example, the negative

effect of simulated herbivory on harvest biomass was

marginal among physiologically integrated shoots but

pronounced among severed shoots (Schmid et al., 1988).

Clonally integrated shoots of S. altissima recovered from

the mechanical damage imposed within days or weeks

(Schmid et al., 1988), comparable to the results obtained

in our study. Similarly, cutting all fronds of Pteridium

aquilinum (L.) Kuhn actually increased above-ground

biomass at the end of the first year at one of two study

sites, but subsequent annual cutting thereafter slowly

reduced above-ground biomass to 10–30% of that on

untreated plots over 7 years (Lowday & Marrs, 1992).

The relationship between shoot density and

reproductive output

Changes in shoot density in the range observed in our

experimental plots (5–50 shoots m22) directly translated

into changes in seed output and hence the long-distance

dispersal. The separation of the 4 high-density plots from

the other 24 experimental plots requires caution in

the interpretation of the density relationship, but obser-

vations from other Russian knapweed patches in Turkey

support our findings that total number of seed heads of

A. repens increased at least up to 200 shoots m22. The

relationship between mean number of seed heads per

shoot and shoot density in A. repens contrasts with the

commonly found negatively density-dependent linear

relationship between shoot density and reproductive

output (Bishop & Davy, 1985; Silvertown & Lovett

Doust, 1993). In our experiment, the low reproductive

output of isolated shoots cannot be explained by

increased interspecific competition because biomass of

the competing vegetation did not differ between the

plots with low or high shoot densities. Rather, the low

investment in sexual reproduction at low shoot densities

may be because of an inherently increased allocation to

vegetative growth rate as long as there is space available

to spread further. The advancing front of A. repens

patches usually consists of widely spaced shoots; this

type of growth form is called guerrilla mode (Lovett

Doust, 1981). The shoots at the outer side of A. repens

1 10 100 1000

0.1

1

10

100

1000

Shoot density / m2

Seed

hea

ds /

m2

1 10 100 10000.0

2.5

5.0

Shoot density / m2

Seed

hea

ds p

er s

hoot

Figure 5 Total number of seed heads per shoot (LHS) and number of seed heads per m2 (RHS) produced by Acroptilon repens at various shoot den-

sities. LHS, left-hand side; RHS, right-hand side.




patches have relatively low seed heads numbers, which

is in agreement with the findings from our study in

which we experimentally manipulated shoot density

inside established patches.

Conclusions

The results from this study indicate that control of

A. repens by nonchemical herbicide-based methods has

to be viewed as a long-term process. The key for the suc-

cessful control of A. repens seems to be an enhancement

of interspecific plant competition to reduce the vigour of

established patches. Selective removal of A. repens bio-

mass without disturbing the competing vegetation ap-

pears to be the most promising nonchemical herbicide-

based strategy to lower the competitive ability and seed

output in A. repens. Attack by the root- and shoot-

mining larvae of the biological control candidate

C. nomadana in early spring leads to the die off of young

A. repens shoots (Schaffner et al., 2001). Provided that

this species has a sufficiently narrow host range, the

introduction of C. nomadana might therefore be an effec-

tive approach to reduce the vigour of A. repens in North

America. Results from the fallowland sites provide some

evidence that the competing vegetation can indeed

profit from the selective removal of A. repens shoots

within relatively short time.

Acknowledgements

We are grateful to H. Muller-Scharer, J. Knight, the editor

and three anonymous reviewers for very helpful com-

ments on earlier versions of this article.

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Response of Acroptilon repens to simulated herbivory and soil disturbance