ABSTRACT.-Spotted knapweed (Centaurea maculosa Lam.) is a major rangeland and roadside weed of the northern Rocky Mountains. It is often found in plant communities dominated by Pseudoroegneria spicatum or Festuca idahoensis, but it rarely invades roadsides dominated by Bromus inerrnis Leyss. Aboveground biomass of the 3 grass species grown in mixture with Centaurea was compared to growth in monoculture at a range of nitrogen input levels. The results suggest that Bromus is capable of suppressing the growth of Centaurea with the degree of suppression increasing with increasing nitrogen levels. The 2 native grasses had no impact on Centaurea under the controlled environment conditions of this study

Lindquist, John L.

Maxwell, Bruce D.

Weaver, T.

English

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Agronomy & Horticulture -- Faculty Publications Agronomy and Horticulture Department 
1996 
Potential for Controlling the Spread of Centaurea maculosa with 
Grass Competition 
John L. Lindquist 
University of Nebraska-Lincoln, jlindquist1@unl.edu 
Bruce D. Maxwell 
Montana State University - Bozeman 
T. Weaver 
Montana State University - Bozeman 
Follow this and additional works at: https://digitalcommons.unl.edu/agronomyfacpub 
 Part of the Plant Sciences Commons 
Lindquist, John L.; Maxwell, Bruce D.; and Weaver, T., "Potential for Controlling the Spread of Centaurea 
maculosa with Grass Competition" (1996). Agronomy & Horticulture -- Faculty Publications. 403. 
https://digitalcommons.unl.edu/agronomyfacpub/403 
This Article is brought to you for free and open access by the Agronomy and Horticulture Department at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Agronomy & Horticulture -- 
Faculty Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Great Basin Naturalist 56(3), © 1996, pp. 267-271 
POTENTIAL FOR CONTROLLING THE SPREAD OF 
CENTAUREA MACULOSA WITH GRASS COMPETITION 
John L. Lindquistl ,2, Bruce D. Maxwell l , and T. vVeaverl 
ABSTRACT.-Spotted knapweed (Centaurea maculosa Lam.) is a major rangeland and roadside weed of the northern 
Rocky Mountains. It is often found in plant communities dominated by Pseudoroegneria spicatum or Festuca idahoensis, 
but it rarely invades roadsides dominated by Bromus inerrnis Leyss. Aboveground biomass of the 3 grass species grown in 
mixture with Centaurea was compared to growth in monoculture at a range of nitrogen input levels. The results suggest 
that Bromus is capable of suppressing the growth of Centaurea with the degree of suppression increasing with increasing 
nitrogen levels. The 2 native grasses had no impact on Centaurea under the controlled environment conditions of this 
study. 
Key words: competition, weed control, Centaurea maculosa, Bromus inermis, Agropyron spicatum, Festuca idahoen-
sis, exotic plants. 
Centaurea maculosa Lam. (spotted knap-
weed) is a major weed associated with spring 
wet-summer dry areas of the northern Rocky 
Mountains (Forcella and Harvey 1981, Tyser 
and Key 1988, Weaver et aI. 1989). Centaurea 
dominates waste places, invades disturbed 
rangeland, and sometimes invades undisturbed 
range (Tyser and Key 1988). In contrast, it rarely 
invades roadsides dominated by Bromus inermis 
Leyss. (Weaver et al. 1989). This suggests that 
it may be excluded from waste places that are 
planted to Bromus before Centaurea invades. 
Alternatively, because planting exotics violates 
the charge of national park managers, one may 
ask whether Centaurea might also be excluded 
from disturbed areas by planting native grasses 
that naturally dominate either relatively dry 
(Pseudoroegneria spicatum [Pursh] Scribner and 
Smith = Agropyron spicatum) or more moist 
(Festuca idahoensis Elmer.) foothill habitats. 
Weed suppression may be accomplished by 
(1) preempting resources with more competi-
tive plant species or (2) using biocontrols or 
herbicides that selectively increase weed mor-
tality, decrease vigor, or prevent reproduction 
(Lindquist et al. 1995). This study considers 
management of Centaurea maculosa by compe-
tition rather than by common herbicide and 
biocontrol methods. This approach deserves 
attention because it may be less expensive and 
more effective than herbicides in the long 
term. 
Our objective was to measure the competi-
tive ability of 3 grass species against Centaurea 
in 2-way interaction experiments in sand cul-
ture. Mixture and mono culture treatments 
were tested for 12 wk at 5 positions on a nitro-
gen gradient to determine whether competi-
tive relations were influenced by differences in 
nitrogen availability. A plant's ability to com-
pete is related to its growth rate or ability to 
gain biomass relative to associated species 
(Harper 1977). We compared aboveground bio-
mass of each species grown in mixture with 
Centaurea to its growth in monoculture. 
MATERIALS AND METHODS 
The rhizomatous exotic pasture grass Bromus 
inermis Leyss. and 2 native bunchgrasses nor-
mally dominating relatively dry foothills (Pseu-
doroegneria spicatum) or moister grasslands 
immediately above and below the conifer zone 
(Festuca idahoensis) were grown in 2-species 
mixtures (replacement series) with C. maculosa. 
Experiments consisted of 3 competition treat-
ments (monocultures of both grass and Centau-
rea, and 50:50 mixture) combined with 5 nitro-
gen addition treatments. Each treatment com-
bination had 10 replicates. Within each experi-
ment, pots were arranged in a completely ran-
domized design on a greenhouse bench and 
rotated weekly to minimize position effects. 
Each experiment was subject to different light 
I Department of Biology and Department of Plant, Soil, and Environmental Sciences, '.Iontana State U niyersity, Bozeman, '.IT 59717. 
2Present address: Department of Agronomy, University of "iebraska, Lincoln, NE 68583-0915. 
267 
Published by Brigham Young University.
268 GREAT BASI~ NATURALIST [Volume 56 
conditions because of its position in the green-
house. A square planting pattern was used 
with 4 plants spaced 5 cm apart. In each pot in 
the mixture treatments, plants of the same 
species were located on the diagonal. 
Seeds were planted at a depth of 1.0 cm in 
1000-cm3 pots filled with coarse washed sand. 
Pots were watered daily for 1 wk to allow 
seedling establishment. Excess seedlings were 
thinned and remaining seedlings allowed to 
grow for an additional week prior to the addi-
tion of nutrients. The basic nutrient solution 
was balanced with respect to all essential nutri-
ents but could be varied to allow the establish-
ment of nitrogen levels from 0%, 1%, 10%, 
30%, and 100% of a standard level (:\1achlis and 
Torrey 1956). Sufficient nutrient solution (200 
ml) was applied to saturate the pot twice weekly 
and water (200 ml) was added once each week. 
Regular watering with nutrient solution and 
alternate washing with tap water held the soil 
solution near the applied level and prevented 
any concentration of the soil solution due to 
evapotranspiration. Experiments were conducted 
during March, April, and May 1988, when 
greenhouse temperatures ranged from 14 0 to 
32 0 C (25 0 C mean). 
Twelve weeks after emergence, plants in 
each pot were clipped at the soil surface, sepa-
rated by species, dried at 45 0 C for 5 d, and 
weighed. 
Nonlinear regression procedures (SAS 1988, 
Gauss-Newton least squares estimation method) 
were used to fit a rectangular hyperbola equa-
tion [1] (Cousens 1985) to mixture and mono-
culture data for each species: 
Ii . N 
B=-----
J.·N 
(1 + _l __ ) 
Ai 
[1] 
where B = aboveground dry biomass (g 
plant-I), Ai = maximum aboveground biomass 
of species i (g plant-I), N = relative nitrogen 
addition level, and Ii = biomass of species i as 
relative nitrogen addition level approaches zero. 
To determine the relative success of Centau-
rea in competition with each grass species, 
estimates of Ai and Ii were compared between 
mixtures and monocultures using the extra 
sum of squares procedure (Ratkowsky 1983, 
Lindquist et al. 1996). In addition, relative 
competition intensity (RCI; Grace 1995) was 
calculated to determine whether competitive 
relationships varied across relative nitrogen 
levels. RCI is calculated as 
where Bmono and Bmix are the aboveground dry 
biomass (g plant-I) for a species grown in mono-
culture and mixture, respectively. A negative 
RCI value indicates that the species performs 
better in mixture than in monoculture. RCI may 
be the best measure for determining species 
displacement under competitive conditions 
across a resource gradient (Grace 1995). Analy-
sis of variance was used to test for differences 
in RCI within a species across nitrogen treat-
ments. Student's t was used to compare RCI 
between species at each nitrogen addition level. 
RESULTS 
A hyperbolic relationship between individ-
ual plant biomass and relative nitrogen level 
was found in all mixtures and monocultures 
(Figs. la-f). Estimates of Ii (biomass at inter-
cept) differed between mixtures and mono cul-
tures only for Centaurea grown in mixture with 
Bromus (Table 1). Estimates of Ai (maximum 
biomass) differed for Bromus and Centaurea 
(Table 1). 
Relative competition intensity was signifi-
cantly negative for Bromus at all nitrogen addi-
tion levels; it varied from negative values at 
low nitrogen to positive values at higher nitro-
gen levels for Centaurea in competition with 
Bromus (Fig. 2). However, RCI did not differ 
from zero in the experiments where P. spica-
tum and E idahoensis were in competition with 
Centaurea (data not shown). 
DISCUSSION 
Growth response of Bromus to nitrogen was 
greater in mixture with Centaurea than in 
monoculture, as indicated by the regression 
lines (Fig. 1a) and the negative RCI values 
across all nitrogen addition levels (Fig. 2). In 
contrast, growth response of Centaurea was 
lower in mixture with Bromus than in mono-
culture (Fig. 1). The increase in Centaurea RCI 
at high relative nitrogen level indicates that 
Bromus is a better competitor in the high 
nitrogen treatments (Fig. 2). Results suggest 
that Bromus is capable of suppressing the 
1996] CO",lPETITIVE SUPPRESSIO~ OF CENTAUREA 269 
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0.0 E: 0.0 
8 
0.0 0.2 0.4 0.6 0.8 1.0 (j 0.0 0.2 0.4 0.6 0.8 1.0 
Relative Nitrogen Addition Level 
Fig. 1. Plot of observed (0) and predicted (.) aboveground dry biomass plant-Ion relative nitrogen addition level 
when grown in monoculture ( ) and mixture (----): a, Bromus grown in monoculture and in mixture with Gentaurea; 
b, Gentaurea grown in monoculture and in mixture with Bromus; c, Pseudoroegneria grown in mono culture and in mix-
ture with Gentaurea; d, Gentaurea grown in monoculture and in mixture with Pseudoroegneria; e, Gentaurea grO\vn in 
mono culture and in mixture with Festuca; f, Festuca grown in mono culture and in mixture with Gental/rea. 
270 GREAT BASIN NATURALIST [Volume 56 
TABLE 1. Estimates of parameter values followed by asymptotic standard errors for maximum aboveground biomass (g 
plant-I) (A), biomass as relative nitrogen level approaches zero (1), and the coefficient of determination (r2) obtained 
from fitting equation [1] to monoculture and mixture data of each species. Variation in 1 and A between competition 
treatments was tested using the extra sum of squares principle, with P value indicating the significance level for the 
comparison of parameter (CoefI) values from the monoculture and mixture regressions. 
Species Coeff Competition treatment Pvalue 
Monoculture Mixture 
Bromus A 1.884(0.115) 3.819(0.361) <0.000 
1 8.769(1.141) 12.893(2.197) 0.283 
r2 0.90 0.84 
Centaurea with A 1.306(0.082) 0.438(0.056) <0.000 
Bromus 1 10.621(1.861) 45.717(35.47) 0.023 
r2 0.85 0.14 
Pseudoroegneria A 1.289(0.237) 0.827(0.087) 0.081 
1 1.758(0.323) 2.490(0.442) 0.410 
r2 0.80 0.82 
Centaurea with A 0.636(0.048) 0.805(0.107) 0.307 
Pseudoroegneria 1 6.770(1.639) 4.617(1.454) 0.665 
r2 0.77 0.60 
Festuca A 0.595(0.029) 0.491(0.056) 0.303 
1 16.747(3.892) 23.921(14.925) 0.857 
r2 0.83 0.39 
Centaurea with A 1.262(0.114) 1.522(0.172) 0.514 
Festuca 1 6.418(1.295) 10.061(2.870) 0.535 
r2 0.77 
growth of Centaurea, the degree of suppres-
sion increasing with increasing nitrogen levels. 
Growth response of Pseudoroegneria and 
Festuca to nitrogen when growing in mixture 
with Centaurea did not differ from their response 
in monoculture. Likewise, growth response of 
Centaurea did not differ between monoculture 
and mixtures with Pseudoroegneria or Festuca. 
Therefore, these results suggest that these 
native grasses are not likely to increase or sup-
press growth of Centaurea, regardless of nitro-
gen addition level. This result is contrary to 
the observed invasion of Centaurea into com-
munities dominated by these grasses. One ex-
planation may be that disturbance (especially 
grazing) in the field creates gaps in the grass 
community where Centaurea can establish itself 
even though it is not a superior competitor for 
resources. 
Competitive interactions were greater be-
tween each grass species and Centaurea at the 
high end of the nitrogen gradient. This may be 
a function of rapid growth. Thus, in nitrogen-
rich environments fast-growing plants may 
rapidly occupy space and usurp resources to 
the exclusion of slow-growing species (Grime 
1979, Radosevich and Holt 1984). Similar com-
petitive effects may be expected to occur on 
other soil resource gradients, assuming adapta-
tions for acquisition of nitrogen and other mobile 
0.66 
nutrients, as well as water, are similar (Grime 
1979, Fitter and Hay 1987). In addition, one 
may hypothesize, based on the resource ratio 
theory (Tilman 1982), that Bromus is a superior 
competitor for nutrients other than nitrogen 
relative to Centaurea. By increasing nitrogen, 
both species should be limited by essential 
nutrients other than nitrogen, and the species 
with the lowest R* (the superior competitior) 
for the other nutrients should displace the 
species with the higher R * for the same nutri-
ents (Tilman 1990). 
The ability of Bromus to out-compete Cen-
taurea in nutrient culture provides one expla-
nation for the observed population dynamics of 
Centaurea in the field. Roadsides seeded with 
Bromus are rarely invaded by Centaurea (Weaver 
et al. 1989). Both field and laboratory observa-
tions suggest that disturbed sites seeded simul-
taneously with Centaurea and the exotic, Bro-
mus, will be dominated by Bromus. The effec-
tiveness of Bromus in suppressing Centaurea 
may be increased with fertilization. Further-
more, it may be expected that established Bro-
mus plants will suppress the growth of Centau-
rea seedlings. The results of this study suggest 
that at the seedling stage Bromus may be used 
to competitively exclude Centaurea. This method 
of weed management merits trial in the field. 
On the other hand, the regional dominants, 
1996] COMPETITIVE SUPPRESSION OF CENTAUREA 271 
> 
."t: * * CI) 
c A 
Q) 0.5 +-' c 
C 
0 
0 .';:; 
.';:; 
Q) 
Q. 
E -0.5 
0 
U 
Q) 
> -1 .';:; 
CO c _ Bromus _ Centaurea 
Q) 
0: 
-1.5 
o 0.01 0.1 0.3 1.0 
Relative Nitrogen Addition Level 
Fig. 2. Relative competition intensity (RCI) of Bromus and Centaurea across 5 relative nitrogen addition levels. Let-
ters above bars indicate whether RCI varies (Duncan's multiple range test, P < 0.05) within species across nitrogen 
level. An asterisk indicates that RCI differs (P < 0.05) among species at that nitrogen level. 
Pseudoroegneria and Festuca, probably would 
not sufficiently suppress Centaurea to decrease 
the potential for invasion. 
Advantages of the competitive method over 
herbicides and biocontrol treatments used to 
manage Centaurea are its long duration and 
low environmental impact. Given these advan-
tages, exclusion of Centaurea with Bromus 
merits trial in environments where the danger 
of invasion exists. 
LITERATURE CITED 
COUSE~S, R. 1985. A simple model relating yield loss to 
weed density. Annals of Applied Biology 107: 239-252. 
FITIER, A. H., AND R. K. Y1. HAY. 1987. Environmental 
physiology of plants. 2nd edition. Academic Press 
Inc., San Diego, CA. 423 pp. 
FORCELLA, F., A~D S. HARVEY. 1981. Migration and distri-
bution of 100 alien weeds in NW USA, 1881-1980. 
Herbarium, Montana State University, Bozeman. 117 
pp. 
GRACE, J. B. 1995. On the measurement of plant competi-
tion intensity. Ecology 76: 305-308. 
GRnlE, J. P. 1979. Plant strategies and vegetation pro-
cesses. John Wiley and Sons, London. 222 pp. 
HARPER, J. L. 1977. Population biology of plants. Academic 
Press Inc., San Diego, CA. 892 pp. 
LIKDQUlST, J. L., B. D. ~1AXWELL, D. D. BeHLER, A:\D J. L. 
Gc:-.;sou.;s. 1995. Modeling the population dynamics 
and economics of velvetleaf (Abutilon theophrasti) 
control in a corn (Zea mays)-soybean (Glycine max) 
rotation. Weed Science 43: 269-275. 
LINDQCIST, J. L., D. A. MORTE~SE~, S. A. CLAY, R. SCHMENK, 
J. J. KELLS, K. HOWATI, A..'\iD P. WESTRA. 1996. Stability 
of corn-velvetleaf interference relationships. Weed 
Science 44: 309-313. 
MACHLIS, L., A~D J. TORREY. 1956. Plants in action. Free-
man, San Francisco, CA. 282 pp. 
R>\DOSEVICH, R. S., A~D J. S. HOLT. 1984. Weed ecology: 
implication for vegetation management. John \Viley 
& Sons Inc., New York. 265 pp. 
RATKOWSKY, D. A. 1983. Nonlinear regression modeling: a 
unified practical approach. Marcel Dekker, Inc., New 
York. 276 pp. 
SAS. 1988. SAS/STAT user's guide, release 6.03. SAS 
Institute, Cary, NC. 1028 pp. 
TILMA~, D. 1982. Resource competition and community 
structure. Princeton University Press, Princeton, NJ. 
. 1990. Mechanisms of plant competition for nutri-
--e-nts: the elements of a predictive theory of competi-
tion. Pages 117-141 in J. B. Grace and D. Tilman, 
editors, Perspectives on plant competition. Academic 
Press, Inc., New York. 
TYSER, R., A~D C. KEY. 1988. Spotted knapweed in natural 
area fescue grasslands: an ecological assessment. 
Northwest Science 62: 151-160. 
WEAVER, T., D. GUSTAFSO~, J. LICHTHARDT, AND B. WOODS. 
1989. Distribution of exotic plants in the Northern 
Rocky Mountains by environmental type and distur-
bance condition. MSU Biology Report 41, Bozeman, 
MT. 91 pp. 
Received 26 September 1995 
Accepted 29 April 1996 


Potential for Controlling the Spread of \u3ci\u3eCentaurea maculosa\u3c/i\u3e with Grass Competition

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Agronomy & Horticulture -- Faculty Publications Agronomy and Horticulture Department 1996 Potential for Controlling the Spread of Centaurea maculosa with Grass Competition John L. Lindquist University of Nebraska-Lincoln, jlindquist1@unl.edu Bruce D. Maxwell Montana State University - Bozeman T. Weaver Montana State University - Bozeman Follow this and additional works at: https://digitalcommons.unl.edu/agronomyfacpub  Part of the Plant Sciences Commons Lindquist, John L.; Maxwell, Bruce D.; and Weaver, T., "Potential for Controlling the Spread of Centaurea maculosa with Grass Competition" (1996). Agronomy & Horticulture -- Faculty Publications. 403. https://digitalcommons.unl.edu/agronomyfacpub/403 This Article is brought to you for free and open access by the Agronomy and Horticulture Department at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Agronomy & Horticulture -- Faculty Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Great Basin Naturalist 56(3), © 1996, pp. 267-271 POTENTIAL FOR CONTROLLING THE SPREAD OF CENTAUREA MACULOSA WITH GRASS COMPETITION John L. Lindquistl ,2, Bruce D. Maxwell l , and T. vVeaverl ABSTRACT.-Spotted knapweed (Centaurea maculosa Lam.) is a major rangeland and roadside weed of the northern Rocky Mountains. It is often found in plant communities dominated by Pseudoroegneria spicatum or Festuca idahoensis, but it rarely invades roadsides dominated by Bromus inerrnis Leyss. Aboveground biomass of the 3 grass species grown in mixture with Centaurea was compared to growth in monoculture at a range of nitrogen input levels. The results suggest that Bromus is capable of suppressing the growth of Centaurea with the degree of suppression increasing with increasing nitrogen levels. The 2 native grasses had no impact on Centaurea under the controlled environment conditions of this study. Key words: competition, weed control, Centaurea maculosa, Bromus inermis, Agropyron spicatum, Festuca idahoen-sis, exotic plants. Centaurea maculosa Lam. (spotted knap-weed) is a major weed associated with spring wet-summer dry areas of the northern Rocky Mountains (Forcella and Harvey 1981, Tyser and Key 1988, Weaver et aI. 1989). Centaurea dominates waste places, invades disturbed rangeland, and sometimes invades undisturbed range (Tyser and Key 1988). In contrast, it rarely invades roadsides dominated by Bromus inermis Leyss. (Weaver et al. 1989). This suggests that it may be excluded from waste places that are planted to Bromus before Centaurea invades. Alternatively, because planting exotics violates the charge of national park managers, one may ask whether Centaurea might also be excluded from disturbed areas by planting native grasses that naturally dominate either relatively dry (Pseudoroegneria spicatum [Pursh] Scribner and Smith = Agropyron spicatum) or more moist (Festuca idahoensis Elmer.) foothill habitats. Weed suppression may be accomplished by (1) preempting resources with more competi-tive plant species or (2) using biocontrols or herbicides that selectively increase weed mor-tality, decrease vigor, or prevent reproduction (Lindquist et al. 1995). This study considers management of Centaurea maculosa by compe-tition rather than by common herbicide and biocontrol methods. This approach deserves attention because it may be less expensive and more effective than herbicides in the long term. Our objective was to measure the competi-tive ability of 3 grass species against Centaurea in 2-way interaction experiments in sand cul-ture. Mixture and mono culture treatments were tested for 12 wk at 5 positions on a nitro-gen gradient to determine whether competi-tive relations were influenced by differences in nitrogen availability. A plant's ability to com-pete is related to its growth rate or ability to gain biomass relative to associated species (Harper 1977). We compared aboveground bio-mass of each species grown in mixture with Centaurea to its growth in monoculture. MATERIALS AND METHODS The rhizomatous exotic pasture grass Bromus inermis Leyss. and 2 native bunchgrasses nor-mally dominating relatively dry foothills (Pseu-doroegneria spicatum) or moister grasslands immediately above and below the conifer zone (Festuca idahoensis) were grown in 2-species mixtures (replacement series) with C. maculosa. Experiments consisted of 3 competition treat-ments (monocultures of both grass and Centau-rea, and 50:50 mixture) combined with 5 nitro-gen addition treatments. Each treatment com-bination had 10 replicates. Within each experi-ment, pots were arranged in a completely ran-domized design on a greenhouse bench and rotated weekly to minimize position effects. Each experiment was subject to different light I Department of Biology and Department of Plant, Soil, and Environmental Sciences, '.Iontana State U niyersity, Bozeman, '.IT 59717. 2Present address: Department of Agronomy, University of "iebraska, Lincoln, NE 68583-0915. 267 Published by Brigham Young University.268 GREAT BASI~ NATURALIST [Volume 56 conditions because of its position in the green-house. A square planting pattern was used with 4 plants spaced 5 cm apart. In each pot in the mixture treatments, plants of the same species were located on the diagonal. Seeds were planted at a depth of 1.0 cm in 1000-cm3 pots filled with coarse washed sand. Pots were watered daily for 1 wk to allow seedling establishment. Excess seedlings were thinned and remaining seedlings allowed to grow for an additional week prior to the addi-tion of nutrients. The basic nutrient solution was balanced with respect to all essential nutri-ents but could be varied to allow the establish-ment of nitrogen levels from 0%, 1%, 10%, 30%, and 100% of a standard level (:\1achlis and Torrey 1956). Sufficient nutrient solution (200 ml) was applied to saturate the pot twice weekly and water (200 ml) was added once each week. Regular watering with nutrient solution and alternate washing with tap water held the soil solution near the applied level and prevented any concentration of the soil solution due to evapotranspiration. Experiments were conducted during March, April, and May 1988, when greenhouse temperatures ranged from 14 0 to 32 0 C (25 0 C mean). Twelve weeks after emergence, plants in each pot were clipped at the soil surface, sepa-rated by species, dried at 45 0 C for 5 d, and weighed. Nonlinear regression procedures (SAS 1988, Gauss-Newton least squares estimation method) were used to fit a rectangular hyperbola equa-tion [1] (Cousens 1985) to mixture and mono-culture data for each species: Ii . N B=-----J.·N (1 + _l __ ) Ai [1] where B = aboveground dry biomass (g plant-I), Ai = maximum aboveground biomass of species i (g plant-I), N = relative nitrogen addition level, and Ii = biomass of species i as relative nitrogen addition level approaches zero. To determine the relative success of Centau-rea in competition with each grass species, estimates of Ai and Ii were compared between mixtures and monocultures using the extra sum of squares procedure (Ratkowsky 1983, Lindquist et al. 1996). In addition, relative competition intensity (RCI; Grace 1995) was calculated to determine whether competitive relationships varied across relative nitrogen levels. RCI is calculated as where Bmono and Bmix are the aboveground dry biomass (g plant-I) for a species grown in mono-culture and mixture, respectively. A negative RCI value indicates that the species performs better in mixture than in monoculture. RCI may be the best measure for determining species displacement under competitive conditions across a resource gradient (Grace 1995). Analy-sis of variance was used to test for differences in RCI within a species across nitrogen treat-ments. Student's t was used to compare RCI between species at each nitrogen addition level. RESULTS A hyperbolic relationship between individ-ual plant biomass and relative nitrogen level was found in all mixtures and monocultures (Figs. la-f). Estimates of Ii (biomass at inter-cept) differed between mixtures and mono cul-tures only for Centaurea grown in mixture with Bromus (Table 1). Estimates of Ai (maximum biomass) differed for Bromus and Centaurea (Table 1). Relative competition intensity was signifi-cantly negative for Bromus at all nitrogen addi-tion levels; it varied from negative values at low nitrogen to positive values at higher nitro-gen levels for Centaurea in competition with Bromus (Fig. 2). However, RCI did not differ from zero in the experiments where P. spica-tum and E idahoensis were in competition with Centaurea (data not shown). DISCUSSION Growth response of Bromus to nitrogen was greater in mixture with Centaurea than in monoculture, as indicated by the regression lines (Fig. 1a) and the negative RCI values across all nitrogen addition levels (Fig. 2). In contrast, growth response of Centaurea was lower in mixture with Bromus than in mono-culture (Fig. 1). The increase in Centaurea RCI at high relative nitrogen level indicates that Bromus is a better competitor in the high nitrogen treatments (Fig. 2). Results suggest that Bromus is capable of suppressing the 1996] CO",lPETITIVE SUPPRESSIO~ OF CENTAUREA 269 C ro ~ ..9 en en ro E .2 CO V) .~ '-Ql .S a::i C ~ .2-..9 en en ro E 0 co E: :::J ~ <..:I '0.. V) ~ c ~ Cl. ~ en en ro E 0 m V) 'iij c:: Ql 0 ~ (1l ~ L( a 5 b 4 3 - -. : 2 2 -i p~-----01 ~/ 01 I"~_mum ___ ~ 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 1.5 i C 1.5 C ~ d ! .2-1.2 ..9 1.2 en en ~ ro 0.9 0.9 E ~ _----e 0 061 CD 0.6 (1l V) 0 0.3 ::; f /, 0.3 ~~/ <..:I 0.0 I (1l 0.0 E: 0.0 0.2 0.4 0.6 0.8 1.0 (j 0.0 0.2 0.4 0.6 0.8 1.0 c 3.0 ro 3.0 e a. 2.5 ~ 2.5 en 2.0 en 2.0 ro E 1.5 0 1.5 m 1.0 (1l 1.0 V) • 8 0 0.5 - ~---!------------------1 ::; 0.5 J: <..:I (1l 0.0 E: 0.0 8 0.0 0.2 0.4 0.6 0.8 1.0 (j 0.0 0.2 0.4 0.6 0.8 1.0 Relative Nitrogen Addition Level Fig. 1. Plot of observed (0) and predicted (.) aboveground dry biomass plant-Ion relative nitrogen addition level when grown in monoculture ( ) and mixture (----): a, Bromus grown in monoculture and in mixture with Gentaurea; b, Gentaurea grown in monoculture and in mixture with Bromus; c, Pseudoroegneria grown in mono culture and in mix-ture with Gentaurea; d, Gentaurea grown in monoculture and in mixture with Pseudoroegneria; e, Gentaurea grO\vn in mono culture and in mixture with Festuca; f, Festuca grown in mono culture and in mixture with Gental/rea. 270 GREAT BASIN NATURALIST [Volume 56 TABLE 1. Estimates of parameter values followed by asymptotic standard errors for maximum aboveground biomass (g plant-I) (A), biomass as relative nitrogen level approaches zero (1), and the coefficient of determination (r2) obtained from fitting equation [1] to monoculture and mixture data of each species. Variation in 1 and A between competition treatments was tested using the extra sum of squares principle, with P value indicating the significance level for the comparison of parameter (CoefI) values from the monoculture and mixture regressions. Species Coeff Competition treatment Pvalue Monoculture Mixture Bromus A 1.884(0.115) 3.819(0.361) <0.000 1 8.769(1.141) 12.893(2.197) 0.283 r2 0.90 0.84 Centaurea with A 1.306(0.082) 0.438(0.056) <0.000 Bromus 1 10.621(1.861) 45.717(35.47) 0.023 r2 0.85 0.14 Pseudoroegneria A 1.289(0.237) 0.827(0.087) 0.081 1 1.758(0.323) 2.490(0.442) 0.410 r2 0.80 0.82 Centaurea with A 0.636(0.048) 0.805(0.107) 0.307 Pseudoroegneria 1 6.770(1.639) 4.617(1.454) 0.665 r2 0.77 0.60 Festuca A 0.595(0.029) 0.491(0.056) 0.303 1 16.747(3.892) 23.921(14.925) 0.857 r2 0.83 0.39 Centaurea with A 1.262(0.114) 1.522(0.172) 0.514 Festuca 1 6.418(1.295) 10.061(2.870) 0.535 r2 0.77 growth of Centaurea, the degree of suppres-sion increasing with increasing nitrogen levels. Growth response of Pseudoroegneria and Festuca to nitrogen when growing in mixture with Centaurea did not differ from their response in monoculture. Likewise, growth response of Centaurea did not differ between monoculture and mixtures with Pseudoroegneria or Festuca. Therefore, these results suggest that these native grasses are not likely to increase or sup-press growth of Centaurea, regardless of nitro-gen addition level. This result is contrary to the observed invasion of Centaurea into com-munities dominated by these grasses. One ex-planation may be that disturbance (especially grazing) in the field creates gaps in the grass community where Centaurea can establish itself even though it is not a superior competitor for resources. Competitive interactions were greater be-tween each grass species and Centaurea at the high end of the nitrogen gradient. This may be a function of rapid growth. Thus, in nitrogen-rich environments fast-growing plants may rapidly occupy space and usurp resources to the exclusion of slow-growing species (Grime 1979, Radosevich and Holt 1984). Similar com-petitive effects may be expected to occur on other soil resource gradients, assuming adapta-tions for acquisition of nitrogen and other mobile 0.66 nutrients, as well as water, are similar (Grime 1979, Fitter and Hay 1987). In addition, one may hypothesize, based on the resource ratio theory (Tilman 1982), that Bromus is a superior competitor for nutrients other than nitrogen relative to Centaurea. By increasing nitrogen, both species should be limited by essential nutrients other than nitrogen, and the species with the lowest R* (the superior competitior) for the other nutrients should displace the species with the higher R * for the same nutri-ents (Tilman 1990). The ability of Bromus to out-compete Cen-taurea in nutrient culture provides one expla-nation for the observed population dynamics of Centaurea in the field. Roadsides seeded with Bromus are rarely invaded by Centaurea (Weaver et al. 1989). Both field and laboratory observa-tions suggest that disturbed sites seeded simul-taneously with Centaurea and the exotic, Bro-mus, will be dominated by Bromus. The effec-tiveness of Bromus in suppressing Centaurea may be increased with fertilization. Further-more, it may be expected that established Bro-mus plants will suppress the growth of Centau-rea seedlings. The results of this study suggest that at the seedling stage Bromus may be used to competitively exclude Centaurea. This method of weed management merits trial in the field. On the other hand, the regional dominants, 1996] COMPETITIVE SUPPRESSION OF CENTAUREA 271 > ."t: * * CI) c A Q) 0.5 +-' c C 0 0 .';:; .';:; Q) Q. E -0.5 0 U Q) > -1 .';:; CO c _ Bromus _ Centaurea Q) 0: -1.5 o 0.01 0.1 0.3 1.0 Relative Nitrogen Addition Level Fig. 2. Relative competition intensity (RCI) of Bromus and Centaurea across 5 relative nitrogen addition levels. Let-ters above bars indicate whether RCI varies (Duncan's multiple range test, P < 0.05) within species across nitrogen level. An asterisk indicates that RCI differs (P < 0.05) among species at that nitrogen level. Pseudoroegneria and Festuca, probably would not sufficiently suppress Centaurea to decrease the potential for invasion. Advantages of the competitive method over herbicides and biocontrol treatments used to manage Centaurea are its long duration and low environmental impact. Given these advan-tages, exclusion of Centaurea with Bromus merits trial in environments where the danger of invasion exists. LITERATURE CITED COUSE~S, R. 1985. A simple model relating yield loss to weed density. Annals of Applied Biology 107: 239-252. FITIER, A. H., AND R. K. Y1. HAY. 1987. Environmental physiology of plants. 2nd edition. Academic Press Inc., San Diego, CA. 423 pp. FORCELLA, F., A~D S. HARVEY. 1981. Migration and distri-bution of 100 alien weeds in NW USA, 1881-1980. Herbarium, Montana State University, Bozeman. 117 pp. GRACE, J. B. 1995. On the measurement of plant competi-tion intensity. Ecology 76: 305-308. GRnlE, J. P. 1979. Plant strategies and vegetation pro-cesses. John Wiley and Sons, London. 222 pp. HARPER, J. L. 1977. Population biology of plants. Academic Press Inc., San Diego, CA. 892 pp. LIKDQUlST, J. L., B. D. ~1AXWELL, D. D. BeHLER, A:\D J. L. Gc:-.;sou.;s. 1995. Modeling the population dynamics and economics of velvetleaf (Abutilon theophrasti) control in a corn (Zea mays)-soybean (Glycine max) rotation. Weed Science 43: 269-275. LINDQCIST, J. L., D. A. MORTE~SE~, S. A. CLAY, R. SCHMENK, J. J. KELLS, K. HOWATI, A..'\iD P. WESTRA. 1996. Stability of corn-velvetleaf interference relationships. Weed Science 44: 309-313. MACHLIS, L., A~D J. TORREY. 1956. Plants in action. Free-man, San Francisco, CA. 282 pp. R>\DOSEVICH, R. S., A~D J. S. HOLT. 1984. Weed ecology: implication for vegetation management. John \Viley & Sons Inc., New York. 265 pp. RATKOWSKY, D. A. 1983. Nonlinear regression modeling: a unified practical approach. Marcel Dekker, Inc., New York. 276 pp. SAS. 1988. SAS/STAT user's guide, release 6.03. SAS Institute, Cary, NC. 1028 pp. TILMA~, D. 1982. Resource competition and community structure. Princeton University Press, Princeton, NJ. . 1990. Mechanisms of plant competition for nutri---e-nts: the elements of a predictive theory of competi-tion. Pages 117-141 in J. B. Grace and D. Tilman, editors, Perspectives on plant competition. Academic Press, Inc., New York. TYSER, R., A~D C. KEY. 1988. Spotted knapweed in natural area fescue grasslands: an ecological assessment. Northwest Science 62: 151-160. WEAVER, T., D. GUSTAFSO~, J. LICHTHARDT, AND B. WOODS. 1989. Distribution of exotic plants in the Northern Rocky Mountains by environmental type and distur-bance condition. MSU Biology Report 41, Bozeman, MT. 91 pp. Received 26 September 1995 Accepted 29 April 1996 

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Potential for Controlling the Spread of \u3ci\u3eCentaurea maculosa\u3c/i\u3e with Grass Competition

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