An Analysis of Competitive Ability in Three Perennial GrassesAuthor(s): A. Mahmoud and J. P. GrimeSource: New Phytologist, Vol. 77, No. 2 (Sep., 1976), pp. 431-435Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2433616 .

Accessed: 12/06/2014 14:52

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org.

.

Wiley and New Phytologist Trust are collaborating with JSTOR to digitize, preserve and extend access to NewPhytologist.

http://www.jstor.org

This content downloaded from 185.2.32.49 on Thu, 12 Jun 2014 14:52:08 PMAll use subject to JSTOR Terms and Conditions

New Phytol. (I976) 77, 43I-435.

AN ANALYSIS OF COMPETITIVE ABILITY IN THREE PERENNIAL GRASSES

BY A. MAHMOUD* AND J. P. GRIME

Unit of Comparative Plant Ecology (NERC), Department of Botany, The University, Sheffield

(Received 8 March I976)

SUMMARY

Competitive interactions between three perennial grasses of contrasted ecology were investigated using an experimental design which permitted measurement of the effects of paired species upon each other. Under productive conditions, each pairing resulted in the total elimination of one species and the order of competitive ability was Arrhenatherum elatius>Agrostis tenuis> Festuca ovina. Under conditions of severe nitrogen stress, the impact of competition upon yield was reduced but the order of competitive ability was unaffected. The results suggest (i) that competitive abilities above and below ground are interdependent and covariable and (2) that the ability to compete for mineral nutrients may be of relatively minor importance in the adaptive physiology of species colonizing very infertile soils.

INTRODUCTION

One of the main problems in formulating a unified concept of competitive ability in vascular plants arises because competition occurs with respect to several different resources, including light, water, various mineral nutrients and space. Hence, it might be supposed that the ability to compete for one resource varies independently from the ability to compete for each of the others. Moreover, the ability to compete for a given resource might be expected to change according to its availability; that is, some plants may be better competitors when a resource is plentiful, others when it is in short supply.

The purpose of the investigation described in this paper was to examine these hypo- theses by comparing the competitive abilities of three perennial grasses in two contrasted environments, one highly productive, the other severely deficient in available nitrogen.

The three grasses studied occur widely in Europe but differ from each other both in growth form and in ecology. Arrhenatherum elatius (L.) Beauv. ex J. and C. Presl is potentially large (to I 50 cm) in stature and is a dominant species in productive meadows, roadsides and derelict land. Pjy contrast, Festuca ovina L. is a small (to 20 cm) tussock species, restricted to nutrient-deficient pastures and other unproductive habitats such as rock outcrops. Agrostis tenuis Sibth. is intermediate between the other two species in its morphology and is most abundant in pastures of moderate productivity.

MATERIALS AND METHODS

Seeds collected from Derbyshire populations of the three species were germinated on * Present address: Biology Department, Faculty of Education, University of Riyad, Riyad, Saudi

Arabia.

43'

This content downloaded from 185.2.32.49 on Thu, 12 Jun 2014 14:52:08 PMAll use subject to JSTOR Terms and Conditions

432 A. MAHMOUD AND J. P. GRIME

nylon cloth moistened with distilled water and seedlings were transferred to the experi- ment io days after germination. Each species was grown either in monoculture or paired in i: i mixtures with each of the other two species in 36-1 square plastic tubs (35 X 35 X 30

cm) filled to a depth of i6 cm with washed silica sand. In the high nitrogen treatment, each container was supplied with 4 1 of Hewitt nutrient solution (Hewitt, I966) con- taining I70 ng/l nitrogen, before planting and, subsequently, at intervals of 3 weeks, throughout the experiment. In the containers representing the low nitrogen treatment, the procedure was identical except that the concentration of nitrogen was reduced to 5 mg/l. Run-off from the tubs was collected in shallow plastic trays. Watering to field capacity, using tap-water, was carried out every 3 days.

In the mixed sowing, ninety-eight seedlings of each of the two species were arranged alternately in rows at intervals of 2.5 cm so that, except at the margin, each seedling was surrounded by four individuals of the other species. In monoculture, the seedlings were arranged in a pattern identical with that in which they occurred in the mixtures and the areas corresponding to those taken up by the other species in the mixture were left unoccupied. This arrangement was preferred to the 'replacement' design (e.g. De Wit, I960; Harper, I96I) in which the total density of plants is maintained constant in all treatments. The monocultures used in the present investigation provide a 'true' control in the sense that they differ from the mixture only with respect to the absence of the other species; hence, a calculation of the effect of each species upon the other is possible by comparing the yields in mixture and monocultures.

The containers were arranged in four randomized blocks in a glasshouse heated to maintain minimum temperatures of 200C by day and I50C by night. A daylength of i8 h was provided throughout by means of supplementary illumination from mercury vapour lamps. Collapse of the shoots at the margin of the containers with tall vegetation was prevented by means of a skirt of plastic netting supported to a height of 8o cm.

The experiment was terminated after I2 months and the living above-ground parts in each container were sorted into component species, dried for 28 h at i 050 and weighed.

RESULTS

The difference in nitrogen status produced two distinct types of vegetation. At the high rate of supply, a dense mass of foliage developed in all the containers and in all treatments involving Arrhenatherum elatius there was a considerable accumulation of leaf litter. The low-nitrogen treatment resulted in a sparse open turf in which individual plants remained identifiable and produced few shoots, most of which were pale green in colour.

In the high-nitrogen treatment, both mixtures and controls supported massive plants of A. elatius, with culms of up to I50 cm. In the control treatment, shoots of Agrostis tenuis reached 50 cm in height and very long stolons were developed. Festuca ovina responded to high fertility by producing compact tussocks, about 20 cm in height and composed of rather succulent leaves. In the controls, the mean dry weight (g) of litter per container in F. ovina (Io0.2+ 6.5) and Agrostis tenuis (47.5 ? I2.4) was considerably less than that produced by Arrhenatherum elatius (I20.7 ? I9.7).

Under high nitrogen supply, both Agrostis tenuis and Festuca ovina were completely eliminated from mixtures containing Arrhenatherum elatius and no significant differences in yield occurred in the latter (Fig. ia). Festuca ovina was also excluded from mixtures initially containing Agrostis tenuis and Festuca ovina. The yield of Agrostis tenuis in

This content downloaded from 185.2.32.49 on Thu, 12 Jun 2014 14:52:08 PMAll use subject to JSTOR Terms and Conditions

Competitive ability in perennial grasses 433 the mixture with Festuca ovina was not significantly different from the yield of the species in monoculture.

Under conditions of low fertility growth in all three species was reduced drastically (Fig. ib). No significant difference could be detected between the yields attained by the three species in monoculture. In the mixtures, the presence of either Agrostis tenuis or Festuca ovina marginally reduced the yield of Arrhenatherum elatius, an effect which was statistically significant only in the case of the former. However, A. elatius caused a marked and comparable reduction in the yield of Agrostis tenuis and Festuca ovina. The yield of Agrostis tenuis when grown with Festuca ovina was not significantly different from that in monoculture. When grown with either Arrhenatherum elatius or Agrostis tenuis, the yield of Festuca ovina was significantly reduced below the control but this effect was less pronounced in the F. ovina :Agrostis tenuis mixture.

(b) (a) 7

500

6

400- 5-

0

4- 4 o 0

200

2-

C +T +F C +F +A C +T+A C +T +F C +A +F C +A +T A. e/atius A. tenuis F ovina A. elatius A. tenuis F ovina

Fig. i. The yield (g) of shoots of Arrhenatherum elatius (A), Agrostis tenuis (T) and Festuca ovina (F) in mixtures (+) and controls (C). (a) Under productive conditions (N 176 mg/i); (b) uinder nitrogen stress (N 5 mg/i). The 950 Y confidence limits are indicated by the vertical lines.

DISCUSSION

Under the productive conditions associated with the high-nitrogen treatment, inter- specific competition was conclusive in that Festuca ovina was totally eliminated by both of the other grasses and there were no survivors of Agrostis tenuis when this species was grown with Arrhenatherum elatius. These results are predictable on the basis of the performance of the three species in the monocultures, where the yield of A. elatius was approximately twice that of Agrostis tenuis and seven times that of Festuca ovina. Ob- servations during the course of the experiment strongly suggested that, at least during the terminal phase of competitive exclusion, the advantage of Arrhenatherum elatius over the other two species depended upon its ability to compete for light and, in this respect, its greater growth in height was of obvious importance.

This content downloaded from 185.2.32.49 on Thu, 12 Jun 2014 14:52:08 PMAll use subject to JSTOR Terms and Conditions

434 A. MAHMOUD AND J. P. GRIME

Additional features of A. elatius which may be implicated in its success were the massive shoot-bases which were impenetrable by shoots of the other two species and the leaves which were sufficiently robust to emerge through the dense layer of litter. The advantage of Agrostis tenuis over Festuca ovina appeared to be mainly related to the ability to produce a taller leaf canopy and also to spread rapidly throughout the turf by means of stolons. It is likely also that the low potential relative growth-rate of F. ovina (Grime and Hunt, I975) placed the species at a competitive disadvantage under the productive conditions.

In the low-nitrogen treatment, all three species suffered a marked reduction in growth and the leaf canopy was so sparse that it is inconceivable that competitive interactions occurred between the shoots. It is therefore significant that the order of competitive ability (Arrhenatherum elatius >Agrostis tenuis >Festuca ovina) remained the same as in the high-nitrogen treatment.

From these results it seems reasonable to conclude that the ability to compete for nitrogen in a severely nitrogen-deficient medium was higher in Arrhenatherum elatius than in Festuca ovina. On first examination, this finding is inconsistent with the ecology of the two species. However, in both, an explanation can be attempted by considering the forms of natural selection which operate in the habitats of the species.

Arrhenatherum elatius

The natural habitats of A. elatius are usually relatively fertile and are characterized by a dense cover of herbaceous vegetation in which the most obvious competition is that occurring above ground. However, it is clear that production of a large biomass of shoot material, a prerequisite for effective above-ground competition, is dependent upon high rates of uptake of both mineral nutrients and water. Consideration of the sequence of events in competition suggests that although, in productive habitats, the phenomenon culminates in above-ground competition for space and light, the outcome may be strongly influenced or even predetermined by earlier competition below ground. Corroborative evidence of the importance of root-competition in productive environ- ments is to be found in the experiment conducted by Donald (I958) in which the competitive advantage of Lolium perenne over Phalaris tuberosa derived from a superiority in both root and shoot competition.

It is suggested, therefore, that although the higher competitive ability of Arrhenatherum elatius over Festuca ovina was maintained to a marginal extent in the low-nitrogen treatment, it is primarily an adaptation to intense competition under fertile conditions.

The logical extrapolation from the results of the present investigation and of Donald (I958) is that the abilities to compete for light, mineral nutrients, water and space are inter-dependent to such an extent that they have been subject to parallel rather than divergent selection (Grime, I973), i.e., competitive ability is recognizable as a family of genetic characteristics which, by maximizing production, facilitate the exclusive oc- cupation of fertile, relatively undisturbed environments. It would appear also that survival on infertile soils is not primarily dependent upon the ability to compete for mineral nutrients.

Festuca ovina From the results of this investigation, there is no evidence to suggest that the

competitive ability of F. ovina increases under conditions of nitrogen stress and it would appear that the ability to compete for nitrogen plays little part in the mechanism whereby

This content downloaded from 185.2.32.49 on Thu, 12 Jun 2014 14:52:08 PMAll use subject to JSTOR Terms and Conditions

Competitive ability in perennial grasses 435 F. ovina is adapted to survive conditions of low soil fertility. A similar conclusion may be drawn not only with respect to nitrogen but also to other essential mineral nutrients from a number of laboratory experiments (e.g., Bradshaw et al., I964; Hackett, I965; Clarkson, I967; Rorison, I968; Higgs and James, I969) in which plants of fertile and infertile habitats have been grown on nutrient-deficient soils and solution-cultures. These studies have provided no convincing evidence that plants indigenous to poor soils are more efficient in the uptake of mineral nutrients when these are present either at high or low concentrations.

An alternative explanation for the survival of plants such as F. ovina under conditions of low soil fertility is that although competition, especially for mineral nutrients and water, occurs in the habitats of these species, its importance is slight in comparison with the ability to persist for long periods in a state of mineral nutrient deficiency and, in this condition, to resist the effects of climate and predation. Evidence in support of this hypothesis is available from a recent field experiment (Grime and Curtis, I976) in which seedlings of F. ovina sown into calcareous grassland severely deficient in nitrogen and phosphorus survived conditions in which seedlings of Arrhenatherum elatius, although outyielding those of Festuca ovina, succumbed to drought.

ACKNOWLEDGMENT

This research was supported in part by the Natural Environment Research Council.

REFERENCES

BRADSHAW, A. D., CHADWICK, M. J., JOWETT, D. & SNAYDON, R. W. (I964). Experimental investigations into the mineral nutrition of several grass species; N Nitrogen level. J. Ecol., 52, 665.

CLARKSON, D. T. (I967). Phosphorus supply and growth rate in species of Agrostis L. J. Ecol., 66, III. DE WIT, C. T. (i960). On competition. Versl. Landbouwk. Onderz., 66, 8. DONALD, C. M. (1958). The interaction of competition for light and for nutrients. Aust. J. agric. Res., 9,

421. GRIME, J. P. (1973). Competition and diversity in herbaceous vegetation-a reply. Natiure, Lond., 244, 310. GRIME, J. P. & CURTIS, A. V. (1976). The interaction of drought and mineral nutrient stress in calcareous

grassland. J. Ecol., 64 (in press). GRIME, J. P. & HUNT, R. (I975). Relative growth-rate; its range and adaptive significance in a local flora.

J. Ecol., 63, 393. HACKETT, C. (I965). Ecological aspects of the nutrition of Deschampsia flexuosa (L.) Trin. II. The effects

of Al, Ca, Fe, K. Mn, N. P and pH on the growth of seedlings and established plants. J. Ecol., 53, 315. HARPER, J. L. (I96I). Approaches to the study of plant competition. Symp. Soc. exp. Biol., I5, i. HEWITT, E. J. (I966). Sand and Water Culture Methods used in the Study of Plant Nutrition. Commonw.

agric. Bur. tech. Commun. No. 22 (revised 2nd edn.), Farnham Royal, Bucks., England. HIGGS, D. E. B. &. JAMES, D. B. (I969). Comparative studies on the biology of upland grasses. I. Rate of

dry matter production and its control in four grass species. J. Ecol., 57, 553. RCRISON, I. H. (I968). The response to phosphorus of some ecologically distinct plant species. I. Growth

rates and phosphorus absorption. New Phytol., 67, 913.

This content downloaded from 185.2.32.49 on Thu, 12 Jun 2014 14:52:08 PMAll use subject to JSTOR Terms and Conditions