Ecophysiological evaluation of intraspecific competition of Cenchrus ciliaris L. (Poaceae) in pots

Evaluación ecofisiológica de la competencia intraespecífica de Cenchrus ciliaris L. (Poaceae) en macetas

A. Vera¹*, C. Medrano², A. del Villar², V. Paz³ y A. Páez³

¹Laboratorio de Ecología, Centro de Investigaciones Biológicas, Facultad de Humanidades y Educación, LUZ. 

²Departamentos de Botánica y de Estadística, Facultad de Agronomía, LUZ. 

³Laboratorio de Ecofisiología, Facultad Experimental de Ciencias, Universidad del Zulia, Apartado 526, Maracaibo 4001-A, estado Zulia, Venezuela. 

*Autor para la correspondencia e-mail: ajvera68@intercable.net.ve

Abstract

Intraspecific competition of buffelgrass (Cenchrus ciliaris L.) planted in pots was evaluated using ecophysiological parameters and the competition coefficient. Experiments were carried out on the University of Zulia campus, adjacent to the Faculty of Sciences, under ecological conditions of a Very Dry Tropical Forest and irrigation. A method of additive density with 2, 4, 8 and 12 plants/pot was used, and a randomized block design with four replications was applied. After transplant, biomass of all plants was harvested twice at 15d and 30d. Plant height, leaf area, root dry weight, shoot dry weight, and total dry weight were significantly greater (P<0.01) in low density treatments (2 and 4 plants/pot) versus high density (8 and 12 plants/pot) treatments. Number of leaves was relatively greater at low plant densities, and flowering was present in the 30d harvest. The coefficient revealed limited competitive capacity for the 15d harvest, but for the second evaluation period, a more intense interaction was detected. Severe intraspecific competition was demonstrated as population density increased. The relation of the coefficients (A₁/A_o) helped collaborate this ecological interaction. We recommend further vegetative competition studies using other weed species of agroecological importance in the Maracaibo Plains, Zulia State, Venezuela.

Key words: buffelgrass, Cenchrus ciliaris, intraspecific competition, Maracaibo, Poaceae, Venezuela, weed.

Resumen

Se evaluó la competencia intraespecífica del pasto bufel (Cenchrus ciliaris L.), en macetas, a través de algunos parámetros ecofisiológicos y el coeficiente de competencia. El ensayo se llevó a cabo en un área de la Ciudad Universitaria de la Universidad del Zulia adyacente a la Facultad Experimental de Ciencias bajo las condiciones ecológicas de un bosque muy seco tropical y con riego. Se utilizó la metodología de densidades de adición (2, 4, 8 y 12 plantas/maceta), y se aplicó un diseño de bloques al azar con cuatro repeticiones. Se realizaron dos cosechas, de la biomasa de todas las plantas, una practicada a los 15, y la otra a los 30 días después del transplante. La altura, el área foliar y el peso seco de raíz, vástago y total fueron mayores en los tratamientos de baja densidad (2 y 4 plantas/maceta), en comparación a los correspondientes de alta densidad (8 y 12 plantas/maceta), revelando diferencias significativas (P<0,01) entre ambos grupos poblacionales. El número de hojas fue relativamente mayor a baja densidad de plantas, y la floración se presentó en la cosecha de los 30 días. El valor del coeficiente reveló una limitada capacidad competitiva para la cosecha de los 15 días, mientras que para el segundo periodo de evaluación resultó una interacción más intensa. Se concluye que existe una fuerte competencia intraespecífica a medida que incrementa la densidad poblacional y la relación de los coeficientes (A₁/A_o) corroboran la presencia de esta interacción ecológica. Se recomienda continuar los estudios de competencia vegetal con otras especies de malezas de importancia agroecológica en la Planicie de Maracaibo, estado Zulia, Venezuela.

Palabras clave: Cenchrus ciliaris, competencia intraespecífica, maleza, Maracaibo, pasto bufel, Poaceae, Venezuela.

Recibido el 24-9-2003 ● Aceptado el 10-10-2005

Introduction

Intraspecific competition gene-rally leads to a decrease in the rate of resource supply per specimen, thus decreasing growth and development of individuals due to changes in the amounts of stored resources. In consequence, intraspecific competition decreases population survival and/or fecundity (2).

Intraspecific competition is reflected by observing changes in biomass in relation to plant density (16).

Methods used to study competition consider proximity factors (plant density, spatial arrangement of individuals, propor-tion of species), as well as growth analysis techniques (8, 4, 17). Regression analysis provides estimates of the degree of intraspecific competition, using population biomass data (16). Combined usage of these valuable tools allows evaluation of the potential competitive ability at the intraspecific level (17).

Cenchrus ciliaris L. (Poaceae), known as buffelgrass, is native to Africa, but is well adapted to tropical and subtropical regions of the Western Hemisphere (18).

From an ecological point of view, buffelgrass is considered an invader weed which displaces native species, modifying to a good extent natural plant communities in California, Texas, Hawaii and Australia (9, 15, 5, 10). Buffelgrass is widely distributed in Mexico, especially in semiarid regions, where it is frequently found as a weed in crop fields (6), giving this species a high competitive ability in agroecological systems.

In Venezuela, buffelgrass is extensively distributed, specifically in the Maracaibo plains, in Zulia State. This region has a Life Zone clasified as Very Dry Tropical Forest (3). However, nothing is known about the behavior of buffelgrass in the Maracaibo plains. Research about intraspecific competition of buffelgrass in this region will provide information about the role of this species as a weed. Therefore, the objective of this research was to evaluate intraspecific competition of buffelgrass in pots, using ecophysiological parameters and the application of a mathematical model.

Materials and methods

Agroecological Conditions of the Experimental Area

Experiments were carried out in an area adjacent to the Faculty of Sciences at the University of Zulia, Maracaibo, Venezuela. The area has climatic conditions of a Very Dry Tropical Forest Life Zone (7) with savannah formations. The introduced buffelgrass (Cenchrus ciliaris) predominates, widely covering the ground. Annual precipitation in the experimental zone is 400 to 600 mm, distributed in two seasons. Mean annual temperature is 28.5ºC, based on data from the meteorological station at the Rafael Urdaneta Airforce Base, Maracaibo (figure 1).

Figure 1. Climogram of the studied area.

Plant Material

Buffelgrass seeds were collected from wild plants belonging to secondary grasslands considered generally as monospecific, with some dispersed individuals of Prosopis

juliflora and Acacia tortuosa located in an area close to the experimental zone. To obtain enough seedlings, Buffelgrass seeds were planted in 50 kg polyethylene bags, at a soil depth of 2-3 cm. Soil was obtained from the Experimental Farm Ana María Campos, Facultad de Agronomía, Universidad del Zulia. Soil texture was sandy, soil pH was 5-6, and the argilic horizon was 20-30 cm deep.

Seedlings were transplanted to pots according to densities described by Patterson (12). Selection of transplanted seedlings was based on: development of first leaf, vertical position, good health and similar heights.

Pot Preparation

Plastic pots (6 kg capacity, 20 cm diameter and 314 cm² area) were filled with a layer of humus and fertilized with 1g of complete formula 12-24-12 (NPK). Fertilizer was added when pots were filled, and soil (humus layer) was similar to that previously described for buffelgrass plantlets. At the experimental site, pots were placed in a protected area to avoid the excess of wind. A barrier was constructed with bricks and wire netting with big holes to avoid plant damage. Plants were roofed with metalic screening with large holes. This construction design protected plants from wind and herbivory.

Experiments and Harvests

Two experiments were performed, the first one between January and March 2000, and the second one between May and July 2000. Plants were watered daily using approximately 2 L of tap water per pot.

Plant evaluations were done 15 and 30 days after transplanting in each experiment. Whole plants were harvested and separated into leaves, stems, inflorescences and roots. Each treatment was repeated four times (2, 4, 8 and 12 plants/pot/treatment/sampling).

Water pressure was used to remove soil and plant debris from the root system. The root system was washed carefully and separated from the shoot.

Ecophysiological Parameters

The ecophysiological parameters analyzed were: plant height, leaf area, shoot and root dry weight, total dry weight, number of leaves and number of inflorescences.

Plant height was measured from the root base to the apex of the flag leaf with a ruler. Leaf area (cm²) was determined using a Delta-T Area Meter MK2 Devices LTD, England. Plants were stored in labeled paper bags and placed in an oven during 72 h at 60ºC to obtain dry biomass.

Individual plant organs (leaves, stems, flowers, roots) were weighted.

Total dry weight was obtained by adding shoot and root dry weights. Number of leaves and inflorescences was determined by counting them on plants.

Experimental Design and Intraspecific Competition Coefficient

A completely randomized block design with four replications (repetitions in space) was used.

The Intraspecific Competition Coefficient was determined according to the method of Spitters (16). The coefficient ratio A₁/A₀ indicates intraspecific competitive stress intensity by estimating the proportional decrease in weight of isolated plants when intraspecific competitor density increases, according to Patterson (12).

Parametric analysis of variance (STATGRAPHICS, version 7.0) was done to evaluate intraspecific competition between buffelgrass plants via the effect of population density (plants/pot) on ecophysiological parameters.

Results and discussion

Significant differences (P<0.01) were detected for plant height, leaf area, and shoot, root and total dry weights, figures 2-5. Number of leaves is presented at the figure 6, and flowering occurred by the 30d harvest.

Figure 2. Height of Cenchrus ciliaris for 15 and 30d harvests. Different letters and numbers indicate significant differences (P<0.01).

Figure 3. Leaf area of Cenchrus ciliaris for 15 and 30d harvests. Different letters and numbers indicate significant differences (P<0.01).

Figure 4. Biomass dry of Cenchrus ciliaris for 15d harvest. Different letters and numbers indicate significant differences (P<0.01).

Figure 5. Biomass dry of Cenchrus ciliaris for 30d harvest. Different letters and numbers indicate significant differences (P<0.01).

Figure 6. Number of leaves of Cenchrus ciliaris for 15 and 30d harvests.  

The equation of the intraspecific competition, for the first 15d harvest, was: 1/Wc = 0.94 + 0.255 Nc

The proportion between the coefficient for the variable N and the intercept, A₁/A₀ for the equations is:

A₁/A₀ = 0.255/0.94 = 0.271

The intensity of competitive stress for the 30d harvest is described according to the equation:

1/WC = -0.0404237 + 0.184873 Nc

If only the absolute value is taken into account, the proportion A₁/A₀ for the equation is: A₁/A₀ = 0.184873/0.0404237 = 4.573. Both equations are given in table 1.

Table 1. Intraspecific competition coefficients for Cenchrus ciliaris for 15 and 30d harvests.

Results for the majority of the evaluated parameters (figures 2-6) allowed us to distinguish two groups. The first group contains treatments 12 and 8 (high density), with the lowest valued variables. In the other group, treatments 4 and 2 (low density), the highest values were obtained, generating highly significant differences (P<0.01). Results were similar in both samples.

The comparison between evaluated variables, in both harvests, shows highly significant differences among treatments (P<0.01) in both periods. These differences were greatest for the 30 d harvest. This result could be explained by the slow growth observed in grasses during their initial developmental stage (6). In addition, greater biomass accumulation and relative increment in the evaluated variables is associated with the aging of plants in the second harvest, due to the greater metabolic demand in their physiological processes (14). However, the impact of competitive stress among plant densities in the treatments in both harvests is proportionally maintained.

Height

Individuals with different heights were observed, even in the same treatment (figure 2). Hereditary differences may exist among individuals of the same species, contributing to non-reciprocal competitive interactions. It has been reported that maize plants (Poaceae) generally shade and genetically suppress different individuals of the species by means of their height (2).

Leaf area

Leaf area values decreased when plant density increased (figure 3), indicating the great intraspecific competitive ability of buffelgrass to interfere in leaf area development, even at low densities. This was observed in both harvests, and could be explained by the semidecumbent growth of this grass species (13).

Shoot dry weight

Shoot dry weight values found in the low density treatments, figures 4-5, could be related to climatic conditions of high temperature and high irradiance, in a very dry tropical forest where the experiments took place. These factors could increase the photosynthetic rate of buffelgrass.

High carbon assimilation capacity has been reported as one of the reasons for the higher growth rate of introduced grass species (1).

Root dry weight

Results for this variable also show an inverse dependence with plant density, figures 4-5, and suggest competition at the root level for space needed for expansion and development. This interference effect is more evident as plants increase per unit space. The allelopatic effect developed by buffelgrass roots may be similar to that reported in Desmanthus illinoensis and Cassia fasciculata (11).

Total dry weight

Strong intraspecific competition in buffelgrass was detected, in relation to individual density, figures 4-5. Biomass production is approximately linearly dependant to resource uptake that limits growth, because biomass distribution within plants varies according to competitive stress (16).

Number of leaves

Individuals in treatments with 2 and 4 plants/pot revealed a relatively greater number of leaves, averaging 4.44 and 4.05 leaves/plant respectively, figure 6. However, in treatments with 8 and 12 plants/pot, the average number of leaves/plant was 3.24 and 2.76 respectively. Presence of a greater number of leaves in plants at low population densities could be explained by the lack of physical interference (overlapping) between them, due to a larger surface area available per individual, figure 6. This, in turn, may increase the capture of solar radiation, thus increasing the formation of vegetative biomass, growth, development and plant productivity, (13).

Number of flowers

Treatments with low densities of individuals (2 and 4 plants/pot) averaged 0.433 and 0.353 flowers/plant respectively, whereas those of high density (8 and 12 plants/pot) did not produce flowers. This result may indicate the high degree of intense competition in this species. In cases of intraspecific competition, a decrease in supply of resources per individual has been related to inhibition of fecundity and reproduction at the population level (2).

Effect of Density on Intraspecific Competition

The A₁/A₀ coefficient ratio, for the 15d harvest, table 1, estimates a proportional decrease of 0.271 g in weight of an isolated plant, when density of intraspecific competitors increases. Patterson (1990) reported an A₁/A₀ ratio of 0.45 g in Anoda cristata, a species of great competitive ability; while Spitters (1983) revealed similar findings in Zea mays (Poaceae). The relatively low coefficient ratio for the 15d harvest of buffelgrass may be related to the slow growth experienced by grasses during their initial developmental stages, thus explaining its limited intraspecific competitive ability.

The 30d harvest revealed an A₁/A₀ coefficient of 4.573, table 1, suggesting that competition was more intense than in the first period. However, the intercept (-0.0404237) does not differ significantly from zero, and with a standard error of 0.109, Table 1; which indicate that A₁/A₀ ratio was not very precise for this harvest. Similar results have been reported for Abutilon theophrasti, an important weed in cotton and soybean crops in the United States (Patterson 1990).

Conclusions

Ecophysiological parameters indicate intense intraspecific competition in buffelgrass (Cenchrus ciliaris) as population density increases. The A₁/A₀ buffelgrass ratio showed relatively low competitive ability on the 15^th harvest day, suggesting that, at this growth stage, there is little agressivity toward other species found in the region.

Recommendations

This research should be extended to other weed species distributed in the Maracaibo Plains.

Acknowledgments

The authors express their gratitude to the Council for Scientific and Humanistic Development of University of Zulia for financial aid, and to the Faculty of Sciences and Center for Biological Investigations, University of Zulia, for allowing access to their experimental area and laboratories during the investigation. Clark Casler provided comments that helped improve the manuscript.

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