Received for publication: 23 October, 2014. Accepted for publication: 30 March, 2015. Doi: 10.15446/agron.colomb.v33n1.46747 1 Weed Management Supervisor, Monsanto Latin America North region. Mexico DF (Mexico). javierramirezsuarez@gmail.com 2 Department of Agronomy, Faculty of Agricultural Sciences, Universidad Nacional de Colombia. Bogota (Colombia). Agronomía Colombiana 33(1), 64-73, 2015 Phytosociology of weeds associated with rice crops in the department of Tolima, Colombia Fitosociología de malezas asociadas al cultivo de arroz en el departamento del Tolima, Colombia Javier Ramírez S.1, Verónica Hoyos C.2, and Guido Plaza T.2 ABSTRACT RESUMEN Phytosociological studies allow for the characterization and descriptive analysis of weed communities in crops. This study aimed to characterize the weed communities associated with weed crops in the “Centro, Meseta, and Norte” zones of the Tolima department. The study was conducted in 96 commer- cial lots, in which a 1 ha area was marked off for the sampling. The development stage, density and cover of the weeds were evaluated. The importance value index, the alpha diversity indices of Shannon-Wiener, Simpson and uniformity as well as the similarity indices of Jaccard, Sorensen and Steinhaus were calculated. For the entire department, 42 weed species were identified, with Echinochloa colona being the principal one in all of the zones. In the Centro zone, 27 species were identified; in the Meseta zone, 31 species were identified; and, in the Norte zone, 38 species were identified. The alpha indi- ces demonstrated that the Meseta zone was the most diverse. The Jaccard and Sorensen indices showed dissimilarity in the weed community for all of the comparisons of the zones. The Steinhaus coefficient registered the highest similarity intensity between the Centro and Norte zones. Los estudios fitosociológicos permiten caracterizar y hacer análisis descriptivos de las comunidades de malezas de cultivos. Este trabajo tuvo como objetivo caracterizar las poblaciones de malezas asociadas a cultivos de arroz de las zonas Centro, Meseta y Norte del departamento del Tolima. Los levantamien- tos fueron realizados en 96 lotes comerciales, en cada lote se demarcó un área de evaluación de 1 ha en la cual se realizaron los muestreos. Se evaluó el estado de desarrollo, densidad y cobertura de las malezas. Se calculó el índice de valor de im- portancia, los índices de diversidad alfa de Shannon-Wiener, Simpson y de uniformidad así como los índices de similitud de Jaccard, Sorensen y Steinhaus. En todo el departamento se identificaron 42 especies de malezas siendo Echinochloa colona, la principal en todas las zonas. En la zona Centro fueron iden- tificadas 27 especies; en la zona Meseta 31 y en la zona Norte 38. Los índices alfa registraron que la zona Meseta fue la más diversa. Los índices Jaccard y Sorensen mostraron disimilitud en la comunidad de malezas en todas las comparaciones de las zonas. El coeficiente Steinhaus registró mayor intensidad de similitud entre las zonas Centro y Norte. Key words: weed communities, importance value index, diversity index, cereals. Palabras clave: poblaciones de malezas, índice de valor de importancia, índice de diversidad, cereales. Introduction Agricultural activities generate changes and filters for biological communities and weeds that are associated with crops are exposed to disruptive factors that make their populations dynamic over time (Booth et al., 2003). In terms of responses to these agents of change, not all spe- cies in an agricultural system are equally important, with differences in frequency, density, and growth habit making some species the principal ones that generate economic and secondary effects that normally do not present problems for yield (Pitelli, 2000). One of the more utilized methods for the analysis of weed communities in agricultural systems is the phytosocio- logical study. Phytosociology is defined as the science that studies plant communities from the f loristic, ecological, and dynamic points of view or as the science that studies plant groupings, their interactions, and their dependence on their environment (Ferriol and Merle, 2006). A quantitative phytosociological study of a weed com- munity in a defined area and time provides a momentary analysis of the plant composition, providing a tool that supplies various inferences for a plant community (Erasmo et al., 2004). The analysis of weed communities can be http://dx.doi.org/10.15446/agron.colomb.v33n1.46747 65Ramírez S., Hoyos C., and Plaza T.: Phytosociology of weeds associated with rice crops in the department of Tolima, Colombia approached with the description of their characteristics, employing tools such as similarity and diversity indices that clarify their performance. Alpha diversity (α) in studies on weed populations measures the amount of diversity within a defined community in a zone (Booth et al., 2003). For weed ecology, the Margalef, Shannon-Wiener, and Simpson indices are more commonly used. The Margalef index focuses on the richness of the spe- cies found in a studied population. The Shannon-Wiener index is based on the proportional abundance of each species and the Simpson index is based on the probability that two individuals in a community sample will be of the same species (Booth et al., 2003). The study of the beta diversity (β) in weeds measures the change in weed species diversity between zones and samples the similarity of the composition of the community between location pairs through the calculation of similarity indices (Booth et al., 2003). The Jaccard and Sorensen index and the Steinhaus coefficient are more common in studies on weeds in agricultural systems. The Jaccard and Sorensen indices only consider how many species are in-common in a pair of evaluated communities and do not take into account the abundance of each species. For its part, the Steinhaus similarity coefficient or index incorporates abundance data into its analysis, taking into account the differences that occur in this data, and so is considered more valuable than the other indices (Booth et al., 2003). For its part, the importance value index (IVI) determines the dominance of the species and the degree of heteroge- neity of the agroecosystem (Pitelli, 2000), allowing for the evaluation of the horizontal structure of the community through the relative dominances, abundances, and frequen- cies (Lamprecht, 1990). The relative density is the number of individuals of a species or absolute density of a species over the total number of individuals or total density of all of the species (Brighenti et al., 2003). Relative frequency is defined as the frequency of a species over the sum of the frequencies of all of the species or the total frequency of all of the species (Mueller-Dombois and Ellenberg, 1976). Rela- tive dominance (relative cover) is defined as the absolute dominance of a species over the dominance of all of the species (Mueller-Dombois and Ellenberg, 1976) or as the cover of a species over the total cover of all of the species, expressed as a percentage (Cantillo et al., 2006). Based on the these considerations and considering the evi- dent importance that this type of study has for the charac- terization and study of plant communities associated with commercial crops, the present study was developed with the aim of identifying the f loristic composition and of charac- terizing the weed populations of commercial rice crops in the Centro, Meseta, and Norte zones of the department of Tolima in four evaluations carried out between emergence and formation of the f lower primordia in the crops. Materials and methods The present study was carried out between July of 2012 and February of 2013 in commercial crops in the department of Tolima, which were divided into three production zones in accordance with the different climatic, topographic, edaphical, and irrigation conditions. In each zone, the municipalities with the larger cultivated areas were used, with 96,319 ha of cultivated rice per year (Fedearroz, 2008), distributed as follows: the Norte zone (26% of the area), the Meseta zone of Ibague (21% of the area) and the Centro zone (53% of the area) with the municipalities of Purificación, Guamo, Espinal and Saldaña (Colombia) (Tab. 1). The sample size was 0.1% (96 ha) of the total area in accor- dance with the following equation (Spiegel, 1988): TABlE 1. Sampling proportional to the area cultivated with rice in the selected municipalities (Tolima department, Colombia). Municipality N (ha) nh (ha) Municipality N (ha) nh (ha) Lerida 5,960 6 Purificación 14,762 15 Ambalema 9,294 9 Guamo 13,620 14 Venadillo 5,265 5 Espinal 12,285 12 Armero 4,711 5 Saldaña 10,472 10 Norte total 25,230 25 Centro total 51,139 51 Ibague 12,484 12 Piedras 4,099 4 Alvarado 3,367 4 Department total 96,319 96 Meseta total 19,950 20 N, cultivated hectares (Fedearroz, 2008); nh, sampled hectares. 66 Agron. Colomb. 33(1) 2015 n = N × Z2 p × qα d2 × (N – 1) + Z2 p × q α (1) Where, n = sample size. N = total of the universal sample (in this case the 96,319 ha cultivated per year). 1.962 with a confidence of 95%. p = expected proportion (5% = 0.05). q = 1-p (in this case: 1-0.05 = 0.95). d2 = precision (in this case 10%). The number of hectares sampled per zone and municipal- ity was distributed in proportion to the stratum, using the cultivated area as the criterion (Tab. 1). The methodology used to distribute the sampling units in the study area resulted in the sampling of an actual area of 384 ha for the entire study (0.4% of the total cultivated area in the selected municipalities) and 0.8 ha in each lot. For the phytosociological study and characterization of the weed communities, a 0.04 m2 sampling square was used, which was thrown randomly five times (5) within the marked off hectare in each lot, following a zigzag pattern. Each hectare represented a commercial lot in accordance with reports from Erasmo et al. (2004) and Plaza and Hernández (2014). Four samples were conducted during the development of the crop: before the application of the first post-emergence control method (7 to 22 days after sowing, das), after the first control method (22-35 das), after the second post-emergence control method (37-52 das) and once the herbicide applications were finished, during the f lower primordia formation stage of the crop (52-65 das). In each sampling, the variables of density and cover were measured for each of the encountered weed species. The species were identified using the studies conducted by Fuentes et al. (2006a), Fuentes et al. (2006b) and Mon- tealegre (2011) as references. The importance value index (IVI) was calculated under the following parameters for each species: absolute density (Da), relative density (Dr), absolute frequency (Fa), rela- tive frequency (Fr), cover (Ca), and relative cover (Cr), in accordance with Curtis and Mclntosh (1950) and Mueller- Dombois and Ellenberg (1976): Da = number individuals per species (2) number sampling total Dr = Absolute density per species (3) Density total for all of the species Fa = Number sites for each species (4) Number total sites Fr = Frequency of each species (5) Frequency total of all of the species Ca = Space occupied by each species (6) Cr = Cover of each species (7) Total cover of all of the species IVI = Fr + Dr + Cr (8) Among the alpha diversity indices (α), the Shannon-Wiener index (H), the Simpson dominance index (D) and the uni- formity index (E) were calculated in accordance with the equations cited by Booth et al. (2003). H = – Σ[pi (ln pi)] (9) D = Σ{[ni (ni - 1)] / [N(N - 1)]}; (10) E = H / ln S (11) Where, pi = proportional abundance of each species ni = number individuals per species N = number total individuals S = number total of species richness H = Shannon-Wiener index On the other hand, of the beta diversity indices (β), the similarity indices of Jaccard (SJ) and Sorensen (SS) and the Steinhaus coefficient (SST) were calculated in accordance with the methodology cited by Booth et al. (2003). SJ = j/(a + b + j) (12) SS = 2j / (a + b + 2j) (13) SST = 2W / (A + B) (14) Where, j = number of species found in both communities a = number of species found only in community a b = number of species found only in community b W = total of the lower abundances A and B = Sum of the lower of the two abundances of each species Results and discussion Floristic composition In the rice production areas of Tolima, 42 weed species were identified, grouped into 2 classes, 20 families, and 31 67Ramírez S., Hoyos C., and Plaza T.: Phytosociology of weeds associated with rice crops in the department of Tolima, Colombia genera. In the zones, the Centro zone presented 27 weed species from 14 families and 21 genera; the Meseta zone had 31 species from 12 families and 23 genera; and the Norte zone contained 38 species from 18 families and 29 genera. In all of the zones, the Poaceae family contributed the highest number of total species: 9 species in the Centro zone, 12 species in the Meseta zone and 12 species in the Norte zone (Tab. 2). The Liliopsida class contributed the highest number of species, grouped into five (5) families, notably Poaceae and Cyperaceae. The Poaceae family contributed 13 species to TABlE 2. Weed species present in the rice crops of the Tolima department (Colombia). Class Family Genus Scientific name Bayer code life cycle Centro Meseta Norte Liliopsida Commelinaceae Commelina Commelina diffusa Burm. f. COMDI Annual/ Perennial X X Liliopsida Commelinaceae Murdannia Murdannia nudiflora (L.) Brenan. MUDNU Annual/ Perennial X X X Liliopsida Cyperaceae Cyperus Cyperus esculentus L. CYPES Perennial X X X Liliopsida Cyperaceae Cyperus Cyperus iria L. CYPIR Annual X X X Liliopsida Cyperaceae Cyperus Cyperus rotundus L. CYPRO Perennial X X X Liliopsida Cyperaceae Fimbristylis Fimbristylis dichotoma (L.) Vahl. FIMDI Annual/ Perennial X X Liliopsida Cyperaceae Fimbristylis Fimbristylis miliacea (L.) Vahl. FIMMI Annual X X X Liliopsida Cyperaceae Torulinium Torulinium odoratum (L.) S.S. Hooper. TOROD* Annual/ Perennial X X Liliopsida Limnocharitaceae Limnocharis Limnocharis flava (L.) Buchenau. LIMFL* Perennial X Liliopsida Poaceae Chloris Chloris gayana Kunth. CHRGA Perennial X X Liliopsida Poaceae Chloris Chloris radiata (L.) Sw. CHRRA Annual/ Perennial Liliopsida Poaceae Cynodon Cynodon dactylon (L.) Pers. CYNDA Perennial X X Liliopsida Poaceae Digitaria Digitaria bicornis (Lam.) Roemer & J.A. Schultes ex Loud. DIGBC Perennial X X X Liliopsida Poaceae Digitaria Digitaria horizontalis Willd. DIGHO* Annual X X X Liliopsida Poaceae Digitaria Digitaria ciliaris (Retz.) Koel. DIGSP Annual X X Liliopsida Poaceae Echinochloa Echinochloa colona (L.) Link. ECHCO Annual X X X Liliopsida Poaceae Eleusine Eleusine indica (L.) Gaertn. ELEIN Annual X X X Liliopsida Poaceae Ischaemum Ischaemum rugosum Salisb. ISCRU Annual/ Perennial X X X Liliopsida Poaceae Leptochloa Leptochloa scabra Nees. LEFSC* Annual X X X Liliopsida Poaceae Leptochloa Leptochloa virgata (L.) P. Beauv. LEFVI* Perennial X X X Liliopsida Poaceae Paspalum Paspalum boscianum Flueggé. PASBO Annual X X X Liliopsida Poaceae Rottboellia Rottboellia cochinchinensis (Lour.) W.D. Clayton. ROTCO Annual X X X Liliopsida Pontederiaceae Heteranthera Heteranthera limosa (Sw.) Willd. HETLI Annual X X X Magnoliopsida Amaranthaceae Amaranthus Amaranthus dubius Mart. ex Thell. AMADU* Annual X X X Magnoliopsida Amaranthaceae Amaranthus Amaranthus spinosus L. AMASP Annual X Magnoliopsida Asteraceae Eclipta Eclipta alba (L.) Hassk. ECLAL Annual/ Perennial X X X Magnoliopsida Caesalpiniaceae Senna Senna obtusifolia L. CASOB Annual/ Perennial X X Magnoliopsida Convolvulaceae Ipomoea Ipomoea triloba L. IPOTR Perennial X X X Magnoliopsida Cucurbitaceae Cucumis Cucumis melo L. CUMMD Annual X X Magnoliopsida Euphorbiaceae Phyllanthus Phyllanthus niruri L. PYLNI Annual X X Magnoliopsida Fabaceae Aeschynomene Aeschynomene rudis Benth. AESRU Perennial X Magnoliopsida Lamiaceae Hyptis Hyptis brevipes Poit. HYPBR* Annual X Magnoliopsida Lythraceae Ammannia Ammannia coccinea Rottb. AMMCO Annual X Magnoliopsida Lythraceae Ammannia Ammannia multiflora Roxb. AMMMU* Annual X Magnoliopsida Malvaceae Gossypium Gossypium hirsutum L. GOSHI* Annual/ Perennial X Magnoliopsida Onagraceae Ludwigia Ludwigia decurrens Walt. IUSDE Annual/ Perennial X X X Magnoliopsida Onagraceae Ludwigia Ludwigia leptocarpa (Nutt.) H. Hara LUDLE* Annual/ Perennial X X X Magnoliopsida Onagraceae Ludwigia Ludwigia linifolia Vahl. LUDLI* X X X Magnoliopsida Portulacaceae Portulaca Portulaca oleracea L. POROL Annual X X X Magnoliopsida Rubiaceae Spermacoce Spermacoce verticillata L. SPEVE* Perennial X X Magnoliopsida Solanaceae Physallis Physalis minuta Griggs. PHYMI* X X Magnoliopsida Tiliaceae Corchorus Corchorus hirtus L. CORHI* Annual/ Perennial X Total of the zones 27 31 38 * Code assigned by the authors. 68 Agron. Colomb. 33(1) 2015 the total number of weed plants in the crop. The Digitaria and Leptochloa genera contributed the higher numbers of species, with 3 and 2, respectively. These results coincide with a report made by Erasmo et al. (2004), evidencing the importance of weed species from the Liliopsida class. Rao et al. (2007) and Erasmo et al. (2004) stated that the more damaging species in this region of Colombia belong to the Poaceae and Cyperacea families. This situation is possibly due to the use of the same cultivation system for several years and to the phylogenetic relationship between the weeds and crops as they share the same requirements for resources (Radosevich et al., 1997; Puentes, 2003; Cobb and Reade, 2010). Considering these facts as well as the revelations of Inoue et al. (2012), the management of weeds in the rice crops of the Tolima department must be directed toward this segment of plants. Importance value index (IVI) The analysis of the IVI for the entire department de- monstrated that 10 species made up 50% of the maximum importance value index, representing the more damaging weeds in the rice production systems of the region (Tab. 3). E. colona was the most important species in the rice crops of Tolima with an importance value index (IVI) of 30.4, a presence in 91.7% of the lots of the departments, a fre- quency of 0.39 and a density of 77.2 individuals/m2 (Tab. 3). TABlE 3. Components and importance value index (IVI) for the species that made up 50% of the maximum IVI value both generally and in each of the sampled zones (Tolima department, Colombia). G en er al I V I Species IVI F Fr (%) D Dr (%) C Cr (%) ECHCO 30.4 0.39 24.5 77.2 2.5 14.2 3.4 DIGSP 17.7 0.001 0.1 362.5 11.9 24.0 5.8 CYPIR 17.1 0.16 9.8 114.5 3.7 15.0 3.6 ISCRU 14.3 0.13 7.9 96.0 3.1 13.5 3.3 MUDNU 13.2 0.14 9.0 74.6 2.4 7.6 1.8 CASOB 11.5 0.01 0.9 200 6.5 16.9 4.1 PASBO 11.3 0.08 5.1 92.4 3.0 13.2 3.2 ROOEX 10.9 0.01 0.8 167.3 5.5 19.0 4.6 DIGBC 10.9 0.1 6.5 71.4 2.3 8.6 2.1 CYPES 9.7 0.07 4.3 52.1 1.7 15.4 3.7 C en tr o Zo ne I V I ECHCO 39.2 0.39 30 62.4 3.4 13.3 5.7 ROOEX 29.3 0.01 1.0 276.9 15.2 30.3 13.1 CYPIR 23.5 0.16 12.6 93.9 5.1 13.4 5.8 ISCRU 21.7 0.13 10.1 97.4 5.3 14.5 6.3 LEFSC* 16.4 0.11 8.1 54.1 3.0 12.2 5.3 DIGBC 14.4 0.08 6.1 83.0 4.6 8.9 3.8 M es et a Zo ne I V I ECHCO 26.5 0.39 20.4 56.7 3.3 9.0 2.9 CYPIR 18.9 0.2 10.3 80.1 4.6 12.5 4.0 ISCRU 15.7 0.16 8.5 74.2 4.3 9.1 2.9 DIGBC 15.6 0.18 9.4 59.7 3.4 8.6 2.8 ECLAL 13.0 0.13 6.8 69.2 4.0 7.1 2.3 TOROD* 12.2 0.03 1.7 61.5 3.5 21.6 7.0 HETLI 11.6 0.02 0.9 85.7 4.9 18.0 5.8 LUDLE* 11.5 0.01 0.5 106.3 6.1 15.3 4.9 CYPES 11.4 0.08 3.9 63.3 3.6 11.9 3.8 MUDNU 11.2 0.09 4.8 65.5 3.8 8.2 2.7 N or te Z on e IV I DIGSP 29.7 0.002 0.1 700.0 20.8 40.0 8.7 ECHCO 28.1 0.39 20.0 123.6 3.7 20.3 4.4 MUDNU 17.8 0.26 13.5 83.0 2.5 8.6 1.9 CYPIR 17.5 0.11 5.5 228.7 6.8 23.8 5.2 PASBO 16.9 0.22 11.0 96.8 2.9 13.6 3.0 CASOB 15.0 0.03 1.6 310.9 9.3 18.9 4.1 CYPES 13.4 0.15 7.8 53.0 1.6 18.5 4.0 ISCRU 11.9 0.09 4.5 123.9 3.7 17.1 3.7 F, absolute frequency; Fr, relative frequency; D, absolute density (individuals/m2), Dr, relative density; C, absolute dominance or cover; Cr, relative dominance or cover (Cr). 69Ramírez S., Hoyos C., and Plaza T.: Phytosociology of weeds associated with rice crops in the department of Tolima, Colombia The level of importance of the species was markedly in- f luenced by the relative frequency, which indicated that it is a species that is adapted to the prevailing conditions of the crops. The importance of this species has been reported by different authors (Holm et al., 1991; Puentes, 2003; Rao et al., 2007; Chauhan and Johnson, 2010a). Erasmo et al. (2004) reported on the importance of this species in weed communities in rice crops of Brazil through the use of phytosociological indices. For importance, the above species was followed by D. cili- aris, C. iria, I. rugosum and M. nudif lora, which presented IVIs of 17.7, 17.1, 14.3 and 13.2%, respectively. These spe- cies together with D. bicornis and P. boscianum offered the higher values of frequency after E. colona (Tab. 3). The important presence of these species coincides with findings for rice crops in Colombia and in different locations in the world and are related to the adaptation of these species to humid conditions (Erasmo et al., 2004; Rao et al., 2007; Chauhan and Johnson, 2009a; Montealegre, 2011). Phytosociological indices and parameters, such as the importance value index (IVI), offer a view of the composi- tion and the distribution of plant species in a community through ecological evaluation methods (Concenço et al., 2013). IVI parameters contemplate the importance of popu- lations within a weed community that, together with the analysis of the number of individuals and produced mass, allow for the inference of which species are more important in terms of infestation (Pitelli, 2000). In the Centro zone, six species represented 50% of the maxi- mum importance value index, representing the principal problem for rice crops in this zone. E. colona was the most important species with an IVI value of 39.2 (Tab. 3). It was reported in 88% of the lots of the zone with a frequency of 0.39 and a density of 62.4 individuals/m2 (Tab. 3), with relative frequency being the most important variable. R. cochinchinensis, C. iria and I. rugosum presented the higher density values (Tab. 3). R. cochinchinensis was the weed with the second highest importance in the Centro zone, with an IVI of 29.3 and a density of 276.9 individuals/m2 (Tab. 3). In Meseta, the analysis of the zone’s data demonstrated that 10 species made up 50% of the maximum importance value index, where E. colona was the most important species of the zone with an IVI value of 26.5 (Tab. 3), a presence in 90% of the lots, a frequency of 0.39, and a density of 56.7 individuals/m2 (Tab. 3); again, the relative frequency was the component with the most inf luence on this level of im- portance. C. iria, I. rugosum and D. bicornis had frequency levels of 0.20, 0.16 and 0.18, and IVI values of 18.9, 15.7 and 15.6, respectively (Tab. 3). The species with the higher den- sities included L. leptocarpa, H. limosa and C. iria (Tab. 3). The IVI in the Norte zone had 8 species that made up 50% of the maximum index. In this zone, D. ciliaris was the most important weed with an IVI of 29.7 (Tab. 3). It was found in 4% of the area with a frequency of 0.002 and a density of 700 individuals/m2. The index for this species was inf luenced by the relative density by a high degree (Tab. 3). S. obtusifolia and C. iria presented density values of 310.94 and 228.70 individuals/m2 (Tab. 3). E. colona was registered in 100% of the cultivated area in the north, with a frequency of 0.39, a density of 123.60 individuals/m2 and an IVI of 28.1 (Tab. 3). There was not a significant difference between the impor- tance indices of E. colona and D. ciliaris due to divergences in the contribution of the components. The absolute density and the relative density were higher in D. ciliaris; while the absolute frequency and relative frequency were more important in E. colona (Tab. 3). Balduino et al. (2005), in sociological studies of tree species, suggested that relative density is the parameter that contributes the most to the importance of relevant species. However, the results of the present study demonstrate that the relative frequency was determinant in the importance of the principal species. The level of adaptation of the species to the ecological conditions of the agricultural environment determine the frequency of weeds in lots and the number of individuals that compete with the crop. This study verified that E. colona is the most important species in the rice zones of Tolima. It is the most frequent weed with an average density of 77 plants/m2. Its negative effect means that it is considered the most problematic weed of the Graminea family in rice crops, with losses due to competition reported at 76% under densities of 280 plants/ m2 (Mercado and Talatala, 1977). Chauhan and Johnson (2010b) suggested that the shading effect caused by the aerial part of E. colona could be the principal mechanism responsible for yield losses. Its level of predominance in rice crops was highlighted in Colombia and Latin America by Plaza and Hernández (2014), Fuentes et al. (2010) and Puentes (2003). This indicates that E. colona has the abil- ity to colonize humid environments where rice crops are developed in the tropics (Puentes, 2003). Adaptive advan- tages such as a high capacity for production and for the germination of seeds under humid conditions (Chauhan and Johnson, 2010a, 2009a) and the plant’s metabolizing of C4 (Halvorson and Guertin, 2003; Montealegre, 2011) 70 Agron. Colomb. 33(1) 2015 facilitate the adaptation and establishment of E. colona populations under the conditions of the agricultural system in Tolima. The importance of E. colona in this region, even with the use of herbicides specific for its control, has been reported by Puentes (2003). Diversity indices Alpha indices analyze the diversity within a weed com- munity. Taking into account the Shannon-Wiener (H), Simpson and Uniformity (E) indices, it was possible to observe that the zones considered in this study were di- fferentiated by their diversity. The Meseta zone was the most diverse (Tab. 4). According to the Shannon-Wiener index, the three evaluated rice zones presented a low species diversity; however, the Meseta zone possessed a proportionally higher species diversity, with the highest value at 2.7, followed by the Norte zone with 2.6 and the Centro zone with 2.3 (Tab. 4). For its part, the lowest value for the Simpson dominance index (0.09) indicated that the weed community in the Meseta zone of Ibague has a low probability of being dominated by few species; therefore, it was more diverse (Tab. 4). For its part, the high value of the Uniformity index was also seen in this zone (0.8) (Tab. 4), suggesting a high species diversity (Booth et al., 2003). TABlE 4. Shannon-Wiener diversity index, Simpson dominance index and uniformity index for the three rice producing zones in the Tolima department (Colombia). Zone Shannon-Wiener diversity index Simpson dominance index Uniformity index Centro 2.3 0.1 0.7 Meseta 2.7 0.09 0.8 Norte 2.6 0.1 0.7 The diversity of the Meseta zone possibly presented itself in response to the fact that the number of individuals of each species was more balanced within the community (Concenço et al., 2013). In a broad sense, the Simpson domi- nance coefficient and uniformity coefficient indicated that the communities were dominated by various species. This could be the explanation for the number of applications and the quantity of active ingredients of the herbicides in use (data not shown) because it is thought that, when a weed community is more diverse, it tends to require complemen- tary control treatments and that the weeds require a high quantity of herbicides due to the differential sensitivity of the species (Kuva et al., 2007). Similarity indices The beta diversity indices of Jaccard and Sorensen facilitate the comparison of areas in terms of composition of the weed communities (Concenço et al., 2012a). According to Felfili and Venturoli (2000), these indices are considered elevated when they are above 0.5 (50%), at which a high similarity can be interpreted between areas. Booth et al. (2003) indicated that the values must be interpreted on a scale of 0 to 1, where 0 indicates total dissimilarity and 1 indicates absolute similarity. The similarity indices of Jaccard and Sorensen seen in the present study showed that the composition was not homogenous, that is, there was dissimilarity for all of the comparisons carried out between the zones (Tab. 5), suggesting dissimilarity in the composition of the weed communities between the Centro, Meseta, and Norte zones despite the fact that underlying similarity factors were seen in the zones, such as the nonexistence of crop rotation and sowing intensification (Erasmo et al., 2004). However, Concenço et al. (2011) suggested that, in zones where a crop is developed in a continuous manner or with rotation over a long period of time, there will be disconnec- tion (dissimilarity). The sampled lots in the department of Tolima have been cultivated with rice for 60 years and, for the most part, have not been the subject of a crop rotation plan at any time of the year. TABlE 5. Similarity indices of the sampled rice producing zones in the Tolima department (Colombia). Index Centro-Norte Centro-Meseta Meseta-Norte Jaccard 0.28 0.28 0.30 Sorensen 0.44 0.43 0.46 Steinhaus 0.80 0.57 0.54 The literature contains results for the Jaccard index that vary in accordance with the climatic and agronomic man- agement conditions; Hyvonen et al. (2003) and Fried et al. (2008) demonstrated homogeneity in weed communities of cereal crops under conditions of a temperate climate in response to the selection effect of the seasons, registering values between 0.5 and 0.8. Under tropical conditions, Concenço et al. (2012a) and Concenço et al. (2013) reported values between 0.2 and 1 due to variations in management. Ramírez (2010), for tobacco crops in the department of Hui- la (Colombia), found high values for this coefficient, close to 1. For the Sorensen index, Concenço et al. (2012b) and Concenço et al. (2011) reported low levels of 0.2 and high levels of 1 for tropical conditions. Furthermore, Erasmo et al. (2004) reported values between 0.22 and 0.75 in rotated rice crops: the low value was found when comparing areas of irrigated rice crops without rotation with areas with a rice-watermelon rotation, while the 0.75 value was found when comparing areas without rotation to areas with a 71Ramírez S., Hoyos C., and Plaza T.: Phytosociology of weeds associated with rice crops in the department of Tolima, Colombia rice-soy rotation. These authors noted the importance of the type of applied herbicide, the application timing, and the abundance of some species. The low similarity between the weed communities was possibly due to the differences in the management of the populations because divergences were seen between the zones at the time of post-emergence herbicide application, in the application equipment, in the volumes of utilized water, and in the provenience of the seeds (data not shown). In this sense, Bernardes et al. (2011) stated that the dis- similarity of weeds between agricultural areas is explained by differences in the conditions of the soil, in the weed control methods (mechanical, cultivation, and chemical) and, mainly, in the utilization of herbicides with different mechanical actions that contribute to the selection of a more diverse f lora. The Steinhaus coefficient calculates the similarity of com- munities, taking into account differences in the abundance of the species, making it more precise (Booth et al., 2003). The results of the Steinhaus index for the present study registered similarity in all of the zone comparisons because values above 0.5 were found in all of the cases. The simi- larity was higher in the comparison between the Centro and Norte zones, indicating a high quantity of in-common species with similar levels of abundance (Tab. 5). This situ- ation was possibly due to the similarity in the temperatures of the zones: the Centro zone registered mean maximum and minimum temperatures of 33 and 23ºC, while the Norte zone registered mean maximum and minimum temperatures of 35 and 23ºC. The climatic differences between the Centro and Norte zones and the Meseta of Ibague zone were clearly observed (mean maximum and minimum temperatures of 27 and 20ºC), determinant factors for the yield of the crops. The average production of the Meseta zone was 8.7 t ha-1, while, in the Centro and Norte zones, it was 7.9 and 7.6 t ha-1, respectively (Fedearroz, 2008). The critical temperatures (extremes), maximum and minimum, facilitated these divergences because they caused serious disturbances in the development of the plants. The average optimal temperature for the development of the rice plants in the vegetative phase was found between 21 and 31ºC (night/day) (Yoshida, 1977). Likewise, Yoshida (1978) and Nakayama (1974) observed that temperatures that are equal to or above 35ºC during the vegetative phase, which are common in the Centro and Norte zones, generate reductions in the tillering, plant height, and subsequent yield. On the other hand, and in agreement with reports from Bernardes et al. (2011), Andreasen and Streibig (2010) and Rao et al. (2007), the repeated use of herbicides, especially with the same action mechanism, may possibly be respon- sible for the similarity reported in the present study due to the fact that, in all of the zones of the department, only 4 action mechanisms are used for the post-emergence active ingredients. The similarity of the weed communities agree with the results of the species inventory because it revealed that the breadth of the weed problem is represented by the same species in all of the zones of the department. In regards to the obtained results, it was concluded that E. colona was the principal weed for the three evaluated zones due to the fact that it presented the highest value for the importance value index and frequency. In order of importance, the following species were observed: C. iria, I. rugosum, D. bicornis, P. boscianum and M. nudif lora. The weed community of the Meseta zone was the most diverse, followed by the community of the Norte zone. The composition of the weed community in the three zones was dissimilar according to the similarity coefficients of Jaccard and Sorensen. However, the Steinhaus coef- ficient demonstrated that the weed communities in these rice producing zones were similar, with the highest level of similarity occurring between the communities of the Centro and Norte zones. literature cited Andreasen, C. and J.C. Streibig. 2010. Evaluation of changes in weed f lora in arable fields of Nordic countries - based on Danish long-term sur veys. Weed Res. 51, 214-226. Doi: 10.1111/j.1365-3180.2010.00836.x Balduino, A.P.C., A.L. Souza, J.A.A.M. Neto, A.F. Silva, and M.C.S. Júnior. 2005. Fitossociologia e análise comparativa da com- posição f lorística do cerrado da f lora de Paraopeba-MG. Rev. Árvore 29, 25-34. Doi: 10.1590/S0100-67622005000100004 Bernardes S., M.B., E.L. Finoto, D. Bolonhezi, W. Carrega, J.A.A. Al- buquerque, and M.Z. Pirotta. 2011. Fitossociologia de plantas daninhas sob diferentes sistemás de manejo de solo em áreas de reforma de cana crua. Rev. Agro@mbiente On-line 5, 173-181. Booth, D. B., S.D. Murphy, and C.J. Swanton. 2003. Weed ecology in natural and agricultural systems. Editorial CABI Publishing, Wallingford, UK. Doi: 10.1079/9780851995281.0000 Brighenti, A.M., C. Castro, D.L.P. Gazziero, F.S. Adegas, and E. Voll. 2003. Cadastramento fitossociológico de plantas daninhas na cultura de girasol. Pesq. Agropec. Bras. 38, 651-657. Ca nt i l lo, E .E ., F.A. L ozada, a nd J.D. Pinzón. 20 06. Diseño metodológico de restauración de la reserva forestal carpatos Guasca-Cundinamarca. Undergraduate thesis. Faculty of http://dx.doi.org/10.1111/j.1365-3180.2010.00836.x http://dx.doi.org/10.1590/S0100-67622005000100004 http://dx.doi.org/10.1079/9780851995281.0000 72 Agron. Colomb. 33(1) 2015 Environment and Natural Resources, Universidad Distrital Francisco José de Caldas, Bogota. Concenço, G., J.C . Sa lton, M.L . Secret t i, P.B. Mendes, R .C. Brevilieri, and L. Galon. 2011. Effect of long-term agricul- tural management systems on occurrence and composition of weed species. Planta Daninha 29, 515-522. Doi: 10.1590/ S0100-83582011000300005 Concenço, G., C.J. Silva, L.A. Staut, C.S. Pontes, L.C.A.S. Laurindo, and N.C.D.S. Souza. 2012a. Weeds occurrence in areas submit- ted to distinct winter crops. Planta Daninha 30, 747-755. Doi: 10.1590/S0100-83582012000400008 Concenço, G., G. Ceccon, R.C. Sereia, I.V.T. Correia, and L. Galon. 2012b. Phytosociology in agricultural areas submitted to distinct wintercropping management. Planta Daninha 30, 297-304. Doi: 10.1590/S0100-83582012000200008 Concenço, G., M. Tomazi, I.V.T. Correia, S.A. Santos, and L. Galon. 2013. Phytosociological surveys: tools for weed science? Planta Daninha 31, 469-482. Doi: 10.1590/S0100-83582013000200025 Chauhan, B.S. and D.E. Johnson. 2009a. Ecological studies on Cy- perus difformis, Cyperus iria and Fimbristylis miliacea: three troublesome annual sedge weeds of rice. Ann. Appl. Biol. 155, 103-112. Doi: 10.1111/j.1744-7348.2009.00325.x Chauhan, B.S. and D.E. Johnson. 2009b. Seed germination ecology of junglerice (Echinochloa colona): a major weed of rice. Weed Sci. 57, 235-240. Doi: 10.1614/WS-08-141.1 Chauhan, B.S. and D.E. Johnson. 2010a. Growth and reproduction of junglerice (Echinochloa colona) in response to water stress. Weed Sci. 58, 132-135. Doi: 10.1614/WS-D-09-00016.1 Chauhan, B.S. and D.E. Johnson. 2010b. Relative importance of shoot and root competition in dry-seeded rice growing with jun- glerice (Echinochloa colona) and ludwigia (Ludwigia hyssopi- folia). Weed Sci. 58, 295-299. Doi: 10.1614/WS-D-09-00068.1 C obb, A .H . a nd J.P.H . Re ade . 2 010. Herbic ide s a nd pla nt physiolog y. 2nd ed. J. Wiley and Sons, Oxford, UK. Doi: 10.1002/9781444327793 Curtis, J.T. and R.P. McIntosh. 1950. The interrelations of certain analytic and synthetic phytosociological characters. Ecology 31, 434-455. Doi: 10.2307/1931497 Erasmo, E.A.L., L.L.A. Pinheiro, and N.V. Costa. 2004. Levanta- mento fitossociológico das comunidades de plantas infestantes em áreas de produção de arroz irrigado cultivado sob diferentes sistemas de manejo. Planta Daninha 22, 195-201. Doi: 10.1590/ S0100-83582004000200004 Fedearroz, Federación Nacional de Arroceros. 2008. III Censo nacional arrocero. Bogota. Felfili, J.M. and F. Venturoli. 2000. Tópicos em análise de vegetação. Comunicações Técnicas Florestais 2, 1-25. Ferriol M., M. and H.B. Merle F. 2006. El Inventario fitosociológico. In: Universitat Politècnica de València, http://hdl.handle. net/10251/16818; consulted: March, 2015 Fuentes, C.L., A.S. Osorio G., J.C. Granados T. and W. Piedrahita C. 2006a. Flora arvense asociada con el cultivo del arroz en el departamento del Tolima-Colombia. Bayer Cropscience; Universidad Nacional de Colombia, Bogota. Fuentes, C.L., A. Fúquene, E.M. Perdomo, and S.C. Pinto. 2006b. Plántulas de especies arvenses frecuentes en la zona centro de Colombia. Universidad Nacional de Colombia, Bogota. Fuentes, C.L., A. Osorio, J.C. Granados, and W. Piedrahita. 2010. Malezas de los arrozales de América Latina. pp. 387-341. In: Degiovani B., V., C.P. Martínez R., and F. Motta O. (eds.). Producción eco-eficiente del arroz en América Latina. Interna- cional Center of Tropical Agriculture (CIAT), Cali, Colombia. Fried, G., L.R. Norton, and X. Reboud. 2008. Environmental and management factors determining weed species composition and diversity in France. Agric. Ecosyst. Environ. 128, 68-76. Doi: 10.1016/j.agee.2008.05.003 Halvorson, W.L. and P. Guertin. 2003. Echinochloa Beauv. spp. U.S. Geological Survey, Southwest Biological Science Center, University of Arizona, Tucson, AZ. Holm, L.G., D.L. Plucknett, J.V. Pancho, and J.P. Herberger. 1991. The world’s worst weeds: distribution and biology. University Press of Hawaii, Malabar, FL. Hyvonen, T., E. Ketoja, J. Salonen, H. Jalli, and J. Tiainen. 2003. Weed species diversity and community composition in organic and conventional cropping of spring cereals. Agric. Ecosyst. Environ. 97, 131-149. Doi: 10.1016/S0167-8809(03)00117-8 Inoue, M.H., B.E. Silva, K.M. Pereira, D.C. Santana, P.A. Con- ciani, and C.L. Sztoltz. 2012. Levantamento fitossociológico em pastagens. Planta Daninha 30, 55-63. Doi: 10.1590/ S0100-83582012000100007 Kuva, M.A., R.A. Pitelli, T.P. Salgado, and P.L.C.A. Alves. 2007. Fitossociologia de comunidades de plantas daninhas em agroecossistema cana-crua. Planta Daninha 25, 501-511. Doi: 10.1590/S0100-83582007000300009 Lamprecht, H. 1990. Silvicultura en los trópicos: los ecosistemas forestales en los bosques tropicales y sus especies arbóreas; posibilidades y métodos para un aprovechamiento sostenido. Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ), Eschborn, Germany. Mercado, B.L. and R.L. Talatala. 1977. Competitive ability of Echinochloa colonum L. against direct-seeded lowland rice. pp. 161-165. In: Proc. 6th Asian-Pacific Weed Sci. Soc. Conf. Jakarta, Indonesia. Montealegre, F.A. 2011. Morfología de plántulas de malezas de clima cálido. Produmedios, Bogota. Mueller-Dombois, D. and H. Ellenberg. 1976. Aims and meth- ods of vegetation ecolog y. Geogr. Rev. 66, 114-116. Doi: 10.2307/213332 Nakayama, H. 1974. Panicle senescence in rice plant. Bull. Hokuriku Natl. Agric. Exp. Stn. 16, 15-17. Pitelli, R.A. 2000. Estudos fitossociológicos em comunidades infes- tantes de agroecossistemas. J. Conserb. 1, 1-7. Plaza, G. and F.A. Hernández. 2014. Effect of zone and crops rotation on Ischaemum rugosum and resistance to bispyribac-sodium in Ariari, Colombia. Planta Daninha 32, 591-599. Doi: 10.1590/ S0100-83582014000300015 Puentes, B. M. 2003. Flora arvense asociada al cultivo de arroz (Oryza sativa L.). MSc thesis. Faculty of Agronomy, Universidad Na- cional de Colombia, Bogota. Radosevich, S., J. Holt, and C. Ghersa. 1997. Weed ecology: implica- tions for management. 2nd ed. J. Wiley and Sons, New York, NY. http://dx.doi.org/10.1590/S0100-83582011000300005 http://dx.doi.org/10.1590/S0100-83582011000300005 http://dx.doi.org/10.1590/S0100-83582012000400008 http://dx.doi.org/10.1590/S0100-83582012000200008 http://dx.doi.org/10.1590/S0100-83582013000200025 http://dx.doi.org/10.1111/j.1744-7348.2009.00325.x http://dx.doi.org/10.1614/WS-08-141.1 http://dx.doi.org/10.1614/WS-D-09-00016.1 http://dx.doi.org/10.1614/WS-D-09-00068.1 http://dx.doi.org/10.1002/9781444327793 http://dx.doi.org/10.2307/1931497 http://dx.doi.org/10.1590/S0100-83582004000200004 http://dx.doi.org/10.1590/S0100-83582004000200004 http://hdl.handle.net/10251/16818 http://hdl.handle.net/10251/16818 http://dx.doi.org/10.1016/j.agee.2008.05.003 http://dx.doi.org/10.1016/S0167-8809(03)00117-8 http://dx.doi.org/10.1590/S0100-83582012000100007 http://dx.doi.org/10.1590/S0100-83582012000100007 http://dx.doi.org/10.1590/S0100-83582007000300009 http://dx.doi.org/10.1590/S0100-83582007000300009 http://dx.doi.org/10.2307/213332 http://dx.doi.org/10.1590/S0100-83582014000300015 http://dx.doi.org/10.1590/S0100-83582014000300015 73Ramírez S., Hoyos C., and Plaza T.: Phytosociology of weeds associated with rice crops in the department of Tolima, Colombia Ramírez, R.A. 2010. Análisis de la composición f lorística de arvenses asociadas al cultivo de tabaco en cinco municipios del departa- mento del Huila. Undergraduate thesis. Faculty of Agronomy, Universidad Nacional de Colombia, Bogota. Rao, A.N., D.E. Johnson, B. Sivaprasad, J.K. Ladha, and A.M. Mor- timer. 2007. Weed management in direct-seeded rice. Adv. Agron. 93, 153-255. Doi: 10.1016/S0065-2113(06)93004-1 Spiegel, M. 1988. Estadística: teoría y 875 problemas resueltos. 2nd ed. McGraw Hill, Madrid. Yoshida, S. 1977. Rice. pp. 57-87. In: Alvim, P. and T. Kozlowski (eds.). Ecophysiology of tropical crops. Academic Press. New York, NY. Doi: 10.1016/B978-0-12-055650-2.50008-3 Yoshida, S. 1978. Tropical climate and its inf luence on rice. Interna- tional Rice Research Institute, Los Baños, Philippines. http://dx.doi.org/10.1016/S0065-2113(06)93004-1 http://dx.doi.org/10.1016/B978-0-12-055650-2.50008-3