PHYToSocIoloGY oF SoWN PASTURE WEEDS UNDER TWo lEVElS oF DEGRADATIoN IN BRAZIlIAN SAVANNA AREAS , MATo GRoSSo Do SUl STATE , BRAZIl 1

Cerrado (Brazilian savanna) is the second biome in Brazil, 2 million km2, approximately 22% of the country (Eiten 2001), covering nearly the whole central-West region. Soils are generally dystrophic, but well structured, often on flat or gently rolled relief, what favors cattle ranching and mechanized crops (Pivello 2006). cattle farming has expanded considerably in the region, after the 1970s, due to low prices of land, finance, and forage ABSTRACT RESUMO

PAlAVRAS-cHAVE: Agroecossistema; Brachiaria; ecologia vegetal; flora; Urochloa.Phytosociology of sown pasture weeds under two levels of degradation in Brazilian savanna areas e-ISSN 1983-4063 -www.agro.ufg.br/pat-Pesq.Agropec.Trop., Goiânia,v. 40,n. 3,jul./set.2010 Macedo & Zimmer (1993), a degraded pasture undergoes a gradual process of vigor, productivity and recovery capacity loss, becoming unable to overcome harmful pests effects, diseases, and weeds, ending up with natural resources degradation.correa et al. (2001) pointed out that pasture degradation in the Cerrado region is often related to exhaustion of soil fertility.Dias Filho (2007) stressed that weed proliferation in pastures is a consequence of the degradation process and not its cause, and that weeds compete for soil nutrients, making them less accessible to forage plants.
The management success of a weed community begins with the survey of weeds, in order to understand their biology and predominant species (Pott et al. 2006).So, floristics and phytosociology are extremely useful tools, helping on practices management and weed control of different crops, the first supplying a list of all weed species and the second supporting the understanding of species structure and dynamics, dominant populations, and the weed community, as a whole.Studies involving sown pastures weed communities were performed by Dantas & Rodrigues (1980), Dias Filho (1990), Mascarenhas et al. (1999), Mitja et al. (2000), Dutra et al. (2004), andTuffi Santos et al. (2004), however, just a few were carried out in the central-West region (Pott et al. 2006, Silva et al. 2006).
The objective of the present research was the phytosociological study of two pastures sown in Cerrado areas, with and without cattle, during the rainy and dry seasons, in Mato Grosso do Sul State, Brazil.

Study areas
Nova Esperança ranch lays in the township of Sidrolândia, micro-region of campo Grande, 37 km South of campo Grande city, Mato Grosso do Sul State, Brazil, with an approximate area of 4,000 ha, including 1,500 ha of swamp, wet grassland, Cerrado grassland and scrub, and Cerrado stricto sensu, used as pastureland for beef cattle.
The Nova Esperança ranch presents a great ecological importance, as it includes seven headwaters and a registered private reserve, near Serra de Maracaju, part of the Pantanal-Atlantic Forest ecological corridor, being one of the few remnants of unbroken native vegetation, with more than 2,000 ha covered with cerradão and semideciduous forest, close to the capital city of Mato Grosso do Sul, campo Grande.
The climate of the region, according to the Köppen system classification, is Aw humid tropical subtype (mean temperature of 24ºc and mean annual rainfall of 1,500 mm), with a rainy season in the Summer (october to April) and a dry one in the Winter.The relief is gently rolled, with predominance of Dystrophic red latosol (Rhodic Ferrasol) (Embrapa 2008).
Two areas (paddocks) of sown pasture were selected, here named area I (AI) and area II (AII).Since the middle 1930s, these areas, originally from Cerrado s.s. and cerradão, have been cleared and burned for pasture development, usual practices in the region.In the 1970s, following a regional trend, African species of Brachiaria (Urochloa) were introduced.

Floristic study
The study was performed from February 2007 to January 2008, during a rainy season (october to April) and a dry one (May to September).A minimum of 12 months is expected for this kind of study, due to variables such as fast vegetative growth and complete flowering, fruit set, and dispersion cycles.Monthly visits were made in order to survey pasture herbaceous, subshrubby and climbing weeds, e-ISSN 1983-4063 -www.agro.ufg.br/pat-Pesq.Agropec.Trop., Goiânia, v. 40, n. 3, p. 312-321, jul./set.2010 collect botanical material, and observe populations, through asystematic walks.Collected and identified material, with flowers and/or fruits, was botanized according to the usual techniques and kept in the cGMS Herbarium, at the Universidade Federal do Mato Grosso do Sul.
Herbs were considered the prostrate or erect non-woody plants, subshrubbs the erect and partially lignified plants, and climbers non-woody plants which use other herbs and subshrubs as support, according to Guedes-Bruni et al. (2002).Species varying from herbs to subshrubs were treated as a separate group.For identification, specialized bibliography and comparison in the herbarium were used and eventually specialists were consulted.Systematics for families follows APG II ( 2003) and for species author names Brummitt & Powell (1992).
Based on literature and field observations, each species was sorted for life form (chamephyte, geophyte, hemicryptophyte, therophyte), after the main groups proposed by the Raunkier system (1934), adapted by Müller-Dombois & Ellenberg (1974).Species which vary from therophytes to hemicryptophytes, hemicryptophytes to geophytes, and chamephytes to phanerophytes were considered as separate groups.Geographic origin was based on bibliography.

Phytosociological study
A set of 42 permanent plots of 0.25 m 2 was distributed in eight (AI) and nine (AII) parallel lines, North-South.Sampling sufficiency was tested through the species-area curve (Braun-Blanquet 1979) and determination of minimum area (Matteucci & colma 1982).Two phytosociological samplings were performed, in July 2007 (dry period) and January 2008 (rainy period), recording all herbaceous to subshrubby Angiosperms present in the plots.For visual evaluation of species coverage, as well as bare ground, dry plant material (straw), and litter (decomposing plant material), these two lumped, the Daubenmire scale was used, following Müller-Dombois & Ellenberg (1974).Based on data of presence-absence and coverage scale, values of relative frequency (RF), relative cover (Rc), and importance value (IV) were calculated.Shannon diversity species index (H') and similarity values based on Euclidian distance were obtained by using the PAST program (Hammer et al. 2001).

RESUlTS AND DIScUSSIoN
The AI and AII floristic survey recorded a total of 25 families, 72 genera, and 104 herbaceous and subshrubby Phanerogams weed species (Table 1).From that total, 76 species are herbs, 14 are subshrubs, nine vary from herbs to subshrubs, and five are climbers, and approximately 49% of the species present some forage potential.only 17% are exotics, however many species considered native only occur in secondary or disturbed natural vegetation, or are from open savanna and favored in the man-made formation, or secondary grassland, which the sown pasture is an example.Many species are considered pantropical weeds, such as the Urena lobata.
concerning life forms, hemicryptophytes predominate in the area, with nearly 41% of the species.Hemicryptophytes present vegetative buds close to the soil surface, where they can escape from grazing.From the total of species, 37 are therophytes and seven geophytes, which, together, represent approximately 42%.The strategy adopted by these species is very common among herbs in the Cerrado area, whereas therophytes do their perpetuation by seeds, dormant in the adverse period, and geophytes present underground buds, protected from disturbances, such as drought and fire, besides herbivory and treading.
The families with the highest number of species were Fabaceae (22 species), Poaceae (17), and Asteraceae (15), which together correspond to nearly 52% of the species found.Similar results were reported by Mascarenhas et al. (1999), lara et al. (2003), and Tuffi Santos et al. (2004), who also pointed out the referred families among those of highest weed species richness in sown pastures.Those families are generally the most numerous in the floristic spectrum of neotropical savannas.
In AI, 66 weed species were found, from 51 genera and 18 families.The richest families were Fabaceae (17 species), Asteraceae (10 species), and Poaceae (10 species).Nearly 1/3, 21 species, are exclusive from this area, with six of them being Fabaceae.Some legumes are potentially valuable as forage resource in tropical regions characterized by low soil fertility (lascano et al. 2001), for example, Arachis and Desmodium, which, in sown pastures, may be favored by grazing.
In AII, 83 weed species were recorded, from 58 genera and 22 families.As well as in AI, families Phytosociology of sown pasture weeds under two levels of degradation in Brazilian savanna areas In both areas, as a mosaic in the cultivated grass sward, extended patches of Paspalum notatum were observed, a native species with good protein content, usually grazed, which protects soil from erosion.According to Pedreira & Pedreira (2006), the species is very competitive and tolerant to adverse conditions, such as drought, waterlogging, intense treading and low soil fertility.
The occurrence of Mimosa spp.and Solanum viarum was common in both areas.According to lorenzi (2000), Mimosa species, in general, are rejected by cattle, due to their spiny and aggressive character.Seeds of S. viarum are spread in cattle dung (Pott & Pott 1994).For both plant groups, the negative selectivity of cattle favors large sizes and/ or large populations.In addition, a large number of scattered individuals or small clumped populations of Pterocaulon lanatum was recorded.In general, the aerial part of Pterocaulon contains cumarine, which acts as a protection against herbivory and can cause phototoxicity (Stein 2005), therefore also probably rejected by cattle.
Shannon diversity values (H') were 4.19 and 4.43 nats, for AI and AII, respectively.Although animal load and management type (period of grazing, exclosure, etc.) are very distinct between both areas, their floristic diversity values are pretty close.These values are considered high, when compared to those obtained by Dutra et al. (2004), in Brachiaria humidicola (1.527 to 2.620) and B. brizantha (1.395 to 2.544) pastures, in the Amazonian Northeastern Pará.
The floristic dissimilarity index between AI and AII, obtained from the Euclidian distance, was 0.7468.This high index indicates that the studied areas are floristically distinct, in other words, present few species in common ( 44%), what can be explained by differences related to cattle, shading by adjacent cerradão, and exposure to wind and sun.
A synthesis of results from recent studies involving survey of sown pastures weed species, in different Brazilian regions, is presented in Table 2. Analysis of floristic dissimilarity among this work and the referred studies revealed values between 0.8825 and 0.9707.This high dissimilarity could be explained by the original vegetation, geographical distance, and regional climatic and edaphic factors, plus differences in type and intensity of anthropic actions, or management, such as forage species, fire, grazing history, and weed introduction and control (herbicide, slashing).
over the dry season, a considerable population reduction of all weeds groups was visually observed, partly due to drought (AI and AII), but also to cattle consumption and treading (AI).This reduction, however, appeared more dramatic among species of Euphorbiaceae, Fabaceae, Malvaceae, and Rubiaceae.on the other hand, Pterocaulon lanatum, besides ungrazed, showed to be drought resistant, due to its xylopodium, practically maintaining the size and density of its populations (table 3).Floristic richness of the AI and AII weed communities responded in a distinct way to the dry season (Figures 1 and 2): in AI, the number of species dropped from 60 to 54 (10%), while, in AII, it increased from 72 to 75 (4%).
The importance value (IV) and relative cover (Rc) for phytosociological data (Table 4) show as main species Paspalum notatum, Brachiaria decumbens, and Sida rhombifolia, in AI, and B. brizantha, P. notatum, and Desmodium incanum, in AII.Similar results were obtained by Tuffi Santos et al. (2004) andlara et al. (2003), who found S. rhombifolia and P. notatum among the species of highest IV.
In AI, the pasture weeds add up an IV of 38.8% (dry period) and 46.3% (rainy period).These values are higher than those obtained for weed species in AII, with IV of 16.1% (dry period) and 15.2% (rainy period).The high IV of the weed community in AI may be related to the presence of cattle, due to treading, grazing selectivity, and dispersion of some weed species.
In AI, B. decumbens reached Rc of 13.4% (dry period) and 21.1% (rainy period), less than for P. notatum, exhibited in both seasons.This may be attributed to the high adaptability of this species to adverse environmental conditions, as previously mentioned.In AII, Rc of B. brizantha stayed high, 38.0% (dry period) and 37.7% (rainy period).
Though in the rainy season, in AII, there was an increase of "straw and litter", overrunning the coverage of B. brizantha, a probable effect of cattle removal, P. notatum also stood out in this area, what confirms its adaptability to different management types.The "straw and litter" and "bare ground" attributes are high in terms of IV and Rc, in both areas and seasons.In AII, "bare ground" reached its maximum Rc (23.5%), in the dry period, while 70.3% of soil was protected by B. brizantha and "straw and litter".
In AI, a considerable amount of bare ground was observed, besides gullies in advanced stage, forming deep ravines and causing relevant loss of vegetation areas and soil.In some patches of nearly bare ground, the presence of Sida rhombifolia, Tridax procumbens, and Acanthospermum australe was observed, indicators of degraded pasture.The last two species, although showing low height, generally being prostrate, form densifications, which give some cover and protection to the soil.Nevertheless, during the dry period, their population was reduced, and, as a consequence, the bare ground area increased considerably.
According to criteria established by Vieira & Kichel (1995), to indicate pasture degradation, AI is considered degraded, due to high weeds infestation, besides unvegetated areas, with soil compaction    These weedy grasses probably remained in AII, due to previous grazing history.Absence or reduced number of individuals of the referred species in AI could be related to competition from B. decumbens, whose basal cover is higher than B. brizantha, once they produce abundant diaspores of a wide range, one being zoochorous (C.echinatus) and some anemochorous (M.repens and D. insularis).
Besides providing comparative information on botanical composition of different grasslands, the gathered floristic and phytosociological data can be useful for further studies, for example, agronomic (management of degraded pastures) and biochemical studies involving pasture weed e-ISSN 1983-4063 -www.agro.ufg.br/pat-Pesq.Agropec.Trop., Goiânia, v. 40, n. 3, p. 312-321, jul./set.2010 species, plus ecological research.Some aspects to investigate could be the separate effects of sown grass species and the presence/absence of grazing, which became confounded.In addition, a longer research time is necessary for a better understanding of the dynamics of pasture weed flora in the micro-region.The pastures, although degraded, are generally intermingled with areas of legal Reserve and Permanent Preservation, and interactions between the agroecosystem and the original matrix have not been adequately studied yet.
There is an obvious need for more investment in time, technology and resources for recovery of degraded pastures, utilization of native forage species (for example Arachis and Desmodium), or regeneration of the original Cerrado vegetation.

Figure 2 .
Figure 2. Total number of species per family and season, in two areas of Brachiaria spp.sown pastures in Cerrado, Nova Esperança ranch, Sidrolândia, Mato Grosso do Sul (Area I = with cattle; Area II = without cattle; DRY = dry period; RAIN = rainy period).

Figure
Figure 1.Number of families, genera and species found in rainy and dry seasons, in two areas of Brachiaria spp.sown pastures in Cerrado, Nova Esperança ranch, Sidrolândia, Mato Grosso do Sul (Area I = with cattle; Area II = without cattle; fam = family; gen = genus; sp = species; dry = dry period; rain = rainy period).

coNclUSIoNS 1 .
The weeds diversity is high, with low proportion of exotic species, in two distinct degraded pastures, in the campo Grande micro-region.The richest families are Fabaceae, Asteraceae, and Poaceae.Hemicryptophytes predominate, followed by therophytes.The main species are Brachiaria decumbens and Sida rhombifolia, in the area with cattle, and B. brizantha and Desmodium incanum, in the area without cattle, plus Paspalum notatum, in both pastures.2. The diversity is considered high, and areas with and without cattle show similar floristic diversity.The floristic dissimilarity index is high between the studied areas and it is even higher in relation to sown pastures in other Brazilian regions.3. The 15-month period rest (without cattle) does not favor the spread of the weed community, on the other hand, it contributes little to the regeneration of the natural Cerrado vegetation.4. The weeds population decreases in the dry season.
two polygon sides are adjacent to little disturbed cerradão.Altitude varies from 509 m to 520 m.Up to September 2006, only six working oxes were kept in this paddock, which stayed without cattle from october 2006 to January 2008, over the study period.

Table 1 .
Weed species of two areas of Brachiaria spp.pastures in Cerrado, Nova Esperança ranch, Sidrolândia, Mato Grosso do Sul.

Table 3 .
Months of occurrence of weed species in two areas of Brachiaria spp.sown pastures in Cerrado, Nova Esperança ranch, Sidrolândia, Mato Grosso do Sul.