Floristic and vegetation structure of a granitic grassland in Southern Brazil

Florística e estrutura da vegetação de um campo granítico no Sul do Brasil

Abstracts

Um levantamento florístico e estrutural de uma comunidade de campo natural foi conduzido no Morro do Osso, um morro granítico em Porto Alegre, RS, Brasil. Os dados estruturais foram obtidos em 39 quadros de 1 m², dispostos em duas grandes áreas de campo. Uma queimada acidental ocorreu em uma das áreas, aproximadamente um ano antes do levantamento, propiciando a análise de diferenças entre os parâmetros dos dois sítios de amostragem. A lista florística contém 282 espécies e, no levantamento estrutural, foram encontradas 161 espécies. As famílias com maior valor de importância acumulado foram Poaceae, Asteraceae e Fabaceae. Os índices de diversidade e a equabilidade foram 4,51 nats ind-1 e 0,86, respectivamente. A análise de agrupamento identificou dois grupos nítidos, que coincidem com os locais atingidos ou não pelo fogo. Uma análise de similaridade entre os dados obtidos e dois outros conjuntos de dados de morros graníticos vizinhos resultou em 28% a 35% de similaridade, com uma equivalente distribuição de espécies por família e muitas espécies dominantes comuns, corroborando a noção de continuidade da flora ao longo dos morros graníticos no sul do Brasil.

campos; diversidade; espécies ameaçadas; fogo; similaridade; vegetação relictual

ARTICLE

Floristic and vegetation structure of a granitic grassland in Southern Brazil

Florística e estrutura da vegetação de um campo granítico no Sul do Brasil

Pedro Maria de Abreu Ferreira^I; Sandra Cristina Müller^II; Ilsi Iob Boldrini^III; Lilian eggersIII, ¹

^IUniversidade Federal do Rio Grande do Sul - Programa de Pós-Graduação em Botânica, Av. Bento Gonçalves, 9500, Bloco IV, Prédio 43433, 91501-970 Porto Alegre, RS, Brazil

^IIUniversidade Federal do Rio Grande do Sul, Instituto de Biociências, Departamento de Ecologia, Av. Bento Gonçalves, 9500, Caixa Postal 15007, 91501-970 Porto Alegre, RS, Brazil

^IIIUniversidade Federal do Rio Grande do Sul, Instituto de Biociências, Departamento de Botânica, Av. Bento Gonçalves, 9500, Bloco IV, Prédio 43433, 91501-970 Porto Alegre, RS, Brazil

ABSTRACT

A floristic and structural survey of a natural grassland community was conducted on Morro do Osso, a granitic hill in Porto Alegre, RS, Brazil. Structural data were surveyed in 39 one square meter plots placed over two major grassland areas. An accidental fire has occurred in one of the areas approximately one year prior to our survey, leading to further analysis of parameters differences between sites. The floristic list contains 282 species, whereas the structural survey has found 161 species. Families with highest accumulated importance values were Poaceae, Asteraceae and Fabaceae. The diversity and evenness indexes were 4.51 nats ind^-1 and 0.86, respectively. Cluster analysis denoted two groups coinciding with the areas distinguished by the fire disturbance. A similarity analysis between our data and two other data sets from nearby granitic hills resulted in 28% to 35% similarity, with equivalent species-family distribution and many common dominant species, corroborating the concept of a continuous flora along the South Brazilian granitic hills.

Key words: campos, diversity, fire, relict vegetation, similarity, threatened species

RESUMO

Um levantamento florístico e estrutural de uma comunidade de campo natural foi conduzido no Morro do Osso, um morro granítico em Porto Alegre, RS, Brasil. Os dados estruturais foram obtidos em 39 quadros de 1 m², dispostos em duas grandes áreas de campo. Uma queimada acidental ocorreu em uma das áreas, aproximadamente um ano antes do levantamento, propiciando a análise de diferenças entre os parâmetros dos dois sítios de amostragem. A lista florística contém 282 espécies e, no levantamento estrutural, foram encontradas 161 espécies. As famílias com maior valor de importância acumulado foram Poaceae, Asteraceae e Fabaceae. Os índices de diversidade e a equabilidade foram 4,51 nats ind^-1 e 0,86, respectivamente. A análise de agrupamento identificou dois grupos nítidos, que coincidem com os locais atingidos ou não pelo fogo. Uma análise de similaridade entre os dados obtidos e dois outros conjuntos de dados de morros graníticos vizinhos resultou em 28% a 35% de similaridade, com uma equivalente distribuição de espécies por família e muitas espécies dominantes comuns, corroborando a noção de continuidade da flora ao longo dos morros graníticos no sul do Brasil.

Palavras-chave: campos, diversidade, espécies ameaçadas, fogo, similaridade, vegetação relictual

Introduction

The study of natural grassland communities, despite its well known ecological importance, has been historically neglected (Overbeck et al. 2007) or limited to dominant grass species used in livestock production (Uys et al. 2004) or grain monocultures. As it is, there is a lack of floristic and structural studies concerning these communities worldwide, which leads, at least partly, to an underestimation of their richness, diversity and conservational value.

Subtropical grasslands, which are also called campos in Brazil (Behling 2002, Focht & Pillar 2003, Overbeck et al. 2007), were dominant in southern Brazilian landscape during late Pleistocene, and its present distribution can be interpreted as a remnant of a drier and cooler climate in the region (Behling 2002, Bredenkamp et al. 2002). Thus, campos configure a relict formation, and the knowledge of its floristic and structural features is vital to understand the processes that lead to modern landscape. Rambo (1954) emphasized both the richness and ecological importance of the southern Brazilian herbaceous flora by indicating that herbaceous species of Asteraceae alone are more numerous than the entire local woody flora.

Despite its ecological and social-economic importance, natural grasslands are a historic target of many anthropogenic disturbances, such as land conversion to grain and wood monocultures, overgrazing and unrestricted fire, being often wiped away from the landscape even before being well known by science. Nevertheless, fire disturbances not always have hazardous consequences for plant communities, such as depletion of soil nutrients and local plant extinction, and there is growing evidence of the importance of both natural and human-driven fires for the maintenance of many natural grassland communities (Quadros & Pillar 2001, Uys et al. 2004, Overbeck et al. 2005, Overbeck et al. 2006, Behling et al. 2007, Müller et al. 2007). Moreover, the acknowledgement of fire disturbance as a crucial environmental factor to the very existence of subtropical grasslands is increasing (Quadros & Pillar 2001, Behling 2002, Behling et al. 2004, Overbeck et al. 2005, Müller et al. 2007).

Granitic grassland formations in southern Brazil includes an area beside the city of Porto Alegre, Rio Grande do Sul, represented by a series of granitic hills known as Morro do Osso, Morro Santana, Morro da Polícia, Morro São Pedro, among others. Although recent floristic and structural surveys concerning southern Brazilian granitic grassland formations are scarce, a historical analysis of the Porto Alegre flora can be found in Rambo (1954), who displayed an extensive species list, albeit outdated nowadays. The most extensive approach was presented by Aguiar et al. (1986) concerning a preliminary floristic survey in ten granitic hills in Porto Alegre region. Boldrini et al. (1998) surveyed floristic and ecological aspects of a grassland community at Morro da Polícia and Overbeck et al. (2006) presented a local species list of Morro Santana, linking floristic composition of burned grassland to environmental factors, such as soil properties and distance from the forest border.

Working in a southern Brazilian granitic grassland, our objectives were: (1) to present floristic and structural data of the local herbaceous flora; (2) to explore the effects of an accidental fire event on floristic and structural patterns and (3) to compare plant diversity of similar hill's formation in the context of grassland vegetation dynamic.

Material and methods

Study area - The study was carried out on Parque Natural Morro do Osso (30°07' S, 51°14' W; 143 m a.s.l.), an area inserted into a chain of granitic hills that surrounds the city of Porto Alegre, Rio Grande do Sul, Brazil. This geological formation, known by the presence of several rare and endemic plant species (Rambo 1954, Aguiar et al. 1986, Boldrini et al. 1998, Brack et al. 1998), configures an intersection point among four different structural provinces that shape the south Brazilian landscape: the Crystalline Shield, the Peripheral Depression, the bottom of the Seashore Plain and the Lagoon System (Menegat et al. 1998). Likewise, this granitic chain is historically thought to be responsible for notorious local plant endemism, acting like a refuge area during ocean transgression times (Rambo 1954). The study site is a Conservation Unit since 1994, covering 127 ha of natural area (Brack et al. 1998).

Natural vegetation at the site comprises a mosaic of grasslands and forests (Atlantic Rainforest), the first occurring mostly on hilltops and north or northwest slopes and the latter mostly along river courses and south or southeast slopes (Rambo 1954, 1956, Müller et al. 2007). Grasslands cover around 40% of the study area and fire, either natural or caused by neighboring human settlements, is a common disturbance to the ecosystem at spaced but non-fixed intervals. There are two major grassland patches in the study site, one is west-oriented and the other is east-oriented, being separated by a relatively large forest/woodland area (figure 1). There is a record of significant burning on the east grassland area in 2005, one year prior to the survey presented in this paper, confirmed by the first collected specimens at the site, which had clear signals of recent burning.

Climate at the study site is subtropical humid, with no dry season (Köppen's Cfa). The average annual temperature is 19.5 °C and the mean annual precipitation is 1348 mm (Nimer 1990). The soil is granite-originated, composed by post-tectonic granitoids dated from 550 m.y.a. and known as Santana granite (Sestren-Bastos 2006).

Data collection and analysis - Data for the structural survey were collected in 39 one square meter plots randomly placed in two major grassland areas (figure 1). Other grassland-covered areas that can be seen in figure 1 comprise tracks, shrublands and surroundings of Park management's buildings, thus not included in the sampling area. In each plot, species cover was registered, applying the Daubenmire scale (Mueller-Dombois & Ellenberg 1974). The survey took place in January 2006, although prior to that, other expeditions were made to search floristic records. Plant identification was done using specific bibliography and contacting specialists when necessary. Family circumscription follows APWeb (2003). A full species list for the Morro do Osso's grasslands is presented based on our own information and previous publications (Brack et al. 1998, Brack et al. 2001).

For each species, absolute and relative frequency (AF and RF, respectively), absolute and relative cover (AC and RC, respectively) and importance value (IV) were calculated. Shannon-Wiener diversity index (H') and the Pielou evenness (E) (Pielou 1969, Magurran 1988) were also obtained. Open soil and litter were evaluated in each plot unit, but omitted in the community indexes calculation.

The raw data matrix obtained for the grassland community (species plus open soil and litter per 39 sampling plots) was submitted to ordination and cluster analysis on Multiv software (Pillar & Orlóci 1993, 1996, Pillar 2006). Both methods were submitted to the bootstrap analysis to verify cluster groups sharpness and ordination axis stability (Pillar 1999a, b). The ordination method was the principal coordinate analysis (PCoA) and, in the cluster analysis, incremental sum of squares was used as the clustering criterion, both based on chord distance resemblance measure between plots (Podani 2000). Variables (species, litter and open soil cover) with axes correlation values greater than 0.45 were included in the ordination scatter diagram. The statistically supported groups obtained from this analysis were then submitted to separate phytosociological analysis with the previously mentioned parameters. Vegetation height, richness, open soil and litter were submitted to variance analysis comparing groups of plots by randomization test with 10,000 iterations (Pillar & Orlóci 1996, Manly 1997).

The species list obtained in the structural survey was submitted to a similarity analysis (Jaccard similarity index) based on structural data from two neighboring granitic hills: Morro da Polícia (Boldrini et al. 1998) and Morro Santana (Overbeck et al. 2006). Only fully identified taxa (specific level) were included in this analysis. A nomenclatural revision was also done on the species lists in order to avoid synonyms and standardize data.

Results

Floristic and structural analysis - The floristic list contains 282 species, distributed in 42 families (table 1). Poaceae (74 species), Asteraceae (63 species) and Fabaceae (25 species) were the most representative families in the floristic survey.

The structural survey carried out has found 161 species, distributed in 30 families. The most diverse families in the structural survey were Asteraceae (45 species), Poaceae (33 species) and Fabaceae (17 species). However, considering the accumulated IV per family, Poaceae had the highest value (33.21%), and five of the top-ten IV-ordered species belong to this family (table 2). The accumulated IV for Asteraceae and Fabaceae was respectively 28.57% and 7.22%. The low value for Fabaceae characterized it as a diverse family with low contribution on frequency or cover in the study site. The species with the highest IV were particularly grasses, the prostrate Rubiaceae Richardia grandiflora and some species of Asteraceae (table 2). The diversity and evenness indexes obtained for the community were H' = 4.51 nats ind^-1 and E = 0.86.

Results from the similarity analysis among Morro do Osso, Morro da Polícia and Morro Santana indicated similar grassland community composition in the three granitic hills analyzed (table 3). Although the values themselves are not remarkably high (average of 33% similarity), they are very close to each other (0.03 standard deviation). There are 57 common species among the three surveys. Many of these species are dominant grasses like Andropogon lateralis, Aristida flaccida and Trachypogon montufarii, suggesting similar community structure and landscape on the different study sites. Moreover, Stipa filifolia, an endemic grass with distribution restricted to Brazilian granitic hills, was also found in the three surveys. Besides grasses, many forb and sub-shrub species of Baccharis, Chaptalia and Eryngium are common among the areas, as well as some rare species like the cactus Parodia ottonis.

Mean species number found per plot (1 m²) was 27.10 (standard deviation = 4.96). Plots with lowest and highest species richness had respectively 15 and 35 species. Mean vegetation height on the sampled area was 28.87 cm (standard deviation 8.62) and open soil and litter were present in all sampling units, the first with 4.07% (standard deviation = 13.11) of mean absolute cover per plot unit and the latter with 0.55% (standard deviation = 0.86).

Clustering and ordination analysis - Cluster analysis denoted the existence of two sharp groups among the sampling units (figure 2). Cluster groups coincided with the two major grassland-covered areas that were distinguished by a natural environmental bias caused by fire disturbance due to recent burn on the eastern slope (axis 1 variation; figure 3). The majority of the east plots formed a compact distribution of points on figure 3, whereas the west plots shaped a more scattered pattern (axis 2 variation). Because of the great coincidence among cluster groups and the set of plots located in the same slope, further analyses were done according to plot location.

Species plotted on figure 3 are related to differences in floristic and structural composition between the two major grassland areas. These differences can be better observed by species performances (figure 4) in each plot group. East plots presented a total of 111 species, being 32 exclusive of this area, whereas for the west plots 129 species were surveyed in total, being 50 exclusive species. Even with these given floristic peculiarities, there are no statistical differences in richness between the areas (east = 27.75 species per plot; west = 26.65).

Grass species such as Schizachyrium tenerum, Trachypogon montufarii, Axonopus siccus and Aristida filifolia had their performance decreased in the east area (compare table 2 and figure 4), whereas Schizachyrium microstachyum, Eragrostis polytricha (grasses), Eryngium horridum, Pterocaulon angustifolium and Vernonia flexuosa (forbs) increased their performance. Andropogon lateralis, Aristida flaccida, Axonopus suffultus (grasses), Vernonia nudiflora (sub-shrub), Eupatorium laetevirens and Aspilia montevidensis (forbs) were dominant species that maintained their relative cover proportion in both areas. The first two grasses, however, had proportionally more cover in east plots (figure 4).

There was no difference considering vegetation height in both grassland areas (east plots: 26.68 cm; west plots: 28.69 cm). In addition, open soil mean cover had no statistical difference between the areas, albeit the slight difference between east and west plots (mean of 1.62 and 1.26, respectively) and the visual difference observed in the field. However, litter presented statistical difference between the areas (mean of 1.06 and 1.39 for east and west plots, respectively; P = 0.03).

Discussion

Diversity in grassland formations presented in studies carried out in Rio Grande do Sul State are quite variable, depending on the method and the region surveyed. In a flat region in the middle of the State, different efforts have found less species when compared to the present study. Focht & Pillar (2003) have found 148 grassland species with vegetation patterns related to relief position and other factors, such as soil moisture, on a study located approximately 35 km west from ours. On the other hand, granitic grassland formations tend to present high species richness, as showed by Boldrini et al. (1998) in a study based on point method on Morro da Polícia, where 189 species were surveyed. In another similar hill (Morro Santana), 165 species were recorded by plot survey in a grassland area that burns in intervals of three to five years (Overbeck et al. 2006).

The survey accomplished at Morro da Polícia by Boldrini et al. (1998) showed more exclusive species (71) and more overall species (171 fully identified species), probably due to a more exhausting sampling process (18 transects with 2829 points). The survey at Morro Santana (Overbeck et al. 2006) was carried out in 48 plots of 0.75 square meters, a sampling process similar to the one we have implemented at Morro do Osso, and resulted in 165 species (64 exclusive) identified to the specific level. Our survey, comprising the smaller total area among the three hills, showed 43 exclusive species. We consider that, among the exclusive species found in our survey, very few are actually exclusive, and their apparent exclusiveness could be simply related to different collection efforts, as the three compared areas are spatially and floristically close to each other.

Considering a broad study of Porto Alegre's hills region, 522 grassland species were pointed out in a survey on ten granitic hills, not including Morro do Osso (Aguiar et al. 1986). The occurrence of many concomitant species among the lists of these studies and ours indicates a rather continuous flora along the granitic hills. In addition, despite the species richness differences estimated by the similarity indexes for Morro Santana, Morro da Polícia and the present study, there is a very similar species composition among these areas, with an equivalent species-family distribution and many common dominant species. This point may corroborate Rambo's idea that these granitic hills were refuge areas during ocean transgression times (Rambo 1954), so that the extant grassland vegetation configures a remnant of different climate conditions (Behling 2002, Bredenkamp et al. 2002).

Presence of endemic and rare species in the study site is considerably high. Rambo (1954), in a classic paper concerning the southern Brazilian flora, emphasized plant endemism to be outstanding in Porto Alegre area, mostly comprising herbaceous species and genera. Although the endemic species list this author presents have greatly diminished at present due to growing collection effort and subsequent increase in the distribution area of many southern Brazilian species, some species deserve attention concerning their particular occurrence on granitic hills, such as Moritzia ciliata and Stipa filifolia. Other species, such as Dyckia choristaminea, have their distribution area limited to Porto Alegre region, whereas Baccharis riograndensis, Sellocharis paradoxa and Eugenia dimorpha have a broader distribution in the State of Rio Grande do Sul. Besides these, Heterothalamus psiadioides and Dyckia leptostachya have their distribution limited to the southernmost Brazilian state, whereas Eryngium horridum and E. ciliatum have the state as their northern distributional limit, also occurring on northern Uruguay and Argentina

Eleven species presented in the floristic list of this study are categorized as extinction-threatened by official governmental list (Sema 2003). Among them, eight are labeled as vulnerable and three as endangered. None of the endangered species (Dyckia choristaminea, Frailea gracillima and Schlechtendalia luzulifolia) and only four of the vulnerable ones (Gomphrena graminea, Mandevilla coccinea, Parodia ottonis and Waltheria douradinha) were accounted for in the structural survey, suggesting scarcity in the area. The other vulnerable species are Gochnatia cordata, G. orbiculata, Moritzia ciliata and Stenachaenium macrocephalum. Only one individual of M. ciliata was sampled, and very few were seen in subsequent expeditions. This evidence of possible population decrease is concerning, as this species is considered, farther than threatened, rare in Brazilian granitic grasslands. The scarcity of S. luzulifolia seen on the field is also concerning, as the distribution of this species has Porto Alegre city region as its northern limit (Mondin & Baptista 1996).

The absolute values of diversity and evenness indexes obtained for the presented community were supposedly higher (4.51 nats ind^-1 and 0.86, respectively) than for Morro da Polícia (4.01 nats ind^-1 and 0.76; Boldrini et al. 1998). Considering that species richness was lower in our area (161 for our study and 189 for Boldrini et al. 1998), the evenness in species performance at Morro do Osso survey should explain the higher Shannon diversity. Another important point is the recent burn undergone in part of the sampling area, which may have contributed to the high diversity index obtained for the community. Overbeck et al. (2005, 2006), working in Morro Santana, have shown that fire disturbance clearly leads to a short term increase in species richness and diversity (Denslow 1985, Harrison et al. 2003), but does not significantly affect overall community species composition. The maintenance of composition, alongside the short-term diversity peak and the regularly break of high competitors tussock species dominance, could explain the diversity and evenness values for the surveyed community, since fire in local area is a long term disturbance event. Forb species may be outcompeted by tussock grass dominance (Laterra et al. 2003, Overbeck et al. 2005). This may be first perceived only at above-ground vegetation, since many species can survive underground as bud-bank or reserve organs waiting for better opportunities, but forb submission may occur if disturbance intervals become long enough to suppress those organs (Rodríguez et al. 2003).

Although cluster and ordination analysis showed a distinct pattern in the studied grassland community, patterns among each clustering group were not uniform. East plots showed more similarity among themselves both in cluster and ordination analysis, suggesting a structural and floristic pattern, probably due to the survival of common fire-resilient species (Müller et al. 2007) and similar post-fire species recruitment (Pillar & Quadros 1997, Uys et al. 2004). On the other hand, west plots do not share a common trait like recent fire and, moreover, are distributed over a larger and more variable environment. Therefore, the pattern drawn by these plots along the second axis on the ordination diagram showed less spatial cohesion, probably illustrating other environmental differences among sampling plots.

Most floristic changes in disturbed areas are a consequence of covering area reduction of dominant species that play a key role on new species recruitment or changes in species performance and consequently in any community diversity modification (Connell 1978, Huston 1979, Olff & Ritchie 1998). Although the fire-driven dominancy reduction that occurred in part of our study area enabled the sprouting of many exclusive species (32) in the east plots, west grassland plots had much more exclusive species (50), possibly due to the removal of fire-vulnerable species in the first and to the higher number of plot units on the western grassland area (23 west plots; 16 east plots). Eryngium pristis, Lantana montevidensis, Trachypogon montufarii and Stipa filifolia are examples of high performance species present exclusively on the west plot units. Overbeck et al. (2005) pointed out that areas with an intermediate time without fire were the most rich of a comparison considering recently (one year) and long time (more than five years) burned places. As it is, the east overall grassland area might reach its species richness peak within few years after this study and our west group plots may be at an intermediate time without influence of fire, presenting more concentration of species per area.

Although fire had no effect on mean vegetation height of sampling units, it did affect floristic and structural composition between the two major areas analyzed. Open soil cover did not differ as well, but litter cover was higher in west plots, clearly as a consequence of burn effect on east grassland area.

Species composition and performance depicted differences between east and west areas. Eryngium horridum, Eragrostis polytricha, Spermacoce verticillata, Noticastrum decumbens, Schizachyrium microstachyum and Pterocaulon angustifolium, which were better represented on our recently burned area, are typically present in early successional stages and altered areas (Eggers & Porto 1994, Heringer & Jacques 2002). Quadros & Pillar (2001), working on a south Brazilian grassland, have also focused on floristic changes after fire and grazing disturbance. They have discussed that vegetation showed greater resilience to fire treatments, albeit remarkably differences were seen in initial observations.

The similarity we have found among the granitic hills, as well as the presence of rare and endemic species among them, denotes a historical continuous flora (at least on grasslands communities) along these formations, corroborating the shared natural history of the area. Besides that, the great diversity observed reflects the ecological importance of this site, as an example of great plant diversity of grassland formations. Nevertheless, further research including more granitic hills and a biogeographical approach are considered to be necessary in order to understand the Porto Alegre's granitic hill chain flora as a whole.

Fire disturbance, with the basic effect of diminishing dominance of some species, thus allowing the rising of others, was partly responsible for the differences in species composition although differences in overall diversity were not recordable. The accidental fire held at the study site showed more effect on performance of species, which is temporary, than on their presence or exclusion of the area. Considering the whole area, this contributes to the high diversity level found for the whole community.

Acknowledgements - The authors are grateful to the Secretaria do Meio Ambiente of Porto Alegre municipality, especially to MsC. Maria Carmen Sestren-Bastos, for the permission of conducting this study. The first author expresses his gratitude for the scholarship given by Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul.

(received: August 07, 2008; accepted: October 28, 2009)

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Publication Dates

• Publication in this collection
  07 May 2010
• Date of issue
  Mar 2010

History

• Received
  07 Aug 2008
• Accepted
  28 Oct 2009