Vascular flora of semi-arid region, São José do Piauí, state of Piauí, Brazil

: The Caatinga biome is located in the semi-arid region of northeastern Brazil and covers about 37 % of Piauí state. The main objective of the present study was a characterization of the Caatinga flora of the farm of Morro do Baixio, in state of Piauí, Brazil (06°51’13” S; 41°28’15” W, at 400 to 540 m above sea level) in view of the fact that very few such surveys were conducted in the state. The flora of the farm was surveyed monthly, during a year, to gather herbs, epiphytes, parasites, sub shrubs, shrubs and trees. We encountered 136 species belonging to 46 families, including a new species of Bauhinia . The richest families were Caesalpiniaceae (15 spp.), Fabaceae (11 spp.), Bignoniaceae and Mimosaceae (both with nine spp.). We observed a higher frequency of typical species from sedimentary Caatinga. However, local conditions favor the appearance of species that occur in Carrasco and Cerrado.


Introduction
The semi-arid of Brazil extends over 800,000 km 2 , approximately 10 % of the national territory within the states of Piauí, Ceará, Rio Grande do Norte, Paraíba, Pernambuco, Alagoas, Sergipe, Bahia and Northern Minas Gerais, delimited by the medium isoieta of 800 mm (Ab'Sáber 1974;Hueck 1972). This region presents heavy rains in some years, prolonged drought periods in others, with irregular and concentrated in a few months, higher evapotranspiration rates and low infiltration capacity of soils (Ab'Sáber 1974;Reis 1976;Kampen 1979).
The Caatinga is the dominant vegetation of the semiarid region (Luetzelburg 1923;Engler 1951) which has been suffering severe environmental degradation in recent decades, mainly as a consequence of rural growth and expansion of agriculture and cattle grazing. Among different types of Caatingas, those located in sedimentary areas have been the focus of very few studies until recently.
The state of Piauí accounts for 37 % of the area of Caatinga biome and according to Sampaio (2002) 118 municipalities within the state are included in the semiarid domain. The lack of such information has motivated the inclusion of sites such as the microrregion of Picos in Piauí as priority area for the conservation of the Caatinga biome (Silva et al. 2004).
In the present study, we aimed provide a check list the vascular flora of the municipality of São José do Piauí, a priority area for conservation, identifying the species which occur in the region in crystalline and sedimentary formations and determining whether the life-forms differ from the normal spectrum of Raunkiaer's system.
The vascular flora was surveyed monthly, during a year in a 2 ha area, to collect herbs, epiphytes, parasites, lianas, sub shrubs, shrubs and trees, throughout the study area. All specimens collected were identified and

Abstract:
The Caatinga biome is located in the semi-arid region of northeastern Brazil and covers about 37 % of Piauí state. The main objective of the present study was a characterization of the Caatinga flora of the farm of Morro do Baixio, in state of Piauí, Brazil (06°51'13" S; 41°28'15" W, at 400 to 540 m above sea level) in view of the fact that very few such surveys were conducted in the state. The flora of the farm was surveyed monthly, during a year, to gather herbs, epiphytes, parasites, sub shrubs, shrubs and trees. We encountered 136 species belonging to 46 families, including a new species of Bauhinia. The richest families were Caesalpiniaceae (15 spp.), Fabaceae (11 spp.), Bignoniaceae and Mimosaceae (both with nine spp.). We observed a higher frequency of typical species from sedimentary Caatinga. However, local conditions favor the appearance of species that occur in Carrasco and Cerrado.
Species were classified as phanerophytes, camaephytes, hemicryptophytes, geophytes, therophytes, lianas, epiphytes and parasites, according Raunkiaer (1934), adaptated by Mueller-Dombois and Ellenberg (1974), to compare the Caatinga life-form spectrum with Raunkiaer's normal spectrum. This classification is based on the meristematic tissue, which remains inactive to growth during unfavorable season (as dry summer or winter), and therefore the location of this tissue is an essential feature of plant's adaptation to climate (Whittaker 1975). To verify if the life-form spectrum shows significant differences to Raunkiaer's normal spectrum, we used a χ 2 test (Zar 1999). For this comparison, lianas were included like phanerophytes, and epiphytes and woody parasites excluded from the statistical analysis.
A comparison of the flora encountered in the presented study with reports from the literature for crystalline and sedimentary formations revealed 33 species (27.96 %) occurring only in the study area, while 85 species (72.03 %) were cited in at least one of the earlier reports. Generally, the more frequent families encountered in this study were representative of the crystalline and sedimentary formations in the semi-arid domain, except for Malpighiaceae. Euphorbiaceae, Mimosaceae, Caesalpinaceae and Cactaceae could be found in crystalline areas (Rodal et al. 2008;Araújo et al. 1995;Ferraz et al. 1998;Alcoforado-Filho et al. 2003). In addition to these species, Bignoniaceae, Fabaceae and Myrtaceae could be found in the sedimentary areas, but not Cactaceae (Araújo et al. 1998;Araújo and Martins 1999;Lemos 2004). Rodal et al. (2008) reported that Euphorbiaceae, Cactaceae and Caesalpiniaceae were the families with the largest number of species in Caatinga. Lemos and Rodal (2002), studying a deciduous thorny vegetation in the state of Piauí, found that, except for Bignoniaceae and Myrtaceae, there was no distinction between families with the largest number of species in crystalline and sedimentary formations. However, analyzing the species distribution of these families, we found that there were differences among crystalline and sedimentary formations. The higher proportion of species in common (32.2 % with 38 species) occurred in Caatinga sedimentary formations (Rodal et al. 2008;Rodal et al. 1999;Figueiredo et al. 2000;Lemos 2004), followed by Carrasco (31.35 % with 37 species) (Araújo et al. 1998;Araújo and Martins 1999), and crystalline formations (29.81 % with 34 species) (Rodal et al 2008;Araújo et al. 1995;Ferraz et al. 1998;Alcoforado-Filho et al. 2003;Lemos 2004, Araújo et al. 2005Rodal et al. 2005;Costa et al. 2007).  Taylor and Zappi (2002) affirm that these species, despite being predominant, also occur in other vegetation types. Aspidosperma pyrifolium also occurred in Cerrado and Commiphora leptophloeos in Carrasco and Cerrado. Bauhinia cheilantha is cited by many authors (Ferraz et al. 1998;Lemos and Rodal 2002) as occurring in crystalline and sedimentary formations.
It is important to note that Raunkiaer's normal spectrum was created for world flora and take into account Mendes and Castro |Vascular flora of semi-arid region, São José do Piauí, Brazil homogeneous climatic conditions (Cain 1950). The χ 2 test showed significant differences of São José flora from the normal spectrum. Phanerophytes and hemicryptophytes were already cited as the main life-forms of Cerrado (Batalha and Martins 2004).
Studies in semi-arid regions of northeastern Brazil are scarce except for recent studies carried out in Ceará and Pernambuco states (Araújo et al. 2005;Rodal et al. 2005;Costa et al. 2007). Therophytes are expected to register higher proportions in high temperature and low precipitation areas, characterizing the life-form spectrum of arid and semi-arid regions (Raunkiaer 1934;Araújo et al. 2005 et al. 2005). This probably must be associated with the precipitation and altitudinal conditions of the study area, as well as the smaller number of species in the herbaceous/sub shrub layer when compared to shrub/woodland layer (99 species, including the woody lianas), a common pattern of sedimentary formations (Rodal et al. 1999;Figueiredo et al. 2000;Araújo et al. 2005). The flora of Morro do Baixio was composed by a high frequency of typical species of sedimentary Caatinga. However, the geoenvironment of the São José municipality within the "cuesta" of Serra Grande (Rivas 1996) is characterized as a region of Cerrado/Caatinga/Carrasco transition, determining the appearance of species that occur in these formations.