THE WOODY FLORA OF CERRADO VEGETATION IN THE STATE OF PIAUI, NORTHEASTERN BRAZIL

Cerrado vegetation covers about 33% of the total area of the state of Piaui, northeastern Brazil, where there are also large areas of transition and contact with other vegetation types. Although the Piaui cerrados are a direct northern prolongation of the central core area, they are considered marginal by almost all authors. There are few previous records of their woody flora. This paper presents a floristic list of woody species based on the survey of 11 localities and updating of two already published lists. The climate belongs to Thornthwaite's subhumid category due to Piaui's position between the semiarid northeastern domain and the superhumid Amazon. The annual total rainfall is similar to that of the major part of the Brazilian cerrado area, but the rainy season is shorter. The soils differ from most cerrado soils in containing a greater proportion of concretions and plinthite with generally lighter colours, which may be interpreted as indicating large fluctuations of the water-table during pedogenesis. There are also widespread indications of seasonal high water-table showing that many of the Piaui cerrados probably belong to Sarmiento's hyperseasonal savannah category. A total of 60 families is represented by 307 woody taxa (including 14 family indet., four genera indet. and 80 species indet.). The lowest floristic diversity (richness) occurred in pure cerrado, the greatest in transition and contact areas.


INTRODUCTION
The state of Piaui is located in the northeastern region of Brazil ( Fig. 1) and has a total area of 25,093,400ha of which about 33. 3% (8,349,759ha) is covered by cerrado vegetation sensu lato, and 14.0% (3,507,107ha) represents transition and contact areas of cerrado with other types of vegetation: caatinga (dry thorn woodland and shrubland), carrasco (dry cerrado-caatinga transition), dry deciduous forest, seasonal subdeciduous forest, Orbignya palm forest, riparian forest and grasslands (CEPRO, 1992). Cerrado constitutes the second most extensive type of vegetation in the state; its area is only exceeded by that of the caatinga.
Although the cerrado area in the state of Piaui is a direct continuation of that of the core area on the Central Plateau of Brazil, it is generally considered marginal (e.g. Rizzini, 1963) since it is located at the northern extreme of the distribution area of the biome. There is little information available about its flora: only five relevant papers have so far been published, of which two (Goergen, 1983;Jenrich, 1989) did not have floristic survey as their main goal. Of the others, Rizzini (1976) produced a preliminary list of 61 species of angiosperms based on a single field survey and an examination of herbarium material, but Barroso & Guimaraes (1980) and Castro (1984) reported intensive field surveys. The present paper communicates the results of surveys of some representative areas in the state.

MATERIALS AND METHODS
Eleven different localities (Iocs. 102 and 121-130, Fig. 1) were surveyed by means of the plot method (Mueller-Dombois & Ellenberg, 1974). The vegetation of all localities fell into the physiognomic class of cerrado sensu stricto (Coutinho, 1978); many were sited in regions of vegetation transition (principally between cerrado and deciduous forest or cerrado and caatinga).
In one locality (loc. 102) the area was surveyed by a systematic grid consisting of thirty 10 x 20m plots, 50m apart, and all woody plants with at least one trunk with diameter at ground level >3cm were recorded. All other localities (see Table 1 and caption of Fig. 1 for details) were sampled by single 20 x 500m (lha) plots on which all woody plants with at least one trunk of > 5cm DBH were recorded.
The species records of Barroso & Guimaraes (1980) from the Sete Cidades National Park (loc. 69, Fig. 1) and Castro (1984) from Urucui-Una Ecological Station (loc. 95) are also included to make the list as complete as possible. The identifications in the latter work have been updated. These authors used the 'preferential collecting' method (Castro, 1994), i.e., a general reconnaissance and collecting, or wide-patrolling or walking-and-gathering method, without setting any specific area or plant size limits. Arboreal and shrubby species have been selected from their lists, which also included species from the lower layers of vegetation.

PHYSICAL ENVIRONMENT
All localities have a fair degree of climatic similarity with average annual temperature between 26.3°C and 27.0°C, precipitation between 1,217mm and 1,709mm, and annual average hydric deficiency between 365mm and 560mm. They all fall into Thomthwaite's dry subhumid climatic type with the exception of locality 126 which just falls into his humid subhumid category (Table 1). The prevalence of subhumid climatic types in the localities studied is a consequence of the geographic position of the state of Piaui. They indicate the transition between the two great groups present in northern and northeastern Brazil, the Wet and Dry climates of Thornthwaite (1948). This transitional climatic character is expected, since the state is under both the northeastern influence, with its semiarid climate, and the Amazonian influence, with its superhumid climate (Azevedo & Caser, 1980). The yearly total precipitation is similar to that of the bulk of the cerrado area in Brazil, and the climate shows a marked hydric seasonality, as it is highly megathermic all the year round and the rains are concentrated in a few months (December to April).
The soil mapping units to which each locality belongs are given in Table 1 and are described in Table 2. Most of the soils show evidence of hydric seasonality in the presence of concretions, plinthite, or light grey or yellow colours (Tables 1 and  2). Concretions and plinthite result most commonly from the interaction between iron, or iron and aluminium, and water in conditions of alternating soaking and drying, as can happen when the water-table shows great variation. Greyish colours may be produced by anaerobic conditions due to the elevation of the water-table, while yellow colours may indicate an excessive humidity with formation of goethite (when iron is present), but may also indicate low iron content and prevalence of kaolinite (Oliveira et al, 1992). The presence of Copernicia and Orbignya palms also indicates the influence of the soil water-table (Rizzini et al, 1988). The prevalence of soils showing the presence of a seasonally high water-table indicates that these Piauian cerrados may be classified closer to hyperseasonal savannahs than to purely   (Sarmiento, 1984). Soils with light colours are not very frequent under cerrado vegetation, where predominantly reddish colours indicative of well-drained soils generally occur . Another important feature is that cerrado is a characteristic vegetation of calcium-poor dystrophic soils, while the presence of deciduous and subdeciduous forest and both Orbignya and Copernicia palms is linked to mesotrophic soils with higher calcium levels. Table 1 gives the number of species and individuals, the co-ordinates, altitude and soil type for each locality sampled. It shows that the three localities surveyed by nonstandardized methods showed the highest number of species. Localities 95 and 69 surveyed by the 'preferential collecting' method without reference to limited area or plant size qualification had 128 and 116 species respectively. Locality 102, where the size qualification used was trunk only >3cm diam. at ground level, contained no less than 2163 individuals of 84 species in 0.6ha. The other 10 areas were all surveyed by the same method (> 5cm DBH on lha plots of the same shape) and are thus directly comparable. In these 10 areas the climate is very similar but there seems to be a relation of species diversity (richness) to soil type. It was greatest (loc. 121, 123, 129) where Plinthosols prevail in the mapping units and least (loc. 124, 126, 127, 128) where Latosols and Quartz Sands predominate, with intermediate levels (loc. 122, 125, 130) where Litholic Soils preponderate. The number of species S was roughly proportional to the absolute density N (Table 1). Therefore, the soil mapping units may influence both the density and the species richness of the vegetation. In the present study the greatest species diversity was found in plots occurring in the Campo Maior complex (a very heterogeneous environment) and in areas of transition to subdeciduous forest, that is in the transition to better (more mesotrophic) soils. The least diversity was found in plots occurring in pure physiognomies of subdeciduous cerrado. Intermediate diversity was found in areas of transition to deciduous forest and to caatinga, that is in the transition to more xeric vegetation types.

RESULTS AND DISCUSSION
The values of N in Table 1 indicate that the absolute density of thick-stemmed woody plants is not as high as that found in typical cerrado sensu stricto in Central Brazil (e.g. , but is closer to that occurring on waterlogged or concretionary and rocky soils.
Of the 307 species encountered in the surveys only 22 occurred in 50% or more of the localities, and only Byrsonima crassifolia, Qualea grandiflora, Q. parviflora, Stryphnodendron coriaceum and Vatairea macrocarpa were present in all 13 surveys, thus indicating a great spatial heterogeneity of species composition in the cerrado vegetation in Piaui. This accords well with the high levels of heterogeneity recorded by Ratter & Dargie (1992) in their survey of 26 areas of the cerrado biome throughout Brazil. Table 3 lists the species and attempts to indicate their geographical distribution where sufficient information is available. Information on geographical distribution can be obtained for only about 60% of the total species listed in Table 3, and is available in a very uneven fashion. Yet, although very speculative, some interpretation can be made of the known geographical distribution. Rizzini (1976) and Heringer et al. (1977) claimed that the cerrados of the states of Maranhao and Piaui have a very different woody flora from the central core area, the two areas having only about 50% of their species in common, and, for this reason, should be considered as a separate floristic or phytogeographic unit. Fernandes & Bezerra (1990) designated the vegetation of these states as the Parnaiba Basin Sector of the Cerrado Province, thus stressing its individuality. Castro (1994) proposed the existence of eight diversity supercentres for Brazilian cerrados, one of them in the northeastern region of the cerrado biome, providing evidence based on the multivariate numerical analyses of many surveys. However, Table 3 shows that many species have wide geographic distribution in neotropical savannahs (Sarmiento, 1983). Some of these species (marked by 'W in Table 3 and representing about 17% of the species listed) have areas that extend far to the south and north. Others (marked by 'S' in Table 3; about 22% of the species listed) have a large area of distribution to the south (Leitao Filho, 1992), with the cerrados of Piaui close to their northern limit. Yet others (marked by 'N'; about 6%) extend further to the north, the Piauian cerrados representing roughly their southern limit. Some species (marked 'C; about 16%) are elements from the shrub-tree caatingas (Rodal, 1992(Rodal, , 1994, while others (marked 'A'; about 5%) also occur in Amazon terra firme (noninundated) forests. Species marked 'R' (about 8%) have their distribution restricted to the cerrados of Piaui and Maranhao. Finally, calcicolous species are indicated by 'Ca' (about 4%) since this soil preference is know to be important in determining species occurrence in cerrado habitats (Ratter et al, 1977(Ratter et al, , 1978. The percentages given above add up to more than the c.60% of species for which information of geographic distribution is available since some species occur in more than one distribution category. Our results show that the Piauian cerrados have some distinct characteristics of their woody flora, not found elsewhere, thus supporting, at least partially, the ideas of Fernandes & Bezerra (1990), Heringer et al. (1977) and Rizzini (1976). The differences can even be observed at family level, e.g. Bombacaceae and Proteaceae, two important families in other areas that have not been found in our surveys in  Cronquist (1988). Code letters: W, wide distribution extending far north and south; S, wide distribution to the south; N, wide distribution to the north; C, also found in shrub-tree caatinga; A, also found in terra firme (non-inundated) Amazonian forest; R, restricted distribution; Ca, calcicolous species. (Available information on distribution is often incomplete so that future research may demonstrate that, for instance, an S may prove to be a W). Total: 60 families; at least 159 genera; 307 taxa.        Piaui. However, our research on the geographical distribution of species does not support the high degree of species endemism claimed by Rizzini (1976) and Heringer et al. (1977). Such claims must be considered against the background of extremely high floristic heterogeneity throughout the cerrado region: Castro (1994) found that about 40% of all woody species occurred at only a single locality out of the total of 145 which he compared. In fact, the number of probably endemic woody species in the Piauian cerrados is no greater than that found by Castro (1994) in other areas of the Brazilian cerrados. This, together with the high proportion of species with wide geographic distribution, suggests that the Piauian cerrados were not isolated from other cerrados areas for any long period during evolution.