Influence of Soil Type and Rocky Outcrops on the Species Distribution in a Woody Plant Community at Brazilian Semiarid

Cerrado is a biome that holds many phytophysiognomies, influenced by the edaphic factors, where the type of substrate is strongly related to the established vegetation. This study aims to verify on how soil physicochemical properties, as well as the presence of rocky outcrops, influence the species distribution of woody plants in a Cerrado fragment located in deep soils and tabular relief at Serra do Boqueirão (289 m elev.) Lavras da Mangabeira municipality (6o72′24′′ S; 38o97′73′′ W), Northeastern Brazil. Every individual with a DNS ≥ 3 cm in 12 sampling units with an area of 12 × 30 m (0.432 ha) was sampled. PAST v. 3.23 software was used in the Main Component analyses. The physical and chemical analyses of the soil were made with samples collected at 0-20 cm and 20-40 cm depths. Differences in diversity, hierarchy and dominance volume of woody plant populations were found between sites with or without the presence of rocky outcrops. The soil has a tendency for water erosion and the aluminum saturation reached values of concentration of 7.9 cmolc dm, with a pH value between 4.1-4.2 in rocky environments. The obtained results suggest that Q. parviflora and A. occidentale probably have some kind of resistance to the toxicity of aluminum in soil with a pH value below 5.0.

of subtypes, as landscapes with typic dense and thin vegetation over deep soils and rocky landscapes with shallow soils and rocky outcrops conditioning the presence of bushes and arboreal vegetation (Ribeiro & Walter, 2008;Mota et al., 2014).
Some of the main influencing factors on the vegetation types of Cerrado are largely related to the edaphic conditions, especially the following: water depth, drainage, actual soil profile depth and texture, as well as the percentage of exposed rocks (Haridasan, 2000;Abreu et al., 2012). These factors influence Cerrado phytophysiognomies as much as anthropic action does (Pivello & Coutinho, 1996;Neri et al., 2007).
The Cerrado soil chemical composition is mostly dystrophic, with low pH, low available calcium and magnesium concentrations and high exchangeable aluminum concentration (Lopes & Cox, 1977;Neri et al., 2007). Depending on concentration level, some metals, such as aluminum, cause soil toxicity, influencing the growth of some native plant species and acting as one of the limiting factors for the growth of some plant species of the Cerrado dominion (Balbino-Miguel et al., 2010). Abreu et al. (2012) suggest that comparative studies on woody vegetation and rocky outcrops in typical Cerrado sites can aid on the evaluation of the effects of edaphic conditions in their floristic and structural compositions.
For the execution of biodiversity conservation projects and sustainable managing plans, the knowledge of the vegetation, its limitations and resilience on the area of interest is necessary (Ferraz et al., 2013). Castro (1994) warned about the lack of knowledge on the floristic heritage of Cerrado, which can negatively influence the supply of genetic variability for the future technological generations, and the high phytodiversity and compartimentalized architecture of the Cerrado biome difficults extrapolation of the results achieved by quantitative floristic surveys.
The few Cerrado regions on Southern Ceará state are restrict to enclaves of small sedimentary reliefs, first identified by Figueiredo and Fernandes (1987), located at Lavras de Mangabeira, Aurora, Granjeiro, Várzea Alegre, Farias Brito, Cedro and Jucás municipalities and over the Chapada do Araripe (Figueiredo, 1997;Moro et al., 2011;Nepomuceno et al., 2016). Little is known on the diversity of fauna/flora, structure and conservation status of those Cerrado fragments.
In face of the demand for scientific knowledge on this biome and looking forward to support recover and preservation actions on it, this study aims to analyze on how the physicochemical composition of the soil, as well as the presence of rocky outcrops influence the patterns of floristic distribution and phytophysiognomic structure of woody species in that fragment of Cerrado strictu senso located in the crystalline Caatinga, Brazilian Semiarid.

Area of Study
Serra do Boqueirão is located at the municipality of Lavras da Mangabeira (6º72′24″ S; 38º97′73″ W) (Figure 1), at an elevation from 282 to 401 m above sea level. This municipality belongs to the semiarid portion of Northeastern Brazil, officialy regionalized as Lavras da Mangabeira microregion and Southern-Central Ceará mesoregion (IBGE, 2010). The local site on study lies near the Lavras da Mangabeira sedimentary basin, a set of three small basins covering circa 60.27 km 2 in a private property, surprisingly well-preserved, without any agropecuary activity. The climate is defined as Warm Tropical Semiarid (Aw), according to the Köppen classification, with two well-defined seasons (dry winters and humid summers), despite the transitory nature of the semiarid climate on Northeastern Brazil (BSh). The average annual pluviosity is 908.9 mm (FUNCEME, 2019), the rain season being from January to April. The average annual temperature is 26.8 ºC (INPE, 2019 In the soil 0-20 and Semiárido To evaluat Principal C an indirec Órloci (19 distributio each envir name of c main (Filfi

Divers
In the pre (without ro sampling u The phyto families. In Note. Area I = Environment without rocky outcrops; Area II = Environment with rocky outcrops; AD = Absolute dominance e Aus. = Absent species for this environment.

Physical/Chemical Aspects of the Soil
The kind of substrate in which the plant community grows, according to the classification of IPECE (2019) for the 1 st cathegoric level are variations of Neosoil and Luvisoils. Granulometric tests, according to textural classification triangle (Lemos & Santos, 1984) (Figure 3) demonstrate a sandy soil with variations between the classes "franco-arenoso" and "areia franca" for environment I and just "areia franca" for environment II. jas.ccsenet.

Figure 3:
Note. a = s environme without ro The result reached at 53.82 to 54        Vol. 12, No. 5; Xavantina, Mato Grosso state (Mid-west Brazil). Lemos et al. (2013), in similar studies in a Cerrado sensu stricto area in Tocantins state (Mid-west Brazil) suggest that A. occidentale and Q. parviflora seem not to be influenced by different kinds of substrates. Ratter et al. (2003) point that taxa of the genus Qualea are well-adapted to different substrates, and thus are widely distributed by the Cerrado biome.
Those facts do not corroborate this study, for as demonstrated, A. occidentale and Q. parviflora significantly lose their representativeness in number of individuals in environments where rocky outcrops are absent, reinforcing the hypothesis that the presence of rocky outcrops is not the only determinant factor that may be influencing the distribution and competitiveness dynamics of the studied plant communities.
There were little variations in soil density between the two environments. Environment I demonstrated a slightly higher density in comparison to environment II, but the density value did not surpass 1.39 Kg/dm 3 . Skopp (2002) and Donagemma et al. (2016) understand that sandy soil density varies from 1.4 to 1.9 g cm -3 , what reflects the occurrence of a greater clustering in sandy materials; in this case, the value of 1.85 g cm -3 is critical for root development.
Converting the maximum value in this study to the values evaluated by Skopp (2002) and Donagemma et al. (2016), the maximum of 1.3 g cm -3 was obtained, whic suggests that soil density is not a limitant factor to the development of the woody species of Serra do Boqueirão. Donagemma et al. (2016) confirm that Quartzarenics Neossolos stand out in the Brazilian System of Soil Classification (SiBCS), having sandy texture in all horizons, reaching 1.50m from the surface or reaching the lithic bed, covering ca. 15% of the Cerrado area in Brazil. Tognon (1991) defends that soil porosity interferes in aeration, water conduction and retention, resistance to penetration and ramification of roots, and consequently in the utilization of available water and nutrients. Pessoa-de-Souza et al. (2015), in studies conducted with Quartzarenics Neossolos corroborate the obtained results, where the total porosity volume did not reach 50%, and by the way evidenced a kind of orthic Neossolo Quartzarênico, for not offering resistance to water percolation, thus allowing a higher physical-hydric dynamics of the terrain, which also contributes to a susceptibility to water erosion due to the high level of porosity. Balbino-Miguel et al. (2010) declared that in soils with pH below 5.0, the high concentration of available aluminum (Al) is one of the factors that cause major toxicity problems, which is a limiting factor to plant growth, since the presence of Al reduces root growth and development, as well as decreases nutrient absorption. Haridasan (2000), Salvador et al. (2000), Echart and CavallI-Molina (2001), and Gomes et al. (2011) corroborated these same trends and help us explain the loss of richness and dominance volume in the environment II. Abreu et al. (2012) confirm a high aluminum saturation associated with environment with characteristics of sandy soils and observe that the physico-chemical analyses of the soil better explain the structural and floristic differences of the vegetation.
However, Q. parviflora and A. occidentale do not seem to be limited in relation to the aluminum concentration in the mentioned environment, indicating a type of tolerance to this element, judging by the fact that the PCA in consonance with the absolute dominance values, demonstrated that these species have a larger domain established in substrates with high aluminum saturation and acid pH. Haridasan (2000) points out that several species of family Vochysiaceae are aluminum accumulators a trait which, therefore, gives them an adaptive advantage in environments with the presence of exchangeable aluminum. Mota et al. (2014), following the same trend of the results obtained in this study, confirmed this hypothesis and indicated that Vochysiaceae and Fabaceae are the most representative families of the Brazilian Cerrado and that, nevertheless Fabaceae, despite having the nitrogen fixation capacity as one of its adaptive advantages for this domain, seems to be more influenced by rocky outcrops than Vochysiaceae.
The resistance of A. occidentale found in the present study may be a reflection of gene mutations, which conferred adaptive success through the limiting factors of the environment. Echart and Cavalli-Molina (2001) explain that Al tolerance can be controlled in different ways, from a single dominant gene to a complex one with additive effect genes acting on different biochemical pathways. In genetic studies conducted by EMBRAPA (2018) in A. occidentale seedlings, aluminum tolerance was pointed out in two distinct genotypes (CCP 06 and BRS 275), in which no symptoms of toxicity were present in presence of this element within 60 days of application. Neri et al. (2007) confirm that most of Cerrado soil has acid pH and low concentration of available calcium and magnesium, with high concentration of exchangeable aluminum, which corroborates the characteristics of most of the soil of the studied area. It suggests that the chemical attributes of the soil associated with high all concentration may be the decisive factors for the establishment of common Cerrado species in the middle of Caatinga of Serra do Boqueirão, since Cerrado species are more adapted to inhabit substrates under these conditions.

Conclusions
The species Anacardium occidentale, Hymenaea stigonocarpa and Qualea parviflora were dominant in the Cerrado environment under study.
There were differences in diversity, hierarchy and dominance of arboreous plant populations between environments with and without the presence of rocky outcrops.
The physical characteristics of the soil suggest that the Cerrado fragment is susceptible to water erosion due to the high level of porosity.
Aluminum in consonance with the rocky outcrops, act as limiting abiotic factor for the establishment and development of woody species in the Cerrado fragment. Q. parviflora and A. occidentale have some kind of tolerance to aluminum toxicity in environments with pH below 5.0.