Pedology and plant physiognomies in the cerrado, Brazil

This study was carried out in Paraopeba National Reserve. It aims to classify and evaluate the soil of the studied area and to verify the influence of soil attributes on vegetation by testing the following hypotheses: 1) under woodland physiognomies (Cerradão) the soil fertility is higher and the Al content lower; 2) open savanna occurs only in areas with high Al contents. For this purpose, representative soils in the Paraopeba National Reserve were mapped, identified, and samples from five profiles were analyzed. The environmental gradient was easily observed by principal components analyses, where the differences between the sites were highlighted. The Spearman correlation was used to verify the hypothesis. The correlation between vegetation (basal area, density, and richness) and soil (K, Ca2+, and Al3+) was statistically significant. The hypotheses were accepted, but hypothesis 1 only partially. Soil features seem to have an influence on the Cerrado phytophysiognomies and structure. Available phosphorous was an important factor for the maintenance of woodland Cerrado. Also, exchangeable Al3+ plays a major role in the establishment of different Cerrado physiognomies in Paraopeba National Reserve.


INTRODUCTION
The Cerrado vegetation is the major neotropical savanna and stretches over one and a half million square kilometers in tropical South America. Cerrado is limited in the north-northwest by the Amazon Forest, in the south and southeast by the Brazilian Atlantic Forest, in the Northeast by the Semi-Arid Caatinga vegetation, and in the west-northwest by the Pantanal wetlands. It stretches from equatorial zones to 23° south latitude (Motta et al. 2002). The Cerrado is usually classified in five physiognomies according to the proportion of trees and grasses. According to Coutinho (1978Coutinho ( , 2006 this biome consists of one grassland (campo limpo), three savannic vegetations (Campo Sujo, Campo Cerrado and Cerrado stricto sensu) and one woodland vegetation (Cerradão). 87-102 88 ANDREZA V. NERI et al.
Deeper Latosols in the Cerrado cover extensive plateaus and gentle slopes, where Fe oxides and gibbsite contents are higher than in wetter Brazilian regions such as the Amazon. However, Cerrado soils can be also very shallow, with nearly no A horizon and often with low permeability in more broken, undulating reliefs, where the underlying substrate generally consists of nutrient-poor pelitic rocks (Resende et al. 2002).
The physical characteristics of Latosols are generally good, with high aggregate stability due to their oxidic character. Clay aggregates are stabilized by high amounts of Fe oxide and gibbsite coupled with organic matter (OM) (Schaefer et al. 2004). This strong stability allows good water and air movement associated with low resistance to root penetration. The soil is also less prone to erosion, with stable aggregates (Motta et al. 2002).
In spite of the good physical characteristics of Latosols, the availability of plant nutrients is low, especially regarding P, Ca 2+ , K and micronutrients. Exchangeable Al 3+ concentration is high in many Latosols, being generally toxic to less tolerant plants. For native vegetation, the low concentration of certain nutrients such as Ca 2+ , P, S and N prevents the use of photosynthetic carbohydrates that accumulate in certain structures (Arens 1963). Due to the morphological properties determined by the edaphic attributes, this vegetation was classified as scleromorphous oligotrophic (Ferri 1977).
Latosols of Cerrado are generally well supplied with water, and groundwater is located few meters below the surface, whereas water deficiency is only observed in the upper two meters during the dry season. As in other savanna ecosystems, the occurrence of an indurated laterite (ironstone) layer is quite common at subsurface or surface level (Rizzini 1997), especially in the plateau borders.
The distribution of plant physiognomies is rather difficult to understand due to generally dystrophic soil properties with very low nutrient concentrations. However, within each community, plant species populations are consistently correlated with aluminum and calcium content, indicating a differentiated competitive potential , Haridasan et al. 1997, Oliveira-Filho 1989, Ratter 1971, Silva Junior et al. 1987.
A number of questions that involve soil and vegetation are quite complex and still unanswered. Some studies emphasize the influence of soil properties on the presence of a particular physiognomy on the one hand, and, on the other, the influence of the floristic composition on soil formation itself (Kellman 1979).
This study was conducted in the Paraopeba National Reserve and aims to classify and to evaluate the soils in the studied area, as well as verify the relationship between the soil and the vegetation. The relationship between soil and vegetation (physiognomy, floristic and structure) was evaluated by testing the following hypotheses: 1) under woodland physiognomies (Cerradão) the soil fertility is higher and the Al 3+ content lower; 2) open savanna occurs only in areas with higher Al 3+ content.

STUDY AREA
This study was conducted in Paraopeba National Reserve, in the state of Minas Gerais, Brazil. FLONA is a Sustainable Use Conservation Unit, according to the Brazilian National System of Conservation Units (SNUC 2000). The geographic coordinates of Paraopeba National Reserve are 19°16'S 44°23'W, and ranges from 734 m a.s.l. in the south to 750 m a.s.l. in the north (Fig. 1). The regional climate is humid tropical, with rainy summers and dry winters (Fig. 2).

VEGETATION SAMPLING AND ANALYSIS
A phytosociological study of the tree-shrub layer was performed employing the fixed area method  plots of 20 m x 100 m were allocated for the sampling, in which all the woody individuals with stem perimeter ≥ 10 cm at ground level were sampled. The plots were distributed in two different physiognomies (Cerrado Stricto Sensu and Cerradão). For statistical analysis the phytossociolocal parameters were calculated for five subplots (20 x 20 m) per site. These subplots represent subdivisions of the big plots (20 x 100 m). The study of the pedo-vegetational gradient was conducted using five environments with rather distinct characteristics: 1) Distrophic Cerradão on Red Latosol; 2) Mesotrophic Cerradão on Red Latosol; 3) Dense Cerrado senso stricto on Yellow Red Latosol; 4) Cerrado senso stricto on typical Yellow Latosol and 5) Cerrado senso stricto on Haplic Cambisol Tb Dystrophic.

SOIL SAMPLING AND ANALYSIS
For this study, five soil profiles were selected and opened (100 cm wide x 150 cm long x 150 cm deep) in different areas, in order to represent all types of 90 ANDREZA V. NERI et al.
Cerrado soils. The soils were classified according to the Brazilian System of Soil Classification (Embrapa 2006). For the chemical and physical analyses, soil samples were collected at every 10cm down to a depth of 30 cm, and from there downwards at every 20 cm to a depth of 150 cm, in total nine samples per profile. Color, detailed morphologic characteristics and total P contents were determined for samples from the layers 0-20 cm and 40-60 cm.
In addition to the five soil profiles, 25 subsurface soil samples (0-10 cm) were collected -five at each site.
The soil samples taken for chemical and physical analyses were air-dried and sieved to 2 mm. In the fraction < 2 mm, all physical and chemical soil properties were determined in the soil laboratory at the Universidade Federal de Viçosa. Soil color was determined by the Munsell color chart, and total P in clay was determined by sulphuric acid extraction.

SOIL AND VEGETATION GRADIENTS
The Spearman Rank Correlation Coefficient (rs) was used to verify the hypothesis that floristic-sociological parameters, such as basal area, species richness and density, in each of the phytophysiognomies are correlated with soil attributes (Al 3+ , Ca 2+ , P, K and the sum of bases). The 25 subsurface soil samples were used for this purpose.
The phytophysiognomies were subjectively ordered according to the density of their vegetation, in order to evaluate its correlation with soil variables, and assigned a number from one to four In addition to testing the hypotheses, a principal component analyses (PCA) was performed in order to gain more insight into the soil characteristics, using soil variables from the 25 subsurface soil samples: pH -active acidity; P -phosphorus; K -potassium; Ca 2+ -exchangeable calcium; Mg 2+ -exchangeable magnesium; SB -sum of bases; t -effective cation exchange capacity; T -cation exchange capacity for pH 7; V -base saturation; m -aluminum saturation; OM -organic matter; and P-rem -remaining phosphorus. The PCA was performed using PCORD (Mccune and Mefford 1997).

RESULTS
In the FLONA of Paraopeba 4,156 individuals were found altogether in the five sites. The richness was 132 species distributed in 47 families. Of the 132 sampled species, 14 were considered indifferent to the pedological conditions, since they were present in all the studied areas: Acosmium dasycarpum  Table I, showing clearly some fundamental differences between the different vegetations, for example regarding to diversity and basal area. A more detailed vegetation analysis can be found in Neri et al. (2012).
The phytophysiognomic gradient observed in Paraopeba National Reserve seems to be directly related to differences in the soil (Fig. 3). In the Mesotrophic Cerradão, where the highest soil fertility is found (Table II), taller plants showed a more closed canopy, associated with the absence of the graminoid layer. On the other hand, the Dystrophic Cerradão exhibited a more open canopy with sparse graminoid layer, associated with lower soil fertility (Table IV) (Tables VIII and X). The most remarkable difference between observations made on the two soils is the dominance of Miconia albicans (Sw.) Steud. in the savannic Cerrado developed on the shallower Cambisol. Another study site was on Red-Yellow Latosol (Table VI) in closed savanna (Cerrado), whose denomination refers to the presence of a higher tree density. In addition to higher tree density, trees are also taller than those in the Yellow Latosol and Cambisol, although both were classified as savanna (Cerrado).

RED LATOSOL UNDER MESOTROPHIC CERRADãO
Mesotrophic Cerradão is a woodland with canopy height varying from 8 to 13 m. Cyperaceae and Poaceae are absent in the understory, where the flora is different from the Dystrophic Cerradão. In the Mesotrophic Cerradão, the most abundant species are Luehea divaricate Mart., Magonia pubescens A. St.-Hill., Myracrodruon urundeuva Allemão and Dilodendron bipinnatum Radlk.
In the Mesotrophic Cerradão, where the vegetation shows a transitional feature between Tropical Dry Forests and savannic Cerrado, the content of available P at soil depth 0-20 cm was the highest of all the soil samples. Available P values steadily decreased down to 70 cm depth, at which the remaining P values were also very low, indicating gibbsite-rich and Fe-oxide mineralogy, with high P adsorption capacity. Available P values in this environment were higher than in the Dystrophic Cerradão on Red Latosol with taller vegetation.    The highest total P value for clay was found in the soil of Mesotrophic Cerradão at a depth of 0-20 cm, whereas in the 40-60 cm layer the value was lower than in Dystrophic Cerradão (Table II).
Organic matter contents were high at 0-10 cm depth, tending to be uniformly high down to 150 cm, indicating intense and deep pedobioturbation by micro and mesofauna (worms, ants and termites) RL pH Available P  (Table II). Ca 2+ contents are high only at the 0-20 cm depth, thereafter decreasing sharply with depth. This suggests that cycling in the surface layer is essential for plant growth, and that these soils do not have any Ca 2+ reserve in the mineral fraction. On the other hand, exchangeable Al 3+ saturation reaches over 85% of the CEC below 30 cm (Table II).
The soil color was 5YR 4/6 in the 0-20 cm layer, and 2.5YR 4/8 in 40-60 cm layer. The soil is clayey throughout the profile, with clay contents varying from 43 dag/kg in the 0-10 cm layer, to 66 dag/kg in the 50-70 cm layer. The high silt contents do not indicate primary minerals, but rather the presence of pseudo-silt, which represents the clay fraction that could not be dispersed by the texture analysis method used here (Table III)  The available P contents in the Dystrophic Cerradão were higher at 0-20 cm depth, decreasing drastically beyond 20 cm depth. Total P contents at both depths were high compared to the other The total P values were obtained at 0-20cm and 40-60cm depths. OM: organic matter; CEC: cation exchange capacity; BS: bases sum; P-rem: remaining P; m=% Al saturation; v: bases saturation. PEDOLOGY AND PLANT PHYSIOGNOMIES IN THE CERRADO soils, indicating greater P reserves (Table IV). The organic matter values were higher than in the Mesotrophic Cerradão, particularly at the 0-10 cm horizon. Similar to the Mesotrophic Cerradão, the CEC values were higher at 0-20 cm and decreased gradually downwards. However, at the same depth, the exchangeable Al 3+ content was higher. In the 10-20 cm layer, the Al 3+ content was also higher than the others, except in Yellow Latosol with savannic Cerrado (Table IV)    The soil of the Dystrophic Cerradão was characterized as Red Latosol with color ranging from 5YR 4/6 at 0-20 cm depth to 2.5YR 4/6 between 40-60 cm. The soil texture is very clayey throughout the profile, with clay contents varying from 74 dag/kg at 0-10 cm to 85 dag/kg 70-90 cm (Table V).

RED-YELLOW LATOSOL UNDER CLOSED SAVANNIC CERRADO
Closed savannic Cerrado is a type of savannic Cerrado with greater density of woody plants to the detriment of the graminoid layer. Platypodium elegans Vogel, Vochysia tucanorum Mart.,

Physical and morphological attributes of Red Latosol (RL) under Dystrophic Cerradão (woodland) in Paraopeba National Reserve.
Color and morphological traits were obtained at 0-20cm and 40-60cm depths.

RL
Coarse Although the Red-Yellow Latosol occurs under denser and taller Cerrado (savanna) vegetation, the available P content was the lowest of the sites, whereas horizontally the highest P content was found at 90-110 cm depth. In spite of the lower available P content in this soil, the total P was higher than in the savanna (Cerrado) on Yellow Latosol, indicating greater non-available P reserve (Table VI). The organic matter amounts were low, and followed the same trend of other soils in the area. Similarly, the CEC values were higher in the 0-30 cm layer, decreasing downwards, and reaching values lower than in other soils, indicating an oxidic minerology. The available Ca 2+ contents in this soil were low and concentrated at the 0-10 cm depth, with values higher than those in the Dystrophic Cerradão. Exchangeable Al 3+ was relatively uniform down to 30 cm, and dropped markedly downwards, due to gibbsite formations and Al 3+ insolubilization.
Color and morphological traits were obtained at 0-20cm and 40-60cm depths.
The available P contents in this environment were very low, even in the surface layer at 0-10 cm depth. The total P contents in this environment were the lowest, in the 0-20 cm layer as well as in the 40-60 cm one, indicating negligible P reserves (Table  VIII), compared with the Red-Yellow Latosol. The Al 3+ content in Cerrado (Savanna) on Yellow Latosol was high at 0-10 cm depth, and decreased down to 90 cm; from this depth downwards, Al 3+ contents increased gradually to 150 cm (Table VIII).
The soil color was 10YR 7/6 at 0-20 cm depth, and 10YR 7/8 at 40-60 cm. The texture class in this environment varied from clay-silt to very clayey (Table IX).

Chemical attributes of Yellow Latosol (YL) under savannic Cerrado in Paraopeba National Reserve.
Color and morphological traits were obtained at 0-20cm and 40-60cm depths. OM: organic matter; CEC: cation exchange capacity; BS: bases sum; P-rem: remaining P; m=% Al saturation; v: bases saturation. Color and morphological traits were obtained at 0-20cm and 40-60cm depths. PEDOLOGY AND PLANT PHYSIOGNOMIES IN THE CERRADO P values dropped markedly, illustrating the strong degree of weathering of these acid soils. Total P contents in the clay fraction were higher than in Yellow Latosol and in Red Yellow Latosol (Table  X). The organic matter contents were higher in the 0-20 cm horizon, as expected, with low and stable amounts from 20 down to 110 cm. This is thought to be evidence of strong pedobioturbation promoted by mesofauna in the Latosols under Cerrado (Schaefer 2001). Consistently, the highest CEC values were found in the 0-10 cm layer, owing to the effect of organic matter, and decreased with increasing depth. The soil was classified as Dystrophic Cambisol. Soil color in the 0-20 cm layer was 10YR 7/6, and 7.5YR 7/8 in 40-60 cm, with a clayey texture down to 70 cm, changing to silt clay at 70-90 cm. From 90 cm downwards we observed the weathered pellitic rock (slate) with increasing silt amounts, ranging from 60 to 74 dag/kg (Table XI) (Tables II, IV, VI, VIII and X). In general, soil and vegetation variables presented a very close correlation (Table XII). In particular, it is possible to highlight the correlation between available P and phytophysiognomies (rs = 0.89) and between available P and basal area (rs = 0.72). Density (number of individuals) and richness present a positive correlation to K, Ca 2+ and SB and a negative correlation to Al 3+ . The results of the PCA using the subsurface soil variables showed a high variance, and a clear separation into three groups (Figure 4). The first group correlated with high soil fertility, Mesotrophic Cerradão; the second group was correlated with organic matter (OM), Dystrophic Cerradão; and the third group correlated with aluminum, the two areas of Cerrado s.s. The percentage of variance accounted for by the first axis was 81% and the second axis was 13.6%.
These results partially contradict the first hypothesis, since Mesotrophic Cerradão appeared to be related to high soil fertility and low aluminum content, whereas Dystrophic Cerradão does not. However the open areas of Cerrado were correlated with high tenor of Al 3+ , confirming the second hypothesis.

DISCUSSION
The soils in all environments of Paraopeba National Reserve were found to be clayey and weathered. In comparison to other soils under Cerrado in Minas Gerais, the low percentage of sand in the soils differs remarkably from the pedoenvironments of medium-texture Dystrophic Red-Yellow Latosols studied in northwestern Minas Gerais, whereas the texture is similar to that of Dystrophic Dark Red Latosols (CETEC 1981).
Available P values found in the soils were low, compared to the values found in the Cerrado vegetation in Biological Reserve of Mogi Guaçu, in the state of São Paulo (Batista and Couto 1990), where values of 17.5 ppm at 0-20 cm and 17.0 ppm at 20-40 cm were found. Oliveira-Filho and Martins (1986) found values for Cerrado on quartzite in the Chapada dos Veadeiros, which are very similar to those found for the areas of Cerrado in Paraopeba National Reserve. The available P values in Paraopeba National Reserve are also lower than the less clayey Latosols of northwestern Minas Gerais (CETEC 1981).
Total CEC values were higher for the two woodland environments, the Mesotrophic and the Dystrophic Cerradão. Comparable values were observed in Cerrado environments on Dystrophic Red-Yellow Latosol and three areas of Dystrophic Dark Red Latosol in northwestern Minas Gerais (CETEC 1981). In other soils studied by CETEC (1981), lower CEC value, associated with yellowish and red-yellowish color, were found. CEC values in the soil under Cerradão in FLONA were closely related to the Cerrado soils studied by Sans et al. (1979), Batista and Couto (1990) and Souza (2004).
The exchangeable Ca 2+ concentrations were very low, with values typically < 0.20 cmol c /dm 3 , in the studied areas, except for the Mesotrophic Cerradão. According to CETEC (1981), Ca 2+ contents in soils under Cerrado were higher in NW Minas Gerais than in the Cerrado of FLONA in Paraopeba National Reserve.
Al 3+ appears to be an important attribute in Cerrado ecosystems, since high concentrations seem to favor some species to the detriment of others (Neri et al. 2012, Furley and. Compared to Cerrado areas elsewhere, a clear difference was observed from Goodland (1969) in Triângulo Mineiro. Goodland found lower values (means varying from 0.71 to 0.76 cmol c /dm 3 ) than were found in Paraopeba National Reserve. These differences are attributable to the clay/silt rich parent material (slate) with high natural Al 3+ content found in Paraopeba National Reserve, unlike soils in Triângulo Mineiro, derived from Al-poor sandstones or basalts, or of northwestern Minas Gerais, derived from sandstones and sandy-clayey sediments.
Ca 2+ , Al 3+ , P and pH contents appear to influence the physiognomy and structure of the vegetation. High Al 3+ concentrations in the soils of Dystrophic Cerradão and Cerrado on Cambisol favored the development of species such as M. albicans. In the Mesotrophic Cerradão, where the Al 3+ concentration was nearly zero in the surface layer (0-20 cm), this species was not found at all. On the other hand, several species such as L. divaricata, D. bipinatum, Tabebuia roseoalba (Ridl.) Sanddwith, Guettarda viburnoides Cham. & Schltdl. were found in richer soils, and they were not detected in the other environments. PEDOLOGY AND PLANT PHYSIOGNOMIES IN THE CERRADO

ACKNOWLEDGMENTS
The authors would like to thank to CNPq and CAPES for support and grants. The authors thank the Paraopeba National Reserve Office -from ICMBio-MMA/Brazilian Environmental Ministry by authorizations for this study and by logistic support.