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Part 4 : The semi-arid north-east
White sand soils in north-east Brazil
Changing aspects of droughtdeciduous vegetation
in the semiarid region of north-east Brazil
Characteristics and utilization of tree species
in the semi-arid woodland of north-east Brazil
Drought, irrigation, and changes in the sertão
of north-east Brazil
White sand soils in north-east Brazil
1
Introduction
2
Site characteristics
3 Distribution of the white sand soils in the
Paraíba-Pernambuco area
4 White sand on the Conde upland, Paraíba
5
Origin of upland white sand
6
The effect of deforestation
References
Eiji Matsumoto
White sand soils are amongst the most infertile of the generally infertile tropical soils. They are composed mainly of quartz sand, and support a distinctive vegetation. This, varying from open savanna to closed forest, is characterized, by pronounced sclerophylly, low diversity, and high endemism. Forests on white sand soils are reported from several humid tropical regions of the world and are variously designated: Amazon caatinga or campinarana in Amazonia (Anderson, 1981); wallaba forest or muri-bush in Guyana, and heath forest, kerangas or padang, in Borneo.
As shown in figure 11.1, extensive areas of white sand soils and related vegetation (white sand formations) are present in Amazonia, occurring in the Rio Negro basin, in Serra do Cachimbo on the Pará-Mato Grosso boundary, on the Chapada dos Parecis in Rondônia and along the Atlantic coast near the mouth of the Amazon, as well as in Maranhão. Small patches are present in many other parts of Amazonia.
Amazonian white sand soils occur under diverse geological and geomorphological conditions. They are found on the low uplands (called terra firme), mainly composed of arenaceous sediments; on natural levees in a floodplain (várzea); sand ridges or dunes in a coastal lowland (restinga); and on plateaux (chapadas) of Cretaceous sandstone, or on hill areas of granitic rocks (Whitmore and Prance, 1987).
White sand soils also cover a considerable area of North-East Brazil, or the Nordeste. The objective of this paper is: (1) to clarify the distribution of white sand soils in North-east Brazil;(2) to examine their characteristics and genetic processes; and (3) to consider the influence of deforestation on their formation.
Broadly speaking, the North-East consists of three distinctive geoecological regions: the zona da mata, the agreste, and the sertão (Andrade, 1980). They are arranged zonally in this order from the Atlantic coast to the inland (fig. 11.2).
The zona da mata do Nordeste is part of the extensive forest zone that stretches along the Atlantic coast from the north-east to the south-east of Brazil. It enjoys a sub-humid climate with marked seasonality. That is, the annual rainfall amounts 1,000 to 2,000 mm, but there is a weak dry season for two or three months when the monthly rainfall is less than 50 mm. Its original plant cover was generally a tropical evergreen seasonal forest, although because of long continued developments in the region, little original vegetation remains.
The geomorphology of the zone da mate is characterized, by low uplands called tabuleiros, although in some parts, such as southern Pernambuco, there are low, rounded hills ("half oranges" or colinas) of deeply weathered crystalline rocks. The tabuleiros are low uplands 30 to 200 metres above sealevel. They have been dissected to a greater or lesser degree by numerous valleys; consequently, some appear as extensive flat uplands; others are only residual fragments (fig. 11.3). They are composed of sandy, permeable, and unconsolidated sediments of Pliocene to Pleistocene age (Barreiras Group). The tabuleiros are largely grouped into three geomorphic surfaces: the higher and older erosional surface; the lower and later depositional surface; and the lowest fluvial terrace surfaces (Matsumoto, 1983). Geologically and geomorphologically, the tabuleiros in the North-East are correlated with the terra firme uplands that predominate in the Amazonian lowland.
The sertão is a semi-arid inland area, and a part of Brazilian plateau (Borborema highland), on which low-relief erosional plains (pediplains) have developed on the crystalline rocks. The annual rainfall in the sertão ranges 500 to 800 mm, and there is a six- to ten-months-long, almost rainless, severe dry season. A type of drought-resistant xerophytic vegetation called caatinga dominates.
The agreste is the transitional zone between the zone da mate and the sertão.
Locally, within the generally semi-arid sertão or the agreste, however, there are some small isolated sub-humid areas covered by forest. Such areas, called brejos, are formed generally on and around isolated heights standing above the plateau surface.
3 Distribution of the white sand soils in the Paraíba-Pernambuco area
The distribution of white sand soils in the states of Paraíba and Pernambuco, part of the North-East, is shown in figure 11.4. As a matter of fact, however, local variations in soils cannot be mapped in detail at such a small scale. White sand soils appear as a component of some soil association units which form a mosaic of various soils, such as red-yellow podzolic soils, redyellow lateritic soils, and so on. Consequently, figure 11.4 shows only the distribution of the soil association units including white sand soils. The areas shown in darker shade are the areas where the white sand soils appear as the most dominant component of the soil association unit; the medium-shaded areas are those where they constitute the second most dominant component; and the lightest shaded areas those where the white sand soils are less dominant.
According to this figure and figure 11.2, white sand soils are principally found in the sub-humid areas. They are distributed mainly in the zone da mate along the coast. In addition, they are found also in the area around the brejos, such as Areia in Paraíba and Garanhuns in Pernambuco. Although the white sand soils in both areas are similar in appearance, they are of a different pedological nature. The white sand around the brejos, developed on the granitic rocks, contains a lot of unweathered minerals (mainly quartz and feldspars) derived from parent rocks. This immature soil is classified as "dystrophic regosol" on the soil maps and explanations by MA and SUDENE (1972, 1973).
On the other hand, the white sand soil in the zone da mate develops on the unconsolidated sandy sediments of tabuleiros, as well as on sand ridges or dunes on the coastal lowland, and is composed of almost exclusively of quartz sand. This type of white sand soil, henceforth simply "white sand," is classified in pedology as "hydromorphic podzol," for the formation of which, in tropical environments, acidic parent material, and shallow groundwater table are considered necessary conditions.
The white sand in the zone da mate is, in turn, classified into two types according to the geomorphological and geological conditions under which it develops. One type, found in strips along the coast, for example, around the cities of Recife and João Pessoa, is the hydromorphic podzol developed on the dune or sand ridge deposits in the coastal lowlands, where the groundwater table is constantly very shallow. This type of white sand has been known widely on the Brazilian Atlantic coast from Amazonia to Rio de Janeiro (Anderson, 1981). Another white sand, here called upland white sand, develops on the surface of tabuleiros. This paper discusses the character and formation of this type of upland white sand. The discussion is based on a detailed field survey on the Conde upland near the city of João Pessoa, the capital of Paraíba state.
4 White sand on the Conde upland, Paraíba
The Conde upland, part of the higher tabuleiro surface, with an altitude of about 110 metres, is located about 15 km south of João Pessoa (fig. 11.4). Most of it has been deforested and converted to farming or left as a low thicket.
The results of field survey on this upland were reported by Matsumoto and Watanabe (1986). This report and subsequent results undergird much of what follows. The distribution of white sand on the Conde upland (fig. 11.5) shows a distinctive pattern: (1) the white sand alternates horizontally with brown-coloured sandy soil (red yellow podzolic soil), here called simply "brown sand;" (2) the two kinds of sand reveal a clear-cut boundary; (3) the white sand occupies a slightly depressed portion of the upland; (4) a dissecting valley follows the downslope of the white sand zone; (5) springs are found near the head of the dissecting valleys.
Richards (1952) suggested that tropical rain-forest rivers flowing out from the white sand area, such as the Rio Negro in Amazonia, always carry "black water." The spring water of the Conde upland
Table 11.1 Chemical properties of spring water of the Conde upland, at W-1 - W- 4 in figure 11.5
| Location no. | Water temperature(°C) | pH | Electrical conductivity(µS/cm) |
| W-1 | 28 3 | 4.67 | 29.5 |
| W-2 | 25.9 | 4.78 | 32.3 |
| W-3 | 27.0 | 4.48 | 33.7 |
| W-4 | 26 0 | 4.20 | 38 4 |
Compared to the grain size of brown sand, the white sand is slightly coarser in mean diameter and better in sorting measure, i.e. the mean diameter in phi units for the white and brown sands is 1.467 (0.36 mm) and 1.664 (0.32 mm), respectively; the sorting measure (standard deviation in phi units) is 0.786 and 1.108, respectively (fig. 11.6). These characteristics are explained by the paucity of silt and clay fractions in the white sand.
Concerning chemical properties (fig. 11.7), the hydrogen ion concentration of soil solution is low (pH = 4.0 to 5.0) in both soils, and the concentration of elements such as sodium (Na), calcium (Ca), potassium (K), and magnesium (Mg) in soil solution is generally poor. Iron (Fe) and aluminum (Al) concentrations in soil solution appear as mere traces in the case of white sand, being high in the upper portion of the brown sand. The content of carbon (C) is low in white sand and moderate in brown sand; that of nitrogen (N) is low in both sands.
The white sand is the result of removal by the action of subsurface water (soil water and groundwater) of clay and various minerals other than quartz from the brown sand. The abundance of subsurface water on the seemingly water-deficient tabuleiros, originally composed of thick and permeable sandy sediments, is explained by the existence of "hardpan" (a lateritic duricrust) formed near the surface. This hardpan acts as an impermeable layer to maintain a shallow groundwater table beneath the tabuleiro surface.
Results of penetration resistance (hardness) tests for the soil layers on the Conde upland (fig. 11.8) may indicate this process. The tests were carried out at several points along a line across the boundary of white and brown sands (A-B in fig. 11.5). In the soil profile of the white sand, a welldeveloped hard layer (hardpan) is found relatively close to the surface (2-3 m in depth), on which shallow groundwater remains. On the other hand, generally no hardpan or groundwater body is evident at a shallow depth in the brown sand profiles. In the soil profile at the narrow transitional zone between the white and brown sands, a thin hard layer develops nearly at the same depth as the duricrust in white sand, indicating the embryonic stage in the formation of the hard layer at a shallow level.
The existence of a shallow hard layer and groundwater under white sand is observed also in many outcrops of upland podzol in the ParaíbaPernambuco area, where groundwater issues as springs at the boundary between the layers of loose white sand and the underlying consolidated, yellow-coloured sand (hardpan).
The formation of the hard layer is related to the precipitation of iron and/or aluminum oxides at the level of the groundwater table or where soil water evaporates. Once such an impermeable hard layer is formed at a shallow level, the groundwater developed on it accelerates the eluviation of elements and clay in the surface soil layer, leading to the formation of white sand.
The formation of white sand may be mainly the outcome of natural processes under existing site conditions, such as geological, climatic, and vegetational ones. However, humans are also believed to accelerate this process through deforestation.
It may be clear from the preceding description that the abundance of subsurface water, maintaining shallow groundwater, should favour the formation of white sand. On sandy soils like the ones on tabuleiros, deforestation may decrease evapotranspiration and increase soil water. Such an outcome leads to the elevation of the groundwater table, which, in turn, results in the formation of the impermeable hard layer near the surface. In this manner, white sands may expand spatially.
The increase in soil water in the deforested terrain was ascertained through the comparison of water content in the soil profiles under the thicket and under the barren land on the Conde upland (fig. 11.9). The soil under the cleared land, both of white and brown sands, dried out only superficially, while, at depth, it contains more soil water than the soil under vegetation. Owing to poor capillary action in the coarse sandy soil, little soil water may evaporate from the soil surface, and the absorption and transpiration through the roots and leaves of plants is the dominant process for the output of water into the air.
Figure 11.9 Soil water content profile under thicket and barren lands on the Conde upland.
This situation was reported also by Kashiwagi (1986) from the sandy soil area near Campina Grande, Paraíba where the soil of a nearly bare cropped field contained more than double the moisture present in the soil under the caatinga.
The zone da mate of North-East Brazil has a history of deforestation of some five hundred years, beginning with the extraction of Brazil-wood (pau brasil; Caesalpinia echinata) in the sixteenth century, followed by clearing for sugar cane plantations and the cutting of firewood for sugar production. Recently, after the establishment of the Pró-Alcool Project, which aims to produce fuel alcohol for automobiles from sugar cane, the sugar cane fields have been extended even onto the surface of tabuleiros. Through such a long continued and repeated deforestation, the soil on the tabuleiros must have been severely modified, especially in the form of the expansion of the white sand areas.
The zona da mata do Nordeste, having many similarities with Amazonia in terms of its natural environment, such as on geology, climate, vegetation, and so on, might be referred to as the region suggesting "Amazonia in the future after development."
Anderson, A.B. 1981. "White-sand vegetation of Brazilian Amazonia." Biotropica 13: 199210.
Andrade, M.C. de. 1980. The land and people of Northeast Prazil. University of New Mexico Press.
Kashiwagi, Y. 1986. "A preliminary report on soil moisture content of bare ground and inside plant community in semi-arid region, Northeast Brazil." Latin American Studies 8: 155-60.
MA Ministério da Agricultura] and SUDENE [Superintendência do Desenvolvimento do Nordeste]. 1972. Levantamento exploratório-reconhecimento de solos do Estado da Paraíba. Rio de Janeiro.
. 1973. Levantamento exploratório-reconhecimento de solos do Estado de Pernambuco. Vol.1. Recife.
Machida, T., M. Inokuchi, and E. Matsumoto. 1976. "Land condition in the Eastern Nordeste Region." Tokyo Geography Papers 20: 9-22.
Matsumoto, E. 1983. "A note on the tabuleiros in the coastal region of the Brazilian
Northeast - A geomorphological approach." Latin American Studies 6: 1-13.
Matsumoto, E., and T. Watanabe. 1986. "Site conditions and formation of white sand in Northeast Brazil." Latin American Studies 8: 31-48.
Richards, P.W. 1952. The tropical rain forest: An ecological study. Cambridge University Press, Cambridge.
Whitmore, T.C., and G.T. Prance (eds.). 1987. Biogeography and Quaternary history in tropical America. Clarendon Press, Oxford.
Changing aspects of droughtdeciduous vegetation in the semiarid region of north-east Brazil
1
Introduction
2
Study sites and methods
3 Results
4
Discussion and conclusion
References
Ichiroku Hayashi
The semi-arid regions of North-East Brazil, the Nordeste, are covered by drought-resistant xerophytic vegetation (caatinga), which have been utilized for ranching and charcoal production. After clearing the vegetation, these areas have also been used for the cultivation of beans, cassava, and maize. Such human activities have degraded lands that now support only a poor harvest of crops and livestock. In order to learn how to conserve the vegetation under present land-use, I have carried out the following investigations:
1. The floristic composition of the caatinga;
2. The successional trend of the caatinga under pressure from
human activities;
3. Quantitative feature of caatinga trees and phytomass
production of the study sites;
4. The nitrogen and carbon content in soils of the study sites.
The results, we hope, should be useful in drawing up plans for rational land management of the area.
The study sites are situated between 7° and 8°S and between 32° and 38°E in the Brazilian North-East (fig. 12.1). Mean annual temperature and total annual precipitation are 20°C and 800-1,000mm, with a severe dry season from October to the following February (Nishizawa, 1976; Rizzini and Pinto, 1964). The soils are classified as solodized solonetz and regosols Ministério da Agricultura e Ministério do Interior, 1971).
Figure 12.1 The study area in North-East Brazil. 1. João Pessoa; 2. Campina Grander 3. Patos.
The vegetation varies from place to place, according to land use, which includes ranching, cultivation, firewood collection, and charcoal Production.
The intensive study sites were located in the vicinity of Campina Grande and Patos, where the vegetation is typical for the region. Nine 10m x 10m quadrats were set out at sites dominated by Mimosa tenuiflora (synonym of Mimosa hostilis) and Caesalpinia pyramidalis. In each quadrat, I measured the diameter of tree stems at 130 cm high (DBH) for each species, and counted the number of shrubs less than 150 cm tall for each species.
For selected specimens of Caesalpinia pyramidalis, Mimosa tenuiflora and Aspidosperma pyrifolium, the weight of stems (Ws:kg), branches (Wb:kg), and leaves (Wl:kg) was taken after first measuring tree height (H:m) and stem diameter (D:cm) at 130 cm high.
The nitrogen and carbon contents of stems and leaves and of the surface soil were determined in the laboratory.
