The physical environment

a. Location

The study area is located in the lowlands of Bolivia (60° to 61°33’ of western longitude and 14°40’ to 16°40’ southern latitude). Politically the area belongs to the Municipality of San Ignacio de Velasco in the Department of Santa Cruz.

Figure 2: Map of the study area

Figure 2 shows the physical boundaries of the study area, as there are: in the north, the southern border of the Noel Kempff Mercado National Park and the Forest Reserve of Bajo Paragua; in the south, the southern political boundary of the San Ignacio-Municipality; in the east, the international frontier between Bolivia and Brazil and, finally, in the west, the roads between San Ignacio and Santa Rosa and between Santa Rosa de la Roca and Florida. This area has been called “Transitional Region Chiquitano-Amazon” (TR-CHA) in this study because it joins the deciduous and semi-deciduous forest with the Amazon forest.

b. Climate

According to Köppen’s empiric classification, the area corresponds to the tropical climate with seasonal summer rains, zone Aw (Bradshaw & Weaver 1993).

However, this classification takes only two factors (temperature and precipitation) into consideration and thus explains the climate of this area only partially. A classification after Brads & Weaver (1993), based on causal factors, describes the climatic conditions much better. It indicates that areas placed between arid-dry and wet equatorial regions, as the transitional region Chiquitano-Amazon, experience an alteration of dry, low-sun seasons and wet, high-sun seasons. These areas are labelled as “tropical seasonal climatic environments”

and also occur in Venezuela, the interior eastern Brazil and Paraguay.

Already based on the few data available for the TR-CHA it becomes obvious that the study area has a transitional climate as Brads and Weaver described. Rainfall increases from south to north, from 1000 mm to 1500 mm (see figure 3). The distribution of precipitation is mono-modal, a maximum of precipitation occurs between January and February and minima in the period of June till August, as can be seen in the climatic diagram of San Ignacio (figure 4).

With regard to temperature, the average varies from 24.5 °C in the southwest to 25.5 °C in the northeast (Fig 3). Maximum and minimum temperatures have the same temporal behaviour as the rainfall (see Fig 4) (Villarpando et al. 2002)

Figure 3: Map of isohyets and isotherms of study area

Figure 4: Climate diagram for San Ignacio de Velasco

0 20 40 60 80 100 120 140 160 180 200

J A S O N D J F M A M J

Month

Rainfall (mm)

0 30 60 90 120 150 180

Rainfall Temp

30 25 20 15 10 5 0

Temperature (°C)

Source: Villarpando et al. 2002, after data from SENAMHI and AASANA

c. Physical geography and topography

The area of study “Transitional Region Chiquitano-Amazon” is part of a valley placed among the Chiquitano, Matto Groso and Caparuch mountain ranges (see figure 5). The relief is composed by western soft hillsides of the Chiquitano mountains ranges and flood plains of the rivers Paragua, Tarvo and their tributaries. These rivers form braided-streams with relatively shallow and wide channels, which are common of tropical seasonal humid regions due to the combined effect of seasonal rainfall and the chemical weathering that produce fine clay soils.

As a consequence, these streams lower the surface across the whole landscape, rather than creating valleys by deep stream incisions (Bradshaw &Weaver 1993).

The topography of the zone presents a west–east variation from the hillsides of the Chiquitano mountain ranges, with gentle slopes, to the flat land near Paragua and Tarvo River, below 220 m, where annual floods occur. Here the Manomo Hill (cerro Manomo) approximate in the centre is the only higher elevation that disturbs this topographic pattern. The altitude changes in direction north and east and varies between on average 400 m in the town of San Ignacio to 174 m in the Community Florida and 190 m in the surroundings of the Marfil lagoon. The mentioned Manomo Hill is the highest elevation with 684 m.

d. Geology and soils

The Chiquitano Mountain ranges and the Brazilian Shield determine the geological characteristics of the area. The Chiquitano Mountains are made up of Precambrian rocks, the oldest rock formation on Bolivian territory. The Brazilian Shield is characterised by flat shapes forming very shallow soils and very old rock outcrops such as granite and basalt (Rafiqpoor & Ibisch, 2004).

The soils of the region are generally classed as having no or only a shallow top horizon and a nutrient-poor subsoil with a high clay content, except for a few sectors with forest coverage, which are rich in organic matter. From the point of view of soil use, the region is characterized by soil classes that put severe limitations on farming. The majority of the soils are classified as Class VI (appropriate for mixed farming cattle-crops, or permanent crops, with erosion and/or fertility limitations) and VII (appropriate for mixed farming or permanent crops, with severe limitations caused by drainage, erosion and/or fertility) (Wachholtz 2002).

Figure 5: Landscape characteristics of the study area.

Figure 6: Diagram of altitude and hydrography of the study area

From the point of view of the potential of the soils for forest management, taking into account forest species diversity, forest development, forest conservation and ecological conditions (soil type, climate, water regime and topography), the region shows a regular to high potential for forestry. Comparing soil suitability for agriculture with forestry and mixed farming, there is a clear indication of suitability for the latter. Mixed farming is, along with logging, the activity with greatest importance in the zone of action. The majority of the flooded savannas, which cover an important portion of the area, are used for mixed farming (Prefectura del Departamento de Santa Cruz, 1996; Ibisch et al. 2002; FCBC online 2004).

e. Watersheds and streams

The majority of the study area (90%) belongs to the Amazon basin draining to the north into the Amazon River; the rest, specifically the areas located in the south, drain to the Plata basin in the south. Four sub-watersheds can be differentiated in the Amazon basin: Tarbo, Paragua, Curichon and Guarayos. The first three watersheds have a greater influence in the study area than the Guarayos basin, which drains to the west. The lagoon Marfil, with a surface of around 130 km² is the largest water body in the area (see figure 6).

Seasonal flooding affects large parts of the study area and has a mayor influence on soil characteristics and vegetation. As a consequence, apart from the climate, both the soils and the flooding regime determine the most important factor geo-diversity which in the literature always shows a high correlation with the pattern of biodiversity (Rafiqpoor & Ibisch, 2004).

The function and importance of the hydro-regulation for this area should be underlined here, because the TR-CHA is the headwater of the large river “Itenez” (Guapore), which plays a similar regulation role in the study area as does the Pantanal to the hydro-system of the Paraguay River.