Vegetation and flora of the savannas in the Brinckheuvel Nature Reserve, Northern Suriname

The Brinckheuvel Nature Reserve includes the most characteristic part of the geomorphological Sabanpasi (or Subgreywacke) landscape (see fig. 4), formed by low, elongate, parallel, gently sloping ridges. The centre of each ridge is crowned by a narrow rib (gravel rib) (see fig. 5). The gravel ribs are in general formed by residual soils of sandy loam covered with a thin layer of quartzite material. The flanks of the ridges are occupied by sandy colluvial soils separated by a layer of gravel from the residual subsoil. At the bases of these flanks there is alluvial soil (see fig. 5 and 9). Generally the residual and colluvial soils bear savanna vegetation, the alluvial soils bear xerophytic wood and mesophytic forest. In the savanna vegetation on the ribs and the flanks of the ridges twelve plant communities can be distinguished (see table I and II). The residual soils are hardly permeable; the colluvial soils, on the other hand, absorb rainwater quickly and soon dry up again. In the Nature Reserve there are three hills; Brinck Hill, Klaiber Hill and Lobles Hill. They rise about 30 m above the level of the vicinity and consist of unweathered subgreywacke rock; their tops are flat and have a cover of 6 m of very permeable, white sand (see fig. 6). In the places where the vegetation was burned in the last few years there is now a savanna vegetation in which fourteen plant communities can be distinguished (see table I and II). The rest of the tops of the hills is covered with xerophytic wood. The rainwater quickly seeps through the white sand cap to the unweathered subgreywacke, runs off sideways, and reappears at the foot of the cap (the so-called source level). On the greater part of the slopes of the hills the subgreywacke crops out, with little gullies and furrows that are filled with erosion material. Ca. 245 species of plants were collected on the savannas of the Brinckheuvel Nature Reserve. Except in places where the influence of fire was noticeable, the presence of a more or less impermeable soil layer at shallow depth is doubtless responsible for the existence of the savannas of the Nature Reserve. This layer makes it possible that the overlying permeable mass is alternately wet and dry, depending on the season. The degree to which this happens depends not only on the thickness of this permeable topsoil but also on the inclination of the surface. This may be illustrated by the following (see fig. 10): 1. Xerophytic forest is found on white sand caps and slopes of the three hills and on very high gravel ribs. In such places the rainwater disappears rapidly, either the soil is well drained (white sand caps), or because most of the water runs off on the surface owing to the sloping of the ground. The soil is never extremely wet. 2. Savanna scrub is found on less high residual ribs and on gently sloping land with a thick permeable topsoil. In both cases the soil can take up more water and hold it longer. The contrast between wet and dry is greater. 3. Savanna hushes and open vegetations are found on the parts with a more level and thin topsoil of permeable material. The topsoil is soon saturated with rainwater that can not or hardly drain off, but dries up relatively soon. The contrast between wet and dry is therefore even greater. 4. Exclusively open vegetations, finally, are found on flat savanna parts with a few cm deep soil situated on uneroded rock, e.g. at the source level of the three hills, beside or near great rock flats and on level residual ground covered with gravel. Here we find the greatest contrast between wet and dry. All this illustrates the fact that in the Sabanpasi savanna area the hydrology of the soil is one of the principal habitat factors. Therefore, more attention should be paid to this factor by a next investigation in the Brinckheuvel Nature Reserve.