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Fig. 8.5A,B Alluvial sand plain with vegetation islands near the northern Carribbean coast of Venezuela at Chichiriviche. A In the rainy season (November 1985) covered with fresh water. B In the dry season (February 1983) covered with a thick salt crust (background). (See Medina et al. 1989)

Fig. 8.6 A Ionic composition of the ground water from pits excavated in the dry season on the sand plain at some distance from (I and II) and close to the open lagoon (III) respectively, of Chichiriviche, Venezuela, as compared to seawater. B Example of a soil pit dug to examine the ground water (Medina et al. 1989). (For the position of the three soil pits see also the transect of Fig. 8.8)

Fig. 8.7A-D Vegetation units of the alluvial plain at Chichiriviche, Venezuela. A Vegetation island on the sand plain. B Halophyte zone with Sesuvium portulacastrum (front) and Batis maritima (middle ground) around a vegetation island with Conocarpus erectus in the background. C Vegetation island with bushes of Conocarpus erectus and Subpilosocereus ottonis. D Grassland with deciduous forest in the background

Fig. 8.7A-D Vegetation units of the alluvial plain at Chichiriviche, Venezuela. A Vegetation island on the sand plain. B Halophyte zone with Sesuvium portulacastrum (front) and Batis maritima (middle ground) around a vegetation island with Conocarpus erectus in the background. C Vegetation island with bushes of Conocarpus erectus and Subpilosocereus ottonis. D Grassland with deciduous forest in the background e) a deciduous forest characterized by species of Capparis, Caesalpinia coriaria, Prosopis juliflora, Jacquinia revoluta, Maytenus karstenii, Erythroxylon cuma-nense, Croton sp. and Pereskia guamacho (Fig. 8.7D).

A transect presenting finer details is shown in Fig. 8.8. In addition to the seasonal differences in the water table the figure also gives an indication of the seasonal

Fig. 8.8 Transect of a part of the alluvial plain of Chichiriviche, Venezuela, showing topographical variations with the major vegetation units (centre), the water tables (centre) and top soil chloride concentrations in the rainy season and the dry season respectively (bottom) and the distribution of the most frequent plant species (top). (After Medina et al. 1989)

Fig. 8.8 Transect of a part of the alluvial plain of Chichiriviche, Venezuela, showing topographical variations with the major vegetation units (centre), the water tables (centre) and top soil chloride concentrations in the rainy season and the dry season respectively (bottom) and the distribution of the most frequent plant species (top). (After Medina et al. 1989)

changes in salt content of the upper 10 cm of top-soil. Overall salt concentrations in the soil as well as seasonal fluctuations tend to be highest in the bare areas and decrease with the sequence of vegetation units as follows: halophyte zone - grassland zone - vegetation islands - deciduous forest.

8.2.2 Dynamics of Vegetation Islands

The most conspicuous feature of these salinas are the small vegetation islands with a diameter of 3 - 10 m and a soil surface 10 - 40 cm higher than the sand plain. Observers have been tempted to consider these islands as a particular stage in a progressive succession, which starts from the bare sand plain, then leads to the halophyte vegetation, followed by island vegetation and finally on to grassland and deciduous forest (Walter 1973).

However, a closer examination extending over several years has shown that there is no such one-way progressive succession towards a stable climax community at the end. There are oscillations between the various vegetation units in time which may be determined by medium-term climatic fluctuations; e.g. the years between 1966 and 1975 appeared to be wetter, and the years between 1976 and 1986 were drier than the long term average (Fig. 8.9). The whole ecosystem is highly dynamic and provides an excellent example of an oscillating mosaic, as opposed to a stable climax equilibrium (see Sect. 3.3.3). By following the development of a given island, we see that islands not only grow into savannas and forest but also die, being eroded and eventually disappearing into the bare sand plain (Fig. 8.10). Thus, these islands appear to be metastable states between the more stable states of the forest and the sand plain, respectively.

TOCUYO DE LA COSTA (20m)

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Renewable Energy 101

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