8.2.1 Formation of Coastal Salt Marshes and Vegetation Physiognomy
Coastal salt marshes at the northern coast of South-America are formed in bays, where the sand-laden waves returning from the beach to the sea are driven back towards the shore by the North-Eastern trade winds. First a sandbar is formed leading
to a sandbank and to separation of a lagoon. Subsequently, this lagoon fills in with sand, drying out and becoming a salt marsh. Both fixed and mobile sand dunes may also form at the coast behind the salt marsh (Fig. 8.3). A typical example are the inland salt-marshes near the northern Caribbean coast of Venezuela first described briefly by Walter (Walter and Breckle 1984) and later studied ecophysiologically in some detail (Lüttge et al. 1989a,b; Medina et al. 1989; Smith et al. 1989). These salt marshes are much more extreme habitats than the restingas (Sect. 8.1) with a very strong seasonality. The most salt-resistant plants found first in such sites are mangroves, which begin to surround the lagoon (Fig. 8.3), an example being Avicennia germinans at the lagoon and salt marshes near Chichiriviche on the Caribbean coast of Venezuela.
The flat alluvial sand plain covering areas previously occupied by the lagoon is subject to marked seasonality because there is a pronounced rainy season in October to December and a strong dry season during the rest of the year interrupted only by a small and short wet period in April (Fig. 8.4). During the rainy season the sand plain may be covered by several decimeters of fresh water, whereas during the dry season the surface is dry and a considerable salt crust may form (Fig. 8.5). The very salty groundwater, with an NaCl-concentration several times that of seawater (Fig. 8.6), percolates upwards to the surface where the water evaporates and leaves behind the dissolved salt. The vegetation of the sand plain can be described by distinguishing five units (Fig. 8.7):
a) the vegetation-free sand and salt flats (Fig. 8.7A), b) a halophyte zone with Batis marítima and Sesuvium portulacastrum as the dominating species (Fig. 8.7B), c) a grass-land zone with Sporobolus virginicus and Oxycarpha suaedifolia as the characteristic plants (Fig. 8.7D), d) vegetation islands with the mangrove associate Conocarpus erectus and the cactus Subpilosocereus ottonis as the physiognomically determinant species (Fig. 8.7A, C), [enumeration continuing on page 274]
TOCUYO DE LA COSTA (20m)
(1963-1985) o Rainfall av. 1020 mm near the northern Caribbean coast of Venezuela close to Chichiriviche for a 22-year period. (Medina et al. 1989)
Fig. 8.4 Average monthly values of rainfall, evap oration and temperature
TOCUYO DE LA COSTA (20m)
(1963-1985) o Rainfall av. 1020 mm
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