In order to explain these phenomena, Ranaivoson (1998) stated
‘‘that while river deposits accumulate seaward, mangrove forest
extends downstream, whereas the soil rises towards the inland
regions. This soil is only flooded during spring tides and then gives
way to a tanne, i.e. prolonged dryness and excess salt due to
evaporation lead to decaying of the mangroves’’. In such
conditions, mangrove establishment becomes difficult, even
unlikely. Only in some barren locations that become less saline
due to sandy clay deposits, can halophytic plants such as
Sporobolus spp. and Arthrocnemum spp. progressively grow.
Conductivity measurements made directly in different water
bodies showed values from 46.6 to 60.9ms cm_1 for Mangoky
tributaries – i.e. around 30–39.7% in terms of salinity – and from
59.5 to 78.4 ms cm_1 for mangrove creeks, i.e. 38.7–51.2%. Higher
values correspond to mangrove zones where A. marina, salt-tolerant
species, is largely dominant and where Rh. mucronata, saltintolerant
one, is clearly decaying. Rasolofoharinoro et al. (1998)
have previously established formangrove stands inMadagascar that
Rhizophora trees are limited in their distribution by mean water
salinities exceeding 30%, and that Avicennia trees could become
stunted when water salinity approched 60%. In Australia, Ball
(1998) established that mangrove species richness analyzed along a
river floodplain was minimal both in high salinity and drying
environments and was the greatest in moderatily saline sites.
Elster’s study in mangroves of Colombia (2000) confirmed that
hypersalinization, increasing sediment deposits and lower water
levels were the main causes of mangrove death. Hoffman et al.
(1985) and Komiyama et al. (1996) reported the role of floods in
Florida, and Cintron et al. (1978) and Jimenez (1990) stated the
importance of salinity and the flowing of rivers in Puerto Rico and
on the Pacific coast of Central America. According to these studies,
it was observed that river floods, one of the necessary conditions
for allowing mangroves to thrive, cannot penetrate anymore into
some parts of the Mangoky delta. In places receiving such a
deficient supply of water during tidal periods, mangrove stands
were decaying, in relation with salinization and drying processes.
Other studies carried out in different parts of Madagascar
(Andriamasinoro, 1993; Ioniarilala, 2000), in Vietnam (Hong and
San, 1993), as well as in other African and Asian countries,
emphasize the same succession mechanism after the deposition of
sediment. That is, Avicennia spp. first becomes established in places
with large tidal ranges that are supplied with large amounts of
alluvium that have high salinity. In some places that are more
exposed to tides, Sonneratia spp. is the pioneer species. After the
substratum has become elevated due to sediment deposition,
other species such as Rhizophora spp. appear on the landward
fringe, where conditions are calmer and where ecological ranges
are lower, specifically relating to salinity and flooding.
In order to explain these phenomena, Ranaivoson (1998) stated‘‘that while river deposits accumulate seaward, mangrove forestextends downstream, whereas the soil rises towards the inlandregions. This soil is only flooded during spring tides and then givesway to a tanne, i.e. prolonged dryness and excess salt due toevaporation lead to decaying of the mangroves’’. In suchconditions, mangrove establishment becomes difficult, evenunlikely. Only in some barren locations that become less salinedue to sandy clay deposits, can halophytic plants such asSporobolus spp. and Arthrocnemum spp. progressively grow.Conductivity measurements made directly in different waterbodies showed values from 46.6 to 60.9ms cm_1 for Mangokytributaries – i.e. around 30–39.7% in terms of salinity – and from59.5 to 78.4 ms cm_1 for mangrove creeks, i.e. 38.7–51.2%. Highervalues correspond to mangrove zones where A. marina, salt-tolerantspecies, is largely dominant and where Rh. mucronata, saltintolerantone, is clearly decaying. Rasolofoharinoro et al. (1998)have previously established formangrove stands inMadagascar thatRhizophora trees are limited in their distribution by mean watersalinities exceeding 30%, and that Avicennia trees could becomestunted when water salinity approched 60%. In Australia, Ball(1998) established that mangrove species richness analyzed along ariver floodplain was minimal both in high salinity and dryingenvironments and was the greatest in moderatily saline sites.Elster’s study in mangroves of Colombia (2000) confirmed thathypersalinization, increasing sediment deposits and lower waterlevels were the main causes of mangrove death. Hoffman et al.(1985) and Komiyama et al. (1996) reported the role of floods inFlorida, and Cintron et al. (1978) and Jimenez (1990) stated theimportance of salinity and the flowing of rivers in Puerto Rico andon the Pacific coast of Central America. According to these studies,it was observed that river floods, one of the necessary conditionsfor allowing mangroves to thrive, cannot penetrate anymore intosome parts of the Mangoky delta. In places receiving such adeficient supply of water during tidal periods, mangrove standswere decaying, in relation with salinization and drying processes.Other studies carried out in different parts of Madagascar(Andriamasinoro, 1993; Ioniarilala, 2000), in Vietnam (Hong andSan, 1993), as well as in other African and Asian countries,emphasize the same succession mechanism after the deposition ofsediment. That is, Avicennia spp. first becomes established in placeswith large tidal ranges that are supplied with large amounts ofalluvium that have high salinity. In some places that are moreexposed to tides, Sonneratia spp. is the pioneer species. After thesubstratum has become elevated due to sediment deposition,other species such as Rhizophora spp. appear on the landwardfringe, where conditions are calmer and where ecological rangesare lower, specifically relating to salinity and flooding.
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