Geoscience Reference
In-Depth Information
directive plans follow one another with increasing speed… This is proof
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12.6 MANGROVE SWAMPS
12.6.1 Introduction
Mangrove swamps are coastal swamps linked to estuaries and
deltas in the intertropical zone. Their characteristics are peculiar and
merit separate treatment. Globally, they cover 25 million ha. They exist
on shores not subject to marine erosion but which, on the contrary,
exhibit sedimentation phenomena: protected bays, coasts protected by
sandbars, etc. They correspond to the intertidal zone. Mangrove swamps
are characterized by mangrove species mostly of the families listed in
Table 12.8.
Table 12.8
Trees of mangrove swamps.
Families
Genera
Avicenniaceae
Avicennia
Rhizophoraceae
Bruguiera
Rhizophora
Sonneratiaceae
Sonneratia
There are about 70 woody species that have variable requirements of
soils and submergence regimes. But anyway, the environment is muddy,
unstable and reducing, which causes two major problems for plants:
growing erect and respiration of roots using, for transmitting oxygen,
a structure that allows air circulation (aerenchyma). There are many
kinds of adaptations:
In
Rhizophora
spp. the roots exit from the trunk and low branches
before curving down and plunging into the water. The tree is
thus propped up. In Brazil, in the indigenous language, the word
for mangrove literally means 'curved tree'.
In
Avicennia
spp. and
Sonneratia
spp. we find
pneumatophores
,
a kind of vertical shaft projected out by the submerged
horizontal parts of the roots radiating from the trunk. These
unusual periscopes do not always rise into the free air but reduce
every day during high tide, the period in which the tree is a
state of 'apnoea'. There could be up to 3000 pneumatophores
per tree. In
Avicennia
spp. they are 30 cm long; in
Sonneratia
spp. the pneumatophores attain 50 cm on average, but some can
go up to 3 m.
