80
WILLLAM MACNAE
by way of estuaries of the small rivers or as a result of seepage through
the base of the chenier, then mangroves encroach and may develop
into dense forests. Mangrove forests of this nature are extensive to
the north and south of Beira, and in the region formed by the vagaries
in the Holocene of the deltas of the Zambezi and neighbouring rivers.
Along the shores of Sarawak and other parts of Borneo mangroves
occur on shores facing the South China and Java Seas. But these
shores, by virtue of their direction, run in such a way that they are
sheltered from the strongest winds that prevail in those seas. Winds of
gale force are rare in the belt within a few degrees of and on either side
of the Equator in the Indo-Australian archipelago.
Elsewhere mangroves are associated with estuaries. Some of the
largest mangrove forests in East Africa are associated with the deltas
of the Zambezi and Rufiji rivers. The most extensive mangals in the
world are associated with the deltas of rivers entering the Bay of
Bengal, the Straits of Malacca-both on the Malayan and Sumatran
sides (the Straits of Malacca, in fact, constitute a drowned estuary)in southern Borneo, in New Guinea, and in Thailand and adjoining
territories t o the eastward.
Mangroves are, therefore, almost always estuarine or else have
developed in lagoons and lows behind cheniers or in the shelter of
fringing or barrier reefs of coral, or of bluffs or islands.
Mangal development, as may be expected, shows a relationship to
the steepness of the shore and to the range of the tides.
The shore exposed between tide-marks, in regions where the tide is
appreciable, includes two distinct sections: (1) a portion exposed a t
low tides, and this does not concern us for it is never colonized by
mangroves, and ( 2 ) a portion entirely covered by water only a t high
spring tides. This is the schorre in international terminology-“ salt
marsh ” in England, America and Australia.
In the tropics and warmer subtropics around the Indian Ocean a
zoned mangal occupies almost the entire schorre : but only in the humid
tropics will the mangrove zonation tend t o be complete. In drier
regions or where rainfall is seasonal only the lower level will be occupied
by mangroves. Here a low cliff-“ a salting cliff ”-often divides the
schorre into two levels: of these the “low marsh” will tend to be
occupied by a more or less well-developed mangal, and the “high
marsh )’ will be either bare or occupied by a variety of halophytes.
Guilcher (1963) records such a pattern in Madagascar and ascribes
it “ to the long dry season and to the high temperature, which lead to
complete desiccation of the surface and crystallization of salt during
neap tides so that no plant is able to grow there ”. Such bare areas are
WILLLAM MACNAE
by way of estuaries of the small rivers or as a result of seepage through
the base of the chenier, then mangroves encroach and may develop
into dense forests. Mangrove forests of this nature are extensive to
the north and south of Beira, and in the region formed by the vagaries
in the Holocene of the deltas of the Zambezi and neighbouring rivers.
Along the shores of Sarawak and other parts of Borneo mangroves
occur on shores facing the South China and Java Seas. But these
shores, by virtue of their direction, run in such a way that they are
sheltered from the strongest winds that prevail in those seas. Winds of
gale force are rare in the belt within a few degrees of and on either side
of the Equator in the Indo-Australian archipelago.
Elsewhere mangroves are associated with estuaries. Some of the
largest mangrove forests in East Africa are associated with the deltas
of the Zambezi and Rufiji rivers. The most extensive mangals in the
world are associated with the deltas of rivers entering the Bay of
Bengal, the Straits of Malacca-both on the Malayan and Sumatran
sides (the Straits of Malacca, in fact, constitute a drowned estuary)in southern Borneo, in New Guinea, and in Thailand and adjoining
territories t o the eastward.
Mangroves are, therefore, almost always estuarine or else have
developed in lagoons and lows behind cheniers or in the shelter of
fringing or barrier reefs of coral, or of bluffs or islands.
Mangal development, as may be expected, shows a relationship to
the steepness of the shore and to the range of the tides.
The shore exposed between tide-marks, in regions where the tide is
appreciable, includes two distinct sections: (1) a portion exposed a t
low tides, and this does not concern us for it is never colonized by
mangroves, and ( 2 ) a portion entirely covered by water only a t high
spring tides. This is the schorre in international terminology-“ salt
marsh ” in England, America and Australia.
In the tropics and warmer subtropics around the Indian Ocean a
zoned mangal occupies almost the entire schorre : but only in the humid
tropics will the mangrove zonation tend t o be complete. In drier
regions or where rainfall is seasonal only the lower level will be occupied
by mangroves. Here a low cliff-“ a salting cliff ”-often divides the
schorre into two levels: of these the “low marsh” will tend to be
occupied by a more or less well-developed mangal, and the “high
marsh )’ will be either bare or occupied by a variety of halophytes.
Guilcher (1963) records such a pattern in Madagascar and ascribes
it “ to the long dry season and to the high temperature, which lead to
complete desiccation of the surface and crystallization of salt during
neap tides so that no plant is able to grow there ”. Such bare areas are