FAUNA AND FLORA OF MANGROVE SWAMPS
133
3. Waterlogging of the soil
Certain species of mangrove only grow in well-drained soils. These
include Xylocarpus spp., Lumnitxera spp. and such mangrove associates
as Osbornia octodonta and Pemphis acidula. Avicennia marina, particularly a t its distributional limits, becomes taller in the better drained
soils at creek edges than farther back.
Also the trees of the barringtonia association show preferences for
well-drained soils, while those of the nypa association grow in waterlogged soils.
To sum up : a complete zonation will only be found in an area with
a considerable intertidal range, with a high rainfall a t all seasons, i.e.
with an excess of water availability over loss by evaporation and
transpiration, and where silt in suspension is available to ensure that
a deposition of mud on the surface of the soil is always raising the soil
level so enabling the mangrove to extend seaward.
4. Influence of gullies, creeks, rivers and channels
These are names for four sizes of waterway in a mangal. A gully
will probably dry out a t low tide and be continuous with a creek which
retains water a t low tide. A river will be larger and like the larger
creeks will have an origin on the land rather than within the mangal.
A channel is a seaway formed by anastomosing of creeks and rivers and
surrounding islands within the mangal. Some of these islands in
Sumatra may be very large, large enough to be included in ordinary
atlas maps of the area.
Gullies and creeks are a natural result of the drainage pattern of all
intertidal flats whether extensive off-shore " waddens " as in North
Holland, or salt marshes or mangals. The larger creeks are the continuation of small streams coming off the land; others have their
origin in gullies which result from the stresses set up in the soil
when the tide ebbs. All creeks tend to cut down to a level close to that
of mean sea-level. As a result they cut below the water table and hence
the drainage in their vicinity is improved. Thus species of mangrove,
characteristic, even diagnostic of well-drained soils, tend t o cluster the
edges of creeks.
At the seaward end Sonneratia alba, 8. apetala or S. griflthii and
Avicennia marina border the mouth of the creek ; upstream they are
replaced by rhizophoras with prop roots arching into the water. These
in turn become interspersed and later replaced by Xylocarpus granatum,
species of Avicennia (A. alba and A . intermedia in South East Asia,
A . marina elsewhere) and towards the limit of saline influence Aegiceras
133
3. Waterlogging of the soil
Certain species of mangrove only grow in well-drained soils. These
include Xylocarpus spp., Lumnitxera spp. and such mangrove associates
as Osbornia octodonta and Pemphis acidula. Avicennia marina, particularly a t its distributional limits, becomes taller in the better drained
soils at creek edges than farther back.
Also the trees of the barringtonia association show preferences for
well-drained soils, while those of the nypa association grow in waterlogged soils.
To sum up : a complete zonation will only be found in an area with
a considerable intertidal range, with a high rainfall a t all seasons, i.e.
with an excess of water availability over loss by evaporation and
transpiration, and where silt in suspension is available to ensure that
a deposition of mud on the surface of the soil is always raising the soil
level so enabling the mangrove to extend seaward.
4. Influence of gullies, creeks, rivers and channels
These are names for four sizes of waterway in a mangal. A gully
will probably dry out a t low tide and be continuous with a creek which
retains water a t low tide. A river will be larger and like the larger
creeks will have an origin on the land rather than within the mangal.
A channel is a seaway formed by anastomosing of creeks and rivers and
surrounding islands within the mangal. Some of these islands in
Sumatra may be very large, large enough to be included in ordinary
atlas maps of the area.
Gullies and creeks are a natural result of the drainage pattern of all
intertidal flats whether extensive off-shore " waddens " as in North
Holland, or salt marshes or mangals. The larger creeks are the continuation of small streams coming off the land; others have their
origin in gullies which result from the stresses set up in the soil
when the tide ebbs. All creeks tend to cut down to a level close to that
of mean sea-level. As a result they cut below the water table and hence
the drainage in their vicinity is improved. Thus species of mangrove,
characteristic, even diagnostic of well-drained soils, tend t o cluster the
edges of creeks.
At the seaward end Sonneratia alba, 8. apetala or S. griflthii and
Avicennia marina border the mouth of the creek ; upstream they are
replaced by rhizophoras with prop roots arching into the water. These
in turn become interspersed and later replaced by Xylocarpus granatum,
species of Avicennia (A. alba and A . intermedia in South East Asia,
A . marina elsewhere) and towards the limit of saline influence Aegiceras