FAUNA AND FLORA O F MANGROVE SWAMPS
131
2 . Xalinity tolerance
Around Darwin and a t several localities in northern Western
Australia the mangroves are creek based and there are extensive bare
areas (at Darwin in spite of a rainfall of over 2 000 mm though this is
concentrated into a few months only in summer). It would appear
that the intense evaporation during the dry months concentrates the
salt in the soil of the flats to levels above that tolerated by most plants
and none but the most resistant will grow.
Salinities of water in soils at Manly, south of Brisbane (Badham,
in litt.) and a t Inhaca (observations by myself and others) often
exceed loo%,. At Inhaca trees of Avicennia marina and Lumnitzera
racemosa grow in soils with a salinity of over 90%, (Fig. 33). I n such
places A . micrinu is dwarfed, forming copses rarely more than waist
height. Such copses are very extensive a t Inhambane (lat. 24"s.
in Moqambique). Ceriops tagal will grow, apparently healthily, but
certainly not as tall bushes, where salinities exceed BOX,. At Inhaca a
copse of elderly gnarled Rhizophora mucronata grows in soil of %%, ,
and similar copses of R. stylosa occur a t Darwin in soils of similar
salinity of ground water.
Avicennia marina appears to have the widest range of salinity
tolerance of all mangroves. It can grow in almost fresh water or in
soils of water salinity exceeding go%,. Under the latter conditions the
trees are dwarfed, with very extensive root systems and with large
numbers of salt-secreting glands. Under conditions of fresh water the
root system is not disproportionately extensive and salt-secreting
glands are absent from the leaves. No doubt this wide salinity tolerance
is responsible for the wide range of this tree. Species with a lower
tolerance range are restricted to a narrow zone with a mangrove.
Sonneratia alba, 5'. apetala and X . griSfithii are all found in the seaward
fringe and would seem to prefer waters of near normal salinity. On the
other hand, 8. caseolaris only grows where the salinity is low, less than
Species of Bruguiera normally grow in those portions of the mangrove with salinities less than as%, . B. parvi$ora reaches its maximum
development around 20%,; B. sexangula prefers soils with a low
salinity, lo%, or less. B. gymnorhiza has a tolerance of from around
Aegiceras corniculatum is always an indicator of the presence of
fresh water. Hence it may be found on the shores of bays along the
seepage line, along the banks of tidal rivers upstream of any other
mangrove-species or where a fresh water swamp merges with a
mangrove.
lo%,.
10-25%,.
131
2 . Xalinity tolerance
Around Darwin and a t several localities in northern Western
Australia the mangroves are creek based and there are extensive bare
areas (at Darwin in spite of a rainfall of over 2 000 mm though this is
concentrated into a few months only in summer). It would appear
that the intense evaporation during the dry months concentrates the
salt in the soil of the flats to levels above that tolerated by most plants
and none but the most resistant will grow.
Salinities of water in soils at Manly, south of Brisbane (Badham,
in litt.) and a t Inhaca (observations by myself and others) often
exceed loo%,. At Inhaca trees of Avicennia marina and Lumnitzera
racemosa grow in soils with a salinity of over 90%, (Fig. 33). I n such
places A . micrinu is dwarfed, forming copses rarely more than waist
height. Such copses are very extensive a t Inhambane (lat. 24"s.
in Moqambique). Ceriops tagal will grow, apparently healthily, but
certainly not as tall bushes, where salinities exceed BOX,. At Inhaca a
copse of elderly gnarled Rhizophora mucronata grows in soil of %%, ,
and similar copses of R. stylosa occur a t Darwin in soils of similar
salinity of ground water.
Avicennia marina appears to have the widest range of salinity
tolerance of all mangroves. It can grow in almost fresh water or in
soils of water salinity exceeding go%,. Under the latter conditions the
trees are dwarfed, with very extensive root systems and with large
numbers of salt-secreting glands. Under conditions of fresh water the
root system is not disproportionately extensive and salt-secreting
glands are absent from the leaves. No doubt this wide salinity tolerance
is responsible for the wide range of this tree. Species with a lower
tolerance range are restricted to a narrow zone with a mangrove.
Sonneratia alba, 5'. apetala and X . griSfithii are all found in the seaward
fringe and would seem to prefer waters of near normal salinity. On the
other hand, 8. caseolaris only grows where the salinity is low, less than
Species of Bruguiera normally grow in those portions of the mangrove with salinities less than as%, . B. parvi$ora reaches its maximum
development around 20%,; B. sexangula prefers soils with a low
salinity, lo%, or less. B. gymnorhiza has a tolerance of from around
Aegiceras corniculatum is always an indicator of the presence of
fresh water. Hence it may be found on the shores of bays along the
seepage line, along the banks of tidal rivers upstream of any other
mangrove-species or where a fresh water swamp merges with a
mangrove.
lo%,.
10-25%,.