124
WILLIAM MACNAE
cultivation in ponds, can they revert to mangrove forests when the
ponds are neglected? So far as I am aware no one has studied the
processes of degradation of mangrove forest soils. But one wonders if
oxidation of peats with consequent loss of fertility may account for the
bare areas so characteristic of the mangals in regions with low or
seasonal rainfall.
I. Control of zonation
While a zonation is normally found in a mangal colonized by more
than one species of tree, the form of zonation described above is not
inevitable, though it covers most of the associations which may be
found intertidally on the upper shore within the tropics around the
Indian Ocean. Macnae (1966) ascribed control of the distribution of
mangrove trees and hence their zonation to the interaction of: (1)
frequency of tidal flooding; (2) salinity of soil water; and (3) waterlogging of the soil. All of these are modified by the presence of creeks,
gullies, rivers and channels ; and the second and third will depend on :
(i) rainfall and/or the supply of fresh water; (ii) evaporation and
transpiration, and (iii) the nature and quality of the soil.
1. Frequency of tidal Jlooding
It is clear that the zones of trees are associated, in the first instance,
with rising ground level and the resulting falling off in frequency of
tidal flooding. This formed the basis for Watson’s (1928) inundation
classes and for de Haan’s (1931) zones. A comparison in tabular form
is given in Table I. De Haan took into account in his scheme the effect
of fresh water, and so his scheme has a slightly wider coverage than
does that suggested by Watson. These zones based on physical features
alone are not universally applicable but apply only in regions where
the mangal covers the whole of its potential area of development. I n
Table I these have been printed in such a way that equivalent zones
are on the same line.
The zones will vary in width with the nature of the slope of the
shore; and with the range of the tide. I n Ceylon where the tidal range
has a maximum of 7 5 cm the mangrove zones are telescoped very
strongly, and during the wet monsoon the tide may give the appearance
of never going out a t all so that the lower, more seaward zones are
continuously under water for weeks on end and even the higher zones
are flooded more frequently than usual. Where the tidal range is great,
and the slope of shore gentle, mangroves may be very extensive and
some miles deep; as, for example, at Port Swettenham and several
localities in Sumatra, where the tidal range approaches 5 m.
WILLIAM MACNAE
cultivation in ponds, can they revert to mangrove forests when the
ponds are neglected? So far as I am aware no one has studied the
processes of degradation of mangrove forest soils. But one wonders if
oxidation of peats with consequent loss of fertility may account for the
bare areas so characteristic of the mangals in regions with low or
seasonal rainfall.
I. Control of zonation
While a zonation is normally found in a mangal colonized by more
than one species of tree, the form of zonation described above is not
inevitable, though it covers most of the associations which may be
found intertidally on the upper shore within the tropics around the
Indian Ocean. Macnae (1966) ascribed control of the distribution of
mangrove trees and hence their zonation to the interaction of: (1)
frequency of tidal flooding; (2) salinity of soil water; and (3) waterlogging of the soil. All of these are modified by the presence of creeks,
gullies, rivers and channels ; and the second and third will depend on :
(i) rainfall and/or the supply of fresh water; (ii) evaporation and
transpiration, and (iii) the nature and quality of the soil.
1. Frequency of tidal Jlooding
It is clear that the zones of trees are associated, in the first instance,
with rising ground level and the resulting falling off in frequency of
tidal flooding. This formed the basis for Watson’s (1928) inundation
classes and for de Haan’s (1931) zones. A comparison in tabular form
is given in Table I. De Haan took into account in his scheme the effect
of fresh water, and so his scheme has a slightly wider coverage than
does that suggested by Watson. These zones based on physical features
alone are not universally applicable but apply only in regions where
the mangal covers the whole of its potential area of development. I n
Table I these have been printed in such a way that equivalent zones
are on the same line.
The zones will vary in width with the nature of the slope of the
shore; and with the range of the tide. I n Ceylon where the tidal range
has a maximum of 7 5 cm the mangrove zones are telescoped very
strongly, and during the wet monsoon the tide may give the appearance
of never going out a t all so that the lower, more seaward zones are
continuously under water for weeks on end and even the higher zones
are flooded more frequently than usual. Where the tidal range is great,
and the slope of shore gentle, mangroves may be very extensive and
some miles deep; as, for example, at Port Swettenham and several
localities in Sumatra, where the tidal range approaches 5 m.