FATJNA AND FLORA OF MANGROVE SWAMPS
129
Each of these levels tends to have its own peculiarities of soil
consistency, mineral content and hence of fauna and flora. Exactly
why beaches level off in this way has not been fully explained : it must
be a function of tidal velocity and of material in suspension. By general
consent (Spooner and Moore, 1940; Emery et al., 1957 ; Guilcher, 1963)
tidal currents in estuaries and off-shore channels are strongest, when,
a t low tide, they are confined within the channels. Although the rate
of rise and fall of the tide is highest when it is crossing the flats, the
fact that it is crossing these flats causes a slowing down of the current
and allows deposition here. When the water has reached the upper
flats within the mangrove the flow is further impeded by the obstruction
caused by the trees and their pneumatophores. The region where these
are most thickly developed is in the seaward fringes of Avieennia and
of Xonneratia and within the rhizophora forests, and these appear to
be the region of greatest deposition. Beyond the seaward fringe the
bank of accretion slopes, sometimes quite steeply, down to mean sealevel. These muds are very soft and ill-consolidated. Behind the
seaward fringe the soils are firm and well consolidated. The boundary
between these soft and firm soils lies towards the upper margins of the
rhizophora forests. The soft, ill-consolidated soils are always waterlogged, and are reached by all or almost all tides. But why does this
belt of soil always slope down as soon as it emerges from the shade and
protection of the seaward mangrove fringe? Are such soils in a state
of flux? Do they flow outwards? They are certainly capable of it and
the area over which a man has walked, or rather wallowed or half swum,
will immediately show signs of downward and outward flowing movements of the soil. It would seem that on some occasions at least, the
soil particles lie on the slope a t a critical angle. The firmer soils within
the mangal are exposed to the air for a few days each fortnight when
the tide does not reach them. This absence of surface water allows
them to be compacted because withdrawal of water brings the particles
closer together. The biological processes described by Schuster (1952)
assist in this compacting.
Figures 8, 11 and 16 indicate the relationships between intertidal
level and zonation of mangroves in several areas under different rainfall
and evaporation-transpiration regimes. Although rainfall figures are
available, figures for evaporation-transpiration are unobtainable and
can only be guessed.
Mangroves occur on tropical shores from the ever-wet to desert ; i.e.
from regions of high rainfall and high humidity to regions of low
rainfall or no rainfall, and of excessive evaporation and transpiration.
129
Each of these levels tends to have its own peculiarities of soil
consistency, mineral content and hence of fauna and flora. Exactly
why beaches level off in this way has not been fully explained : it must
be a function of tidal velocity and of material in suspension. By general
consent (Spooner and Moore, 1940; Emery et al., 1957 ; Guilcher, 1963)
tidal currents in estuaries and off-shore channels are strongest, when,
a t low tide, they are confined within the channels. Although the rate
of rise and fall of the tide is highest when it is crossing the flats, the
fact that it is crossing these flats causes a slowing down of the current
and allows deposition here. When the water has reached the upper
flats within the mangrove the flow is further impeded by the obstruction
caused by the trees and their pneumatophores. The region where these
are most thickly developed is in the seaward fringes of Avieennia and
of Xonneratia and within the rhizophora forests, and these appear to
be the region of greatest deposition. Beyond the seaward fringe the
bank of accretion slopes, sometimes quite steeply, down to mean sealevel. These muds are very soft and ill-consolidated. Behind the
seaward fringe the soils are firm and well consolidated. The boundary
between these soft and firm soils lies towards the upper margins of the
rhizophora forests. The soft, ill-consolidated soils are always waterlogged, and are reached by all or almost all tides. But why does this
belt of soil always slope down as soon as it emerges from the shade and
protection of the seaward mangrove fringe? Are such soils in a state
of flux? Do they flow outwards? They are certainly capable of it and
the area over which a man has walked, or rather wallowed or half swum,
will immediately show signs of downward and outward flowing movements of the soil. It would seem that on some occasions at least, the
soil particles lie on the slope a t a critical angle. The firmer soils within
the mangal are exposed to the air for a few days each fortnight when
the tide does not reach them. This absence of surface water allows
them to be compacted because withdrawal of water brings the particles
closer together. The biological processes described by Schuster (1952)
assist in this compacting.
Figures 8, 11 and 16 indicate the relationships between intertidal
level and zonation of mangroves in several areas under different rainfall
and evaporation-transpiration regimes. Although rainfall figures are
available, figures for evaporation-transpiration are unobtainable and
can only be guessed.
Mangroves occur on tropical shores from the ever-wet to desert ; i.e.
from regions of high rainfall and high humidity to regions of low
rainfall or no rainfall, and of excessive evaporation and transpiration.