FAUNA AND FLORA O F MANGROVE SWAMPS
149
Of these the starred species are probably incapable of proper development in the shade of any tree, the others are more tolerant and may
germinate and develop in the shade or in the open. Normally the
pioneer on sheltered sea shores is Avicennia alba or A . marina or under
special circumstances Xonneratia sp. Seedlings of these may be found
far out on the beach but the level at which they occur will lie between
mean sea-level and high water of the lowest neaps. Most of these
seedlings become coated with silt and this acts, in the same way as
shade, to kill off the seedlings. Those at higher levels develop into
saplings, long and slender, giving each other support. A few a t the
outer edge, from the beginning, develop as well-shaped, young trees.
Chapman ( 1940) states that " successful colonization by these seedlings
depends either upon a very shallow sea-where there is only a small
tidal rise-otherwise there must be a considerable tidal rise and fall so
that they are exposed for some hours during the day ". Once established the avicennias and the species of Sonneratia cause accretion by
impeding water movement, the soil level rises, and others, spp. of
Rhixophora, Bruguiera and Xylocarpus, germinate from stranded seeds
or seedlings and become established. All these trees tend t o grow taller
than the pioneers, overtop them and these then die off. Hence one
rarely finds a well-grown tree of Avicennia marina in a rhizophora or
bruguiera forest. Occasionally an old stag-headed tree of Sonneratia
alba persists in these forests. Such trees are quite common in the
Rufiji delta. The " nurse " effect of Avicennia marina at Richards
Bay, Natal, has been mentioned by Macnae (1963).
Against this Professor A. W. Bayer of the University of Natal,
Pietermaritzburg, has suggested to me that in marginal areas such as
Natal colonization of new banks of sand or of mud in a suitable estuary
seems to depend on which species happens to be producing seedlings
when the banks form. If it is A . marina then this species will colonize,
if it is R. mucronata or B. gymnorhiza then one of these will be the first
to become established.
An extension of this principle may explain some of the vagaries in
distribution of Avicennia alba, A. marina, Xonneratia alba and S.
gri&thii in Malaysia. The species developing on an available mud bank
will depend on the seeds which become grounded. But why one or
sometimes another of these species develops in a pure stand is difficult
to account for.
Lumnitzera racemosa normally develops under light shade of
landward fringe avicennias or in the shelter of grasses and rushes.
Acanthus ilicifolius is an opportunist. It will develop on any type of
clearing and once established forms dense thickets which may prevent
149
Of these the starred species are probably incapable of proper development in the shade of any tree, the others are more tolerant and may
germinate and develop in the shade or in the open. Normally the
pioneer on sheltered sea shores is Avicennia alba or A . marina or under
special circumstances Xonneratia sp. Seedlings of these may be found
far out on the beach but the level at which they occur will lie between
mean sea-level and high water of the lowest neaps. Most of these
seedlings become coated with silt and this acts, in the same way as
shade, to kill off the seedlings. Those at higher levels develop into
saplings, long and slender, giving each other support. A few a t the
outer edge, from the beginning, develop as well-shaped, young trees.
Chapman ( 1940) states that " successful colonization by these seedlings
depends either upon a very shallow sea-where there is only a small
tidal rise-otherwise there must be a considerable tidal rise and fall so
that they are exposed for some hours during the day ". Once established the avicennias and the species of Sonneratia cause accretion by
impeding water movement, the soil level rises, and others, spp. of
Rhixophora, Bruguiera and Xylocarpus, germinate from stranded seeds
or seedlings and become established. All these trees tend t o grow taller
than the pioneers, overtop them and these then die off. Hence one
rarely finds a well-grown tree of Avicennia marina in a rhizophora or
bruguiera forest. Occasionally an old stag-headed tree of Sonneratia
alba persists in these forests. Such trees are quite common in the
Rufiji delta. The " nurse " effect of Avicennia marina at Richards
Bay, Natal, has been mentioned by Macnae (1963).
Against this Professor A. W. Bayer of the University of Natal,
Pietermaritzburg, has suggested to me that in marginal areas such as
Natal colonization of new banks of sand or of mud in a suitable estuary
seems to depend on which species happens to be producing seedlings
when the banks form. If it is A . marina then this species will colonize,
if it is R. mucronata or B. gymnorhiza then one of these will be the first
to become established.
An extension of this principle may explain some of the vagaries in
distribution of Avicennia alba, A. marina, Xonneratia alba and S.
gri&thii in Malaysia. The species developing on an available mud bank
will depend on the seeds which become grounded. But why one or
sometimes another of these species develops in a pure stand is difficult
to account for.
Lumnitzera racemosa normally develops under light shade of
landward fringe avicennias or in the shelter of grasses and rushes.
Acanthus ilicifolius is an opportunist. It will develop on any type of
clearing and once established forms dense thickets which may prevent