136
WlLLIAM MACNAE
t o be shut in from wave action by the development of cheniers or some
other form of sand dune.
But all this may change as a result of vagaries of currents either in
the rivers or in tidal channels just off shore. A shore of accretion may
become a shore which is being actively eroded. What, then, is the
effect on the mangal? Carter (1959) has made a short study of these
problems on the western shore line of Malaya. She indicates that species
of mangrove react differently. Species of Avicennia and of Xonneratia
are more vulnerable than species of Bruguiera and these more vulnerable than species of Rhizophora. This bears a direct relationship to the
depth of rooting of the trees. The soils of the seaward fringes are
ill-consolidated and so clearly very susceptible t o slight changes of
currents, changes which tend to remove and not deposit soil. Hence
these fringes will soon succumb to wave action. It will take a slightly
longer period to breach the rhizophora forests, but once this is done
the remainder of the mangal is quickly destroyed merely because the
soil is washed from under the trees.
It is this sort of process which has been responsible, according
to van Bemmelen (1949), Umbgrove (1949) and Obdeyn (1941a-c,
1942a,b, 1943, 1944)) for the widening of the Straits of Malacca and for
the reorganization of shore lines on either side of these straits and
southwards along the north shore of Sumatra.
Erosion of a shore often, if not always, follows on the removal by
man of a mangal. Certain of the areas studied by Carter (1959) show
erosion as a result of human interference, as for example at Pontian in
south Johore where cultivation comes right up to the shore. Here
attempts have been made to recolonize with species of Avicennia
planted behind a shelter of fishing stakes and brushwood. This has been
to some extent successful (Rep. For. Adm. Malaya, 1956).
111. ADAPTATIONS SHOWN BY THE FLORA
A. Adaptations to growing in ill-consolidated muds
Very few species of mangrove are deep rooted, or have persistent
tap roots. Almost all are shallow rooted but the root systems are often
extensive and may cover a wide area. Rhizophoraceous trees have
seedlings with a long radicle which would seem well suited t o develop
into a tap root., but as soon as the seedling becomes established in the
mud the radicle develops little further. I n Rhizophora (Figs. 35 and 36)
its function is taken over by a system of bunches of roots developed at
the end of the prop roots about a foot underground. These roots are
thick and air filled and give rise to nutritive roots which penetrate the
WlLLIAM MACNAE
t o be shut in from wave action by the development of cheniers or some
other form of sand dune.
But all this may change as a result of vagaries of currents either in
the rivers or in tidal channels just off shore. A shore of accretion may
become a shore which is being actively eroded. What, then, is the
effect on the mangal? Carter (1959) has made a short study of these
problems on the western shore line of Malaya. She indicates that species
of mangrove react differently. Species of Avicennia and of Xonneratia
are more vulnerable than species of Bruguiera and these more vulnerable than species of Rhizophora. This bears a direct relationship to the
depth of rooting of the trees. The soils of the seaward fringes are
ill-consolidated and so clearly very susceptible t o slight changes of
currents, changes which tend to remove and not deposit soil. Hence
these fringes will soon succumb to wave action. It will take a slightly
longer period to breach the rhizophora forests, but once this is done
the remainder of the mangal is quickly destroyed merely because the
soil is washed from under the trees.
It is this sort of process which has been responsible, according
to van Bemmelen (1949), Umbgrove (1949) and Obdeyn (1941a-c,
1942a,b, 1943, 1944)) for the widening of the Straits of Malacca and for
the reorganization of shore lines on either side of these straits and
southwards along the north shore of Sumatra.
Erosion of a shore often, if not always, follows on the removal by
man of a mangal. Certain of the areas studied by Carter (1959) show
erosion as a result of human interference, as for example at Pontian in
south Johore where cultivation comes right up to the shore. Here
attempts have been made to recolonize with species of Avicennia
planted behind a shelter of fishing stakes and brushwood. This has been
to some extent successful (Rep. For. Adm. Malaya, 1956).
111. ADAPTATIONS SHOWN BY THE FLORA
A. Adaptations to growing in ill-consolidated muds
Very few species of mangrove are deep rooted, or have persistent
tap roots. Almost all are shallow rooted but the root systems are often
extensive and may cover a wide area. Rhizophoraceous trees have
seedlings with a long radicle which would seem well suited t o develop
into a tap root., but as soon as the seedling becomes established in the
mud the radicle develops little further. I n Rhizophora (Figs. 35 and 36)
its function is taken over by a system of bunches of roots developed at
the end of the prop roots about a foot underground. These roots are
thick and air filled and give rise to nutritive roots which penetrate the