130
WILLIAM MACNAE
Only in the former do they reach their maximum development. Only
the hardiest survive on desertic shores-Lewinsohn (1963 in Zitt.) tells
me that Avicennia marina still occurs a t the southern tip of the Sinai
Peninsula, a locality from which Ascherson described its occurrence in
1903.
Only in areas with a rainfall above 2 000 mm is a complete succession of mangroves found. On the Queensland coast the succession is
complete only on that stretch between the Hinchinbrook channel and
Mossman where the rainfall is over 2 000 mm and at Tully and Innisfail
reaches 4 000 mm tapering off both to north and to south. On the coast
of eastern Africa there is no comparable belt with a complete succession.
Similar interruption in the zonation occurs in the drier parts of India
and in Timor and various localities in eastern Indonesia.
These figures also indicate that frequently one zone, usually either
one of the variants of the bruguiera forests (e.g. either the bruguiera
gymnorhiza forest or ceriops thickets) or one of the variants of the
landward fringe, is dominant. At Port Swettenham and Cairns the
bruguiera gymnorhiza forests are highly developed; a t Magnetic
Island, ceriops thickets; a t Townsville, St. Lawrence, Inhaca and
Tanga the bare areas exposed for days on end dominate the upper
intertidal areas.
De Haan’s scheme is based on the number of days on which the
tide reaches certain levels on the mangrove area. Reference to Figs. 8,
11 and 16 will indicate the importance of this. On the transect across
the mangal on Magnetic Island only the seaward fringe and part of the
rhizophora forest are immersed at each of the sixty tides per month.
A maximum of twenty-five tides per month reach the bare area, in
some months none reaches this level. Between four and twenty-one
tides per month reach beyond the upper margin of the ceriops thickets
and only twenty-two tides per year or a maximum of seven in one
month reach high water mark. For several months on end no tides reach
this level. As a result of the high rainfall (ca 1 500 mm) and run-off from
the mountain the landward fringe receives an ample water supply and
supports a forest.
The Townsville transect indicates that all areas which are wetted by
fewer than 117 tides per year (varying from four to twenty-one per
month) are bare of vegetation. Not even saltworts or halophytic
grasses occur. When these areas have been wetted by the tide the
water drains away; but much is drawn up by capillary action and
evaporated to leave a glittering crust of crystalline salt. There is
insufficient rain and insufficient seepage through the soil to wash the
salt away.
WILLIAM MACNAE
Only in the former do they reach their maximum development. Only
the hardiest survive on desertic shores-Lewinsohn (1963 in Zitt.) tells
me that Avicennia marina still occurs a t the southern tip of the Sinai
Peninsula, a locality from which Ascherson described its occurrence in
1903.
Only in areas with a rainfall above 2 000 mm is a complete succession of mangroves found. On the Queensland coast the succession is
complete only on that stretch between the Hinchinbrook channel and
Mossman where the rainfall is over 2 000 mm and at Tully and Innisfail
reaches 4 000 mm tapering off both to north and to south. On the coast
of eastern Africa there is no comparable belt with a complete succession.
Similar interruption in the zonation occurs in the drier parts of India
and in Timor and various localities in eastern Indonesia.
These figures also indicate that frequently one zone, usually either
one of the variants of the bruguiera forests (e.g. either the bruguiera
gymnorhiza forest or ceriops thickets) or one of the variants of the
landward fringe, is dominant. At Port Swettenham and Cairns the
bruguiera gymnorhiza forests are highly developed; a t Magnetic
Island, ceriops thickets; a t Townsville, St. Lawrence, Inhaca and
Tanga the bare areas exposed for days on end dominate the upper
intertidal areas.
De Haan’s scheme is based on the number of days on which the
tide reaches certain levels on the mangrove area. Reference to Figs. 8,
11 and 16 will indicate the importance of this. On the transect across
the mangal on Magnetic Island only the seaward fringe and part of the
rhizophora forest are immersed at each of the sixty tides per month.
A maximum of twenty-five tides per month reach the bare area, in
some months none reaches this level. Between four and twenty-one
tides per month reach beyond the upper margin of the ceriops thickets
and only twenty-two tides per year or a maximum of seven in one
month reach high water mark. For several months on end no tides reach
this level. As a result of the high rainfall (ca 1 500 mm) and run-off from
the mountain the landward fringe receives an ample water supply and
supports a forest.
The Townsville transect indicates that all areas which are wetted by
fewer than 117 tides per year (varying from four to twenty-one per
month) are bare of vegetation. Not even saltworts or halophytic
grasses occur. When these areas have been wetted by the tide the
water drains away; but much is drawn up by capillary action and
evaporated to leave a glittering crust of crystalline salt. There is
insufficient rain and insufficient seepage through the soil to wash the
salt away.