118
WILLIAM MACNAE
G. Variations
Variation in development of these zones is to be expected, yet in
each and every mangal visited by the author they are recognizable.
The succession is only complete when there is an excess of available
fresh water over loss by evaporation and transpiration.
Fresh water may be supplied to a mangal from two sources : either
it falls as rain or it is kirought down into the area by rivers flowing
from an area with a higher rainfall. I n the deltas of both the Rufiji
and Zambezi rivers the rainfall is insufficient to account for the dense
and well-developed mangrove forests. Over much of these areas the
rainfall is around 750 mm and seasonal, but the rivers bring down
enough water to influence the water table and give enough to enable
proper development of the trees and allow of a complete succession
and zonation.
When evaporation and transpiration exceed the available fresh
water to the extent that the soil water becomes hypersaline, the
zonation becomes interrupted. This interruption appears first in the
ceriops zone (Fig. 16, p. 104). Towards the centre of this zone the bushes
become stunted, rarely growing above waist height. They die off and
recolonization does not follow. With increasing excess of evaporation
this bare area expands both t o landward and to seaward. A few ceriops
bushes grow alongside the trees in the landward avicennia fringe and a
few occur just above the level of the bruguiera forest, narrowed in its
turn to an insignificant remnant behind the line of Rhizophora mucronata or R. stylosa along the edges of creeks. This may be seen in Figs.
8, 11 and 16 which represent mangroves in localities differing from one
another only in rainfall and in excess of evaporation and transpiration.
When the substratum is composed of a well-drained sand rather
than a mud, and no silt is availabIe for conversion into mud the entire
facies may change. Three mangals of bhis sort are known to me ; one
is at Kosi Bay on the Natal-Mopambique border, another at Inhambane
some 400 km to the north at latitude 23"s. (Fig. 14), and the third
within the Bako National Park at the entrance to the delta of the
Sarawak river in Borneo. The streams draining into these areas come
from hinterlands which are to a large extent uncultivated with porous
sandy soils of recent origin, soils which seem to require much supplementing to grow crops successfully. No silt, then, enters the bays.
At Kosi Bay Lumnitxera racemosa has become dominant in the
entire mangal behind a seaward fringe of Avicennia m r i n a . Rhizophoraceous mangroves play a minor role alongside creeks where
Rhixophora mucronata and Bruguiera gymnorhiza grow on soils enriched
WILLIAM MACNAE
G. Variations
Variation in development of these zones is to be expected, yet in
each and every mangal visited by the author they are recognizable.
The succession is only complete when there is an excess of available
fresh water over loss by evaporation and transpiration.
Fresh water may be supplied to a mangal from two sources : either
it falls as rain or it is kirought down into the area by rivers flowing
from an area with a higher rainfall. I n the deltas of both the Rufiji
and Zambezi rivers the rainfall is insufficient to account for the dense
and well-developed mangrove forests. Over much of these areas the
rainfall is around 750 mm and seasonal, but the rivers bring down
enough water to influence the water table and give enough to enable
proper development of the trees and allow of a complete succession
and zonation.
When evaporation and transpiration exceed the available fresh
water to the extent that the soil water becomes hypersaline, the
zonation becomes interrupted. This interruption appears first in the
ceriops zone (Fig. 16, p. 104). Towards the centre of this zone the bushes
become stunted, rarely growing above waist height. They die off and
recolonization does not follow. With increasing excess of evaporation
this bare area expands both t o landward and to seaward. A few ceriops
bushes grow alongside the trees in the landward avicennia fringe and a
few occur just above the level of the bruguiera forest, narrowed in its
turn to an insignificant remnant behind the line of Rhizophora mucronata or R. stylosa along the edges of creeks. This may be seen in Figs.
8, 11 and 16 which represent mangroves in localities differing from one
another only in rainfall and in excess of evaporation and transpiration.
When the substratum is composed of a well-drained sand rather
than a mud, and no silt is availabIe for conversion into mud the entire
facies may change. Three mangals of bhis sort are known to me ; one
is at Kosi Bay on the Natal-Mopambique border, another at Inhambane
some 400 km to the north at latitude 23"s. (Fig. 14), and the third
within the Bako National Park at the entrance to the delta of the
Sarawak river in Borneo. The streams draining into these areas come
from hinterlands which are to a large extent uncultivated with porous
sandy soils of recent origin, soils which seem to require much supplementing to grow crops successfully. No silt, then, enters the bays.
At Kosi Bay Lumnitxera racemosa has become dominant in the
entire mangal behind a seaward fringe of Avicennia m r i n a . Rhizophoraceous mangroves play a minor role alongside creeks where
Rhixophora mucronata and Bruguiera gymnorhiza grow on soils enriched