_ 
ae Ne et 
wae. SS eT 
674 
places. Among the trees, scattered specimens of Dolichandrone spa- 
thacea, Xylocarpus granatum, Heritiera litoralis are clearly elements 
of both the Barringtonia and the Nipa formations, but they are not 
present in the best expression of the mangrove swamps. In small 
rivers, the mangrove vegetation itself is wanting or poorly developed ; 
thus, at the mouth of the Lamao River, on the Reserve, only isolated 
species of Rhizophora conjugata, Avicennia officinalis, and Sonneratia 
pagatpat were noted. 
While no further stages in the development of mangrove swamps 
behind sandy beaches are present at Lamao, yet in many parts of ‘the 
Philippines these are characteristically the feature of the vegetation at 
the mouths of large rivers, behind bars along exposed coasts. Their 
best expression shows the vegetation to be distributed in zones; Nipa. 
and its associates occupying the upper limits of high tide, with none 
of the elements of the mangrove formation, and the latter situated in 
the more deeply submerged parts of the tidal area. Irregular topog- 
raphy, made by the smaller tidal channels, leaves hummocks at the 
base of which the elements of the Nipa formation may be encountered, 
on top are representatives of non-halophytic vegetation, while the channel 
itself is occupied by species of Rhizophora or Bruguiera. 
(b) Mangrove and Nipa formations at the mouths of rivers not 
behind beaches—The mouth of the Alangan River is in a re-entrant 
across which a subaqueous bar is being formed, which causes the breaker 
line to be a considerable distance from the shore (see map) and in this 
the water is comparatively speaking, quiet, especially on its south or 
more protected side. Here a considerable development of the mangrove 
formation which is advancing and which in time will fill up the re- 
entrant, is seen near the mouth of the river. Considerable hydrodynamic 
force is present, so that much mud has not as yet been accumulated. The 
substratum is consequently sandy, and this is especially true at the base 
of the cliffs which will be discussed below. 
While the best expression of mangroye swamps is found in muddy 
deltas yet, as is often assumed, there is no reason to believe that such 
a soil is requisite for their development. It is far more true that the 
hydrodynamic factors which prevent the deposit of mud, also establish 
a too strenuous habitat for the seedling stages of the mangrove species. 
Other things being equal, the trees can thrive in sandy soils. 
(a) Mangrove formations at the mouths of rivers.—These factors 
being combined at the mouth of the Alangan River, there is but a poor 
expression of the mangrove element in that situation. The species 
present are mostly Sonneratia pagatpat and Avicennia officinalis. Out- 
side the Reserve, the best representation of this vegetation is found on 
delta regions at the heads of protected bays. Here the hydrodynamic 
elements are such as to allow the rapid capture of the newly formed 
mud flats. The greater the tidal area, the more extensive the advance 
