20 MANGROVE SWAMPS 
by year; its area diminishing or increasing as the lands drained 
or filled in by the action of the river are of greater or less 
extent than those newly formed. The draining and filling in 
of the lands on the upper limits of the swamp is very gradual, 
so that although the change from mangrove to dry forest is 
characteristic of these areas the process is extremely slow and 
less noticeable than the advance of the forest over newly formed 
flats on the sea edge of the swamp. 
The mangrove forests may contain trees 1.35 meters in diam- 
eter; and when fully stocked, with mature timber, compare 
favorably with the commercial forests of the land. Areas with 
650 cubic meters per hectare are found in the older swamps. 
These forests are not swamps and marshes, as we think of them 
in temperate regions, where trees grow in wet places that are 
periodically covered with standing water; but are literally 
forests of the sea with their roots in a stratum in which salt 
water is always present. For the greater part of the time the 
roots and even the lower part of the trunks of the trees are 
submerged in from 0.5 to 1 meter of salt water, while at high 
tide the lower limbs and foliage of the trees on the edges of 
the swamp are often submerged for a short time without injury 
(Plate II, fig. 1) ; conditions of life that would absolutely destroy 
ordinary forest trees. 
Their character as forests of the sea is emphasized by the 
fact that when they form narrow strips, coral and sand beaches 
are often found back of the swamps on exposed coasts. The 
vegetation on these mud flats can be divided into two classes; 
mangrove swamps, in which large trees are present, and nipa 
swamps, which are characterized by a growth of the stemless 
palm, Nipa fruticans. 
Mangrove-swamp forests, or “mangles,” as they are called 
locally, are usually made up of thick stands of medium-sized 
and even-aged trees. Normally they are very free from under- 
growth other than seedlings, and are characterized by the pres- 
ence of roots showing on or above the surface of the ground 
(Plates I, III, IV, V, XVI, XXXVIII, and XLI). Depending 
upon the species in question, these may take the form of erect 
roots, knees, high prop roots, or mere swollen roots with side 
branches extending along the surface of the ground. The air 
roots have a spongy texture and absorb air which serves for 
the aération of the root system. These peculiar roots are one 
of the most distinguishing characteristics of mangrove swamps. 
When the mud flats are not covered with water, the roots give 
a very peculiar appearance to the vegetation. 
