Evening star., 1938-12-08
Of Stars, Men
And Atoms
Notebook Of Science Progress
In Field, Laboratory
And Study
By Thomas r. Henri.

By Thomas r. Henry.
A new type Of land-building which Is
increasing the territory Of Uncle Sam has
been found by scientists of the Carnegie
Institution Of Washington.
Its agents are red and black mangrove
trees which grow on low, tropical coasts
such as those of Southern Florida. Most
of these trees have peculiar root systems
which enable them to grow in soft soils
and survive the floodings Of tidal areas.
The mechanisms by which they build up
land out Of the sea are shown in an ex-
hibit prepared for the annual meeting
of the Carnegie trustees here tomorrow.
Even where the trees are not actually
active as land builders, according to the
Carnegie botanists, they aid materially
by holding the shores against severe
erosion.
The vigorous young mangroves, &C
cording to the explanation of the exhibit
issued by the institution, advance into
the shallow bays and sounds and soon
grow such mass of roots that the loose
soil materials are held In place. Debris
from their growth, other organic mate
rials and sands become deposited in the
shallow, slack waters over these swamps,
where they accumulate in such guam
titles as to build up the soil level above
the average high tide.
Many acres of new land already have
been created by this method, the botanists
claim. This is shown by comparing old
surveys with those made recently, and
then visiting the areas where increases
are shown to determine what has caused
them. Between Key West and the main
land, 1t was found, several hundred new
acres have been created.
One of the largest mangrove swamps
in the world is in the southwestern Ever
glades. In places the forest extends five
or more miles inland. The soils here are
mostly raw, fibrous peat, frequently over
II feet deep, indicating that organic
debris has accumulated over long pc
riod. 1t is probable, says the Carnegie
report, that many thousands Of formerly
small mangrove islands of the region
grew together to form mangrove land,
thereby pushing further and further into
the Gulf of Mexico.
The stout, arching prop-root systems Of
the red mangroves and the long, horizon
tai roots Of the black mangroves with
their finger like projections not only pre-
vent the normal invasion Of the land by
the sea, but retard the extreme erosion
caused by hurricanes which wash high
waters over these swamps. II the soil
was not held by the mangroves much of
the land would be washed away even by
normal waves and tides. Engineers have
encouraged growth Of the trees along the
embankments and fills.
The swamp says the Carnegie report.
develops in an orderly sequence of plant
communities. The pioneer plant along
the outer edge is the red mangrove. 1t
is the most active in starting new land
areas for it is capable Of growing in water
always submerged by tides. 1t spreads
great distances by remarkable seedlings
that germinate on the parent plant and
develop long, dart-like roots. When these
seedlings are released they fall into the
water or upon soft soils. Some take root
on the spot, but most of them float away
in currents to become stranded in the
shallow waters over shoals or along low
coasts. Thus a new mangrove swamp Is
started
The rate and direction of dispersal of
these seedlings from Southwestern Flor
ida toward the Tortugas Islands, 60
miles out in the Gulf of Mexico from
Key West. have been studied by Car
negie scientists. The experiments indi-
cate that they can travel rapidly and
survive long journeys without losing their
ability to grow. They may even cross the
the same species Of mangroves are found
in West Africa.
The red mangroves are followed by
black mangroves. The pioneer thickets
build up the soil until the water becomes
shallow. Then the black mangroves and
other salt marsh plants add more debris
to this swamp until, when sufficient
height is reached, inland plants are able
to invade the area. Thus there is more
or less regular succession of mangrove
zones from open water to upland vege-
tation. Further evidence of this process
is given by the soil deposits under some
of the inland forests near mangrove
areas, which are layered with mangrove
peat.
1T seems" says the Carnegie report,
that much Of Southern Florida is pro-
tected from erosion by mangroves and
some Of 1t is being built up by the active
mangrove growth. Experimental plant
ings Of mangroves on many types of soil
indicate that this process could be
speeded up and land formed in desired
places. One Of the coral fragment reefs
at the Tortugas Keys is being so planted
to test the method in an exposed location.
Without mans aid mangroves are quite
ehcient. With human agencies they may
prove even more valuable"