and Physiological Adaptations
environment where the water is filled with high
concentrations of dissolved salts, water levels are
constantly changing, and in oxygen deprived
sediments would certainly exclude most plants.
However, mangroves thrive in these conditions. They
have evolved certain morphological and
physiological responses, which allow them to avoid
the pitfalls of these harsh conditions.
are facultative halophytes which means salt water
is not a physical requirement for growth. Most can
grow well in fresh water, but mangrove communities
are not usually found in strict freshwater
environments. There are two possible explanations.
Most strict freshwater habitats exist where tidal
inundation does not occur. Although not a direct
physical requirement, tidal fluctuation plays an
important indirect role in mangrove distribution.
Tidal fluctuation results in the reduction of
competition due to alternating wetting and drying,
transportation of relatively clean water and
nutrients in, exporting wastes, detritus, and
sulfur compounds, effective dispersal of
propagules. Where evaporation is very high, tidal
fluctuation wash excess salt away preventing
excessively high soil salinity concentrations. Due
to the above factors, mangrove systems reach
greatest development around the world in low-lying
regions with relatively large tidal fluctuations.
In Red mangroves,prop roots extend above most high
tide levels. Black mangroves are excluded as water
depths increase. In freshwater communities other
species may out compete the mangroves for space.
are found where salinity ranges from 0-90ppt. Red
mangroves are found where soil salinities range
60-65ppt. Black and White have been recorded in
soil salinities greater than 90ppt. In restricted
bays and flats water salinities often range over
40ppt. For most species of plants these conditions
would inhibit growth. On the other hand mangroves
have evolved physiological responses to utilize
these specific conditions to out compete other
species under these usually harsh
species utilize two major methods of internal ionic
regulation. Salt excluding species do not take salt
water internally. The Red mangrove is a salt
excluder separating freshwater at the root surface
by creating a type of non-metabolic ultra
filtration system. Transpiration at the leaf
surface creates negative pressure in the xylem.
This causes a type of "reverse osmosis" to occur at
the root surface. The salt concentration of xylem
sap in the red mangrove is about 1/70 the salinity
of surrounding seawater, but this is l0 times
higher than in normal plants.
and White mangroves regulate ionic concentration by
excreting salt through glands
on the leaf
This temperature sensitive enzymatic process
involves active transport with energy expended.
Xylem sap is 1/7 concentration of salt water. This
is l0 times the concentration of the salt
three species exhibit to a small degree a
combination of both methods of salt regulation. The
Red mangrove stores and disposes of excess salt in
the leaves and fruit. Black and White mangroves are
capable of limited salt exclusion in the
most plants anaerobic sediments are not a problem
and even assist in reducing competition.
and spongy tissue in roots and modified branches
facilitates gaseous exchange. In Black mangroves,
(up to l0,000 per tree) extend up to 20 cm above
the sediment. Prop roots in the Red mangrove
possess many lenticels which allow O2 diffusion
with passage to underground roots by means of open
passageways (aerenchyma). In the White mngrove,
lenticels in the lower trunk obtain 02 for
and pneumatophores may be present when found in
oxygen deprived sediments.