how plants cope in the mangroves: dealing with salt water

CONTENTS

Fauna index

How plants cope in the mangroves

Peter K L Ng and N Sivasothi (editors)

The problems of salt water

Plants experience many problems living in or near sea water which is 'physiologically dry' because most plant and animal tissues are more dilute than seawater. For osmosis to occur, water must move from a more dilute to a more concentrated compartment. This is why when one waters a normal plant with sea water, the plant will wither and die as the salty soil now extracts water from the plant instead of replenishing it!

The fluctuating salinities experienced in mangroves due to tides, dessication and weather make the situation more unpredictable and worse. Depending on its location, soil quality may vary from well-aerated peat, anaerobic clays or sandy soils. In Singapore, most mangroves are found on clays which constitute the 'mangrove mud' usually associated with this community.

During the rainy season, rivers emptying into mangroves become swollen and may deposit large quantities of sediment. In view of this unstable and harsh environment, it is no wonder that mangroves have a low diversity of species compared to the terrestrial tropical rain forest types as few plants have their special adaptations. In Singapore, for the flowering plants, there are only about 60 mangrove and mangrove associate species whereas there are about 890 species in Bukit Timah Nature Reserve, which is representative of tropical evergreen rain forest!

Sungei Buloh Besar drains into
the Western Johor Straits c. 1988.
The cleared area just right of the river
is the site of the present Visitor Centre
of Sungei Buloh Nature Park

A view of Bukit Timah's
primary forest, host to almost
900 species of flowering plants

However, many mangrove species can grow well in freshwater. A Bruguiera cylindrica forest is found in Christmas Island growing exclusively in freshwater high on a plateau, left behind by failing sea levels over the millenia. Bruguiera sexangula has been grown in an inland freshwater stream at the Bogor Botanic Gardens (Java) for more than a hundred years!

Bruguiera gyrnnorhiza and Acanthus species have been cultivated in freshwater tanks at the Department of Biological Sciences, The National University of Singapore for more than a decade. What stops them spreading into freshwater areas is their inability to compete with the freshwater species - they can grow in higher salinity environments because they have adaptations for this which the freshwater species lack.

Bruguiera gymnorhiza treelet
growing in a fresh water tank
of the garden of the Department
of Biological Sciences,
National University of Singapore

So the high level of salinity in the mangrove environment will lead to high concentrations of salt in tissues which will severely damage metabolic processes, leading to death. How do mangrove plants prevent this?

Salt secretors
Some mangrove plants like Api-api (Avicennia species), Jeruju (Acanthus species) or Kacang-kacang (Aegiceras corniculata) are salt secretors. The common salt concentration in the sap is high at about one-tenth that of sea water. Salt is partially excluded by the roots and the salt is excreted by the salt glands by the plant expending energy.

The concentrated salt solution evaporates near the gland, becomes crystals which are removed by wind or rain. One can taste the salt by licking the leaves of these species to confirm this!

Salt crystals on the leaves of
Acanthus ilicifolius

Ultrafiltrators
Other mangrove plants like Bruguiera, Lumnitzera, Rhizophora or Sonneratia species are non-secretors. They can selectively absorb only certain ions (electrically charged atom(s) andlor group of atom(s) which a salt becomes on going into solution) from the solutions they come into contact with by a process called ultrafiltration. However, even with this, exclusion is not complete. Some salt is lost by transpiration through the leaf surface or accumulates in some cells of the leaf. Suggestions have even been made that some species deposit a good part of the excess salts in the old leaves which are shed.

Other adaptations to cope in the mangroves:
Breathing roots
Vivipary

What is mangrove?
Introduction

The Ecosystem
Abiotic
Biotic

Value
Intro
Products
Indirect uses
Potential uses

About Mangroves
in Singapore
History
Mangroves to visit
Conservation

From "A Guide to Mangroves of Singapore", Peter K. L. Ng and N. Sivasothi (editors)
Volume 1: The Ecosystem and Plant Diversity and Volume 2: Animal Diversity
Authors: Kelvin K. P. Lim, Dennis H. Murphy, T. Morgany, N. Sivasothi, Peter K. L. Ng,
B. C. Soong, Hugh T. W. Tan, K. S. Tan & T. K. Tan
BP Guide to Nature Series published by the Singapore Science Centre, sponsored by British Petroleum
© 2001 Raffles Museum of Biodiversity Research, The National University of Singapore & The Singapore Science Centre