Interviewed: Alison Kim Shan Wee, molecular ecologist (kimshan.wee[at]nottingham.edu.my)
(Photo: Awash in sea water twice daily, mangrove trees have adapted to the challenging environment | Pic by Alison Kim)
HOW ARE mangroves, which grow by tropical and subtropical coasts, the same as plants in the deserts or the Arctic? Well, they are all extremophiles — a group of organisms that are able to thrive in extreme environments.
According to molecular ecology researcher Alison Kim Shan Wee, mangrove trees can live in soil of high salt levels, which could kill most other trees.
Coastal soil can harbour 10-times the salinity of inland soil. So, how do mangrove trees survive such a hostile environment?
Mangroves are a group of trees and shrubs that grow where the sea and land meet. Together with coral reefs and seagrass, they form the intertidal zone.
This zone usually contains high salt concentrations due to the tides that bring in seawater twice a day.
Mangroves developed three ways to tolerate salt: salt exclusion (roots), excretion, and accumulation (leaves).
Malaysia has four main groups of mangroves species, namely Bruguiera parviflora, Avicennia species (locally known as Api-api), Rhizophora species (Bakau Minyak), and Sonneratia species.
These trees have eye-catching roots: the stilt and prop roots of Rhizophora that seem to lift the trees, and the pencil roots of Avicennia that jut out of the ground.
“Stilt and prop roots grow like a tongkat (canes) to prop the Rhizophora trees upright. Such roots descend from a different plant lineage than the pencil roots of Avicennia trees,” says Alison.
But they have all evolved similar adaptations to thrive in the salt-rich intertidal zone. “That’s called convergent evolution,” she says — just like ducks and frogs having evolved webbed feet to swim better.
The stilt roots of Rhizophora trees play an important role in regulating their salt level. While other plants use glands in their leaves to remove salt through secretion, Rizhopora are ‘non-secretors’ and they lack the salt glands.
Rather, Rhizophora trees use their roots and reduce salt intake via ultrafiltration, Alison explains. Layers of microscopic pores in the roots exclude salts.
On the other hand, Avicennia, a salt-secretion species, have leaves that play a remarkable role in removing salts from the tree. These plants have salt glands in their leaves. The glands filter salts out of water in the plant and secrete them.
The secreted salts then form salt crystals under tiny hairy leaves. They taste salty if you lick them.
Some species accumulate salt in their leaves and barks. When these parts are old and ready to shed, they drop off the mother tree. The salts eventually return to the sea.
These adaptations – in the roots or leaves – matter when we select which species to plant in coastal sites. A better understanding of these adaptations would improve the survival of saplings and efficiency in a restoration project.
[Edited by YH Law]
G.G. Jiang et al. 2017. Salt management strategy defines the stem and leaf hydraulic characteristics of six mangrove tree species. Tree Physiology 37: 389-401.
R. Reef et al. 2015. Regulation of water balance in mangroves. Annals of Botany 115: 385–395.
H.T. Nguyen et al. 2015. Growth responses of the mangrove Avicennia marina to salinity: development and function of shoot hydraulic systems require saline conditions. Annals of Botany 115: 397-407.
M. Griffiths et al. 2008. Differential salt deposition and excretion on leaves of Avicennia germinans mangroves. Caribbean Journal of Science. 44: 267-271.