What if there was a tree that liked salt?

Science Illustrated - - CONTENTS -

Most of the so-called higher plants - land plants with trunks and branches, fruit and flow­ers - don’t do so well in salt wa­ter. Even el­e­vated salin­ity in the soil is enough to kill many species of tree, and salin­ity is a ma­jor prob­lem in in­land Aus­tralia as wa­ter-ta­bles fall in times of drought (or through ex­ces­sive ir­ri­ga­tion).

One ex­cep­tion to the “salt is bad” rule is the grey man­grove. This is the species found in Aus­tralia, and it’s one of the tough­est trees on the planet.

Usu­ally, Aus­tralian plants and an­i­mals are re­stricted into bands more or less as­so­ci­ated with cli­mate and rain­fall. Up north, we get the re­mains of an­cient rain­forests. Down south, more tem­per­a­ture species and the “dry scle­ro­phyll” eu­ca­lypt forests. In­land, hardy desert plants.

But grey man­groves are found in ev­ery state, even in Western Aus­tralia where there are huge gaps in pop­u­la­tions. A man­grove for­est at the Al­bro­hos is­lands is 500km from the next group of trees at Bun­bury, and 300km from one at Shark Bay.

The se­cret to the grey man­grove’s suc­cess is of course its tol­er­ance for salt wa­ter. A unique adap­ta­tion al­lows it to take sea wa­ter in, and ex­crete the salt from its leaves. But the grey man­grove can also tol­er­ate less salty con­di­tions, such as in times of flood.

An­other chal­lenge liv­ing in es­tu­ar­ine con­di­tions is the lack of oxy­gen in the soil. The grey man­grove deals with this via its dis­tinc­tive pneu­matophores, or aerial roots.

As well as act­ing as an­chors against the tide, the pneu­matophores are able to ab­sorb oxy­gen di­rectly from the air. All plants breathe oxy­gen through their roots, but usu­ally un­der­ground.

The fi­nal chal­lenge for a tree that lives in an en­vi­ron­ment that’s fre­quently un­der­wa­ter is in ger­mi­nat­ing off­spring. Seeds can sink and rot. So the man­grove uses a struc­ture called a propag­ule.

About the size of a wal­nut, the propag­ule floats in a nat­u­ral ori­en­ta­tion that points the even­tual roots to­ward the ground and the stem to the sky. It al­lows the new tree to get enough light, while also find­ing nu­tri­ents.

Be­cause of their re­la­tion­ship with salt wa­ter, grey man­groves are an ex­cel­lent in­di­ca­tor of wa­ter qual­ity, cli­mate change, and the over­all health of a coast­line.

This grey man­grove propag­ule has an­chored it­self in the sand and turned it­self up­right.

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