Monday, 23 June 2014

Seeing whats Under the Mud

Mangroves are famous for their aerial root systems but until recently, I did not know how much of the root system was hidden below the surface.  Cyclone Yasi, which hit the Queensland Coast at Cardwell in 2011, completely stripped away some of the smaller, more exposed swamps and revealed the substrate.  To my surprise, the ground was largely composed of mangrove roots rather than mud.

Mangrove swamp destroyed by Cyclone Yasi
Remains of a stand of mangroves, Cardwell
Broken mangrove roots and mud
Shredded remains of roots cover much of the surface
Rhizophora mangroves
What the stand would have looked like before the cyclone
Mangrove mud is actually toxic to mangrove trees as many toxic compounds occur in the anoxic mud. Mangrove trees pipe air down into the mud to create an oxyzone around each root that destroys the toxic compounds and that is why mangroves have stilt roots.  The roots contain aerenchyma, which is a tissue with air spaces in it to allow the flow of air.  Mangrove roots also follow crab holes which also introduce air into the mud and create non-toxic zones.  Knowing that mud is hostile to roots, I assumed that the roots would taper off quickly.  Many wetland plants have small root systems due to the lack of oxygen in the soil.  When Cyclone Ita came along in 2014, it exposed some mangrove roots on the sandy mangrove coast at Cooya Beach allowing their structure could be seen.
Rhizophora root system
New roots growing out from the stilt root
Where the root strikes the ground, there is a star burst of lateral roots.  The roots give rise to a finer set of roots four times which is to say that the roots have forth order branching.  The effect is almost fractal like a snowflake.  Each of the rootlets resembles a small pine tree.

Fine roots, Rhizophora

This root structure is entirely different to that of trees that live on the same sand just above the tidal zone.  Most species of trees have roots that are like cables and these roots run for distances of many metres and branching is generally simple dichotomous branching where the root divides into two similar sized branches.  Strand trees have shallow root systems which run through the top soil which is where the fertility is.  Their roots also seem to stay above the water table which can be close to the surface near a beach.  Calophyllum trees, beach she-oaks and paperbarks have this pattern.

Root structure of a Calophyllum inophyllum
A giant Calophyllum tree toppled by Cyclone Yasi reveals cable-like roots
Another root system design is to have strong scaffold roots which radiate from the trunk.  These are elliptical in section and composed of stiff timber to resist flexing.  Secondary tap roots descend from the radiating scaffold roots.  Coral trees, Beach Almonds and Bloodwood trees have this pattern.

Roots of Terminalia catappa
Scaffold roots and secondary tap roots of a Beach Almond (Terminalia catappa)
So what does all this mean?  Roots keep trees alive so it is important to understand roots.  Mangrove roots are special as they have an additional function that roots of trees on land do not have, the piping of air into the soil. The shape of trees and tree leaves is largely controlled by their ability to pipe moisture to their leaves - the water potential theory.  Mangrove tree root systems may be optimised for piping air rather than seeking moisture and fertility.  The old theory that stilt roots are needed to prevent trees from falling over in the soft mud is clearly rubbish.  In no case have I seen a mangrove tree topple over and pull up a root ball as rainforest trees often do.  In every case, the mangrove trees roots have snapped at surface level and the tree has been torn away so the ground is stronger than the tree.

Mangroves are also putting a huge amount of carbon below the ground surface where it may be retained indefinitely. Growing mangroves may be a way of stripping some carbon from our atmosphere.  Sewage plants usually discharge into the mangroves and in many coastal towns, the surrounding mangroves have grown taller and denser as a result of the lower salinity as treated sewage dilutes the seawater and irrigates the mangroves in the dry season.  Some carbon is already being captured due to this accidental process. Perhaps we can use mangroves to sequester carbon?  Conversely, clearing mangroves releases carbon into the atmosphere.  When cane farmers cleared mangroves to grow cane, the soils firstly became acid and poisonous but as the carbon in the soil oxidised and disappeared into the atmosphere, the ground surface fell sometimes by a few metres and some areas ended up useless and permanently underwater.  Clearing of mangroves is no banned in Australia for this and other reasons.

More info on carbon sequestration in mangroves

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