Mangrove Plants 1: Getting to the root of things

Often, I get besieged by botanists (including closet botanists and ornithologists) due to my botanical inaptitude. So here I am atoning for my zoological bias and aversion to all things from kingdom Plantae.

To start off, the swampy mangrove is a good place to introduce and highlight how highly adapted plants can be since there are less than 30 true mangrove plant species in Singapore and many have developed unique adaptations for the habitat.

Here, conditions on the ground is mostly muddy and unstable with low oxygen and high salt content from the sea. This poses various challenges to plants - imagine living in salty quick sand!

Hence, from ground up, plants that have evolved roots for support and coping with low oxygen and salt immediately gain a competitive advantage over those that do not.

One such group is Rhizophora. If asked how does one increase stability? Rhizophora's answer would be to grow more legs.

Members in this group develop prop or stilt roots that branch and loop from the trunk and branches. The result is a network of charmingly grotesque gothic pillars at the bottom of the tree.

When exposed during low tide, these long roots also help with oxygen intake.

These roots also have an additional trick that blocks salt from seawater entering the roots in a process called ultrafiltration.

In Bruguiera and Ceriops, the trees send their roots out far and wide, and increases anchorage by having bent, kneed roots at intervals that resemble the legs of people doing sit-ups on the ground.

This curious structures not only adds stability by increasing surface area of attachment and binding more sediment, but the parts that stick out also helps obtain oxygen.

In Bruguiera, the roots are also said to be able to perform ultrafiltration to remove salt.

Avicennia and Sonneratia also spread their roots far and wide for stability, but have spike-like breathing roots in place of kneed roots.

In spy or adventure movies, characters sometimes hide underwater and breathe using straws or tubes poking out of the surface. Breathing roots, or pneumatophores, work in a similar way.

The roots of Sonneratia are capable of ultrafiltration, but not in Avicennia. The latter, has other tactics which will be discussed in a future post.

But what is most interesting is how unrelated (or only distantly related) groups have somehow acquired similar adaptations at root level for mangrove living. Rhizophora, Bruguiera and Sonneratia can exclude salt when taking in seawater (ultrafiltration), while Avicennia and Sonneratia solve oxygen shortage with spike-like breathing roots.

This phenomenon is what scientists term as convergent evolution. Another example would be how dolphins (order Cetacea) and manatees (order Sirenia) have separately evolved fluked tails for swimming. Oops, mentioned mammals in a plant post.

Further reading:

1. Ng, P. K. L. and Sivasothi. N. 1991. How plants cope in the mangroves. A Guide to the Mangroves of Singapore Volume 1. Singapore Science Centre.