In tropical and subtropical regions, on the intertidal mudflats of the coast, the growing bushes are called "mangroves". At high tide, some of these trees are completely submerged, some have only a partial canopy exposed to the water, and at low tide, the trees and the silt under their feet are completely exposed.
Image source: tentree.com
In this unique forest that is often soaked by the sea, whether it is the fish, shrimp and snail crabs that grow under the trees, the water birds that live in the trees, or even the trees themselves, they have developed a special set of survival skills to cope with the rise and fall of the tide.
Monster tree roots
Muddy beaches are not suitable for tree growth compared to solid soils inland — the silt is too soft to support a tall canopy; the microbes in the silt consume so much oxygen that there's not much left for the roots to breathe. Not to mention that every day at high tide, the roots and even half of the trunk will be soaked in salt water and subjected to the impact of the waves. In order to adapt to this high humidity, high salt and lack of oxygen environment, the trees in the mangrove forest have grown various forms of strange roots.
At high tide, the trees in the mangrove forest are partially submerged by seawater (above), and after the low tide, the trees are re-exposed to the air (bottom), at which point the animals in the mangrove also begin to become active.
The first thing to be solved, of course, is the big thing of breathing. There is too little oxygen in the silt, and these trees expose part of the root system in the air and breathe fresh air directly, which is called "air rooting". The root of the air is loose in texture, with countless gaps in the middle, and the root skin is covered with stomata.
After the roots of some trees grow in the mud for a while, they begin to pluck onions in the mud and rush out of the mud vertically. The breathing roots of the typical mangrove plant, the wood olive, are more interesting, and its roots first grow upwards, protrude from the ground and then grow down, re-rooted in the mud. So repeated many times, there is an additional knee-like arch on the ground, called "knee root".
To support the canopy on muddy ground, special root structures are also required. Some trees have the lower stems that expand outward and flatten out to become plate-like, while others grow horizontally under the soil to form a network. There are also breathing + support dual function metamorphosis roots - the main trunk and side branches of the red sea olive grow a lot of air roots, like banyan trees, one by one hanging down into the soil, not only can provide support for the canopy, carrying can also play a role in breathing.
Plants are also born
In the mangroves, it is also difficult to reproduce offspring. The soil under the trees is often flooded by the tide, and the seeds may drift into the sea with the water, and even at low tide, there are flocks of crabs and snails, waiting for the delicacy to fall from the sky. The trees' countermeasure is to "raise the seed into a bud" on the branch and then let it leave the mother plant, which is quite similar to the birth of mammals.
After the fruit of the mangrove plant matures, it does not fall off directly, but will germinate on the mother plant, absorb the nutrients provided by the mother tree, and slowly grow. The seeds of the tung tree and the white bone soil will quietly grow into small seedlings in the fruit, and only after the fruit has landed, the young shoots will break through the skin. The seeds of autumn eggplant and red sea olive will quickly elongate on the mother plant and become pen-shaped fetal seedlings.
In many kinds of trees in mangroves, the fruit will gradually develop into slender strips (above). These fruits are not in a hurry to fall, but grow and develop on the plant until after germination, leaving the mother plant, falling into the soil, and directly becoming a new sapling (below).
These fetal seedlings are heavy at the upper end and light at the lower end, and if the tide is low and the seedlings fall, they will root down, the buds will go up, and they will be inserted on the mudflat, as if they were artificially planted. The body of the fetal seedling is rich in tannic acid, and the taste is very bad, even the small crabs and snails that eat fallen leaves are dismissive of it. The seedlings escaped many threats, began to pull out real leaves, and eventually became a member of the mangrove forest.
Diners in the mud
After the low tide, the mangrove ground became lively, and countless animals of all sizes burrowed out of the cave. Mangroves drop a large number of leaves, dead branches, and residual flowers every day, and the subtropical heat and seawater at high tide quickly break down these fallen leaves into easily digestible organic matter. The animals, gathered in the mud under the trees, also got busy and prepared to eat.
The leaves, flowers, and fruits of trees fall to the mud and often become a good meal for crabs.
Crabs are regular visitors to muddy terrain, crawling out of their burrows at low tide and wandering around the soft mud in search of food. The most common crab under the mangrove forest is the tide crab, the high-profile guy holding a large claw is a male crab, and the female tide crab is much more low-key, and both claws are equally small and inconspicuous. They eat the sediment along with the organic matter in it, and then expel the sediment, and if they can pick up a fallen leaf or residual flower, it is considered a big meal. At high tide, they burrow into the hole and block the hole with mud blocks. Even if the sea floods the beach, the crabs can still be in a hole in the air and wait for the next low tide.
Tidal crabs. Image credit: 123rf Gallery, photo dave montreuil
The star in the mud is nothing more than mudskippers. The shape of the mudskipper is very cartoonish, with a large head and a small body, and its large eyes protrude like a toad. They bounce around quickly on mudflats after low tide, and the combined suction cups of their ventral fins also allow them to easily climb on almost vertical roots or branches.
Mudskipper raw has a big head and a pair of large eyes, and the shape is very cartoonish. Their ventral fins are specialized into suction cups, which allow them to "climb" onto the stems of trees.
Mudskippers not only breathe with their gills, but also exchange air with moist skin, they live on land most of the time, and if they stay in the water all the time, they may drown! Therefore, at high tide, mudskippers will gang up and avoid the sea on the trunks of trees.
There are holes in the top of the trees
Compared with the residents under the mud on the bottom floor, the animals living at the top of the trees do not care much about the rising and falling tides. At the top of the mangrove forest, the heron birds nest collectively to give birth, and the egrets, oxback herons and night herons will grab the "homestead" on the canopy, noisy and noisy.
Egrets nesting in mangroves
It's not that there's anything special about the canopy of mangrove plants, but that nesting here has an incomparable advantage: proximity to the "canteen". On the beach under the mangroves, there are countless sand silkworms, snail shells, shrimp and crabs and small fish, although humans will have a hard time walking in the mud, but the water birds with slender legs and feet walk easily in this canteen.
The blue-gray heron, known as a water bird but can't swim, can only carefully stand in the shallow water, staring closely at the water, once the target is determined, it will strike like lightning. The stocky blue-footed sandpiper wanders through the water, sometimes plunging its head into the water to catch a fat mudskipper or a small crab with teeth and claws, and feasts on it.
Written by Peng Peng
Photography / Chen Liangjun
Magazine Editor/Zhang Yu
WeChat Editor/Zifan
This article is compiled from the magazine Naturalist