It is obviously green, why is it called red tree?

Whenever the tide rises, a romantic scene is staged in Xiamen Coastal Wetland Park - the woods on the three islands are submerged by sea water, and only the crowns of some taller tree species are exposed above the water, like a "forest on the sea". If you look down from a high altitude, you will find that this is actually a painting composed of hearts and five-stars.

Mangrove landscape of Binhai Wetland Park on the side of Xiamen Xiatanwei Torch Bridge

(Photo source: School of Environment and Ecology, Xiamen University)

But seeing this, some inland friends may be curious: There are actually trees that can survive in sea water and even grow and reproduce normally. Isn’t this too magical?

That’s right, this magical tree is the mangrove.

01

The leaves are obviously green, so why are they called red trees?

Specifically, mangroves refer to a type of woody plant that grows in the intertidal zone of tropical and subtropical coasts, not a specific species. As for why they are called mangroves, as the name suggests, it is not because its leaves are red, but because this type of plant will appear red in some cases.

Many members of the mangrove plants belong to the Rhizophoraceae family. The bark of these Rhizophoraceae species contains a substance called tannin, which will be oxidized to red after contact with air. In this way, isn’t it "mangrove"?

The tannins in the bark of mangrove plants are oxidized to red when exposed to air.

Image source: environment.bm

Mangroves can be divided into true mangroves and semi-mangroves according to their characteristics. True mangroves generally refer to species that are only distributed in the intertidal zone of the coast, while semi-mangroves are species that can grow in the intertidal zone and survive on land. But whether it is true mangroves or semi-mangroves, they have one common feature, that is, they can survive in an environment where seawater is periodically flooded.

The tide rises and floods the mangroves. Photo credit: Photo by the author

This is unimaginable for many plants. As we all know, if you water your plants with salt water, your favorite flowers may die in a few days. The high-salt and oxygen-deficient environment caused by the periodic inundation of intertidal seawater is very harsh for plants. Not only that, the high temperature and scorching sun at the seaside in tropical and subtropical areas is also a great challenge for them.

The magic of mangroves lies in the fact that they have evolved many interesting traits, such as unique root structure and special reproduction methods to adapt to such harsh environments.

02

Not only does it have strong rooting ability, but it can also "spit" salt?

Let's first take a look at the unique root structure of mangrove plants. Common root types of mangrove plants include: prop roots, shoot roots, surface roots, plate roots, finger roots and knee roots, etc. Based on these names, it should be easy for everyone to imagine the general appearance of these special roots. These special roots contain rich aeration tissues, which can help mangrove plants better exchange gases.

The prop roots of mangrove plants. Image source: asknature.org

In addition, in order to cope with the high-salt environment, mangrove plants have also developed a series of salt avoidance and salt secretion measures. On the one hand, they protect cells from the effects of high salt by controlling the intracellular salinity below the threshold; on the other hand, they excrete salt from the body through organs such as salt glands.

Salt secretion phenomenon of mangrove plants. Image source: ocean.si.edu

03

Do viviparous plants really exist?

Compared with the above tricks, mangrove plants have another trick to cope with the extreme environment of the intertidal zone: viviparity! Speaking of "viviparity", you probably don't think it's anything special, after all, it's too common for mammals. But it's not hard to believe that the so-called "viviparity" phenomenon also exists in mangrove plants.

Of course, this is not the same as the other, but the common point is that the offspring begins to develop on the mother's body and will not leave the mother until it matures to a certain stage. This kind of "viviparity" also exists in rice, but it is called panicle germination. The rice seeds on the panicle will germinate immediately when they encounter suitable environmental conditions. This will have a negative impact on agriculture. If the seeds have germinated, what can we eat?

Back to the topic, for many mangrove plants, the "viviparous" phenomenon means that their seeds germinate before leaving the mother's body, then grow into rod-shaped hypocotyls, and finally fall from the mother's body into the mud in the intertidal zone.

Hypocotyl of a mangrove plant. Image source: mangroveactionproject.org

Viviparity is a special reproductive mechanism derived from mangrove plants to cope with the intertidal environment. Its importance to the survival, reproduction and expansion of mangrove plants is self-evident. Compared with adult individuals, the survival of seeds in harsh environments is undoubtedly more challenging. At this time, if they can germinate on the mother body and rely on the mother's nutrients to develop to a certain stage, the survival rate of these "seeds" can be increased.

Hypocotyl rooted in the intertidal zone Image source: References

Moreover, the development of seeds into hypocotyls on the mother body can not only increase the survival rate of these "mangrove babies" in the intertidal zone, but also allow them to cover themselves with a solid layer of "armor" when they travel across the ocean.

During the development process, the hypocotyl accumulates a certain amount of tannin, which is the substance mentioned above that makes the mangroves "red". Tannin is a natural preservative. When the hypocotyl drifts along the ocean current, the tannin can provide a certain degree of protection to the hypocotyl, making it less likely to be corroded by seawater. It must be said that mangrove plants are really a magical group. During the long evolutionary period, they have tenaciously and successfully coped with the adjustments brought about by the harsh environment.

04

Can the Coast Guard continue to hold its ground?

Due to reclamation, improper development and other reasons, the global mangrove area has been declining year by year since the 21st century. The Food and Agriculture Organization of the United Nations (FAO) Global Forest Resources Assessment Report (2015) shows that the global mangrove area in 2015 was 14,752 kha, a decrease of 3.98% from 2010.

Changes in the area of ​​mangroves in the world from 1990 to 2015 Image source: self-made by the author

In China, the growth territory of mangroves has been encroached upon due to factors such as aquaculture, real estate development, and construction of ports and dock facilities. Moreover, sewage discharge and eutrophication caused by aquaculture and livestock and poultry farming have also caused a certain degree of damage to mangroves.

Fortunately, as the ecosystem service value of mangroves has been increasingly valued, the country has strengthened the protection of mangroves and carried out ecological restoration of mangroves, and the area of ​​artificial forests has increased year by year.

Mangroves have existed for tens of millions of years. When the tide is high, they are flooded by sea water, like green islands in the water. When the tide is low, their intertwined roots stand in the intertidal zone like coastal guards. Who would want the "forest in the sea" to disappear?

Produced by: Science Popularization China

Author: EVEE School of Life Sciences, Peking University

Producer: China Science Expo

References:

Alongi, DM (2012). Carbon sequestration in mangrove forests. Carbon management, 3(3), 313–322.

Amos, W., & Harwood, J. (1998). Factors affecting levels of genetic diversity in natural populations. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 353(1366), 177–186.

Hoque, RR, & Balachandran, S. (2019). Handbook of environmental materials management. CM Hussain (Ed.). Cham, Switzerland: Springer.