Mangroves-response

SJR
Dec 4, 2020
9 min read

Hello there friends! Welcome back, I'm happy you are here!

We spent the last few weeks learning about mangroves! I hope you found the content engaging and interesting and if you didn't have time to read any of it - let this post serve as a place for you to touch base with the information then expand from here.

Let's learn the basic idea of what a mangrove is and then we'll review some special features of mangals (mangroves, mangrove forest, mangal = interchangeable). Before we wrap it up, we'll review, as always, why we should care - why it matters for the larger ecological picture of Earth. We've got a lot to cover, so let's get to it! :)

Mangroves are groups of trees and shrubs that live in what is known as the coastal intertidal zone. These forests are exclusively located in tropical or subtropical latitudes on Earth, however as climate change is creating warmer temperatures, some species are being found farther North and South than historically seen. You have likely seen an image of a mangrove somewhere, looking like a tangle of roots and branches seemingly just floating in the water or strangely elevated from the soil. Each tree or shrub starts as an individual plant however, over time, they grow in associations that can be a few miles thick off a coastline or in a riverine environment - or upto 3,860 square feet as seen in the Sundarbans Forest, an eerie and mystical UNESCO World Heritage Site.

So, a mangrove forest is located at the intertidal zone - what does that mean? That is the juncture between terrestrial environments and aquatic environments. Imagine a sandy-edged beach, a rocky shore, a shallow part of a reef, a sandy riverine floodplane and you have the beginning of a good home for a mangal. The location seems unlikely, right? We're used to considering trees and shrubs as strictly land organisms - and that is one of the features that makes mangroves interesting and unique.

Setting up shop in an intertidal zone is no small task.

-You have sandy or rocky, often nutrient deficient soil

-The lack of soil aeration creates a problem for any plant that needs to find oxygen

-The tide rushing in and out would topple over many plants, drowning them

-The salt water - wouldn't that just kill a tree?

I have come to admire these botanical beasts - they know how to adapt for sure! Let's look a bit more closely at the specifics of how they do this. Let me preface this by saying that mangrove species are organized a bit differently than, say OAK TREES. Mangroves are organized by their adaptive qualities and geographical locations more than their strict genetic makeup. Because of this, the +/- 80 species of mangrove may look quite different than one another in many ways, but they are still classified as mangroves because, let's say - their lifestyle.

A tree growing in sandy or rocky soil that is frequently submerged in water is a special phenomenon. Think about trying to stay rooted with constantly shifting soil beneath you and with the tides rushing in and out up to twice a day. Even more protected mangroves that aren't directly facing a raging tide are affected by rising and falling water levels; or in the case of rivers - currents and tidal fluctuations at the same time! To deal with this environmental pressure, mangroves have established a complex system of roots that help to stabilize them against the forces of the moving water. These root systems are what give mangroves their typical tangled-look. Some roots arch outwardly from the trunk and plunge down towards the water, rooting themselves in the soil away from the main trunk. These roots are called prop roots or stilt roots. Other roots, called knee roots, jut up from the soil and then re-submerge themselves again - thus looking like a knee.

Imagine being at the beach yourself, trying to stay standing as the tide comes in and out - - maybe that's a bit challenging. Now imagine if a circle of 5 people were holding hands and bracing themselves together against the tide - that would be more stable. In this same way, the complex branching aerial roots give mangroves a more stable foundation to survive the shifting waters.

It is easy to forget that trees and other plants require oxygen. We often think of them as making oxygen for us, but let's reorient that thought process with a little reminder that trees use photosynthesis to produce energy. The process of photosynthesis requires CO2, creating glucose. At this point, the cells of the tree require Oxygen to further use that glucose and make it available for the plant. This process is cellular respiration and it requires OXYGEN and the biproduct of this process is CO2. It just happens that plants 'exhale' more O2 than CO2 and that is certainly a convenience for us Oxygen-loving types.

Now that we've established a tree's need for oxygen, let's talk about how trees get it. Tree bark, roots and some fruits are covered in small speckly-looking cells known as lenticels. These are tiny openings in the surface material of the plant - that allow for gas exchange - bringing in Oxygen to the sublayers of tissue. Mangrove trees and shrubs have these like any other trees, however as we know - the roots and lower branches are often entirely submerged in water, so you would think they would not get enough oxygen. Many mangroves have evolved to have what are known as pneumatophores. These are vertical roots that jut up from the submerged roots of the tree or shrub. They range from straw to cone shaped and are hugely variable in size - ranging from 8 inches to 10 feet tall! These vertical roots are themselves covered in lenticels and are able to capture air and deliver it to the entire tree body. The pneumatophores also contain substances which are hydrophobic, which allow them to repel water and protect themselves from drowning if covered.

Interestingly, if you see the surface of an apple or a pear or a twig, you may see speckles - these are lenticels........you are seeing the mechanism for the gas exchange on the surface of plants and their fruits etc!!!! That is a pretty cool phenomenon!

With stabilizing root formations and lenticels+pneumatophores bringing in oxygen for respiration, the tree has some of what it needs. It also needs fresh water and other nutrients that may be deficient in the soil. Let's take a look at how some of that happens. For me, one of the most fascinating attributes of mangrove varieties, is their ability to survive in water-logged environments that have over 100x the salt content most plants could deal with. Although our first inclination may to assume they LOVE salt, the reality seems to be that they have evolved some work-arounds to avoid the death-by-salt scenario. Generally, mangroves are categorized as either 'SECRETORS' or 'BLOCKERS,' These categories have to do with how they deal with their saline environment. Some species, such as the black mangrove, are able to push salt through pores or salt glands on their leaves. The water eventually evaporates, leaving salt crystals on the surface of the leaves which may blow or wash away. These species often have an almost frosty look to them. Other species, such as the red mangrove, create barriers to the salt, never allowing it to enter the root system. In these cases, over 90% of the salt is prevented from ever entering the plant's vascular system.

There is another phenomenon that has been noted by some scientists, where salt water will enter the plant, but will be pushed into certain leaves. The high concentration of salt in these leaves will attract more water to them, giving them an enhanced succulent thickness. This process will eventually cause those leaves to fall off - taking the excess salt with them. This observation does not seem to have a strong prevalence from what I have read; most plants specialize in one of the other techniques of anti or de-salination.

Over time, silty build-up occurs at the base of the trees and shrubs as the tides move in and out. The water brings with it many tiny sediments that get trapped in the chaos of mangrove branches and roots and settle at their base.

These trapped sediments:

-build up around a mangrove - this can even mean extending a coastline or

a shallowing of a deeper area of water

-help to clear the water around mangroves, which is especially useful coral reefs,

which benefit from the increased sunlight available in clearer water

A few other distinctive features that can be found on mangroves include, waxy leaves, small hairs on the leaves and somewhat succulent leaves. All of these features serve similar purposes in preventing water loss through exposure to the sun or wind, as well as storing extra water.

Now that we know what a mangrove is and how it can miraculously survive its chosen environments, let's take a quick look at a list of some of the wildlife found in mangroves around the world. You will note the variety of wildlife that mangroves support, partially because of their aerial structure, partially because of their subaquatic structure. Because of the diversity of surfaces and micro-environments within one mangal, you will find an equally diverse array of wildlife present.

Our world's mangroves are home to:

-microbes and fungi (these feed on dead leaves, producing nitrogen/phosphorous/sulfur and iron that return to the mangroves

-underwater sponges/snails/worms/anenomes/barnacles/oysters

-crabs/shrimp/fish (especially in their early stages; some fish stay up to 6 years before they leave a mangal and head for the reef or open ocean!)

-birds!!!!!!!!! so many birds!!!!!!

-ants/termites/scorpions/spiders/fireflies

-bees/butterflies/moths/bats (essential pollinators)

-proboscis monkey

-snakes/lizards/frogs/crocodiles/alligators

-bengal tigers (unique in that they eat land and sea prey)

-fiddler crap/mud lobster/mudskipper

-manatees are found in the seagrass environments that are often at the edge of a mangrove

It is hard to overstate how integral mangroves are to so many other species. Serving as a nursery for many aquatic species, mangroves create a safer place than the open sea or the coral reefs, and many creatures nest here until they are strong enough. Not only are mangroves vital landscapes, but they also serve as strong carbon sinks (in addition to salt marshes and seagrass environments). Carbon sinks draw carbon down out of the air and into a sequestered environment, so as you are continuing to learn about mangroves and other fascinating landscapes, you may run into the term BLUE CARBON. This is a reference to carbon that has been sequestered (stored) in an aquatic environment. Mangroves are extremely important here, as they are able to sequester upto 1450 pounds of carbon a year per acre of mangrove. That is roughly the amount of fuel one car would use to drive 7,650 miles.

Mangroves are at risk of being deforested for development or for the creation of shrimp farms. Shrimp farming is, in fact, the main threat to mangroves. Not only does this affect the home of all the creatures you learned about above, it reduces the blue carbon we are able to capture. Additionally, shrimp farming has a high rate of toxifying the soil as a result of the shrimp feces and other harmful substances (such as pesticides) that leach into the sand. Because they are difficult to sanitize and clean, shrimp farms are often abandoned, leaving a rundown plot of soil that has been contaminated.

Mangroves have been shown to play a significant role in stabilizing coastlines. Their ability to break the force of the tide not only helps prevent erosion of the coastline, but it helps reduce or prevent storm surges from impacting the coastline. This extremely important role, among the many other roles a mangrove can play - has caused many countries to endorse reforestation efforts. It seems that it has been quite a learning curve to plant new mangrove forests.

Unlike most plants that germinate after the seed leaves the parent plant, mangroves germinate fully living plants straight from the parent tree. This is known as VIVIPARY and the propagules (basically, ready to live trees!) can float on the surface of the ocean for days upto a year until they sink and take root away from the parent plant. This phenomenon is unique in each variety of mangrove, so if you'd like to learn more, perhaps start by looking up the variety of mangrove you are interested in and proceed from there.

This special way of reproducing has made reforestation a bit more of a tricky process than it would be for strictly terrestrial trees, as the saplings of mangroves has specific wet/dry time needs as they grow and become stronger. As reforestation efforts are being learned and honed, professionals are doing amazing work to successfully re-instate mangroves on coasts where they have been lost. The coast of Senegal is particularly impressive in the projects that it has put forth to fortify its coastal rice paddies by replanting mangroves. Take a look!

I'd like to take one last minute here to state that mangroves in places like Hawaii and Tahiti, are actually considered invasive species. It is important to remember that most things are not black and white and although there are numerous amazing attributes of mangroves, restoration and rehabilitation of any environment must be looked at holistically and take a larger perspective. It could be said that nothing is a neutral species everywhere. Ecosystems are robust and yet fragile in many ways. Take a minute to remember the other side of any story you are interested in; it will offer you a more dimensional perspective.

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Thank you for learning about mangroves with me! Let's take a moment to review all that information!

MANGROVES:

-trees/shrubs: live in tropical/subtropical latitudes

-live in intertidal zones

-specific adaptations to environment:

-specialized root systems for stability

-salt prevention or extraction (up to 90%)

-pneumatophores covered in lenticels to get oxygen

-waxy/hairy/succulent leaves to prevent water loss

-amazing environment for many other creatures/fungi/bacteria // especially as a nursery

-traps sediments; stabilizes the coastline / cleans the water