How And Why Mangroves Can Grow In Freshwater

Mangrove trees are a species of trees specially adapted to salty, marine environments where no other trees can survive. They grow in tropical and sub-tropical environments along coastlines around the globe. Mangrove trees are the only trees that can survive in harsh saline water, but why can mangroves also grow in freshwater?

Mangrove trees aren’t picky when it comes to salt or freshwater. While they certainly grow in either freshwater or saltwater, mangrove trees grow best in water that is a mix of 50% saltwater and 50% freshwater. This mixture of freshwater and saltwater is called brackish water.

Most plants, especially trees, can’t tolerate much salt in the ground, but mangrove trees have adapted to these climates. They grow very well in salty, tidal areas as well as in brackish water where most other plants wouldn’t survive the high salt levels. Read on to find out more about these amazing trees!

Can Mangroves Grow In Freshwater?

As long as mangrove trees are not completely submerged in water they can grow in freshwater or saltwater. You won’t typically find many mangrove trees growing in freshwater areas though because there is a lot of competition in these environments compared to the minimal competition in saltwater areas.

Mangrove trees also rely on tidal fluctuations for seed dispersal. The tide coming in and washing out helps to spread seeds far and wide instead of staying in one place. 

Mangrove trees can certainly grow well in freshwater environments but there are a few reasons why you won’t usually find many mangrove forests along lakes, riverbeds, or swamps. Let’s expand on these reasons.

You can lean more in our other article about the most common trees that grow in freshwater here!

Too Much Competition For Mangroves To Grow In Freshwater

These trees have carved themselves a niche as the only trees that can survive in the salty environment that they prefer. The only competition for space in these areas is from other mangrove trees

There are many more species of trees that can survive in freshwater, which makes it harder for mangrove trees to come in and get a foothold.

Just a few freshwater trees that thrive along riverbanks, and freshwater swamps include cypress trees, gum trees, willow trees, aspen trees, birch trees, and a few species of oak trees. These trees have been growing in these areas for centuries!

Tidal fluctuations are required for mangrove seed dispersal and since these seeds won’t flow upstream to freshwater locations, mangrove trees won’t settle in these areas on their own.

No Tidal Fluctuation In Freshwater Locations

In coastal areas where mangrove trees flourish the fluctuating tide provides several benefits. Seed dispersal is dependent on these tidal surges. The tide helps to remove waste and bring in clean water. In freshwater environments, there is usually very little fluctuation in water level.

Unless the climate plays a role in water levels, the waterline of freshwater bodies takes much longer to rise or fall. Droughts can shrink the waterways over time, while periods of heavy rains can make the banks swell, but the ocean tides fluctuate daily.

The seeds on mangrove trees have adaptations that help them grow in the harsh salty environment these trees call home. For one, they germinate while still hanging in the branches. They can stay on the tree for up to three years, growing a long root so that they have an early start when they reach solid ground.

Mangrove seeds will also float when they drop into the water. This trait lets some seeds travel miles away before they reach solid ground. Once there, they establish quickly because they’ve already started growing roots.

Less Competition In Saltwater

If mangrove trees were growing along riverbanks, when the seeds fall, they would end up getting washed along the banks where other trees are already growing. This would make it tougher for them to begin growing because there isn’t much real estate.

That is until the seeds get washed into brackish waters where other plants can’t tolerate the salt.

It’s easier for mangrove trees to establish and continue to grow where there is little to no competition. No other trees can grow in brackish or salty water, so these areas are where mangrove trees really thrive.

Few plants have ways to deal with saltwater. Other than mangrove trees, seaweeds, seagrass, algae, and a few types of ferns, can withstand saline water. Mangrove trees are the only tree species that have ways to deal with saltwater.

If you’d like, you can read more about the most common trees that grow in saltwater here!

Why Can’t Most Plants Survive Saltwater?

First off, saltwater will dehydrate plants. Salt in the soil absorbs water and then creates drought-like conditions for the plant and it dries out. When a plant absorbs salty water, it essentially gets dehydrated.

We all know we shouldn’t drink salt water because it speeds up dehydration in humans. In the same way, we get dehydrated by saltwater, plants also dry out because of salt in the water.

Another reason salt water is an issue for plant life is that when salt is dissolved in water, the sodium and chloride ions split up and separate. The roots of plants will readily absorb chloride ions, which are then transported to the leaves. There, these ions will accumulate to toxic levels and eventually lead to quick degradation the plant.

According to Purdue University, salt is bad for plants. Not only are chloride ions bad for plants, but sodium ions can also damage plants. Sodium ions in the ground will replace essential nutrients that plants need such as potassium, magnesium, and calcium.

Mangrove Trees Are Unique

What makes mangrove trees unique, is they have ways to remove up to 90% of the salt from the water. They can either separate and store the salt away by storing it in older leaves and bark. The bark is then replaced with new, and the old leaves drop to the ground, taking the excess salt with them.

Other mangrove trees can excrete salt from pores in their leaves. Some species have small pores at the base of the leaves, while others excrete the salt along the sides and top of the leaves. These leaves may even have crystals of salt on them.

The last way mangrove trees deal with excessive salt is to block it from entering completely. Red mangrove trees can block out over 90% of the salt that comes in contact with their roots. These adaptations make mangroves unique and allow them to grow in salty conditions.

Mangroves Can’t Deal With Cold Temperatures

Cold temperatures and mangrove trees don’t mix. Mangrove trees love the hot temperatures of tropical areas. Cold weather will stunt mangrove growth or freeze them out altogether.

Many freshwater-loving tree species will simply go dormant when winter sets in. They lose their leaves and slow down their growth rate so that they can survive cold winters, mangrove trees can’t deal with freezing temperatures.

These trees do well in USDA Hardiness Zones 9-12 where the temperatures tend to stay warmer year-round. Mangrove trees can tolerate a lot of salt, but they can’t deal with freezing temperatures.

Instead if cold-weathered areas, you can read our detailed guide on the most common places to find mangrove trees here!

Freshwater Mangrove Forest Found In Mexico

Down in Mexico, along the Yucatan Peninsula, scientists have recently made an exciting discovery that had previously been hidden from the modern world. Along the San Pedro Mártir River, in Tabasco, they discovered a 50-mile stretch of mangrove trees!

A Monumental Find For Mangrove Trees

What’s special about this forest of mangroves, is that they were found over 70 miles away from the nearest saltwater coast, and some 85 to 120 feet above sea level.

Meaning in 2016, the first freshwater mangrove forest was discovered. As you can plainly see—yes, mangrove trees can really grow in freshwater, and they do.

This find is significant because it can give us a glimpse into an ecosystem that has been untouched by modern hands for approximately 110,000 years ago.

This freshwater mangrove ecosystem is thought to have been created when the last ice age caused sea levels to rise. As the ocean slowly receded and was replaced by freshwater, the mangrove trees remained and adapted to a new life on the river.

Not only did we know that mangrove trees could in fact grow in freshwater, but we have recently found irrefutable proof that freshwater mangrove forests can, and do exist.

The Importance Of Mangroves In The Environment

Mangrove forests are extremely important to the environment for a number of reasons.

For one, they create a vast, biodiverse home for fish and other wildlife who depend on these areas. Mangrove trees also help the environment by cleaning pollutants in the water and sequestering carbon from the air.

These trees also help protect coastlines from erosion, they create a buffer from storm surges and tropical storms.

Mangroves Are Natural Fish Hatcheries

The tangle of roots from red mangroves reaches into the salty water, down into the muddy bottom creating places for small fish to hide from larger predators.

Many adult species of fish come to the mangrove roots to spawn, including sharks and rays. Once these fish get bigger, they move out of the mangroves into larger bodies of water and then repeat the process.

Mangroves also take back in the nutrients from fish as well! You can read more about what mangrove trees actually eat here.

Mangroves Help Clean The Environment

According to the Florida Department of Environmental Protection, mangrove forests remove pollutants before they can make it to coral reefs and seagrass habitats. Mangrove peat also helps to absorb excess water during periods of heavy rain, thus reducing the probability of flooding along the coast.

Mangrove trees are also adept at removing carbon dioxide from the atmosphere. These trees pull in carbon dioxide, then store it in the leaves, trunks, and roots of the tree. When the leaves fall or the tree reaches it end, that carbon is then deposited into the ground, this is called carbon sequestering.

According to work published in GreenBiz, mangrove forests sequester about 24 million metric tons of carbon from the environment every year. Some studies suggest that mangrove forests—pound for pound— can store away 4 times as much carbon from the atmosphere as rainforests can. This makes mangrove forests important not only for wildlife but for the entire planet.

Grow Mangrove Trees At Home

These trees are at home in tropical settings where it’s hot and humid, and they like their roots in salty water. I don’t mind dipping my toes in warm tropical waters too! But did you know you can grow them at home wherever you live?

Mangrove trees can grow really tall, but if you keep them trimmed they will stay short. They are easy to make into dwarf trees. This especially is helpful in colder climates when you have to take them inside before the winter sets in.

If you don’t have a greenhouse you can keep mangrove trees in pots inside your house when cold weather arrives. They need a lot of sunlight—about 10-12 hours worth—and they need to be kept very warm. A seed mat will help with these temperatures, you can try the Seedling Heat Mat.

This heat mat increases the temperature of the rooting area anywhere from 10-20℉ above the ambient air temperature, which makes it perfect for indoor gardening, and allows for better plant growth, and flower seed germination.

Mangrove Trees As Aquarium Plants

Mangrove trees, especially the red mangrove variety adapt well to aquarium and pond lifestyles. It doesn’t matter the type of water either, be it saltwater, freshwater, or brackish, these plants will do well as long as they are kept away from freezing temperatures!

Freezing temperatures will ruin your mangrove trees.

As these plants are fast-growing, they will need a moderate amount of trimming to keep them small and compact. If you would like to grow these amazing plants at home you can get seedlings like these 8 Healthy Strong Red Mangrove Seedlings.

That’s All For Now!

Mangrove trees are fascinating specimens that can withstand the harsh salty waters in tropical climates all around the globe. They are essential for a lot of species of fish, mollusks, birds, and much more. These trees thrive in salty and brackish waters because there is so little competition in this kind of environment.

And we now know that mangrove trees can and do grow in freshwater. Because of climate, competition, and tidal flows, mangrove trees don’t usually grow in freshwater. When the conditions are right, mangrove trees do grow well in freshwater.

Thanks for sticking around with us and learning why mangrove trees can live in freshwater!

References

Aburto-Oropeza, O., Burelo-Ramos, C. M., Ezcurra, E., Ezcurra, P., Henriquez, C. L., Vanderplank, S. E., & Zapata, F. (2021). Relict inland mangrove ecosystem reveals Last Interglacial sea levels. Proceedings of the National Academy of Sciences, 118(41).

Adame, M. F., et al. “Carbon stocks and soil sequestration rates of riverine mangroves and freshwater wetlands.” Biogeosciences Discussions 12.2 (2015).

Jiang Jiang, Douglas O. Fuller, Su Yean Teh, Lu Zhai, Hock Lye Koh, Donald L. DeAngelis, Leonel da Silveira Lobo Sternberg, Bistability of mangrove forests and competition with freshwater plants, Agricultural and Forest Meteorology, Volume 213, 2015, Pages 283-290, ISSN 0168-1923. 

Liangmu Wang, Meirong Mu, Xiaofei Li, Peng Lin, Wenqing Wang, Differentiation between true mangroves and mangrove associates based on leaf traits and salt contents, Journal of Plant Ecology, Volume 4, Issue 4, December 2011, Pages 292–301.

Sepúlveda-Lozada, A., Mendoza-Carranza, M., Wolff, M. et al. Differences in food web structure of mangroves and freshwater marshes: evidence from stable isotope studies in the Southern Gulf of Mexico. Wetlands Ecol Manage 23, 293–314 (2015).