Plants are found across the world in a wide range of environments, each with their own unique challenges for sustaining life. Go ahead and picture a tree in your head… what do you see? Most people would talk about bark on a tree trunk, a big shading canopy, grassy dirt underfoot, etc., but what about more extreme habitats? Mangroves, for example, are a group of plants found on coastlines that use both the land and sea to survive. Comprising 80 different species, mangroves are a broad category comprised of shrubs and trees that grow in tidal estuaries, salt marshes, and along various tropical and subtropical coastlines worldwide. Temperature is the largest limiting factor to where mangroves can grow: a temperature fluctuation of 12 degrees Celsius (10 degrees Fahrenheit) is enough to damage the plant, and a freezing period for even a few hours can be deadly. To survive in these unique ecosystems, a handful of adaptations are required beyond that of typical plants.
Starting with the most notable physical characteristic, many mangroves have stilt protrusions growing above the water which aid in supporting the plant, holding sediments, and bringing oxygen to the roots underwater. By stabilizing sediments, mangroves are able to take in nutrients from the ground, stay steady in the soil, and absorb water from the ground. Despite growing in or near water, mangroves still need air to breathe. The soil along shores is commonly low in oxygen, and mangrove roots may be submerged for extended periods of time as tides fluctuate. While plants do have a circulatory system to transport water and nutrients to where it is needed, it is not able to transport oxygen; this means plants need to absorb oxygen nearby where it must end up. With terrestrial plants, the roots can absorb oxygen from the soil and it can diffuse to where it is needed, but in water, the diffusion of gases is ~10,000 times slower than air. This led to mangroves evolving unique ways of bringing in oxygen. For example, black mangroves (Avicennia germinans) have roots that grow close to the sea surface and spread farther outwards to increase the stability of the plant in the uneven soil. Additionally, some of their roots will grow perpendicular to the soil which allows them to absorb oxygen from the air; these vertical roots are called pneumatophores. Another instance of this is prop roots, used by red mangroves (Rhizophora mangle), which grow above the surface horizontally and then downwards. These increase the amount of surface area that can absorb oxygen and aid in stability.
Breathing and remaining still are only the first few hurdles that mangroves face while growing in these environments, their next challenge stems from drinking salt water all day. High levels of salt are harmful for most plants, so to thrive in coastal communities, each species of mangrove has adapted into one of the two following categories: secretors, which remove excess salt, or non-secretors, which block the salt from entering entirely. Secretors have specialized pores that saltwater exits from, and over time salt crystals will form as it builds up. On the other hand, non-secretors either use specialized filtration to block salt from entering the roots, or they direct all excess salt to individual sacrificial leaves which eventually fall off. Leaf cells hold more water than other cells in the plant, so over time the salt will build up there. As these leaves age, salt and water continue to build up until the entire sacrificial leaf falls from the tree, taking all that excess salt with it.
Mangrove forests play a large role in the lives of humans, animals, and other plants. They act as a barrier between the land and sea, protecting property and people during storms. They also provide habitats for fisheries, filter out pollutants, and the wood can be used in construction. Mangroves can also build up land for plants that are less tolerant of salty and wet conditions, as their roots can hold substantial amounts of substratum. The extensive prop roots support a diverse ecosystem as they allow juvenile organisms to hide from predators, and the canopy is home to many unique endangered organisms such as the mangrove hummingbird (Amazilia boucardi). Additionally, mangrove forests improve nearby water quality by absorbing nutrients from runoff that might otherwise have caused algal blooms, in turn protecting fragile ecosystems such as coral reefs. While mangrove forests are critical, they are facing many threats.
Even though mangroves are keystone species (meaning they have a disproportionately large influence on their ecosystem compared to their abundance), these plants are disappearing at a frightening rate worldwide. Coastal development, aquaculture, sea level rise, and invasive species are the leading causes of mangrove forests disappearing. It is expected that coastal areas worldwide will have more human development, which will displace the mangrove forests and other ecosystems. Aquaculture is the largest threat facing these forests, especially shrimp aquaculture. Due to the increasing popularity of shrimp in developed countries (namely China, Japan, and the United States), we are seeing an increased need for shrimp farms. These farms typically require the removal of mangrove forests, but even if left alone, the pollution those farms release is still destructive. Additionally, climate change is leading to rising sea levels which can drown mangrove forests. Although climate change will increase the range of these forests in many parts of the world, they may be blocked from retreating inland if human development is in the way. Further, invasive species threaten mangrove forests by encroaching on their habitat and consuming the plants at uncontrollable rates. While mangrove forests are in danger, people are fighting back.
Mangrove forests can be protected, and studies are showing that the best approach may be to step back and let nature do the work. In the past, mangrove restoration was treated the same as a terrestrial forest: simply plant the seeds of one species in neat rows and move on. However, using this approach, only 20% of the seedlings usually survived. These coastal habitats are constantly changing, and if the conditions are not right for the seedlings then they suffer. Furthermore, mangrove forests are composed of many different plant species- not just one. It is possible to restore a mangrove forest, but it takes more steps than most ecosystems. Often, it is better to step back and let the trees repopulate the area and manage seed dispersal on their own. This approach is promising, but invasive species and the flow of water can hamper or block mangrove forest restoration. Since mangrove seeds spread through the water, if the coast does not deposit the seeds where they can grow, then a forest cannot be established. Moreso, invasive species can also prevent mangroves from growing by overtaking coastal resources. One rehabilitation method that has shown promising results starts with removing the invasive species and then altering the coastline to allow the tide to flow easily through, which will deposit the seeds. Although, this assumes that healthy mangrove forests are close enough to the sites to spread these seeds, which is not always the case.
As with many conservation and restoration efforts, pushing for further research and educating the public on their importance are the first steps in protecting mangroves. Restoring mangrove forests is important, and just as vital is protecting the current mangrove forests. At the community level, some actions we can take to prevent further destruction are establishing buffer zones between development and the coast, adding fencing to prevent livestock access, and restoring natural tidal flows by removing barriers. At the individual level, you can help protect mangrove forests by properly disposing of your litter, as well as avoiding walking, biking, or driving through areas they inhabit. Mangrove forests play a vital role in coastal ecosystems by supporting and protecting humans, animals, and other plants alike. Even if you don’t directly interact with mangroves, understanding their role and becoming an advocate for their protection is putting your best foot (or root) forward.
Cavanaugh, K. C., Kellner, J. R., Forde, A. J., Gruner, D. S., Parker, J. D., Rodriguez, W., & Feller, I. C. (2013). Poleward expansion of mangroves is a threshold response to decreased frequency of extreme cold events. Proceedings of the National Academy of Sciences of the United States of America, 111(2), 723–727. https://doi.org/10.1073/pnas.1315800111