Science Blog: Marveling at Mangroves

31 May 2021
Marveling at Mangroves

In slightly half a century from 1970, carbon emissions have increased by 90 percent, global wildlife populations decreased by nearly 70 percent, and Earth Overshoot Day, which measures the date when annual humans’ ecological footprint has outpaced the planet’s capacity to provide these ecological resources, moved in that brief period from Winter to Summer. Just to put that into context. Suppose we put the 200.000-year-old history of homo sapiens on a 24-hour clock. Then the past fifty years would last a mere 20 seconds. The world will soon become uninhabitable if we continue on the same course. But the critical ‘seconds’ to act on climate change are rapidly running out.

However, there are also reasons for optimism. Support for climate mitigation and conservation have gained immense political and societal momentum. Calls for sustainability are changing life radically, from lifestyles, to business and governance. Carbon offsetting is one of the revolutions making a positive impact on the climate by compensating emitted CO2, enabling climate mitigation aside from large reductions of carbon emissions. That is exactly the kind of service that Regreener is facilitating.

Regreener has begun planting mangrove trees in collaboration with the renowned reforestation organization Eden Reforestation Projects. Their collaboration enables carbon offsets in the coastal wetlands of Mozambique in Eastern Africa. But what are mangroves actually and how do these trees contribute to mitigating climate change? In this first science blog, we unravel the mystery of mangroves and examine their impact on not only climate mitigation, but also on human welfare and biodiversity.

The name ‘mangroves’ refers to approximately 110 tree species that are able to survive in saline or brackish water and are found in intertidal zones along tropical and subtropical coasts around the world. Adapted to long-term flooding, intense heat, tsunamis, tropical cyclones, mud and high salt concentrations, there are many things to admire about these hardened trees for nature-lovers. Take, for example, their unique dispersal and colonization mechanism. Instead of being carried by the wind or eaten by animals and thus dispersed, mangrove seeds remain attached to the parent tree and evolve into seedlings with their own leaves capable of photosynthesis. Like the parent tree, the seedlings use carbon dioxide and water to produce sugars for growth and oxygen. Thereafter, these seedlings, so-called propagules, are dropped into the water below the parent tree and remain floating before rooting in suitable soil. Propagules have the capacity to float for up to months as they are transported by the currents along the coasts and even across oceans to find a spot to call home. Furthermore, tree roots require oxygen for respiration, which is scarce in the waterlogged and oxygen-stricken soil of mangrove forests. Mangroves have evolved to cope with this lack of oxygen through having breathing (or aerial) roots that allow for aboveground oxygen uptake when the water level lowers. These spectacular root systems form a closely-knit and nutrient-rich labyrinth. Ideal conditions for wildlife to flourish! It is no wonder then that these magnificent ecosystems are crucial to ocean life as mangroves are the nurseries of one-third of all marine fish species in need of food and protection from predators in early life. Hence, mangroves are essential to commercial fisheries. Mangrove forests also synergistically protect corals and seagrass: the cornerstones of marine biodiversity. The mangrove ecosystem of Maputo Bay, Mozambique, where the planting projects of Eden Reforestation are located, even harbors many iconic tetrapod species such as dugong, dolphins, five species of sea turtles, and a third of bird species native to Southern Africa, like the mangrove kingfisher. Replanting mangroves in threatened and depleted mangrove forests is thus essential to safeguarding biodiversity for future generations.

But that’s not the primary reason for Regreener to collaborate with Eden Reforestation. Mangroves are actually indispensable to human welfare. The concept of ecosystem services, in wider use since a few decades, is used to evaluate the services of nature provided to humans. If ranked on ecosystem services, mangroves would definitely score a five-star-rating. Mangrove is the best studied ecosystem in the paradigm of Blue Carbon which studies the carbon storage and sequestration of oceanic and coastal ecosystems. Research has revealed that mangrove forests are champions for climate mitigation, being up to four to five times more efficient at carbon storage than tropical rainforests. Not only do the mangrove trees themselves sequester CO2 from the atmosphere through photosynthesis, the root systems also capture sediment from tidal floodings and store organic material in the mud, thus preventing massive releases of CO2. Each mangrove tree planted by Eden Reforestation in Mozambique removes over 308kg of CO2 over its 25-year lifespan, or 12 kg of CO2 per tree per year. And that’s just the trees! Mangroves in Mozambique store around 62 tonnes of carbon dioxide in sediment per ha. When destroyed, these forests become immense CO2 emitters.

Importantly, investment in mangrove protection and restoration is key to helping developing countries meeting their National Determined Contributions of the Paris Agreement. As Mozambique is one of Africa’s most vulnerable countries to climate change, mangroves are too essential in climate adaptation. Coastal populations in Mozambique will be affected through sea level rise, saltwater intrusion into agricultural fields, and severe cyclones and erosion. Mangroves are the critical ecosystems that coastal human populations need in an increasingly threatening climate. Mangrove forests function as storm barriers, thereby breaking waves, halting disastrous coastal erosion and coastal flooding. The maintenance of mangrove forests is five times more cost-effective than human-built infrastructure for coastal protection. Mangroves also filter water from pollutants, support local food security, as well as the economy through ecotourism and the sustainable use of fish and forest products such as medicinal plants. Regreener facilitates the projects of Eden Reforestation wherein local communities are engaged in reforestation and financially supported. Likewise, involving local communities in conservation efforts decreases pressure on the mangrove forests.

Combined, these annual ecosystem services of mangroves are estimated to be worth $194.000 per hectare. Unfortunately, mangroves, covering less than 1% of the world’s surface, are threatened ecosystems with a loss of over a third of the global cover compared to pre-1980 levels. Despite their value in terms of climate mitigation, mangrove cover still is decreasing annually by around 2%. Maputo Bay has seen the highest loss of mangroves in Mozambique over the past decades. But luckily their extent is again increasing in recent years through much needed restoration efforts.

Small efforts coupled together can thus make a big impact for the planet. The mangrove forests of Maputo Bay that are restored and maintained will keep wildlife and people safe from harm. The planting project in Mozambique showcases how reforestation not only tackles climate change, but also improves human welfare and biodiversity.
That’s one more reason for optimism.

Bob Kreiken is currently pursuing a Master's degree in Forest and Nature Conservation at Wageningen University and Research. As science advisor he gives recommendations to the Regreener team with regard to investments in natural climate solutions.

Tree planting in Mozambique © Eden Reforestation Projects