Unexpected good news about mangrove forests

Unexpected good news. Decade after decade, mangrove forests worldwide continued to decrease in area, but that decline appears to have reversed. There has been a slight increase since 2010.

This was reported by researchers led by Daniel Friess (Tulane University, New Orleans). last week Science. They themselves seem surprised by the results, because in the headline above their article they speak of an “unexpected expansion” of these forests. In addition, they also see that leaf cover in existing mangrove forests is gradually increasing, an indication for recovery of damaged forests.

Mangrove forests occur in tropical and subtropical areas, on the coast, in intertidal zones. They are adapted to life in swampy, oxygen-poor soil with high salt concentrations. Their root systems have suckers (aerial roots) that extend above the ground or water surface and obtain oxygen directly from the air. In addition, they have mechanisms to manage high salt concentrations – they know how to keep salt out of their cells, or they actively remove salt from their cells. For local populations, mangroves provide food, income (for example from fishing and tourism) and protection against storms by dampening rough waves. Much mangrove forest has been lost in recent centuries as a result of logging, agricultural expansion (including palm oil plantations), construction, aquaculture and pollution.

That the outcome is so different from previous trend analyses, such as that of the Global Mangrove Watchaccording to the authors, mainly has to do with the data used. Previously, radar images were used for this. This mainly captures “the roughness of the surface,” writes first author Zheng Zhang of Tulane University in an email. This method makes it difficult to distinguish young mangrove forests because their vegetation, Zhang writes, is “short and sparsely seeded” and the surface roughness is limited. For their now published article, the researchers used optical images that are based on reflected infrared light. “This provides information about the vegetation and the water content of the leaf cover,” writes Zhang.

According to him, there were already indications from other research that mangrove forests were expanding again in East Asia and Australia, among others. “But the scale of the recovery worldwide, and the trend break in Southeast Asia, among others, has surprised us.”

For their research they used data from the 1980s to 2023. It turns out to be a balance of pluses and minuses. Until 2010, there was so much deforestation in Southeast Asia that it continued to tip the global balance to the negative side. Parts of Indonesia – Kalimantan, Sulawesi, Sumatra – and Myanmar stand out in particular. But since 2010 the trend has reversed. In Central and West Africa, the researchers saw the opposite: a net increase in mangroves until 2014, after which a decrease. In northern Australia, western Mexico and the Middle East, mangrove forests have increased steadily since the 1990s. In 2023, the total measured area of ​​mangroves worldwide was 153,961 km²again almost comparable to the figure from the early 1980s (154,810 km²).

The researchers further point out that existing, damaged forests often recover faster than expected if they are left alone. They write that more attention should be paid to this in policy. This policy now focuses strongly on the planting of new mangrove forests.

Martin Zimmer, affiliated with the Leibnitz Center for Marine Tropical Research in Bremen and not involved in the research, calls the publication “very good news.” But he also has caveats. He mainly wonders how robust the observed expansion is. This expansion can partly be traced back to the countless restoration projects that have been started in various places in recent years, based on a growing realization that planted mangroves can eventually protect coastal areas against the accelerated rise in sea levels due to global warming. In addition, mangroves can help reduce CO2₂concentration in the atmosphere. Mangroves are known for their ability to capture a relatively large amount of carbon, especially in oxygen-poor soils, because the decomposition of biomass is slowed down there. “But many of those recovery projects are poorly thought out,” says Zimmer. “Around 80 percent fail.”

Zimmer also has reservations about the researchers’ conclusion that mangroves, as they write, are an “underestimated resilienceIt is a term that is often translated into Dutch as ‘resilience’. “But it is a complex concept that has not yet been clearly defined,” says Zimmer. In any case, this concept does not include: colonization of new areas and resettlement as a result of recovery projects. “The authors do.”

Bregje van Wesenbeeck, scientific director at the Deltares knowledge institute and professor of natural solutions for water safety and climate adaptation at TU Delft, also has her reservations. For example, measurements have only been taken since the 1980s. “Before that time, a lot of mangrove had already disappeared,” says Wesenbeeck. In addition, mangroves prefer to expand inland. But that option is closed in many places due to advancing humans and their activities. “The expansion now observed appears to take place mainly seaward.” Mangroves are more vulnerable there, says Van Wesenbeeck. Moreover, this seaward expansion has another side. This is possible because upstream rivers transport more sediment to the coastal areas. But what causes this? “It is often because forests upstream have been cut down, or because rivers have been canalised.” For example, the expansion of mangroves comes at the expense of other nature elsewhere.

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