What restoration of the forests of Mayotte, a global biodiversity hotspot, after Cyclone Chido?

The Mahoran forests suffered considerable damage following the passage of Cyclone Chido. Restoring them is essential today, to preserve the exceptional biodiversity they contain and so that the Mahorais can continue to benefit from the protective role of forests during, for example, floods, landslides or in the face of siltation of the lagoon. . Three members of the Scientific Council of Mayotte's natural heritage explain The Conversationwhat it would take to do this and what not to do.

Seen from the air, the Mayotte archipelago gave the impression of a green island until recently. But the passage of Cyclone Chido on December 14, 2024 completely disrupted the landscape, changing this color to brown, with in certain sectors most of the trees knocked down or broken, and completely defoliated, stripping the ground bare and subjecting it to the pluvial erosion on slopes. At a time when we are rightly talking about “rebuilding Mayotte”, what place and what future for the forests of this global biodiversity hotspot? Because reconstruction should not be done to the detriment of the natural heritage and exceptional biodiversity of this territory.

To answer this question, let's start by looking at what the forests of Mayotte look like.

These represent 38.6% of its territory. These forests play an essential ecological role, in particular for the regulation of the local climate and for limiting the impacts of natural hazards, thanks to their role in the water cycle (interception of precipitation, evapotranspiration, regulation of river flow, moderation of flooding by slowing down the flow of water, recharging of groundwater, etc.), but also for the reduction of soil erosion and protection against landslides in order also to restrict siltation of the lagoon.

Forests also constitute a natural heritage rich and irreplaceable in biodiversity for Mayotte. Low altitude forests mainly constitute “agroforests” with large fruit trees (coconut trees, mango trees, breadfruit trees, jackfruit trees) and cultivated undergrowth (banana, cassava, citrus fruits, etc.) having a deceptive natural appearance, because natural biodiversity has been greatly depleted by cultivation. It does not in fact remain at the beginning of the 21st^e century only about 5% of true natural forests in Mayotte.

Unique forest ecosystems in the world

However, these natural forests are of exceptional interest for the conservation of remarkable biodiversity in such a small territory (374 km²).

Due to their very strong decline elsewhere in the Comoros archipelago, these forests correspond to the last remnants of forest ecosystems of this very original and unique archipelago in the world.

They are also home to very rich biodiversity, since the latest assessment (2020) of the spontaneous vascular flora of Mayotte reports 1,015 species present, including 719 natives. Among these we find 119 species endemic to the Comoros archipelago (that is to say present only in this territory), including 59 species (8% of the native flora) strictly endemic to Mayotte.

A certain number of these Mahoran endemics only exist at the summit of Mount Choungui, a peak resulting from the release by erosion of an ancient volcanic chimney filled with viscous lava, which benefits from more arid climatic conditions. There are also species supposedly new to science and not yet described, sometimes with very restricted populations. This local endemism gives this site exceptional scientific interest.

The fauna of Mayotte also presents very remarkable species, such as the white crabeater (Ardeola idae), heron in danger of extinction worldwide, which is the subject of a national action plan in Mayotte, or the drongo of Mayotte (Dicrurus of the Waldenses), endemic bird and classified vulnerable in this territory.

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A degraded forest with highly threatened biodiversity, but partly protected

With such unique but highly threatened ecosystems, Mayotte is located in a global biodiversity hotspot.

It is estimated that 43% of the 610 native species known and evaluated to date are threatened, including many tree species such as the Madagascar baobab (Adansonia madagascariensis), Madagascar erythrin (Erythrina madagascariensis), the Comores cotea (Ocotea comoriensis), etc.

The surface area of ​​Mayotte's forests has also decreased by 24% over the last 30 years, in particular due to illegal forest clearing.

This situation led the French committee of the IUCN (International Union for Conservation of Nature) to send an open letter to local and national authorities on June 4, 2020, asking them to implement measures to stop this degradation. .

This cry of alarm was also taken up on December 16, 2020 by the Scientific Council for Natural Heritage of Mayotte and the National Council for Nature Protection.

These observations and threats led to the creation of two national nature reserves (NNR), the first is that of the Mbouzi islet on 142 ha (including 82 ha on land), created in 2007 and the 2^e is that of the forests of Mayotte in 2021 over 2,800 ha (i.e. 7.5% of the surface area of ​​Mayotte), grouping into six zones the best preserved groups of the forests of the summits of Grande Terre.

These two RNNs are supplemented by a network of numerous coastal sites protected and managed by the Coastal Conservatory. Special attention is paid to mangroves and back-mangroves, highly threatened ecosystems in Mayotte, in particular back-mangroves which are of particular, often unknown, interest.

The forest devastated by the cyclone

On December 14, 2024, tropical cyclone Chido devastated Mayotte with wind gusts of more than 220 km/h, which led to unprecedented damage to human settlements on the archipelago. But this cyclone also had very significant impacts on natural environments and in particular forests which were largely degraded by the cyclone.

It is currently still difficult to estimate the extent of the direct damage, but the photographs taken suggest that it could be very significant.

This damage to the trees has also greatly altered the habitats of the species that grow on these trees, such as orchids, ferns, moss, mushrooms and also the fauna that nests there. In particular, the heronries of the white crabeater, a highly endangered species which nests in trees, such as that of the Ironi Bé mangrove in Dembeni, have been heavily damaged.

Furthermore, we are already observing very worrying secondary damage in areas devastated by the cyclone, such as the installation of illegal food crops, slash-and-burn crops and charcoal production areas, which are seriously depleting forests already weakened by the cyclone. This must be addressed appropriately so that there are no irreversible alterations to these ecosystems.

What forest restoration should be implemented after the passage of the cyclone?

This cyclone constituted a disturbance of historic magnitude, which disrupted forest ecosystems and initiated a natural dynamic of repopulation by plants.

Rather than wanting to reconstitute the Mahorais forest through massive, systematic and undifferentiated replanting of areas devastated by the cyclone, the principle must be, in environments that are not too degraded, to promote the natural resilience of existing populations from the plant covers which resisted the cyclone and seeds present in the soil, whose germination is facilitated by the opening of the undergrowth.

Dead trees also constitute important habitats for certain species of fauna, fungus and flora, it is also important to preserve control zones in the two RNN without active restoration intervention, particularly in the Mbouzi islet RNN more isolated from human interventions, in order to follow the dynamics of natural regeneration of the forest after a cyclone.

But work to clear overturned trees, mechanical control of invasive exotic species, as well as planting native species (from nurseries and local seeds) proven to be present in the sites before the passage of the cyclone will have to be carried out where the spontaneous renewal of indigenous populations does not appear sufficient.

To achieve all this, it will be necessary to proceed in stages, by first carrying out a precise inventory with estimation and mapping for each plot of the damage suffered (importance for each species of toppled trees, broken trunks, partially broken crowns, etc.).

This inventory will make it possible to establish for each plot a field diagnosis of the regeneration potential and the risks of invasion of exotic species, leading to the development of a restoration strategy for each forest group, which must be examined and validated, to the two RNN, through their scientific advice, that is to say the Scientific Council of Natural Heritage of Mayotte, before being implemented over a period of at least 10 years, with monitoring of the restoration of the populations and biodiversity.

This restoration therefore requires meticulous work to diagnose and support the natural healing processes of the forest, by a sufficient number of personnel trained for this.

“Emergency rescue plans” for animal and plant species endangered by the passage of the cyclone will probably also have to be put in place in parallel.

For the forest RNN, this restoration will be logically integrated into the management plan which must be developed and whose implementation has just begun in 2024. For that of the Mbouzi islet, the second management plan of which, currently in course, corresponds to the period 2018-2027, it can be part of a new management plan to be developed in advance.

The authors would like to warmly thank Mr. Thani Mohamed Ibouroi, curator of the RNN of the Mbouzi islet, Mr. Djamadar Saindou, director of the RNN of the forests of Mayotte, and Ms. Amélie van Gemert, president of Gepomay (Group of studies and protection of birds in Mayotte) for their information and photographs on the damage from Cyclone Chido.