Tiny liverworts and lava forests

Deep in the southeast of La Réunion, a tropical island shaped by its volcanos, lies the Forêt de Mare Longue—one of the last remaining lowland rainforests of the Mascarene Islands. This forest is a hotspot for endemic diversity in the archipelago. Among them is a tiny liverwort: Capillolejeunea mascarena

Lejeuneaceae, a large group

The genus Capillolejeunea is the only Lejeuneacae endemic to Africa (Pócs et al., 2011). Lejeuneaceae are a large group of leafy liverworts that dominates the moist tropics with hundreds of species.
Capillolejeunea mascarena is restricted to the Comoros, Madagascar, Mauritius and Réunion. It grows on rotting wood, corticolous and epiphyllous in lowland and submontane (100-800 m) forests. It forms delicate mats of overlapping, bilobed leaves just a few millimeters wide.

I encountered it growing on Agarista buxifolia, one of the native tree species occuring only on Réuion and Madagascar, during a bryological excursion in the Forêt de Mare Longue on 11.10.2023.

Capillolejeunea mascarena

Forêt de Mare Longue, a special forest

The Forêt de Mare Longue is one of the last pockets of lowland rainforest on La Réunion. Located near the town of Saint-Philippe, it sits at the foot of Piton de la Fournaise, one of the world’s most active volcanoes. The forest grows directly on young basaltic lava flows, where thin soils and harsh conditions make plant life a challenge.

Piton de la Fournaise

But Mare Longue is astonishingly biodiverse. Many endemic vascular plants grow here (Strasberg et al., 2005), alongside countless mosses, ferns, lichens, and liverworts. The "bois de couleur" grow here—a term locals use to describe native broadleaved trees with diverse wood textures and colors.

From vulcanic eruption to forest

The origin of Mare Longue’s forest lies in volcanic activity. After an eruption, bare lava fields stretch across the landscape—hostile, black, and hot. Yet within a few years, pioneer species like Agarista buxifolia begin to appear. These hardy shrubs can take root in cracks in the lava, starting the slow process of soil formation and ecological succession (Vidal et al. 2019).

But Agarista does not disperse to new lava fields alone. The seeds are often dispersed by frugivorous birds, which fly across the island carrying the forest in their guts. These birds eat the fruits of native trees and drop the seeds—along with natural fertilizer—in new locations. This process is what kickstarts the return of native forest after volcanic destruction (Lavergne et al., 2005).

However, many of these bird species are now extinct or declining. The loss of frugivores has serious consequences for forest regeneration: fewer seeds are spread, recovery slows, and exotic plants can more easily take over (Lavergne et al., 2005; Thibault & Mérot, 2013). Without birds, there are fewer trees. Without trees, there's no forest formation. And without a forest, delicate liverworts like Capillolejeunea mascarena cannot survive.

Primary succesion on lava field created by Piton de la Fournaise. The white lichen is Stereocaulon vulcani, the green fern is Nephrolpepis cordifolia

Conservation is difficult...

The Forêt de Mare Longue is protected within Réunion National Park and recognized as a UNESCO World Heritage Site. But that doesn't make it safe. Alien species, particularly Psidium cattleianum and Cinnamomum verum, significantly alter native vegetation on Réunion Island, especially in lowland and montane ecosystems (Caujapé-Castells et al., 2013). Invasive species rapidly colonize new volcanic lava flows and thereby prevent the establishment of native species during primary forest succession (Thibault & Mérot, 2013, Caujapé-Castells et al., 2013). The spread of invasives leads to a decline in native flora, diminishing biodiversity and altering ecosystem services. Effective management strategies are necessary to control invasions and support native species regeneration, especially after volcanic disturbances (Caujapé-Castells et al., 2013).

...but essential

Preserving ecosystem on Réunion means more than fencing off conservation areas. It means restoring bird populations or manually dispersing seeds, and especially controlling invasives. Theres still large areas of high conservation value relatively free from invasive species. There is high potential for restoration and conservation with proper management of those invasives and prioritization of invasive free sites (Fenouillas et al., 2022).

Agaristes buxifolia in Forêt de Mare Longue

Fissidens ah-pengae in Forêt de Mare Longue, the species is only known from this forest

me taking pictures of Fissidens ah-pengae

Pandanus spec. in the forest

Link to the inaturalist observation of C. mascarena

References

Vidal, C., Peltier, A., & Peltier, J. (2019). Dating young (<1000 yr) lava flow eruptions of Piton de la Fournaise volcano, Réunion Island, using the size of Agarista salicifolia (Labill.) Pers. Journal of Volcanology and Geothermal Research, 378, 48-54. https://doi.org/10.1016/j.jvolgeores.2019.05.008

Lavergne, C., Rivière, J.-N., & Boullet, V. (2005). Les forêts de La Réunion: Composition, structure, fonctionnement et dynamique. Office National des Forêts / Parc National de La Réunion.

Pócs, T., Schäfer-Verwimp, A., & Kürschner, H. (2011). An annotated checklist of the bryophytes of Madagascar. Bryophyte Diversity and Evolution, 33, 1–76.

Strasberg, D., Rouget, M., Richardson, D. M., Baret, S., Dupont, J., & Cowling, R. M. (2005). An assessment of habitat diversity and transformation on La Réunion Island as a basis for identifying broad-scale conservation priorities. Biodiversity and Conservation, 14(12), 3015–3032. https://doi.org/10.1007/s10531-004-0315-y

Thibault, J. C., & Mérot, C. (2013). Effects of invasive plants on primary succession and native forest recovery on Réunion Island. Journal of Vegetation Science, 24(3), 425-436. https://doi.org/10.1111/jvs.12038

Caujapé-Castells, J., et al. (2013). Effects of alien plant invasions on native vegetation remnants on La Réunion, Mascarene Islands, Indian Ocean. Biological Conservation, 167, 195-208. https://doi.org/10.1016/j.biocon.2013.08.006

Fenouillas, P., Ah-Peng, C., Amy, E., et al. (2022). A research-action process to implement priority areas for alien plant clearing on Réunion Island. Biological Invasions, 25(5), 1165-1179. https://doi.org/10.1007/s10530-022-02971-8

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