The atoll vegetation

Marie José Langlade, IRD

(a review from the books referenced)

 

Summary of the plants  which has been encountered  on the atolls

 

The plant species growing on the Tuamotu atolls do not show a high degree of diversity. In addition, in the case of the atoll of Tikehau for example, only 43 out of the 165 plant species recorded up to now have resulted from a natural process of colonisation. The others were introduced.

The natural process of colonisation

A set of hydro-climatic anomalies favoured the arrival of seeds which were carried, mostly from the Indo-Malayan peninsula, by the wind and the sea.

The archipelagos were gradually colonised by plant species which were transported in the form of seeds and spores by the sea currents (hydrochoria), the winds (anemochoria) and the sea-birds (zoochoria). In this way, coconut palms, pandanus, ferns and orchids took root on the atolls.

Low diversity

The isolation of the atolls and the harsh climatic conditions no doubt explain why the species growing on the atolls show so little diversity.

• Isolation

Since the atolls are located so far from the continental masses, very few plant species have been able to travel this far. Due to random events occurring during the transportation of the diaspores, the range of plants growing on the atolls is not the same as that to be found on the continents from which they originated.

• Harsh environmental conditions

In addition to the lack of water, saline environments can be tolerated by only a small number of plant species such as halophiles (those living on salty soils) and xerophiles (those capable of surviving drought).

A high rate of endemic species

These  environmental constraints have given rise to specific modes of adaptation, such as highly developed root systems and hairy leaves.

7% of the atoll flora are endemic.

The cyclones have greatly disturbed the pattern of plant growth on the atolls. In the coconut groves, where the ground is covered with a thick layer of coral debris, some rare species such as Laportea ruderalis (red tufts), Achyranthes velutina and Guetarda speciosa have developed.

The "feo" are fossilised coral structures present in the middle of the "motus" which show that the sea level used to be much higher than it is today. These features provide the vegetation with rather  particular conditions of growth: here the trees are mainly Pandanus tectorius with their stilt-like roots, and the bushes tend to be Pipturus argenteus with their pale green leaves, whereas the herbaceous flora show a greater degree of diversity.

The main species growing on the atolls

• On the ocean shores

The soils covering the coarse substrates on the ocean shores are rather poorly developed, and in addition, these sites are directly exposed to the wind and the sea-spray. Only a small number of species have therefore developed here.

 

In this bush environment, herbaceous species are rare. Ligneous plants are more common, however: these are descendants of older herbaceous plants which have adopted forms which are better suited to these particular ecological conditions. The plants are often separated by large stretches of coral debris.

• On the lagoon shores

The internal lagoon slopes, on the contrary, are often covered with fine sand, on which only a discontinuous layer of xerophilic and halophilic shrubs are able to grow. Here the conditions are less extreme and a wider variety of species is to be found.

Trees, bushes and climbing plants such as Suriana maritima and Scaveola maritima dominate the grassy layer consisting mainly of Triumfetta procumbens and Lepturus repens, a graminaceous plant forming a thick carpet.

• In the heart of the "motu"

These sites, which are less highly exposed to the sea spray and the wind, benefit from the underground water-head, which is not far from the surface. The soils in these places are more highly developed than on the shores.

 

Pisonia granbdis ou "ngatae" These natural forests have often been sacrificed to make way for the coconut groves. The pisonia trees are sometimes 20m high and 1m in diameter. This tree, which has a grey bark and roots running along the ground, is one of the sea birds' favourite nesting places. The organic matter with which the birds enrich the soil has favoured the development of a dense layer of shrubs and grasses.

Pandanus tectorius ou "tima" Inland from the coastal bush growing on the poorest substrates (consisting of gravel and stones), the pandanus forest has developed, showing the most highly balanced pattern of tree development (there is a perfect balance between the trees, the climate and the soil) to be found on the "motus". The pandanus can reach heights of up to 6 or 8m. Its male flowers, the "hinanos", have a remarkably suave perfume.

The Pandanus forests consist of open stands, with which Guettarda speciosa and Tournefortia argentea are also mixed. As one moves further towards the interior, the Pisonia grandis  are joined by other ligneous species such as Calophyllum inophyllum.and "tamanu" or "'ati", Cordia subcordata or "tou", Cocos nucifera or coconut palm. The Pisonia dominate a layer of shrubs such as Pipturus argenteus and Morinda citrifolia or "nono", the fruit of which are edible but have a sickening taste. The herbaceous layer is more varied, forming a continuous carpet of ferns or "nanue". Monospecific colonies of cyperaceae grow in the hollows.

The above herbaceaous species are home to many wild animals including crabs, reptiles and rats such as Rattus exulans (the Polynesian rat) and Rattus rattus (a traveller on the European ships).

References

Bonvallot J, Laboute P, Rougerie F, Vigneron E (1994), Les atolls des Tuamotu, ORSTOM, 296p.

Florence J, 1993, la végétation de quelques îles de Polynésie française in  Atlas de Polynésie Française, ORSTOM Editions, 54-55