The influences of warm temperatures, high rainfall, isolation, fauna, soil depths and types, and geological history have fused to develop Christmas Island’s unique plant life.
The island is home to 237 native plant species, including at least 17 endemic species found nowhere else in the world. About half the island’s plants are not known anywhere else in Australia.
Major vegetation types
The natural vegetation on the island is categorised in three main community types.
Primary rainforest
On the plateau and terraces where soils are deep is stunning evergreen tall rainforest, with a closed uneven canopy around 40 m in height. Some trees emerge up to 10 m above the canopy and it’s here that the endemic Abbott’s booby nests. The trees are prolific with ferns and orchids, and on the ground are stands of young palms, ferns and lilies.
Marginal rainforest
The shallower soils of the slopes and terraces down to the coast - and some plateau areas -support semi-deciduous forest, with smaller trees (15-30 m) and thicker patches of young palms. Land crabs - particularly tens of millions of red crabs - feed on understorey plants, giving the forest its bare understorey.
Scrub forest
Scrub forest is also redferred to as open forest and vine forest. Deciduous scrub survives on the steep slopes and inland cliffs with very little soil. Here are deciduous trees, scrambling scrub and many vines. On the more exposed areas between the scrub and coastal cliffs are patches of herbland, with prostrate herbs, sedges and grasses.
Herbland
Along the coastal cliffs, small pockets of herbland vegetation can be found on very shallow soils. Often exposed to salt spray from crashing waves, there are a few species of low herbs, sedges and grasses adapted to these harsh conditions.
Other vegetation types
Some other types of natural vegetation are restricted to small areas on Christmas Island. For example, areas with surface water such as the Dales, and parts of the Eastern Terraces, often have a special assemblage of plants. At Hosnies Springs- some 100 m inland from the coast, two species of mangroves (Bruguiera species) exist- remnants from a time when this part of the island was in contact with the ocean. Also, in small pockets on limestone cliffs or boulders around the island can be found unique ‘lithophytic’ communities. These sometimes include the rare endemic ferns Tectaria devexa var. minor and Asplenium listeri.
Endemic Plants |
Habit |
General Location |
Abutilon listeri |
shrub |
coastal fringe and terraces |
Arenga listeri |
tree palm |
plateau and terraces |
Asplenium listeri |
fern |
terraces |
Asystasia alba |
herb |
coastal fringe and terraces |
Brachypeza archytas |
orchid |
terraces |
Colubrina pedunculata |
shrub |
terraces |
Dicliptera maclearii |
herb |
coastal fringe and terraces |
Dendrocnide peltata var. murrayana |
tree |
terraces |
Flickeringia nativitatis |
orchid |
plateau |
Grewia insularis |
shrub/tree |
terraces |
Hoya aldrichii |
vine |
plateau |
Ischaemum nativitatis |
grass |
coastal fringe |
Pandanus christmatensis |
shrub/tree |
terraces |
Pandanus elatus |
tree |
plateau and terraces |
Phreatia listeri |
orchid |
plateau |
Peperromia rossii |
herb |
plateau |
Zehneria alba |
vine |
coastal fringe and terraces |
Zeuxine exilis |
orchid |
plateau |
Exotic plants
Over 230 Christmas Island plant species are exotic having been introduced by humans over the last century. About 80 of these exotic species are now categorised as noxious weeds, threatening species, or common alien invaders of natural areas on mainland Australia, Pacific Islands and tropical America.
Numerous exotic trees, shrubs and vines have established in the settled areas and disturbed areas throughout the island. Troublesome vines include Mikania micrantha, Antigonon leptopus and several members of the Fabaceae. Invasive trees include Leucaena leucocephala, Clausena excavata (coffee bush), Spathodea campanulata (African tulip tree), Delonix regia (royal poinciana), Aleurites moluccana (candlenut), and Cordia curassavica (black sage), to name just a few. Some of these, especially Clausena excavata, have been found invading undisturbed forest. The threats posed by these plants are attrition of the forest edges or the interruption of natural succession. Some exotic species such as poinciana and candlenut were once used in rehabilitation areas and now require control to prevent them spreading into the native forest.