Plants

 

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.