Plants

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.

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.

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.