Nothofagus discoidea is endemic to New Caledonia. It is restricted to ultramafic soils and is found in just five locations. This species was thought to be more widespread but now has a restricted extent of occurrence (EOO) of 2,598 km². The species has experienced historical population decline largely due to climate change but decline has been exacerbated by nickel mining and fire. All of these factors still threaten the species and is suspected to be causing a decline in the species EOO, area of occupancy (AOO) and quality of habitat with fire being, perhaps, the greatest threat. There is a growing concern of a lack of regeneration of the species due to the need for large scale disturbance and specific growth conditions which are becoming rarer due to climate change; with more forest turning into woodland or maquis following fire. This shows a decline in the species native habitat range, there is also anticipated decline in mature individuals. The species is globally assessed as Endangered.

Aire géographique

This species is endemic to New Caledonia (Jaffré 1999). The species is known from just five locations; four in the south (Thy, Mois de Mai, Faux Bon Secours, Rivière des Pirogues) and one in the north (southern slope of Tonine near Bopope tribe) (L. Barrabé pers. comm. 2018). Nothofagus discoidea is found from 100 to 800 m asl. Its current estimated extent of occurrence (EOO) is 2,598 km² and its area of occupancy (AOO) is not predicted to be much larger than 44 km². Both EOO and AOO are in decline due to threats from fire and mining.

Population

Subpopulations of Nothofagus are largely made up of older individuals of a similar age (Veblen et al. 1996). The N. discoidea population is made up of a few small subpopulations, with the southern subpopulations separated by up to 20 km from each other and the northern location being over 100 km away. The highly disjunct distribution, on both a local and a regional scale, suggests that all species of Nothofagus were either previously more widespread, or that isolated subpopulations have been mobile following climatic shifts, although N. discoidea and N. baumanniae may never have extended far beyond their present geographical range (Veblen et al. 1996). Recent pollen analysis suggests a much more widespread distribution of the genus; which further suggests that climate change has been a long contributing factor to the decreasing population of the species. Knowing that this species has a disjunct distribution and has never extended much further than its current range, suggests that this species has been declining for a long time. It is suspected that since European settlement of New Caledonia the reduction of mature individuals has sharply declined leaving the species scattered in five locations.

Habitat

This tree species is restricted to ultramafic soils where they tend to dominate various forest associations in the lowlands (Holloway 2012). It occurs in wet dense forest and forms a stunted forest in regions with persistent moisture and cloud cover (Veblen et al. 1996). Indications are that climatic deterioration and increasing aridity led to the decline of cloud forest cover and close extant species (Holloway 2012). According to Read et al. (2005), Nothofagus discoidea and N. baumanniae experience the narrowest climate, the former occurring at the warmer and drier end of the range. It is thought that the Brassii group (which includes all species within New Caledonia) are fairly poor dispersers as the seeds are not readily windborne and sink in sea water (Holloway 2012). This coupled with fractured and distant subpopulations and its poor seed dispersal restricts the species from expanding and becoming more prosperous. In general Nothofagus species require large scale disturbance of the forest canopy to successfully regenerate however seedlings are known from sites and there is some regeneration from seed.

Within a Nothofagus forest, there is often little overlap between species stands. The under canopy of trees typically occur 5–10 m lower often containing species that comprise the upper canopy in mixed rain forest. This layer commonly includes several genera in the families Podocarpaceae, Araliaceae, Cunoniaceae, Lauraceae, Myrtacee, Sapindaceae, Euphorbiaceae and Apocynaceae along with many species (Veblen et al. 1996).

Menaces

There is little literature available regarding threats to Nothofagus species. The species is potentially threatened by three pressures fire, climate change and nickel mining which are having a devastating effect on much of New Caledonia’s flora. The degree to which this impacts the species specifically is not known. The region is experiencing rapid economic growth, rising population and the constant mining for nickel which is causing many species to become fragmented and extinct in parts of the island. Many of the Nothofagus species habituate to lower montane forests (600–800m) which suggests that these species are unlikely to be able to shift in response to a changing climate due to poor wind dispersal and the need for specialised soil conditions.

There are further concerns of a reduced regeneration potential for Nothofagus forests within New Caledonia especially following fires where rainforest is often replaced by xerophytic woodland or maquis. Fire is the greatest threat to this species, due to its occurrence at low elevations. The species is known to suffer from repeated fires, on a yearly basis in some parts of its range (L. Barrabé pers. comm. 2018). As there is growing incidence of fire on the island this could become a greater threat to the species as its native habitat is lost. Nothofagus has not been known to invade these new habitats produced from fire and at present it is not known the extent to which this change impacts N. discoidea (Veblen et al. 1996).

Conservation

BGCI Plant Search (2017), reports no ex situ collections of this species and it is advised that ex situ collections of this species should be made. The species is found within five protected areas including Thy, Mois de Mai, Faux Bon Secours in Parc de la Rivière Bleue.

Bibliographie

Munzinger J., Morat Ph., Jaffré T., Gâteblé G., Pillon Y., Tronchet F., Veillon J.-M., & M. Chalopin 2016. FLORICAL: Checklist of the vascular indigenous flora of New Caledonia. vers. 22.IV.2016.

Endemia.nc 2016. Faune et Flore de Nouvelle-Calédonie.

Experts

Assessor(s): Baldwin, H.

Reviewer(s): Read, J., Barrabé, L., Tanguy, V.

Contributor(s): Barstow, M., Rivers, M.