Homalium polystachyum is a tree endemic to the north-west of Grande Terre in New Caledonia. Within its genus, H. polystachyum is a discrete taxonomic entity. It is is found in shrubland and humid forest on ultramafic substrate at an altitudinal range 450-550 m asl. The main threats are linked to ongoing mining activities on Tiébaghi area, and bushfires on Art island. Mining extraction projections for the coming 25 years would lead to the extinction risk of the main subpopulation. Homalium polystachyum is therefore considered as Critically Endangered (CR) under criterion A4c with an inferred population reduction of more than 80% over three generations (75 years) starting in 1970 and projected to 2045.

Geographical area

Homalium polystachyum is a tree, endemic to the north-west of Grande Terre in New Caledonia, only known from two localities: Tiébaghi and Ile Art, the latter from a single herbarium specimen collected in 1975.

Population

The population size is not precisely known, but based on information from experts, it is estimated that the Tiébaghi subpopulation numbers around 1,000 mature individuals and that this appears to be the largest subpopulation. However the mining extraction projections for the coming 25 years suggests that more than 80% impacted subpopulation may be affected.

Habitat

Homalium polystachyum is found in shrubland and humid forest on ultramafic substrate at an altitudinal range 450-550 m asl.

Threats

The main threat to this species is linked to ongoing mining activities in the Tiébaghi area. New Caledonia contains 20 - 30% of the world’s nickel resources. Intense mining activities since the late 19th century have generated soil erosion (1.2% of bare ground mapped by SPOT5 in 2007). Based on the distribution of metal-rich soils and site accessibility, these threats are predictable and planned (there was a rise in nickel production from 60,000 to 200,000 tons per year between 2013 and 2015). Additionally, Ile Art may be severely damaged by uncontrolled bushfires. A fire outbreak could affect the entire subpopulation.

Conservation

The genus Homalium is protected by legislation but does not occur in any protected area. Through a partnership in 2015 with IAC (Institut Agronomique néo-Calédonien) and SLN (Société le Nickel) a monitoring programme was set up, but only a few seed collections have been made so far. This species would benefit from in situ and ex situ emergency conservation actions. It is recommended to ensure the monitoring of existing populations and to preserve viable populations within delimited areas at Tiébaghi. It is moreover advised to conduct propagation tests in order to design a replanting programme and to search actively on Ile Art.

Bibliography

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

L'Huillier L.; Jaffré T. and Wulff A. 2010. Mines et Environnement en Nouvelle-Calédonie : les milieux sur substrats ultramafiques et leur restauration.

Morat, P.; Jaffré, T.; Tronchet, F.; Munzinger, J.; Pillon, Y.; Veillon, J.-M. and Chalopin, M. 2012. Le référentiel taxonomique Florical et les caractéristiques de la flore vasculaire indigène de la Nouvelle-Calédonie [The taxonomic database « FLORICAL » and characteristics of the indigenous flora of New Caledonia]. Adansonia sér 3(34) , p.177-219.

Wulff AS, Hollingsworth PM, Ahrends A, Jaffré T, Veillon J-M, L’Huillier L, et al. 2013. Conservation Priorities in a Biodiversity Hotspot: Analysis of Narrow Endemic Plant Species in New Caledonia.

Experts

Assessor(s): Tanguy, V., Veillon, J., Amice, R., Barrière, R., Fleurot, D., Gailhbaud, P., Garnier, D., Suprin, B.

Reviewer(s): Vandrot, H.

Contributor(s): Wulff, A.

Facilitator(s): Chanfreau, S., Tanguy, V.