Forest and tree species distribution on the ultramafic substrates of New Caledonia

About a third of New Caledonia is covered with ultramafic soils (UM) which hosts a rich flora (endemism > 85%) threatened by mining activities. This combination makes the ultramafic vegetation a floristic hotspot within a biodiversity hotspot. UM soils are distributed from sea level to 1618 m elevation with about two-thirds forming a large continuous unit while the remaining forms numerous relatively small isolated units. Here, we provide a synthesis of the distribution of forest and tree species across 22 UM units. We compiled an extensive tree occurrence dataset (109,896 occurrences and 1,065 species) and a new expert-based forest map at a 1:3000 spatial resolution. Only 10% of these species represented more than 50% of the occurrences, while 10% of the species had only one or two occurrences. A quarter of the UM area did not contain any occurrences, and we estimated that on average a quarter of the species remained to be inventoried in the 22 units. Forest covers about one-third of the UM areas with forest coverage ranging from 1.7% to 72.3% in the different UM units. Forest coverage increased from 14.6% on sea level to 93.3% at 1,200 m of elevation. About 30% of the forest and 90% of the species were located within mining concessions, while 14.5% and 73%, respectively, were located within protected areas. We recommend setting up new protected areas on ultramafic substrate, specially in the Northern province, to protect more forest and the diversity it harbours.