The vegetation on ultramafic rocks in New Caledonia

New Caledonia, an archipelago in the Southwest Pacific, is a biodiversity hotspot (Myers 1988; Myers et al. 2000) whose rich and unique flora has generally been associated with the diversity of geological substrates. New Caledonia has one of the largest extent of ultramafic rocks in the world (Garnica-Díaz et al. 2023). Ultramafic rocks are igneous rocks that are poor in silicon and rich in iron and magnesium. When weathered, they produce soils that impose difficult conditions for plant growth (Kazakou et al. 2008), and they have often been referred to as “ultrabasic rocks” or “serpentine” in the literature. The peculiarity of New Caledonia was noted by its earliest European visitors. In his journal, James Cook wrote that “our botanists did not complain for want of employment at this place; every day bringing something new in botany or other branches of natural history” (Cook 1777, p. 124). Although they did not visit ultramafic outcrops, the naturalists accompanying James Cook were also struck by the geological distinctiveness of New Caledonia. For instance, Georg Forster (1777, p. 576) anticipated that “there was some probability of meeting with rich and useful minerals upon this island, which, as far as we saw, distinguished itself from all those we had hitherto examined in the South Seas, in being entirely destitute of volcanic productions”. Georges’ father, Johann Forster (1778, p. 34), similarly wrote that “the mountains of New Caledonia are the most likely to contain the richest metallic veins”. Later, the geologist Jules Garnier explored more extensively the main island of New Caledonia noting the large areas of “terrains serpentineux” (serpentine terrains) and revealing the abundance of nickel ores (Garnier 1885). This dramatically changed the future of the island where the mining industry now plays a major role. The earliest European naturalists exploring New Caledonia focused on inventorying the diverse flora of the archipelago, and had limited interest or time to study its ecology, including plant–soil relationships. Robert Virot made the first substantial contribution to the ecology of the New Caledonian flora with his thesis “La végétation canaque” (Virot 1956). After spending several years on the island, he was able to give an overview of the biological forms, plant formations, and phytosociology of the flora. His work prefigures that of Tanguy Jaffré. In 1980, Tanguy Jaffré published his PhD thesis entitled « Etude écologique du peuplement végétal des sols dérivés de roches ultrabasiques en NouvelleCalédonie » (« Ecological study of the plant population of soils derived from ultrabasic rocks in New Caledonia »). Although it was published in French by ORSTOM (now IRD) with limited dissemination, this work has attracted a large and steady number of citations over the years (Figure 1). With this in mind, we thought it would be useful to make this work more accessible to the scientific community as a whole. Thanks to the support of the Société Botanique de France, an English translation is published in this special issue. The three thesis chapters, « les formations végétales des terrains ultrabasiques » (“Plant formations on ultramafic rocks”), « les groupements végétaux des maquis » (“Plant communities of the maquis”), and « phytogéochimie » (“Plant mineral nutrition”) representing about 73% of the pages of the original text, are published as three articles in this special issue. The figures were reformatted and are published in colour for greater clarity. The plant taxonomy was updated following the FLORICAL database (Morat et al. 2012; Munzinger et al. 2023) and the soil classification follows the World Reference Base (IUSS Working Group WRB 2015). Minor alterations to the text were made to avoid anachronisms with the current state of the art on the ecology of ultramafic vegetation and the New Caledonian flora. The first article entitled “Plant formations on ultramafic rocks in New Caledonia” (Jaffré 2023a) starts with a description of three contact zones between ultramafic rocks and other substrates. These suture zones have received little attention since, although they are interesting systems to study. They are known as areas of micro-endemism (Pintaud and Hodel 1998; Pillon et al. 2008; Gâteblé and Munzinger 2018). There, species with distinct soil preferences can co-occur and hybridize, which may trigger speciation (Pillon et al. 2009). The second part of the article describes the main plant formations occurring on ultramafic rocks in New Caledonia. Contrary to many other areas in the world (particularly temperate regions), ultramafic substrates of New Caledonia can harbour dense and diverse forests. The forests of New Caledonia have received significant attention (e.g. Jaffré and Veillon 1995; Ibanez et al. 2014; Birnbaum et al. 2015). The article also mentions several cases of monodominant formations, where one tree species represents a large proportion of the individuals reaching the canopy. A wide diversity of species can be dominant in New Caledonia (Pillon et al. BOTANY LETTERS 2023, VOL. 170, NO. 3, 333–337 https://doi.org/10.1080/23818107.2023.2234432