Abstract
The biodiversity hotspot of New Caledonia is globally renowned for the diversity and endemism of its flora. New Caledonia's tropical rainforests have been reported to have higher stem densities, higher concentrations of relictual lineages and higher endemism than other rainforests. This study investigates whether these aspects differ in New Caledonian rainforests compared to other high-diversity rainforests in the Southwest Pacific. Plants (with a diameter at breast height ≥10 cm) were surveyed in nine 1-ha rainforest plots across the main island of New Caledonia and compared with 14 1-ha plots in high-diversity rainforests of the Southwest Pacific (in Australia, Fiji, Papua New Guinea and the Solomon Islands). This facilitated a comparison of stem densities, taxonomic composition and diversity, and species turnover among plots and countries. The study inventoried 11 280 stems belonging to 335 species (93 species ha-1 on average) in New Caledonia. In comparison with other rainforests in the Southwest Pacific, New Caledonian rainforests exhibited higher stem density (1253 stems ha-1 on average) including abundant palms and tree ferns, with the high abundance of the latter being unparalleled outside New Caledonia. In all plots, the density of relictual species was ≥10 % for both stems and species, with no discernible differences among countries. Species endemism, reaching 89 % on average, was significantly higher in New Caledonia. Overall, species turnover increased with geographical distance, but not among New Caledonian plots. High stem density, high endemism and a high abundance of tree ferns with stem diameters ≥10 cm are therefore unique characteristics of New Caledonian rainforests. High endemism and high spatial species turnover imply that the current system consisting of a few protected areas is inadequate, and that the spatial distribution of plant species needs to be considered to adequately protect the exceptional flora of New Caledonian rainforests.
Highlights
Endemism hotspots, areas that harbour a disproportionally high number of species restricted to that region, are considered of high importance in the fields of biogeography, conservation and evolution (Carnaval et al, 2009; Kier et al, 2009; Myers et al, 2000; Sandel et al, 2011)
High stem density, high endemism and a high abundance of tree ferns with stem diameters ≥10 cm are unique characteristics of New Caledonian rainforests
Species density per plot in New Caledonia was not significantly different from other countries in the Southwest Pacific (Wilcoxon rank sum test, P = 0.25), but family density per plot as well as endemism rates were significantly higher in New Caledonia (Wilcoxon rank sum test, P < 0.001)
Summary
Areas that harbour a disproportionally high number of species restricted to that region, are considered of high importance in the fields of biogeography, conservation and evolution (Carnaval et al, 2009; Kier et al, 2009; Myers et al, 2000; Sandel et al, 2011) Their continued relevance to various fields is illustrated by a recent volume on Endemism Hotspots in Annals of Botany (Harrison and Noss, 2017, and references therein). Wetter and less seasonal (more stable) climates than mainlands, probably due to the effects of the surrounding ocean (Weigelt and Kreft, 2013) Provided they are geologically old and topographically complex, they can harbour a considerable number of endemic species (Kier et al, 2009; Keppel et al, 2016). Due to their isolation from the mainland, islands often have unique species, such as large, flightless birds and evolutionary relicts (Carlquist, 1972; Trewick et al, 2007)
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