Abstract
Blechnum opacum is a small forest fern indigenous to New Caledonia and Vanuatu. After examining the considerable morphological variation in New Caledonian populations, along with phylogenetic analysis of chloroplast DNA sequences, we conclude that B. deplanchei should be reinstated for some of the plants in New Caledonia. Blechnum deplanchei is readily recognisable, with pinnatisect fronds that are usually larger, pinnae that are numerous and oblong, and with the distal lamina segments reduced in size. Based on two chloroplast loci, the three sampled populations of B. deplanchei comprised a distinct and well‐supported lineage. The genetic and morphological distinctiveness of B. deplanchei was maintained over a reasonably large area, including areas of broad sympatry with B. opacum . Blechnum deplanchei is largely confined to the mountains in southern Grande Terre, in areas underlain by ultramafic substrates. It generally occurs at higher elevations, and is assessed with a conservation status of Near Threatened. Even with the recognition of B. deplanchei , B. opacum remains a variable species. A few plants are morphologically intermediate. We provisionally retain them in B. opacum , but they require further investigation. The morphologically intermediate samples in the phylogenetic analyses were the most distantly related among the ingroup, contributing to the paraphyly of B. opacum with respect to B. deplanchei . The morphologically intermediate plants are unlikely to be first generation hybrids or allopolyploids, given their genetic distinctiveness, the regularity and comparable size of their spores, and because some occur far to the north of the distribution of B. deplanchei ; furthermore, Vanuatu plants have a similar morphology. It is possible that they may simply be unusually large plants of B. opacum , or a distinct, third species. Resolving their status requires genetic analysis of additional samples from northern Grande Terre and Vanuatu. With the circumscription adopted here, we assess B. opacum as Least Concern.