Overview
At the time of the Te Papa visit, five fern species were known on the Snares Islands and four were photographed (see images to the side of main image). There is a habit shot followed by a close-up of a fertile frond for each species. Species from top to bottom are: Asplenium scleroprium, Asplenium obtusatum, Blechnum durum, Polystichum vestitum. The one species not imaged was Histiopteris incisa.
North East Island, the main island of the Snares group, slopes gently downhill from the tall, tussock covered western cliffs towards the forested east coast, creating four small catchments, which drain into Boat Harbour and Hoho Bay. This combination of greater moisture and westerly wind protection results in higher numbers of ferns on the east coast of the island.
Polystichum vestitum is a common, distinctive fern on the Snares, dominating through much of the forest understorey.
On North-East Island, Blechnum durum occurs throughout forest and shrubland, in gullies and on slopes.
The infertile fronds of Blechnum durum are pinnate, narrowly elliptic and up to 60cm long. The fertile fronds are much shorter and have an exceptionally shriveled appearance due to the restricted lamina surface and close set pinae.
Asplenium obtusatum is common under the forest and shrub canopy and also seen beyond the bush edge in the Poa tussock lands, in coastal sites and, occasionally, as an epiphyte on Olearia lyallii (a tree daisy). This species has blunt or rounded pinnae apices and sori which do not reach pinna margins.
Asplenium scleroprium is closely related to Asplenium obtusatum. Intermediate forms between these two species were evident through the forest. Asplenium scleroprium and the intermediate forms were restricted to a smaller section of the east coast and South Promontory. Asplenium scleroprium is distinguished from Asplenium obtusatum primarily by the tapering apices and sori reaching the pinnae margins at indentations.
Asplenium scleroprium looks very similar to A. obtusatum because the former species is a so-called allo-polyploid, derived from an event involving chromosomal doubling and hybridisation between A. obtusatum and something like A. flaccidum. So in short, A. scleroprium is ‘half’ A. obtusatum.
Most fern hybrids are sterile, but chromosomal doubling restores fertility, with the new allopolyploid being able to reproduce itself. However, with twice the chromosomes, the allopolyploid can no longer breed (successfully) with either of its parent species, so a new species is ‘born’.
This is common in Asplenium ferns - other New Zealand examples are A. gracillimum (derived from chromosomal doubling and hybridisation between A. bulbiferum and A. hookerianum) and A. lyallii (A. hookerianum and c. A. oblongifolium). Polystichum neozelandicum (P. oculatum and P. wawranum) is a non-Asplenium New Zealand example.