This blog explores the fern flora of the Snares Islands/Tini Heke where I was one of four Te Papa Science staff to visit the islands in Nov-Dec 2013. There are currently five recognised species of fern on the Snares Islands, the closest sub-antarctic island group to New Zealand.
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 protection results in higher numbers of ferns on the east coast of the island.
The most common ferns are Polystichum vestitum (prickly sheild fern/puniu) and Asplenium obtusatum (shore spleenwort/parako). Polystichum vestitum occurs beneath Olearia canopy and dominates the composition in the gullies. From my encounters with this species on the mainland, I expected to see more of it in the tussockland, but it rarely extends into tussock on the Snares. The Snares Islands Polystichum vestitum also looks different to the mainland version of this species. It often occurs as very robust specimens beneath the canopy and the scales on the underside of the frond are pale brown. Mainland specimens appear less robust and exhibit dark scales with a light brown margin.
While common under the forest and shrub canopy, Asplenium obtusatum is seen beyond the bush edge in the Poa tussock lands, in coastal sites and, occasionally, as and epiphyte on Olearia lyallii (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, both in its true form and intermediate forms, was restricted to a smaller section of the east coast and South Promontory, but its range was not thoroughly investigated. Asplenium scleroprium is distinguished from Asplenium obtusatum primarily by the tapering apices and sori reaching the pinnae margins at indentations.
The fern genus Blechnum is distinct in that most species have fertile and infertile fronds that are markedly different. The fertile fronds appear, to the untrained eye, like a shriveled up frond. This is the effect of restricted lamina surface and, in Blechnum durum, very close-set pinnae.
The infertile fronds of Blechnum durum are pinnate, narrowly elliptic and up to 60cm long. The fertile fronds are shorter. This species occurs throughout forest and shrubland, on North East Island, in gullies and on slopes.
That’s only four fern species, I hear you say. Well, the only vascular plant we failed to locate on our Snares Islands visit was the fern Histiopteris incisa. This fern often colonises areas of new slips or canopy gaps and, in its southern limits, is deciduous (dies off in winter). This seems to be the species with the most limited distribution on the Snares Islands. Our search was not comprehensive, as we had other research priorities to keep us busy.
Over much of the island, it is hard to imagine juveniles of any plant species surviving the vigorous seasonal activity of millions of seabirds. This activity also corresponds with the peak growing season. However, it is unlikely that this fairly weedy fern species has died out on the Snares. It would be good to hear of recent observations and obtain more detailed Snares location information for this species. If you have an observation of Histiopteris from the Snares, leave a comment, or add an observation to the Snares Islands Flora project page on Nature Watch NZ.
Other blogs related to the Te Papa Snares Islands field work: