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 shield 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 an 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.
I was one of four Te Papa Science staff to visit the Snares Islands, Tini heke in Nov-Dec 2013.
We wish to thank the Department of Conservation, Hokonui Rūnaka, Te Rūnanga o Awarua, Te Rūnanga o Ōraka-Aparima & Waihōpai Rūnaka, who made our fieldwork possible.
Other blogs related to the Te Papa Snares Islands field work:
Snares Island Flora – an introduction
Were broad-billed prions from the Snares part of the massive die off-off this species in 2011?
Life through a burrowscope lens (part 3) – subterranean Snares Islands
Snares Islands – 1947 and 2013 – In the footsteps of Edgar Stead (Part 11)
Western Chain, Snares Islands – 1929 and 2013 – In the footsteps of Edgar Stead (Part 10)
Hi Ant, both blogs are great. I was interested to see the A. scleroprium pictures and to read Leon’s explanation of where it came from. I’m 90% sure I’ve seen it in coastal gullies under Hebe elliptica scrub between Colac Bay and Riverton.
Also interested to know that the Snares flora is so small – I’d never have imagined that to be the case.
Thanks for your feedback. Asplenium scleroprium could easily be growing in that habitat and location. We have collections at Te Papa from Bluff and Sandy Point. Your discovery, if correct, would be an expansion of it’s range. If the population is large enough, and you have written permission from the landowner, we would be interested in receiving a specimen for the collection. I’ll send instructions via email. According to Leon, it’s possible that your plant could be a hybrid between Asplenium obtusatum and A. flaccidum. Leon will be able to check this by looking at the spores under the microscope.
I was also interested and perplexed at the small number of species on the Snares Islands/Tini Heke. The islands are only c.100 km from Stewart Island/Rakiura. I think the activity of so many seabirds and marine mammals limits the colonisation ability and spread of new plant arrivals.
There’s a good reason why Asplenium scleroprium looks very similar to A. obtusatum. 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.
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.
So in short, A. scleroprium is ‘half’ A. obtusatum.
Thanks Leon. An extremely thorough explanation.