Polystichum cystostegia - the alpine shield fern


Scientific name: Polystichum cystostegia (Polystichia meaning ‘many rows’, referring to the rows of ‘spore case’ on the underside and cystostegia referring to the bladder-like tissue covering the spore capsules)


Common Name: alpine shield fern (The name shield fern derives from the circular, shield like, protective covering (indusium) over the spore capsules)


Family name: Dryopteridaceae


Colonies of the alpine shield fern (Polystichum cystostegia) are prominent in some of the higher altitude rocky and stony places (boulderfields) of Mt Taranaki yet this fern is absent from the remainder of the North Island. Alpine shield fern is however widespread in similar habitats of the South Island and also occurs on the Stewart Island and the Auckland, Chatham and Campbell Islands.


This tough little fern is generally found growing in sheltered crevices amongst rocks where it spreads by a branching underground stem. On Mt Taranaki these rocky places occur mainly at the eroded bases of old lava flows, such as Humphries Castle and Warwick Castle, and in valley and ravine bottoms. These “natural rock gardens” are generally partially carpeted with mosses and many different herbs, grasses and ferns; including the alpine shield fern in the higher reaches of this habitat.


The Polystichum genus of ferns may have first reached New Zealand by long distance dispersal of their spores, across the Tasman Sea, some 13 million years ago. The ancestral Polystichum have since undergone significant adaptive changes and have been able to occupy a wide range of habitats from coastal to alpine zones.

Polystichum cystostegia was originally described as the ‘Egmont Fern’ from specimens collected by Ernst Dieffenbach (1811-1855), a naturalist employed by the New Zealand Company, and the first European to climb Mt Taranaki in 1840. There has been some confusion as to the origin of the specimen having at one point been ascribed to Mount Tongariro, which Dieffenbach did not visit. In fact there are no substantiated records of the fern from Mt Tongariro.


One of our highest altitude ferns, alpine shield fern has brownish-green fronds that die off in winter and reappear in November and December. The many orange-brown scales, small stout, harsh to the touch, fronds, and thinly raised circular protective covers (indusia) found on the spore capsules on the underside, are distinctive features.


As noted earlier, the alpine shield fern is of particular interest in Taranaki as one of a limited number of species for which Egmont National Park is the only North Island location. Although most of the indigenous plants on the mountain probably originated from elsewhere in the North Island, the alpine shield fern is one of a few that almost certainly came from the South Island. Despite the recent volcanism and the isolation of Mt Taranaki, the alpine shield fern’s light wind dispersed spores enabled successful colonisation.


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