ARTICLE : Polystichum "richardii"

 The fern Polystichum “richardii” around Wellington

Being partly responsible for the recent splitting of Polystichum richardii (= pikopiko = the common shield fern) into three species (Perrie et al., 2003: Evidence for an allopolyploid complex in New Zealand Polystichum. New Zealand Journal of Botany 41: 189-215), I have been asked to write an identification guide for the Wellington region.

The three species replacing P. richardii are P. oculatum, P. neozelandicum, and P. wawranum.   Polystichum richardii is a later synonym of P. neozelandicum, and hence is not a correct name for any of these newly circumscribed species.   The following table outlines the distinguishing characteristics of these three species.   The scales from the rachis (upper frond axis or stem) and stipe (lower frond axis or stem) are especially important for identification.

Polystichum oculatum and P. wawranum are both tetraploid, each with four sets of chromosomes.   They are morphologically quite different.   However, Polystichum neozelandicum has an intermediate appearance, and could be confused with either of the other two species.   Polystichum neozelandicum is octoploid, with eight sets of chromosomes, which is reflected in the bigger size of its spores.   It appears to have originated from an allopolyploid event involving hybridisation between P. oculatum and P. wawranum – hence its intermediate appearance, spore size aside – and the doubling of chromosome number.

Polystichum oculatum and P. neozelandicum both appear to be fairly common around Wellington.   For instance, both were seen on the recent Carey Gully trip, both occur at Makara, and I have seen both while running around Mount Victoria.   Polystichum oculatum has a mostly eastern distribution from East Cape to Banks Peninsula, but it extends west around Cook Strait, as far north as about Pukerua Bay.   Polystichum neozelandicum is more widely distributed and there are two subspecies: subsp. neozelandicum north of Rotorua, and subsp. zerophyllum south of Taupo.   Polystichum wawranum is not (yet?) known from Wellington City.   The most southerly records of P. wawranum that I know of are from Otaki Forks (the Waiotauru Track) in the west and Pongaroa in the east.

There are three other Polystichum species native to New Zealand: P. cystostegia, P. silvaticum, and P. vestitum.   Polystichum cystostegia is widespread and fairly common in alpine areas of the South Island, but in the North Island it is only known from Mount Taranaki.   It has quite large, pale-orange stipe and rachis scales, and its indusia are markedly convex (i.e., the outside margins of the indusia are closer to the frond than the main bodies of the indusia).

Polystichum silvaticum and P. vestitum are both found around Wellington.   They can be distinguished from the other species by their rachis scales, which are bicolourous with a pale brown margin completely surrounding a dark brown centre.   (Note that the other species, particularly P. oculatum, can have both pale and dark brown patches, but never with a dark centre surrounded by a pale margin.)

The best way to distinguish Polystichum silvaticum and P. vestitum is to look for indusia with a hand-lens or microscope.   Polystichum vestitum has indusia, while P. silvaticum does not, even when the spore producing structures are young.   (Be wary as the other species can lose their indusia with age.   Therefore, in trying to make a definite identification of P. silvaticum, old fronds should be avoided.   Look for fronds with either young or mature spore producing structures; these will look greenish-white and black, respectively.   Old spore producing structures look brown.)   Furthermore, P. silvaticum is usually confined to dark, wet habitats (i.e., alongside forest streams), while P. vestitum can be found in both shaded and open habitats.

A factor complicating identification amongst the native species of Polystichum is hybridisation, with P. vestitum being especially promiscuous.   Hybrid plants are sterile and have aborted (abnormally formed) spores that can be seen with a microscope, but their recognition in the field can be difficult and requires familiarity with both parents.   Like most hybrids, those between the native species of Polystichum combine the characteristics of both parents.   For instance, the rachis scales of the hybrids P. oculatum × P. vestitum and P. neozelandicum × P. vestitum have the pale brown margins as found in P. vestitum, and the cilia-like marginal projections found in P. oculatum and P. neozelandicum but unknown in North and South Island P. vestitum.   Distinguishing P. oculatum × P. vestitum and P. neozelandicum × P. vestitum can, however, be difficult.

A handful of adventive Polystichum species also occur in New Zealand.   They can be separated from the native species by their production of bulbils (Brownsey & Smith-Dodsworth, 2000:   New Zealand ferns and allied plants.   2nd ed. Auckland, David Bateman Ltd.).

The following key will, I hope, assist with identifying native Polystichum from the Wellington City region.   I have not included P. cystostegia or P. wawranum (see above for their distinguishing characteristics).   The hybrids most likely to be encountered, P. oculatum × P. vestitum and P. neozelandicum × P. vestitum, are included, but others are not (e.g., P. silvaticum × P. vestitum, P. oculatum × P. neozelandicum).

Although now living in Wellington, I cannot (yet) describe the local distribution of each species as well as I would like.   However, readers might be interested in Perrie (2003: Polystichum in the Lower North Island.   Manawatu Botanical Society Newsletter 30: 3–4), which lists sites for each species around Palmerston North, where I was based during my study.   I am more than happy to assist/confirm identifications of Polystichum and/or supply additional information.

I can be contacted by mail (Leon Perrie, Te Papa, P.O. Box 467, Wellington), phone (04 381 7261), or e-mail: leonp@tepapa.govt.nz

Leon Perrie