Focus on a Fern (Taxon of the Week: Polystichum vestitum)

The New Zealand fern Polystichum vestitum. Photograph by Alan Liefting.

For only the second time, the Taxon of the Week is going to be a single species. But while my earlier attempt at writing a Taxon of the Week post was hampered somewhat by a shortage of information about the species concerned, I'm happy to say that's not so much of a problem this time. And I'm also happy to say that for this post, I'm going home.

Polystichum vestitum (Forst.) Presl 1836, the prickly shield fern, is one of New Zealand's most abundant fern species. It's found in almost every corner of the country, including the Chatham and subantarctic islands, and even reaches as far south as Macquarie Island*. It is, however, restricted to the New Zealand biogeographic region - references in early sources to its presence in South America seem to represent confusion with Polystichum chilense (Looser, 1948). Polystichum vestitum is able to handle a greater deal of direct sun than other forest ferns, and is able to persist in cleared areas (Olsen, 2007). Mature specimens are about a metre in height and have a semi-tree fern growth habit, with a short trunk formed by the upright rhizome. Polystichum species are known as "shield ferns" because the stipes of the leaves are covered with glossy scales.

*Macquarie Island represents a obscure but interesting piece of the Great Trans-Tasman Rivalry. A small, windswept island halfway to Antarctica, almost untroubled by humans since the declines of sealing and whaling removed pretty much every reason why anyone would ever want to go there, Macquarie is biogeographically related to other subantarctic islands belonging to New Zealand, but is itself owned by Australia (in fact, it's technically part of the state of Tasmania, making Tasmania the third-longest Australian state north to south after Western Australian and Queensland). As a result, it's often covered in natural history works (such as bird field guides) for both countries. Despite having no trees, Macquarie Island is also notable for having been home to the world's southern-most parrot species, the parakeet Cyanoramphus erythrotis, until the effects of introduced animals caused their sudden decline and extinction in the late 1800s (according to Taylor, 1979, they survived dogs, they survived cats, but they were eventually undone by the rabbits**).

**The arrival of rabbits meant that the island was able to support higher populations of cats and also-introduced weka than it had previously, increasing the amount of predation by those species on parakeets beyond what the parakeet population could handle.


Another shot of Polystichum vestitum from NZ Plant Pics.

The scales of Polystichum vestitum are quite variable, and some authors have suggested that more than one species might be concealed under this name. Specimens found on the main islands of New Zealand have teardrop-shaped scales with broad bases and smooth edges, and with a glossy dark brown central region surrounded by a light brown margin. In many specimens from the Chatham and subantarctic islands, the scales become much longer, with a long trailing tip to the teardrop, and the dark brown centre disappears to leave an entirely light brown scale. In many Chatham Island specimens, the scales also develop notable marginal projections. However, these divergent morphologies are not universal in the outlying populations - instead, the populations vary from specimens with fully divergent morphologies to ones almost indistinguishable from mainland individuals. Analysis of the variation within Polystichum vestitum by Perrie et al. (2003b) failed to find clear divisions between the variants. When the variants were analysed using AFLP*** data, the fully divergent specimens from the Chatham Islands did cluster together, but with only low support, while the remaining specimens (including less divergent Chatham Island specimens) did not. Perrie et al. therefore recommended against recognising the divergent specimens as a distinct species or variety. However, it is remarkable that the level of variation in the small area of the Chatham Islands should be greater than that seen through mainland New Zealand. Perhaps an early population of P. vestitum became established on the Chathams and was partway into evolving into a new species but a second wave of colonisation from the mainland slowed things down? Multiple colonisations of the Chathams from the mainland have been demonstrated for another fern species, Asplenium hookerianum (Shepherd et al., 2009).

***Amplified Fragment Length Polymorphism - a method of observing variation in the sizes of the fragments that extracted DNA is chopped into by restriction enzymes. AFLP data is arguably a much rougher means of molecular analysis than full sequence comparison, but it has the distinct advantages of being much quicker and having a fraction of the cost, and hence also allowing comparison of a greater number of genes/alleles and individuals than would often be feasible with full sequencing.


The underside of a Polystichum vestitum leaf, showing the bicoloured scales. Photo by Larry Jensen.

In fact, the whole question of fern dispersal is an interesting one - as in, how much of it goes on? Ferns, of course, reproduce by means of spores which, being very small and light, could easily be carried long distances - perhaps even across oceans. It has therefore been suggested that distance has not been a major barrier in fern evolution. Brownsey (2001) suggested that most New Zealand ferns were derived from recent and common dispersals between Australia and New Zealand. In contrast, an AFLP analysis of Polystichum by Perrie et al. (2003a) found that the New Zealand species clustered in a clade, suggesting that they could possibly be derived from a single dispersal event. Interestingly, the closest relatives of the New Zealand clade were species from Lord Howe Island, which is positioned between Australia and New Zealand. Perrie et al. (2003a) also found specimens of another New Zealand species, Polystichum silvaticum, clustered together but were nested within specimens of Polystichum vestitum. This is in contrast to the results of Perrie et al. (2003b), which found a large distance between data from P. vestitum and P. silvaticum (but without data from other Polystichum species to provide a root). P. silvaticum shares the character of bicoloured scales with P. vestitum, but differs from it (and other Polystichum species) in lacking an indusium, a membrane that covers and protects young developing spores. Is it possible that P. silvaticum represents a derivative of P. vestitum?

And as a final aside, let me return to those subantarctic populations of Polystichum vestitum. On the Snares Islands, clumps of P. vestitum are apparently the preferred cover for nests of the Snares Island snipe, Coenocorypha huegeli (Miskelly, 1999). Why, you may ask, do snipes prefer to nest under ferns? As it turns out, birds on the Snares that nest higher up apparently lose a lot of eggs or chicks to petrels. Petrels don't eat the other birds, but they also nest under cover in the area - and petrels are notoriously bad at making landings. Touchdown for a petrel seems to basically involve throwing itself at the ground and hoping that there is enough vegetation to cushion its descent. Any nest in the way of a plummeting petrel is turned into kindling. In this situation, a nice sturdy fern is a ground-nesting birds friend, catching the petrels before they scramble your eggs.

REFERENCES

Brownsey, P. J. 2001. New Zealand's pteridophyte flora — plants of ancient lineage but recent arrival? Brittonia 53 (2): 284-303.

Looser, G. 1948. The ferns of southern Chile (conclusion). American Fern Journal 38 (3): 71-87.

Miskelly, C. M. 1999. Breeding ecology of Snares Island Snipe (Coenocorypha aucklandica huegeli) and Chatham Island Snipe (C. pusilla). Notornis 46: 207-221.

Olsen, S. 2007. Encyclopedia of Garden Ferns. Timber Press.

Perrie, L. R., P. J. Brownsey, P. J. Lockhart, E. A. Brown & M. F. Large. 2003a. Biogeography of temperate Australasian Polystichum ferns as inferred from chloroplast sequence and AFLP. Journal of Biogeography 30 (11): 1729-1736.

Perrie, L. R., P. J. Brownsey, P. J. Lockhart & M. F. Large. 2003b. Morphological and genetic diversity in the New Zealand fern Polystichum vestitum (Dryopteridaceae), with special reference to the Chatham Islands. New Zealand Journal of Botany 41: 581-602.

Shepherd, L. D., P. J. de Lange & L. R. Perrie (in press, 2009). Multiple colonizations of a remote oceanic archipelago by one species: how common is long-distance dispersal? Journal of Biogeography.

Taylor, R. H. 1979. How the Macquarie Island parakeet became extinct. New Zealand Journal of Ecology 2: 42-45.