Impressive and Intriguing: Norfolk Island’s diverse ferns

From tall tree ferns to the tiny filmy ferns, Norfolk Island has a diverse range of ferns and fern allies. There are 44 indigenous taxa on the island, representing 24 percent of all native plants found on the island. Seven ferns are endemic to Norfolk, and eight ferns, not all of which are endemic, are listed as threatened because of their low population on the island. Three species of fern have Norfolk in their specific name.

The good rains in 2011 and 2012 have provided ideal conditions for fern growth. Surveys in 2012 found that many ferns, including some very rare species, have proliferated in the past two years. Ferns of many species are abundant throughout the forest and one species thought possibly extinct on the island has reappeared.

It is not surprising that a remote island in the middle of the ocean supports many fern species. Ferns propagate by dispersing very small spores that can be blown on the wind for very long distances. Thirty-eight (86 percent) of the native ferns are shared with Australia, New Zealand, Kermadec Islands, New Caledonia, Fiji and/or regions much further afield in Asia. Wide distribution of many species demonstrates the success of fern dispersal.

The tallest fern in the world occurs on the island, growing to 20 metres in height. The endemic Norfolk Island Tree Fern Cyathea brownii, locally known as ‘Farn’, can be seen throughout the island, reaching its best development in terms of height and abundance in the valleys of the National Park. This tree fern was given its current botanical name in 1929, although it was originally collected on the island in 1804 by Ferdinand Bauer and has had several previous names. Less obvious to the casual observer is a second, less common, species of tree fern, known as the Rough Tree Fern Cyathea australis subsp. norfolkensis. This tree fern is an endemic subspecies, closely related to the common eastern Australian species Cyathea australis subsp. australis. Rough Tree Fern is largely restricted to the valleys in the National Park. Other than its shorter stature, it differs from the more common Norfolk Island Tree Fern by the retention of frond bases below the fresh fronds, the greater number of fronds and the smaller frond scars on the trunk.

Quaker James Backhouse recognised the two types of tree fern on the island during his 1835 visit, which was published in 1843: “I explored some of the gullies on the south of Mount Pitt. Here two tree-ferns, Alsophila excelsa [Cyathea brownii] and Cyathea medullaris [Cyathea australis subsp. norfolkensis] were very fine; the former measured 40 feet [12 metres] and the latter 20 feet [6 metres]; both had magnificent circular crests of fronds; those of the Cyathea were 11 feet [3.3 metres] in length.”

At the other extreme in terms of size, are three very small species from a group known as filmy ferns. The smallest of these is less than 25 mm across. All three species are restricted to moist rocks and soil banks in the deep valleys in the National Park, most often near waterfalls. The common names reflect the relative size of these species; Large Filmy Fern Cephalomanes bauerianum, Middle Filmy Fern Crepidomanes endlicherianum and Small Filmy Fern Crepidomanes saxifragoides. These are very delicate ferns, with frond segments only one cell in thickness. Because they are very susceptible to desiccation, these ferns are only found to the wettest places in the forest.

Eleven ferns are primarily epiphytic, that is they mostly grow on dead or living plants, or are lithophytes, growing on rocks. These ferns only use the plants they are on for support; they are not parasitic upon the host plant. The largest of these is the Birds Nest Fern Asplenium australasicum, the same species that is well known to gardeners in eastern Australia. This species is moderately common in some parts of the National Park and is often seen in gardens around the island. You may also come across a strange looking Birds Nest Fern with crinkled fronds. The Crispy Birds Nest Fern has been given the name Asplenium australasicum forma robinsonii. Now only known in cultivation, it is probably a hybrid with another Asplenium species, possibly Asplenium polyodon. This fern, known as Sicklefern, is a relatively large epiphyte often growing on tree ferns or fallen and decaying trunks of trees and treeferns. The long, arching fronds can be one metre long.

Felt Fern Pyrrosia confluens is the most common of the epiphytes, climbing rhizomes (stems) form large colonies on rocks and tree trunks and specimens may be seen high on the upper trunks of pines. Other epiphytes include the endemic fern ally Hanging Forkfern Tmesipteris norfolkensis. This small species is related to other species in the western Pacific, from as far afield as New Guinea and Tahiti. Forkfern grows on the fibrous trunk of tree ferns, primarily in sheltered valleys.

Kangaroo Fern Microsorum pustulatum, so called because the shape of the frond is much like the foot of a kangaroo, is very common throughout the forest. This robust, climbing fern grows across the ground, over rocks and high into the tallest pines and palms, where it grows as a true epiphyte.

Another Asplenium is possibly the most beautiful ground fern on the island. Commonly known as Twofrond Fern and botanically as Asplenium dimorphum, this endemic and common fern, as both the common and botanic names suggest, produces two distinct types of fronds on the same plant. The delicate lacy fronds of this fern make it unmistakable on the forest floor.

The King Fern Marattia salicina has a bulky, woody base from which grow long fronds well over two metres in length. This species, which also occurs in New Zealand, is largely restricted to a few valleys in the National Park. This is an ancient fern with relatives in many countries, particularly in the southernhemisphere. A large specimen can be seen in the Botanic Garden. This giant fern much impressed James Backhouse in 1835; his journal records his admiration for the size of the fern: “The most remarkable object that arrested our attention was Marattia elegans, a fern of great beauty, having fronds 14 feet [4.2 metres] in length, 7 feet [2 metres] of which were destitute of branches; of these it had 8½ pairs which were again branched and clothed with leaflets, five inches long; and three-quarters of an inch broad.”

Many ground ferns are seen along the walking tracks in the National Park and Botanic Garden. Although these may not be distinctive to the casual observer, some have interesting names. These include the difficult to pronounce Pteris zahlbruckneriana, known as Netted Brakefern, which was named in 1833 by Austrian Stephano Endlicher after countryman Johann Zahlbruckner, a prominent botanist. The derivation of some of the manes of Norfolk’s ferns is also interesting and sometimes quite mysterious; some are named after people who never visited Norfolk Island. Other species have unusual names, such as Bristly Cloakfern
(Cheilanthes distans), Binung (Christella dentata), Bats-wing Fern (Histiopteris incisa), Pop Rock Fern (Nephrolepis flexuosa) and Adders Tongue (Ophioglossum petiolatum). One ground fern is named after Norfolk Island; the Norfolk Island Water Fern Blechnum norfolkianum. Although named after Norfolk Island, this fern is not restricted to the island but is also found on some tropical islands such as Vanuatu and on the Kermadec Islands.

The Elkhorn Platycerium bifurcatum, commonly seen growing on trees around Burnt Pine and in island gardens, is not native to Norfolk. As with another six species of fern, this epiphyte is not indigenous, but was brought to the island as a garden plant and has spread from garden to garden, but has not yet colonised the forest.

A good place to start looking for Norfolk’s ferns is the Palm Glen Track in the National Park. This easy circuit walk is about 900 metres in length and traverses a gully containing many fern species. The Botanic Garden also contains many ferns and is easily accessible from Burnt Pine. Although you may not know the names of the ferns, see how many different types you can spot while on the walking tracks in the forest or out and about around the island.

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Image Credit: Robin Nisbet
www.robinnisbet.com

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Article content disclaimer: Article first published in YourWorld, Volume 03 Issue 01, 2013. Please note that details of specific travel, accommodation and touring options may be outdated. References to people, places and businesses, including operating days and times may be have changed. References to Government structure and Government businesses/entities may no longer be applicable. Please check directly with businesses and/or Government websites directly rather than relying on any information contained in this article before you make travel arrangements.