The Silver fern
Written by Anne Braithwaite (Lincoln University)
Edited by Michael Hudson, Lincoln University, September 2006
- Scientific name: Cyathea dealbata (G.Forst.)
- Synonyms: Alsophila tricolor
- Popular Common names: Silver fern
- Other common names: Silver King, Ponga, Punga, Bungie, Kaponga, kátote
- Kingdom: Plantae
- Phylum: Pteridophyta
- Class: Filices (the ferns)
- Order: Cyatheales
- Family: Cyatheaceae (tree ferns)
- New Zealand Status: native (endemic)
- Taxonomy sources: Alan, 2000; Everett, 1981; Large et al., 2004; Metcalf, 2005.
Tree ferns were present in Carboniferous swamps, and by 275 Million years ago they had abundant forms similar to what we have today (Large et al., 2004).
The silver fern (Cyathea dealbata) is easily recognised with its distinctive silver/white waxy secretions on the underside of the fronds (Jones, 1987) (Figure 1). This colouring is only gained by each frond after two years of life (Metcalf, 2003). The silver fern can grow to a height of 10 metres and up to 8 metres in diameter. The trunk is distinctive as it shows the visible remains of where the old fronds attached giving an uneven, rough look (Metcalf, 2003). The fronds spread horizontally from the top of the trunk, are 2–4 metres in length and between 60 centimetres to 1.2 metres wide (Figure 2). During the life cycle of the silver fern, different features can be used for identification. The fertile fronds are dotted with brown sori on the underside. Sori are numerous sporangium which are the capsules holding the fern spores (Metcalf, 2003). The juvenile silver fern has no trunk. The trunk is relatively slow growing compared to the fronds (Jones, 1987). At this stage they are therefore harder to recognise (personal observation).
Figures 1–2. The New Zealand silver fern, Cyathea dealbata. (Click on an image for the full caption.)
The tree fern Cyathea milnei resembles C. dealbata. Cyathea milnei grows up to 8 metres in height with fronds 4 metres long. The distinctive difference between the two ferns is that C. milnei lacks the frosted silver underside that C. dealbata is famous for (Large, 2004).
Full natural and naturalised range
The silver tree fern is endemic to New Zealand, meaning it is found nowhere else (Large et al., 2004).
New Zealand range
The silver fern is widely distributed across New Zealand. Found throughout the North island, the silver fern extends as far south as the Catlins on the east and Karamea on the west coast of the South Island. In addition the silver fern is also found on the Three Kings and on the Chatham Islands (Metcalf, 2003). Often found in the sub canopy of drier forests and also in open scrub (Large et al., 2004), C. dealbata prefers lowland to montane forests (Metcalf, 2003).
The silver fern is found throughout Canterbury on Banks Peninsula and in Peel Forest (Lucas et al., 2000; Molly, 1983) (where there is suitable habitat).
Natural History in Canterbury
The silver fern grows best in shady, wet, well-drained areas – predominately a forest environment. Often found in gully and stream side ecosystems the silver fern can be found at elevations between 5 and 650m above sea level (Lucas et al., 2000). Also found in fairly open habitats, the fronds usually become tatty and wind swept. The silver fern has also found favour as a popular garden plant as it can withstand drier conditions than most other tree ferns; it also has a very attractive appearance with the iconic white/silver underside (Fisher, 1984).
Cyathea dealbata is also found on Banks Peninsula in kahikatea and matai forests. Only remnants of these forests and their accompanying silver ferns now remain. The kahikatea forest is mainly a swamp forest ecosystem; wet and at fairly low elevation. In addition these kahikatea forests are also found in gullies with moist well drained soils that are composed of volcanic materials and wind swept loess from the Canterbury plains. In contrast the matai forest ecosystem is at a higher elevation (120–200 metres) on less sheltered slopes that cover a more general area (Lucas et al., 2000). Being a moist forest ecosystem the matai forest grows in deep loess and volcanic soils that are on moderately well drained slopes (Lucas et al., 2000).
Silver ferns like very moist conditions; change in their environmental conditions can cause severe damage. They are prone to frost damage and over heating (Large et al., 2004).
The life cycle of the silver fern has two different stages: sporophyte and gametophyte. The adult leafy plant produces the spores on the underside of the fronds. These are held in small packages called sporangia. This diploid plant (meaning each of its cells contains two sets of chromosomes in the nucleus) is called the sporophyte. Cells in the sporangium undergo meiosis and haploid spores (one set of chromosomes) are released. A spore germinates forming a gametophyte; small, thin and heart shaped. The gametophyte produces the sex organs, each which produces sperm and eggs. Fertilisation takes place producing a diploid zygote. As a developing sporophyte the zygote is partially parasitic on the gametophyte (Large et al., 2004).
Silver ferns play an important role in the ecosystem of the New Zealand bush. Firstly they support many twiners, rootclimbers, scramblers and tendril climbers. Secondly the fronds drop off leaving the stipes. These then decay and assist in the formation of a suitable environment for epiphytes. Epiphytes are plants that grow off the ground using other plants for support but are not parasitic (Pope, 1925-26).
Silver ferns also provide shelter for the rejuvenation of later succession tree species within a forest. An example of this is seen when roads are cut through scenic reserves; Cyathea dealbata and Cyathea medullaris act as a windbreak to the forest interior (Pope, 1924-25).
The dead trunks of silver ferns also play a significant role in a forest as they create a place for new seedlings to grow and also provide a home for a number of invertebrates (Ogle et al., 2000).
The rhizome (or trunk) of the silver fern contains a large central area called the pith and an outer region called the cortex. This outer area of soft cellular material stores starch. The cortex is also punctuated by vascular tissue composed of cells specialised in the transport of water, and nutrients that have been absorbed by the roots. The fronds work to photosynthesise sugars and also are specialised for reproduction (Large et al., 2004).
Fertilisers that are often used in cultivation to help tree ferns grow are blood and bone, well-rotted manure, fish meal, and seaweed. High nitrogen favours frond growth where as high phosphate favours spore production (Large et al., 2004).
In general the silver ferns are susceptible to several bacterial and fungal diseases. Crown rot is a bacterial infection that may occur in cultivation if ferns are watered from the top. An excess of water to builds up in the centre of the crown causing the fronds to die off and the centre to go soft. Root rot is another common fungal disease and may be indicated by whitish fungal mould on the stem and also mushroom like fertile growths at the base of the trunk (Large et al., 2004).
Pests are a common cause of damaged fronds. Sap sucking insects such as, leafhopper's (Hemiptera), scale insects (Homoptera), thrip (Thysanoptera), and mites (Arachnida) infest the young growth and cause frond damage. Other pests such as slugs (Gastropoda), snails (Gastropoda), caterpillars (Lepidoptera), stick insects (Phasmatodea), and grasshoppers (Orthoptera) also cause frond damage (Large et al., 2004).
How to find the Silver fern
The silver fern can be found between an elevation of 5 and 650 metres (Lucas et al., 2000). Very distinctive with the silver/white underside of the fronds, the silver fern can be found in areas of forest throughout it’s natural range (Metcalf, 2003).
Peel forest is an ideal place for ferns with its mild, moist climate. The silver fern can be found there along with many others species (Molloy, 1983).
Abundance and Conservation Status
- New Zealand: abundant
- Canterbury: abundant (within range)
The silver fern is the most abundant fern throughout New Zealand. They are very common in the North Island and also found in the South Island, Chatham Islands and on the Three Kings (Metcalf, 2003).
With the colonisation of New Zealand a lot of the native bush was lost to fires and clearance for farming. This resulted in the indiscriminate and widespread destruction of the silver fern habitat. As well as the human started fires, ancient fires caused a certain degree of forest removal on the Canterbury plains. Evidence from fallen logs, buried charcoal and other soil traces all indicate that most of the South Island below the tree line was covered in forest. The loss of the forest habitat would have had a huge impact on the amount of tree ferns covering the area (Williams, 1988). However in the areas of bush that remain, the silver fern is abundant and therefore there is no conservation threat.
Significance for people
Traditional Maori significance and uses
Maori made use of the ponga by cutting the fronds for sleeping on. In addition the fronds were also used as route markers in times of war. Making a track through the bush at night, Maori warriors would lay the silver fern fronds white side up, tips pointing in the direction of travel. The track could be seen at night by the apparent glow of the white frond contrasted against the dark background; enabling an attack of the enemy at first light (Fisher, 1985). In general Maori used silver ferns for the construction of food storage houses. The thick walls were effective at preventing the rats getting in. Trunks also formed the walls of cooking sheds and lined storage pits for kumara. In addition the silver ferns were used as a food source. As well as the pith being a good source of starch,the young fronds were also eaten (Large et al., 2004).
Cyathea dealbata had a few medical uses: the rhizome hairs were used as a wound dressing and the pith as a poultice for cutaneous eruptions. In addition ponga powder was used by the early New Zealand settlers for the reduction of fever (Large et al., 2004).
Today, in general, silver ferns have many uses. In construction the trunks are used for the creation of fences and shade houses. In the garden, the fibre from silver fern is used in potting mix, and the trunks are used as borders. Cyathea dealbata trunks are used especially for unusual vases, coasters, and a variety of boxes (Large et al., 2004). The silver fern have also become a popular garden plant (personal observation).
The silver fern holds great significance for New Zealanders: the distinctive silver underside has now become the New Zealand icon. A number of New Zealand sports teams have it as their national emblem and others have adopted the name. An example of the latter are the 'Silver Ferns', the name of the woman's national netball team (personal observation).
The silver fern is a very well known tree fern recognisable by the silver/white underside of the fronds. Having an extensive natural range the silver fern is found in the north and south islands of New Zealand as well as the Three Kings and Chatham Islands. Although large areas of natural habitat have been destroyed by human activity, the silver fern is an abundant species under no threat of extinction. Found in Canterbury in areas of lowland, montane forest C. dealbata is also a popular gardening plant. It is no wonder that this beautifully distinct fern has become New Zealand's national emblem.
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