Dicksonia antarctica
FAMILY: Dicksoniaceae
BOTANICAL NAME: Dicksonia antarctica
COMMON NAME: soft treefern
CONSERVATION SIGNIFICANCE:
None recorded
Richard Barnes
Description
The soft treefern has a palm-like appearance. It is large and robust, 2–12 m in height,
with an erect, thick trunk and large, evergreen fronds that radiate out from the crown.
The trunk is rough and fibrous as it is covered with coarse hairs as well as the old
frond stem bases and a mat of adventitious roots that causes the stem to thicken
toward the base. The old stem bases eventually crumble and become hidden by the
mat of adventitious roots. The lower frond stem (stipe) is short and stout, dark brown
and smooth. The hairs covering the stipe bases are 2–3 cm long, coarse and brittle,
shiny and red-brown.
The upper frond stalk (rachis) is pale (fawn to straw-coloured) with numerous hairs on
its upper surface, while the lower surface is more sparsely covered with hairs. The
fronds (croziers) are dark green, growing up to 4 m long. The leafy portion (lamina)
of the frond is divided three times into fairly narrow divisions. The main divisions
(pinnae) decrease in length along the frond, from 30–40 cm near the mid-frond to
10 cm at the lower end, and are attached by their full width to the rachis, forming a
wing along the main stalk. The primary (pinnae) and secondary frond divisions
(pinnules) are oblong with pointed tips, and virtually stalkless. The obvious spore
clusters (sori) are numerous, globular and located, one per lobe, along the margins of
the pinnae and pinnules, with each spore cluster (sorus) protected by a green, curvedback marginal lobe and a fawn, membranous, inner true covering (indusium).
Confusing species
Soft treefern looks very similar to Cyathea species. However it can be easily
distinguished by its smooth stipe bases, which are covered with hairs not scales, the
fibrous matted roots that cover much of the trunk, and the marginal position of the
spore clusters and their two-valved structure.
Flora of Tasmania
Dicksonia antarctica
Distribution and Habitat
Distribution of Dicksonia antarctica in Tasmania 2004 data
In Australia, Dicksonia antarctica has a broad distribution, with its natural range
spreading across south-eastern Australia including Tasmania, Victoria, New South
Wales and Queensland. It appears to no longer occur in South Australia.
In Tasmania, soft treefern is common and widespread, ranging from sea level to
1000 m above sea level. It is mostly absent from extremes of altitude (above 900 m),
such as the Central Plateau, and from low rainfall areas such as the Midlands. It is
most prolific and widely distributed in areas where the soils are fertile and the rainfall
is over 1000 mm per year. As the annual rainfall decreases, the local distribution of
soft treefern becomes confined to moist gullies and sheltered southerly slopes. Where
the rainfall falls below 600 mm per annum, it occurs in very isolated stands in only
the most sheltered sites.
Soft treefern grows in numerous types of plant communities, and is particularly
abundant in wet forest communities. It occurs in forest types ranging from rainforest
to sheltered gullies within dry sclerophyll forest and subalpine forest. It becomes less
common where the forest canopy is dense and reaches its best development in wet
gullies and forests where the canopy is partly broken.
Ecology
Soft treefern is a very robust, long-lived species, living from up to 1000 years. It is an
obvious and important component of the Tasmanian bush. It is very hardy, tolerating
temperatures from –10 °C to 40 °C, and prefers fertile organic soils, moist, humid
conditions and some shade.
Soft treeferns remain evergreen and produce new fronds year-round. A large adult can
support between 30–35 fronds. The funnel-like shape of the whorl of fronds may
facilitate harvesting of rainfall, which collects in the stem apex and rehydrates the
aerial adventitious roots at the base of each frond. Such a strategy may eliminate the
need for soil moisture competition with surrounding tree species. The tender, fleshy
croziers (young, uncurling fronds) are an occasional food source for native animals
such as possums and parrots. The old fronds tend to form a skirt around the plant and
probably protect the vulnerable growing apex from invasion and smothering by
climbers and epiphytes.
Flora of Tasmania
Dicksonia antarctica
The growth rate of the soft treefern falls within the range of 1 to 10 cm per year,
depending on site conditions. The trunk, or caudex, of these ferns is formed by the
accumulation of the persistent frond bases and aerial roots, with no true wood or bark.
This structure has good moisture-retaining properties and also has considerable
longevity and stability as a substrate, at least compared to a bark surface. The trunk
diameter is increased by the outgrowth of fibrous rootlets along almost the entire
length of the trunk. It is these rootlets that enable treeferns to survive without a
subterranean root system and allow them to easily re-establish and grow if they fall
over or are cut off at the base.
Soft treeferns play an important role in the ecology of many plant communities. The
trunk, with its thick mantle of matted rootlets, is an ideal surface for germination of
the seeds or spores of various rainforest species and ferns and provides an important
substrate for epiphytes, which can almost entirely clothe the trunk. Certain epiphytes
have a strong preference for soft treefern trunks as a substrate, which is thought to be
related to the physical characteristics of the caudex. The importance of Dicksonia as a
substrate for pteridophytes, in particular the filmyferns (Hymenophyllaceae) and
forkferns (Tmesipteris spp.), is well documented in both Victorian and Tasmanian
forests. It has been found that soft treefern trunks are the main substrate for 12
epiphytic fern species in Tasmania. A further four species use it as a minor host and
15 normally terrestrial species have been observed occasionally as an epiphyte.
However to support a healthy population of epiphytes, the trunks need to be a least
1.5 m high (approximately 40 years old). It is thought that this is because the skirt of
dead fronds tends to suppress growth of epiphytes, which tend to cover most of the
trunk until the trunk reaches this height.
Soft treeferns do not reach reproductive maturity until they are 23 years old or
approximately 95 cm in height (based on an average growth rate of 3.5–5 cm per
year). When reproductively mature, spore production is prolific, occurring mainly in
late summer. Over 750 million spores per frond are produced annually but ferns under
stress do not produce spores. Spore release can occur at any time but most are
released in late summer/early autumn. Germination can occur within seven days of
spore release, with the first fronds of the true fern emerging within 6 to 7 months. It
takes 3–4 years for the soft treefern to be fully developed. It requires adequate light
and moisture for the prothallus to establish, grow and subsequently develop into a
mature fern. It has been observed that young treeferns developing beneath the crowns
of large plants rarely grow to maturity, indicating that germination, development and
maturation of immature treeferns is inhibited by the close proximity of a mature plant.
Although the released spores have a short dispersal range, because of the huge
number produced, the potential dispersal range is much greater. Distances of 500–800
km appear to be no barrier to the dispersal of this species. Consequently, recruitment
of this species at a site does not rely on the on-site presence of the species. As long as
there is a source population within 500–800 km, soft treefern can potentially occupy
suitable sites. Thus its distribution is not limited by spore dispersal but rather by the
environmental conditions that occur during the first two stages of its development. As
long as appropriate conditions exist and there is a source population within range, soft
treeferns can rapidly colonise areas where gaps and some disturbance occur. This is
one of a few ferns that are the first to recolonise areas disturbed by activities such as
logging or road construction. Soft treeferns can also establish in suitable
microenvironments of otherwise unsuitable harsh alpine, coastal or arid areas,
although individuals tend to be scattered, poorly formed and stunted in these areas.
Flora of Tasmania
Dicksonia antarctica
Soft treeferns are hardy enough to recover from quite severe disturbances such as
forest fires. The growing apex is well-protected and can regrow if the mature fronds
are removed. In addition, the stem is protected by the thick mantle of adventitious
rootlets, which are able to regrow. If the fern is provided with an appropriate
substrate, it will easily re-establish, as the crown will quickly regenerate and the
treefern will continue growing. However, soft treeferns have a low survival rate in
areas where there has been substantial disturbance, such as clear-fell logging
operations that involve intense regeneration burns of the coupe area post-harvest.
Severe disturbances such as logging and burning also have a major effect on the
epiphyte communities that rely on soft treefern trunks as a substrate. It is not yet
known how long it takes for the full diversity of epiphytes to re-establish on the trunks
in regenerating forests.
Potential for Cultivation
The potential for cultivation of this species is moderate because it is very slow
growing and takes many years to grow to a size that is acceptable for sale. However it
is an easy species to propagate and grow, if given the right conditions, and is
attractive as a small fern even before it forms a significant trunk. Currently most
supplies of mature ferns are sourced from the wild, where they are mainly salvaged
from areas about to undergo a significant land use change, such as conversion from
native forest to plantation, and the land is covered by a Forest Practices Plan. In
Tasmania, harvesting is currently done under a tagging system overseen by the Forest
Practices Authority and guided by a Tree Fern Management Plan.
Soft treefern is very popular and highly prized by landscape gardeners due to its
natural beauty, ease of establishment, minimal maintenance, hardiness and resilience
to a range of conditions, as well as its versatility. It is very suited to garden planting
and landscaping purposes, as well as for indoor use, and is the second most commonly
planted treefern in the world.
When harvested, the whole fern, including a substantial portion of the trunk, is cut off
from the base and the fronds are removed. The trunk can then be stored for several
months provided it is kept moist, making it ideal for long-distance transport. To
replant the soft treefern requires a hole large enough to support the plant and secure it
with soil. Plants become self-supporting after about 12 months. In drier areas, the
ferns need to be provided with adequate moisture to re-establish successfully.
Soft treefern was once used as a food source for Tasmanian Aborigines. The top part
(0.5 m) of the stem was split and the starchy pith scooped out and eaten raw or
roasted.
Currently, soft treeferns are not only used as garden plants but the trunks are also used
as craft material for making items such as plant pots, baskets and bowls. The trunks
are quite resistant to decay and often used as boarding for walking tracks in boggy
areas. The fronds have potential for use as floral fillers in flower arrangements.
Information Sources
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Honours Thesis, University of Tasmania, Hobart.
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University of Queensland Press, Brisbane.
Flora of Tasmania
Dicksonia antarctica
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Management Plan – December 2001. Department of Natural Resources and
Environment, Victoria.
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South Australia. Melbourne University Press, Carlton, Victoria.
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Transporting or Trading of Dicksonia antarctica in Tasmania, July 2001. Forest
Practices Board, Tasmania.
Forest Practices Board (2003) Tree fern identification and management. Flora
Technical Note Series No. 5. Forest Practices Board, Tasmania.
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Flora of Tasmania