The Tree-fern is native to Australia but cultivated in the milder parts of Britain where it sometimes occurs in the wild. It doesn’t like being dry, and temperatures below -5 Celsius are said to be fatal. I look for it when I’m in the south-west of Scotland, but I’ve so far only met it at the Logan Botanic Garden, Castle Kennedy and a few private gardens near Stranraer (Dumfries and Galloway). Sheltered spots on the west of Scotland have the reputation of mild winters and some people call the climate ‘subtropical’. However, in especially harsh winters, the temperatures in SW Scotland do fall below -5 Celsius.
In Australia I’ve seen it flourishing in damp woodland gullies in New South Wales and Victoria, and in the temperate rain forest of the Huon River Valley in Tasmania, where it forms part of the understorey. It thrives in dappled shade but survives in bright sunshine too. Australians call it ‘Soft Tree Fern’.
Many specimens of Dicksonia antarctica have been planted at the Logan Botanic Garden in Dumfries and Galloway, SW Scotland. Some of them have been damaged in unusually severe winters but these large ones are doing well. The wickerwork dinosaur is by Trevor Leat. Photo: John Grace.
Ferns first appear in the fossil record 360 million years ago, in the late Devonian period. Dicksonia looks ‘primitive’ but compared with several other families of ferns, the tree ferns are not so old, a mere 140 million years. The genus Dicksonia may have emerged only 65 million years ago. Visitors to the Logan Botanic Garden will have seen the brilliant wickerwork art by Trevor Leat depicting a dinosaur among the tree ferns, but whether Dicksonia antarctica co-existed with dinosaurs is questionable. The nearby plaque strikes a cautionary note (the Age of Dinosaurs was 245-66 million years ago).
Dicksonia is named after James Dickson (1738-1822) a plantsman who came from Traquair in Peebleshire. He was a ‘self-made’ man, making plant-collecting tours of the Scottish Highlands between 1785 and 1791 and marrying the sister of Scottish explorer Mungo Park. He achieved eminence despite humble origins and was one of the first members of the Linnean Society of London. The species name antarctica does not mean that the fern belongs to the Antarctic continent, in this context it simply means the plant comes from the extreme south.
D. antarctica in the temperate rain forest of the Huon River Valley in Tasmania. Photo: John Grace.
Tree ferns came to Britain by accident, arriving here in the 19th century as ballast in cargo ships from Australia. The ‘logs’ were discarded in the dockyard, but in mild and damp conditions the logs sprouted fronds. Interest in tree ferns for British gardens then began.
The ‘logs’ are nothing like logs from Angiosperm trees. Ferns have no secondary xylem (‘wood’). The logs are composed of the dead bases of the fronds, forming a medium for rooting. Some authors speak of Dicksonia as having a ‘rhizome’ inside the trunk, but rhizomes are usually defined as underground organs. It isn’t therefore a rhizome but simply a root, embedded in fibrous and decaying dead material which stores water. In a young plant the root reaches the soil but in old and large plants it may not. These ‘logs’ are shipped from Tasmania, having been harvested from the wild, and can be purchased from specialist nurseries here in the UK (see the footnote for more about wild-harvesting). When planted in soil (the right way up, of course) the living tissues will sprout fronds.
Cross section of the stem of D. antarctica. Attribution: Muséum de Toulouse, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
The fibrous tissues help to protect the species from wildfires. An Australian study mentions D. antarctica as one of the species in Tasmanian old-growth forest that is especially resistant to fire (Hickey 1994). I have read that some individuals may be long-lived, attaining a height of 6 metres and living for 1000 years. The funnel-shape of the rosette of fronds is thought to be efficient at trapping precipitation and conveying water to the growing tissues.
Global distribution of Dicksonia antarctica, according to GBIF.
I have found only one study on the ecophysiology of this species (Hunt et al 2002). Most ferns and ‘living fossil’ angiosperms like Ginkgo have low rates of photosynthesis. The accepted narrative is: “they evolved in times when the CO2 concentration was much higher than today, and therefore needed fewer stomata – most ferns and Ginkgo still have relatively few stomata and this means their rate of photosynthesis is limited under modern-day CO2 concentrations”. However, Dicksonia antarctica has a moderately high rate of photosynthesis, and the stomata respond to humidity in much the same way as Angiosperm trees (Hunt et al 2002). We presume this is because the species is relatively modern, not a ‘living fossil’. There is another study which looks at water transport of ferns in general (Pitterman et al 2011). The conclusions also point to Dicksonia being somewhat more ‘modern’ than many other fern species.
Distribution of Dicksonia antarctica in Britain and Ireland, from BSBI records. The recent records for West Cornwall seem to be mostly from wild places rather than in gardens (in the BSBI recording system, records from gardens are permitted).
Image from below the canopy. Melbourne Botanic Garden. Photo: John Grace.
Indigenous Australians took the live pith from the top part of the stem for food. They split the stem, scooped out the starchy material and ate it raw or roasted in ashes.
Footnote
The exploitation of Tasmanian forests became an issue in the 1980s, fuelled by the publicity of David Bellamy’s visit to Tasmania and his campaign to stop the Franklin River being dammed and its rainforests drowned. There remain ethical and conservation issues surrounding the trade of Dicksonia ‘logs’. As far as I can ascertain, those sold in Britain are harvested from the wild Tasmanian forests. Some may come from Victoria. They grow slowly, and so it is not practical to raise them in Australian nurseries. Until recently, importation of all plant material to Britain has been controlled by the EU regulation which states “plants exported to EU countries must be sourced from nurseries…”. Does this mean that imports have been illegal? And what of the post-Brexit future? Readers may be interested in an old journalistic article from the Guardian in 2002, called ‘A frond in need’. For more down-to-earth browsing, see the exchange of views here.
References
Hickey JE (1994) A floristic comparison of vascular species in Tasmanian old-growth mixed forest with regeneration resulting from logging and wildfire. Aust. J. Bot. 42, 383–404.
Hunt MA et al (2002) Ecophysiology of the Soft Tree Fern, Dicksonia antarctica. Austral Ecology (2002) 27, 360–368.
Nitta JH et al (2022) An open and continuously updated fern tree of life, Frontiers in Plant Science 13 https://doi.org/10.3389/fpls.2022.909768
Pitterman J et al (2011) Structure-function constraints of tracheid-based xylem: a comparison of conifers and ferns. New Phytologist 192, 449-461.
©John Grace
Botany in Scotland 