Cyatheales

Tree fern (Cyatheales)
Dicksonia antarctica
Scientific classification
Kingdom:	Plantae
Division:	Pteridophyta
Class:	Polypodiopsida /  Pteridopsida (disputed)
Order:	Cyatheales
Families and Genera
Thyrsopteridaceae Thyrsopteris Loxsomataceae Loxsoma Loxsomopsis Culcitaceae Culcita Plagiogyriaceae Plagiogyria Cibotiaceae Cibotium Cyatheaceae Alsophila Cyathea Gymnosphaera Hymenophyllopsis Sphaeropteris Dicksoniaceae Calochlaena Dicksonia Lophosoria Metaxyaceae Metaxya

The order Cyatheales, which includes the tree ferns, is a taxonomic division of the fern class, Polypodiopsida. No clear morphological features characterize all of the Cyatheales, but DNA sequence data indicate the order is monophyletic. Some species in the Cyatheales have tree-like growth forms, but others have rhizomes. Some species have scales on the stems and leaves, while others have hairs. However, most plants in the Cyatheales are tree ferns and have trunk-like stems up to 10 metres (33 ft) tall. It is unclear how many times the tree form has evolved and been lost in the order.

Description

In general, any fern that grows with a trunk elevating the fronds (leaves) above ground level can be called a tree fern. However, the plants formally known as tree ferns comprise a group of large ferns belonging to the families Dicksoniaceae and Cyatheaceae in the order Cyatheales.

Tree ferns are found growing in tropical and subtropical areas, as well as temperate rainforests in South Africa, Australia, New Zealand, and other island groups nearby; a few genera extend further, such as Culcita in southern Europe. Like all ferns, tree ferns reproduce by means of spores developed in sporangia on the undersides of the fronds.

Dicksonia antarctica at Kew

The fronds of tree ferns are usually very large and multiple-pinnated, but at least one type has entire (undivided) fronds. The fronds of tree ferns also exhibit circinate vernation, meaning the young fronds emerge in coils that uncurl as they grow.

Unlike flowering plants, tree ferns do not form new woody tissue in their trunk as they grow. Rather, the trunk is supported by a fibrous mass of roots that expands as the tree fern grows.

Some genera — for example Dicksonia and Cibotium, and some Cyathea — can be transplanted by severing the top portion from the rest of the trunk and replanting it. If the transplanted top part is kept moist it will regrow a new root system over the next year. The success rate of transplantation increases if the roots are dug up intact. If the crown of the Tasmanian tree fern Dicksonia antarctica (the most common species in gardens) is damaged, it will die because all new growth occurs there. But other clump-forming tree fern species, such as D. squarrosa and D. youngiae, can regenerate from basal offsets or from "pups" emerging along the surviving trunk length. Tree ferns often fall over in the wild, yet manage to reroot from this new prostrate position and begin new vertical growth.

Tree fern frond ("fiddlehead") by the Akatarawa River, New Zealand: These unopened fronds are edible, but must be roasted first to remove shikimic acid.

The number of tree fern species is likely to be around a thousand. Although new species are discovered in New Guinea with each botanical survey, many species throughout its range have become extinct in the last century as forest habitats have come under pressure from human activity.

Threats to tree ferns

While many ferns are able to achieve a widespread distribution because of their spore reproduction, tree fern species tend to be very local. This makes their species much more susceptible to the effects of local deforestation. Why species are not more widespread is unknown, especially considering they have sufficient height to have a greater chance of getting spores into the wind stream.

Where feral pigs are a problem, as in some Hawaiian rainforests, they will knock over tree ferns to root out the starchy pith, killing the plant.

A lone tree fern next to the Peak Tram line on Hong Kong Island

Non-Cyatheales tree ferns

Outside of the Cyatheales a few ferns in other groups could be considered tree ferns, such as several ferns in the family Osmundaceae that can achieve short trunks under 1 metre (3.3 ft) tall. A few species in the genera Blechnum, Leptopteris, Sadleria and Todea could also be considered tree ferns in a liberal interpretation of the term.

The families that constitute Cyatheales have been relatively firmly established as a clade by DNA sequencing and morphological studies. The order Plagiogyriales, which contains the family Plagiogyriaceae, is most closely related to the Cyatheales, not to the Osmundales as had been previously supposed.

Transplanted Dicksonia antarctica tree ferns at Combe Martin Wildlife and Dinosaur Park, Devon, England

Cyathea medullaris can grow up to 20 m tall in its native New Zealand.

Phylogeny

The following phylogram shows a possible relationship between the Cyatheales families and their genera.

	Cyatheales Thyrsopteridaceae Thyrsopteris Loxsomataceae Loxsoma Loxsomopsis Culcitaceae Culcita Plagiogyriaceae Plagiogyria Cyatheaceae Sphaeropteris Gymnosphaera Alsophila Cyathea Hymenophyllopsis Cibotiaceae Cibotium Dicksoniaceae Lophosoria Dicksonia Calochlaena Metaxyaceae Metaxya Polypodiales

References

• Pryer, K.M., A.R. Smith, and J.E. Skog. 1995. Phylogenetic relationships of extant ferns based on evidence from morphology and rbcL sequences. American Fern Journal 85: 205-282.
• C.Michael Hogan. 2010. Fern. Encyclopedia of Earth. National council for Science and the Environment. Washington, DC
• Large, M.F. and J.E. Braggins Tree Ferns. Timber Press (2004).
• Smith, A.R., K.M. Pryer, E. Schuettpelz, P. Korall, H. Schneider & P.G. Wolf 2006. A classification for extant ferns. PDF (420 KiB) Taxonomy 55(3): 705-731.

External links

• Community: Care and propagation of Treeferns (German/English)
• Fern Files: Tree Ferns