Monday, 26 January 2009

An Urban Epiphyte

An epiphyte is a plant which grows on another plant without parasitising it(1). In the best known examples the host plant is usually a tree. Epiphytes are particularly common in tropical rain forests and many different types of plants can be involved, for example, ferns, orchids, bromeliads and even cacti. An epiphyte has some advantages over plants growing on the ground, for example less competition and increased light levels. On the other hand such a plant is likely to experience lower levels of nutrients and water.
Because of the danger of drought and drying out, epiphytes tend to be found in areas with high rainfall. In Britain and Ireland most epiphytes are found among the so-called ‘lower plants’ (i.e. mosses and liverworts) but some ‘vascular’ plants (i.e. plants with stems) are involved, particularly ferns and particularly ferns in the genus Polypodium (often referred to as ‘polypodies’). As a result of the higher rainfall requirements you are more likely to see epiphytic polypodies (and other epiphytes) in Ireland and the western side of the island of Britain than you are on the drier eastern side(2). Nevertheless, epiphytic polypodies are usually confined to old woods in remote rural locations such as parts of Ireland, Wales, western Scotland and western England. A famous location is Wistman’s Wood on Dartmoor. This is an ancient oak wood in which, “... [the oaks’] crooked branches [are] festooned with mosses and lichens, with ferns and even young trees sprouting from their clefts”(3). Although polypody ferns are reasonably common on old walls in South Manchester (for example the walls of Withington Sewage Works near Hardy Farm), until recently the only vaguely epiphytic one that I had seen was growing on a, decidedly dead, elder bough in one of the compounds on Chorlton Ees.
Then last year (2008) I received an email from local conservationist, Leon Patsalides (the founder of the Friends of Hough End) telling me of a fern that he had spotted growing on a street tree on Upper Chorlton Road; he called it a “sky-walking fern”(4). Next time that I travelled along that road, on the top deck of the number 86 bus, I kept my eyes open for it, and eventually spotted three examples. All three were Polypodium species growing on London Plane trees at between about 3.5 to 7.0m above street level and usually in the crotch of the tree at the point at which the main trunk begins to branch. The photograph above shows, what is probably, the best example on the corner of Wood Road and Upper Chorlton Road.
I mentioned this discovery to Audrey Locksley of the Manchester Field Club and she said that she had seen the same phenomenon on Urmston Lane(5). Subsequently, I walked the length of Urmston Lane, from the centre of Urmston to Stretford, and spotted seven more examples. Nevertheless, the phenomenon doesn’t appear to be particularly common and I have now examined dozens of plane trees, on main roads, side streets and parks and have not spotted any more.

A note on the species involved is in order here:

The ferns are just out of reach (and I tend not to carry a ladder around with me!) – so I have not been able to obtain a specimen for identification. My educated guess is that they are most likely to be Intermediate Polypody (Polypodium interjectum) as this is the species present on the walls of Withington Sewage Works(6) and they look very similar. The tree is invariably London Plane (Platanus x hispanica) which is a man-made hybrid between the Oriental Plane (P. orientalis) and the American Plane (P. occidentalis)(7). London Plane was first grown in Britain at the end of the 17th century and became a preferred street tree, first in London, then in other cities in the UK, because of its resistance to pollution and its general hardiness. In the wilder corners of Britain you are most likely to see epiphytic polypodies on oak trees (Quercus sp.)(2) but, in South Manchester, I have not seen a single fern on a single oak.
In conclusion, this may be a genuinely new urban phenomenon but it’s difficult to give any sort of explanation. And it’s particularly difficult to explain why only plane trees appear to be involved and why it only seems to occur on certain busy main roads.
If anyone spots any more epiphytic ferns, on any more local trees, I hope that they will let me know.

Dave Bishop, January 2009
1.For more information on epiphytes see: www.kew.org/ksheets/epiphytes.html

2. ‘Woodlands’ by Oliver Rackham, Collins 2006.

3. ‘The Wild Woods’ by Peter Marren, David & Charles, 1992.

4. Personal communication from Leon Patsalides, February 2008.

5. Personal communication from Audrey Locksley, March 2008.

6. Letter from Dr. F. Rumsey (Natural History Museum) to Ms. P. Tolfree re: ‘The Identification of Polypodium Ferns from the walls of Withington Sewage Works, near Brookburn Rd., Chorlton’, 2nd February, 2006.

7. ‘A Field Guide to the Trees of Britain and Northern Europe’ by Alan Mitchell, Collins 1974.

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