[NatureNS] More on Fairy Rings

Date: Fri, 11 Oct 2013 13:49:19 -0300
From: "Stephen R. Shaw" <srshaw@Dal.Ca>
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Still interested in the origin of the 'Sensitive Fern' rings on the Mt  
Uniacke estate, on a recent visit to UK I mentioned this to a  
colleague. He had been in S. Africa with a biologist there, who in  
turn had had some involvement with the Namibian fairy ring phenomenon.  
  In a low rainfall zone in a remote Namibian desert, there are  
thousands of these circular depressions of bare sand up to 30 feet  
across in 'colonies', each surrounded by a fringing ring of desert  
grass that is more luxuriant than that in the clumps lying between the  
rings.  Ring origin is uncertain but one recent research suggestion is  
that certain sand termites eat the grass from the middle of the ring  
outwards and kill it, so the centre retains no grass at all.  The  
centre therefore is able to collect and retain precious rainfall water  
just below the surface for use by the termites, water that elsewhere  
gets taken up rapidly by root systems of the grasses.  The grass at  
the edge of the circle has partial access to this reservoir so becomes  
lusher and available as termite food.  The rings can last for up to 75  
years but eventually each disappears, individually.  If you?re not  
aware of these fascinatingly bizarre structures (I wasn?t), type  
?Namibian fairy rings? into Wikipedia or Google.  These particular  
termites are stylized by researcher Juergens as even more effective  
water engineers than beavers, which might raise a few eyebrows here.

The S. African biologist e-mailed back that he did not know about  
fairy rings involving ferns, but had turned up a reference to a N.  
American fern that I?d missed:

http://www.blueplanetbiomes.org/lady_fern.htm

This site refers to the Northern Lady Fern (Athyrium filix-femina), a  
common native species and ornamental, which according to this site  
often forms fern rings, though no further reference for this is cited.  
Trying to pursue this on Google turned up several interesting  
medicinal uses by native peoples, but failed to reveal any other  
citations of Athyrium ring formation or speculation about the  
mechanism.  Have any of the botanists on this list run across any more  
information, for Athyrium, or for Osmunda mentioned below?

Somehow this led to the Interrupted Fern (Osmunda claytoniana).  In  
Wikipedia, it is said to form small, dense colonies, each spreading  
locally through a very large rhizome, and it is said also to often  
form fairy rings.  So at least in this case, there is some possibility  
of a mechanism, in that an extended rhizome might use up essential  
resources (nitrogen?) at its centre.  It might then give rise to  
above-ground fronds in a ring but only at the still-resourced edge of  
the rhizome, as I think Dave Webster originally suggested.  If this is  
correct, the consequence would be that each fern ring actually is a  
clone of genetically identical individuals.  Apparently this is true  
for other ferns with rhizomes, and for some plants like goldenrod that  
also spread through rhizome colonization. Perhaps everyone but me knew  
this already.

Steve (Halifax)


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