Cystopteris fragilis

Smith, Alan R. & Kessler, Michael, 2018, Prodromus of a fern flora for Bolivia. XXVIII. Cystopteridaceae, Phytotaxa 344 (1), pp. 75-79 : 77

Cystopteris fragilis (L.) Bernh. in Schrader, Neues J. Bot. 1(2): 26, t. 2. f. 9. 1806 [1805].

Range: —Nearly cosmopolitan; Greenland, throughout North America, Mexico, and Mesoamerica, but sparingly in Andean South America in Venezuela, Colombia to Bolivia (CO, LP); specimens from southern Argentina and Chile, from near sea level to 3500 m, appear to be mostly, or entirely, C. fragilis s.l. (but see Arana & Mynssen 2015, who exclude C. fragilis from this area, treating the taxon there as C. apiiformis ); Europe, Middle East, and Asia; apparently absent in Australasia and Polynesia, but said to be naturalized in New Zealand ( Brownsey & Smith-Dodsworth 1989). Numerous varieties have been described, especially in Europe. This species is often said to be the world’s most widespread and common fern, but the reality is that it is a polyploid complex, comprising great morphological variation and multiple cytotypes.

Ecology: —Common; terrestrial and occasionally epiphytic in humid to semihumid high-montane forests and alpine habitats, usually in rocky, locally moist and shady habitats, but also on rocky roadside banks, along streams and ditches, and in scrub; 3400–4400 m (lower in temperate regions of South America and on the Peruvian coast).

Notes: —A variable species with respect to blade dissection, and segment margins, with veins ending in teeth; the teeth may be sharply acute or more rounded. It differs from C. diaphana (Bory) Blasdell , with veins ending in emarginations, and often more dissected blades; other helpful distinctions, at least in Bolivia, are mentioned in the key, but the applicability of these characters to specimens of the two species outside Bolivia needs further investigation. There are also differences in spore ornamentation ( Pearman 1976, Arana & Mynssen 2015), as well as indusial size, shape, and degree of apical incision; these differences require further study over a broad geographical range. Detailed studies will likely show that this is a species complex of reproductively isolated, but morphologically very similar cryptospecies, “unique genomic combinations [that] would require new names if recognized as species” ( Rothfels et al. 2014). Allopolyploidy, confirmed by Rothfels et al. (2014, 2017), is rampant in this group, and variants examined by them suggest many independent origins of tetraploid cytotypes. Such a complex situation is certainly apparent among the North American representatives of Cystopteris fragilis and allies, and is probably also operative among South American representatives.