The familial taxonomic framework here applied follows Smith & al. (2006). Additional, more recent phylogenetic molecular studies by Schuettpelz & al. (2016) considerably modified this taxonomy, mostly in adopting a much narrower familial and generic concept and thus accepting more families and genera than before. For instance, Woodsiaceae was accepted as a monogeneric family with Athyriaceae (e.g. native Athyrium) and Cystopteridaceae (e.g. native Cystopteris and Gymnocarpium) segregated from it. There is a lot of controversy whether or not too many fern genera are accepted now (compare Schuettpelz & al. 2018 and Christenhusz & Chase 2018).
The Belgian pteridophytes, native as well as non-native, are here classified as follows (familial circumscriptions that differ from Lambinon & al. 2004 are preceded by an *):
Isoetaceae: Isoetes (only native taxa);
Lycopodiaceae: Diphasiastrum, Huperzia, Lycopodiella, Lycopodium (all with only native taxa);
*Aspleniaceae: Asplenium (incl. Ceterach) (only native taxa);
Blechnaceae: Blechnum (only native taxa);
Dennstaedtiaceae: Pteridium (only native taxa);
Hymenophyllaceae: Hymenophyllum (only native taxa);
Marsileaceae: Pilularia (only native taxa);
Ophioglossaceae: Botrychium, Ophioglossum (all with only native taxa);
Osmundaceae: Osmunda (only native taxa);
*Thelypteridaceae: Phegopteris, Thelypteris (incl. Oreopteris) (all with only native taxa);
1. Plants free-floating on water === Salviniaceae
1. Plants not free-floating on water, rooted on solid substratum === 2
2. Stems jointed (articulated). Leaves scale-like, rudimentary, in whorls and fused into sheath at each node. Sporangia in terminal strobili === Equisetaceae
2. Stems not jointed. Leaves not whorled at nodes. Sporangia variously arranged === 3
3. Leaves filiform, grass-like === 4
3. Leaves never grass-like: scale- or needle like or with a distinct blade === 5
4. Plant tufted. Leaves forming basal rosette, straight at apex (not circinnate). Sporangia borne singly in swollen leaf bases (native) === Isoetaceae
4. Plant rhizomatous. Leaves on short-creeping stems (never in a basal rosette), circinnate at apex (at least when young). Sporangia numerous in globose, hairy sporocarps (native) === Marsileaceae
5. Leaves inconspicuous, needle- or scalelike, less than 20 mm === 6
5. Leaves with distinct blade === 7
6. Leaves needle-like, entire or indistinctly serrate at base, eligulate. Sporangia all similar (homosporous) (native) === Lycopodiaceae
6. Leaves scale-like, distinctly serrate along most of margin, with a ligule on the upper side (at least when young) (Fig. 3). Sporangia dissimilar, of two sizes (heterosporous) === Selaginellaceae
7. Leaves (at least sterile ones) entire === 8
7. Leaves not entire === 9
8. Leaves dissimilar: fertile ones (sporophores) a spike (sporangia borne on erect non-laminate stalks), sterile ones (trophophores) ovate, rounded at apex, abruptly tapering at base, ca. 10 x 4 cm (native) === Ophioglossaceae p.p. (Ophioglossum)
8. Leaves all alike: linear-lanceolate, cordate at base, 10-60 cm long, with a distinct stalk (native) === Aspleniaceae p.p. (Asplenium scolopendrium)
9. Leaves flaccid, membranous and translucent (not more than one cell thick) (native) ==== Hymenophyllaceae
9. Leaves never translucent and flaccid === 10
10. Fertile leaves (sporophores) inserted near the base of sterile leaves (trophophores), long-stalked. Plants less than 30 cm (native) === Ophioglossaceae p.p. (Botrychium)
10. Leaves not with a fertile part branching off from the base of the sterile part === 11
11. Leaves strikingly dissimilar: fertile segments distinctly narrower than sterile ones === 12
11. Leaves not strikingly dissimilar: all identical or only (slightly) differing in size === 15
12. Leaves parsley-like, 4 pinnate. Small annuals or perennials, less than 30 cm === Pteridaceae p.p. (Anogramma, Cryptogramma)
12. Leaves (at least sterile ones) 1-2 pinnate === 13
13. Leaves large, up to 200 cm, sterile below and fertile above. Sporangia without an annulus (native) === Osmundaceae
13. Leaves usually smaller, never sterile below and fertile above. Sporangia with an annulus ===14
14. Sori not covered by inrolled leaf margin, long and linear, occupying almost entire length of the pinnae of the fertile leaves. Blade of sterile leaf (ob-) lanceolate, more than 3x as long as wide. Margin of the pinnae entire to crenate (native) === Blechnaceae
14. Sori covered by inrolled leaf margin. Margin of the pinnae lobed to pinnatifid === Onocleaceae
15. Margins of fertile fronds revolute (i.c. sori covered by reflexed margin of pinnae) === 16
15. Margins of fertile fronds more or less plane, not revolute === 17
16. Indusium of two kinds: an outer (pseudo-)indusium (i.c. incurved margin of pinnae) and an inner (true) indusium (often obscure). Rhizomes long-creeping. Leaves 3-4 pinnate, often very large (native) === Dennstaedtiaceae
16. Indusium of one kind (pseudoindusium). Rhizomes short. Leaves 1-3 pinnate, usually small === Pteridaceae p.p.
17. Sori elongate along veins, never marginal, usually with linear indusium or leaves entirely covered with scales beneath (native) === Aspleniaceae p.p.
17. Sori variously shaped, often round, sometimes marginal (if elongate along veins then without indusium). Leaves never entirely covered with scales beneath === 18
18. Fronds pinnatifid. Indusium absent === Polypodiaceae
18. Fronds pinnate. Indusium present or not === 19
19. Indusium absent (native) === Thelypteridaceae p.p., Woodsiaceae p.p.
19. Indusium present ===20
20. Indusium reniform or peltate, margins entire === Dryopteridaceae
20. Indusium not reniform or peltate (if more or less rounded then margins toothed or fringed with hairs)=== 21
21. Sori submarginal(native) === Thelypteridaceae p.p.
21. Sori superficial (native) === Woodsiaceae p.p.
In addition to the fern species treated here several others are in cultivation in Belgium and might occur as escapes. One of the commonest taxa indoors surely is Nephrolepis (especially N. cordifolia C. Presl and N. exaltata (L.) Schott; Hovenkamp & Miyamoto 2005). Nephrolepis has been accommodated in a separate family (Nephrolepidaceae) or in Dryopteridaceae or Polypodiaceae, although recent phylogenetic studies rather support inclusion in Lomariopsidaceae (Smith & al. 2006). Nephrolepis superficially resembles Polypodium but fronds are longer with much more pinnae, lighter green, always 1-pinnate and sori are always covered by an indusium. Nephrolepis cordifolia and N. exaltata are increasingly found as garden escapes in southern Europe (see for instance Marchetti 1999, Simões da Silva & Domingues de Almeida 2000) and might occur in suitable places where frostbite is unlikely in Belgium as well (basement walls,…). In recent times several other non-native ferns from other genera have been recorded as escapes in neighbouring countries. Clement & Foster (1994) cite for instance species of Phymatodes, Davallia, Cyathea and Dicksonia but these are generally confined to climatically more favourable locations (chiefly in the Channel Islands). See also Grenfell (1983) for an account of exotic ferns running wild in the British Isles. More interesting is a recent record of Doodia aspera R. Brown on a London wall (Edgington 2005). In the Netherlands Marsilea quadrifolia L. (Marsileaceae) recently occurred along river Waal (Drok & Weeda 1999), probably as an escape from cultivation although native status has been suggested. It is unmistakable with its 4-foliolate leaves and long-creeping rhizomes. Osmunda regalis L. is a locally frequent native species in woodlands and heaths on peaty soil. It is also cultivated for ornament and at least part of the Belgian populations is probably non-native. Their exact identity possibly requires additional research since several other taxa of Osmunda are involved. Page & Bennell (1986) cite the more or less distinct Osmunda cinnamomea L. and O. claytoniana L. (the latter has been reported from the Netherlands; van Ooststroom 1970). Finally, cultivated populations of Osmunda regalis could represent non-native varieties and should be checked carefully. Botrychium lunaria (L.) Swartz is a very rare, endangered native species. Two additional taxa of this genus have been reported from Belgium: Botrychium matricariifolium (A. Braun ex Döll) Koch, an extinct native and B. simplex E. Hitchc. The latter is represented in Belgium by a single, enigmatic record in 1908 in the sea dunes in Knokke, possibly introduced with planted Swedish pines (De Langhe 1989) or as an exceedingly rare, extinct native (Leten 1992). Finally, several additional pteridophytes have recently been recorded as escapes in the (heated) greenhouses of the Botanic Garden in Meise. Psilotum nudum (L.) Beauv. (Psilotaceae) is a common pot weed and probably occurs in other greenhouses in Belgium. Cyclosorus dentatus (Forssk.) Ching. (syn.: Christella dentata (Forssk.) Brownsey et Jermy, Thelypteris dentata (Forssk.) St John) (Thelypteridaceae) also behaves like a weed and occurs in greenhouses where it is not cultivated. Further records include Pellaea rotundifolia (Forst.) Hook. (Pteridaceae) and Microsorum pteropus (Blume) Copeland (syn.: Polypodium pteropus Blume) (Polypodiaceae).
Christenhusz M.J.M. & Chase M.W. (2018) PPG recognises too many fern genera. Taxon 67(3): 481-487. [available online at: http://www.ingentaconnect.com/contentone/iapt/tax/2018/00000067/00000003...
Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.
De Langhe J.E. (1989) Werd de Kleine maanvaren (Botrychium simplex Hitchcock) inderdaad te Knokke gevonden? Dumortiera 43: 28-29.
Drok W.J. & Weeda E.J. (1999) Marsilea quadrifolia L. (Klaverbladvaren) nieuw voor Nederland. Gorteria 25: 89-103. [available online at: http://www.repository.naturalis.nl/document/567231]
Edgington J.A. (2005) Exotic ferns on London walls. BSBI News 100: 42-44. [available online at: http://archive.bsbi.org.uk/BSBINews100.pdf]
Grenfell A.L. (1983) Adventive ferns. I. BSBI News 35: 12. [available online at: http://archive.bsbi.org.uk/BSBINews35.pdf]
Hovenkamp P.H. & Miyamoto F. (2005) A conspectus of the native and naturalized species of Nephrolepis (Nephrolepidaceae) in the world. Blumea 50: 279-322. [available online at: http://www.repository.naturalis.nl/document/565638]
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Leten M. (1992) Botrychium simplex te Knokke? Dumortiera 50: 1-4.
Marchetti D. (1999) Nephrolepis cordifolia C. Presl (Nephrolepidaceae, Pteridophyta) « naturalizzata » nella Toscana nord-occidentale. Ann. Mus. Civ. Rovereto, Sez. Arch., St., Sc. Nat. 13: 163-166. [available online at: http://www.museocivico.rovereto.tn.it/UploadDocs/45_art07_marchetti.pdf]
Page C.N. & Bennell F.M. (1986) Osmunda. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1: 12. Cambridge University Press, Cambridge.
Schuettpelz E. & al. (2016) A community-derived classification for extant lycophytes and ferns. Journ. Syst. Evolut. 54(6): 563-603. [available online at: http://onlinelibrary.wiley.com/doi/10.1111/jse.12229/pdf]
Schuettpelz E., Rouhan G., Pryer K.M., Rothfels C.J., Prado J., Sundue M.A., Windham M.D., Moran R.C. & Smith A.R. (2018) Are there too many fern genera? Taxon 67(3): 473-480. [available online at: http://www.ingentaconnect.com/contentone/iapt/tax/2018/00000067/00000003...
Simões da Silva A. & Domingues de Almeida J. (2000) Nephrolepis exaltata (Nephrolepidaceae), naturalized in Europe. An. Jard. Bot. Madrid 58(1): 182. [available online at: https://www.researchgate.net/publication/280742300_Nephrolepis_exaltata_...
Smith A.R., Pryer K.M., Schuettpelz E., Korall P., Schneider H. & Wolf P.G. (2006) A classification for extant ferns. Taxon 55(3): 705-731. [available online at: http://www.fieldmuseum.org/sites/default/files/smith-et-al-taxon-2006.or...
Van Ooststroom S.J. (1970) Aanwinsten voor de Nederlandse adventief-flora, 10. Gorteria 5(1): 1-9. [available online at: https://www.narcis.nl/publication/RecordID/oai%3Anaturalis.nl%3A527120]