Fig. 1. Pteridium aquilinum ssp. ×subaquilinum nom.
prov., mid-June aspect (Finland, Lohja, Paavola, 1990).
– Photo: Harri Harmaja.

Notes on Pteridium

[Abstract] HARRI HARMAJA: Notes on Pteridium. – As a result of intensive field observation and studies on herbarium specimens with traditional methods it is stated that, contrary to the prevailing view, the bracken, Pteridium aquilinum (L.) Kuhn s. lato (Dennstaedtiaceae) comprises at least three sympatric taxa in Finland. At present, the taxonomy of the genus Pteridium Gleditsch ex Scop. in a worldwide scale is at the state of a very active analysis including molecular and genetic methods, especially by John Thomson (Sidney, Australia). Pending the final synthesis of that work, the following names are applied to our brackens: 1) a widely distributed indigenous taxon having fastly expanding leaves with a deltoid, more or less horizontally positioned lamina is P. aquilinum (L.) Kuhn ssp. pinetorum (C.N.Page & R.R.Mill) J.A.Thomson (P. aquilinum ssp. latiusculum (Desv.) Hultén ex R.T.Clausen sensu auct.), 2) another taxon that has slowly expanding, large leaves with an elongated, obliquely erect lamina and a pronounced weed character by its habitat ecology, is provisionally called as P. aquilinum ssp. ×subaquilinum nom. prov., and 3) plants that resemble P. aquilinum ssp. pinetorum and the recently described "P. pinetorum C.N.Page & R.R.Mill ssp. sibiricum Gureyeva & C.N.Page" but seem to represent a distinct taxon, are called as the "third taxon". Taxon no. 2 is supposed to be the hybrid between ssp. aquilinum and ssp. pinetorum. Plants that appear intermediate between ssp. ×subaquilinum nom. prov. and ssp. pinetorum also occur in places and may represent backcrosses between them. Additional variation, not readily fitting with these taxa, is discussed and the characteristics of an apparent fourth morph are presented. Typical P. aquilinum ssp. aquilinum, as it is found in e.g. central and western Europe, does not seem to occur in Finland. A table showing the diagnostic characters is presented for the comparison of P. aquilinum ssp. ×subaquilinum nom. prov. and ssp. pinetorum. Photographs of all the 3-4 taxa are presented. It is reminded of the neglected fact that the existence in Finland of the genome of aquilinum s. str. was confirmed on molecular basis already in 1995.

The bracken, Pteridium aquilinum (L.) Kuhn, passed until fairly recent years as the single but very variable species of its genus, possessing an almost cosmopolitan distribution. This concept, or tradition, dates back to the world-monograph on the genus (Tryon 1941). In Finland, the bracken never awoke essential taxonomic interest: it was considered uniform, without any distinguishable taxonomic units, or races. After Tryon´s contribution our bracken got an exact intraspecific name: it was considered to represent var. latiusculum (Desv.) Underw. ex A.Heller. However, my personal preliminary observations suggested that two races would occur in Finland: besides ssp. latiusculum, the main race, also plants which tentatively were assigned to the more southern ssp. aquilinum (Harmaja 1990). However, this paper did not arouse much interest. The publication of Wolf & al. (1995) which confirmed the occurrence in Finland of both aquilinum and pinetorum (as latiusculum) genomes on molecular basis, was unknown to the Nordic botanists for a long time. Unfortunately, the Finnish localities from where the samples originated, were not mentioned in the paper (and still seem to remain unknown).

Meanwhile, especially in Britain, an active taxonomic research on Pteridium has taken place: e.g., Wolf & al. (1995, a molecular analysis) and Page (1997, on traditional basis). Actually, the British pteridologists have had lively discussions as to how many races of the bracken exist in western Europe and adjacent areas, and how to name them.

Finally, we got a new monograph-like treatment in which Tryon´s taxa were subjected to a morphometric analysis and DNA fingerprinting (Thomson 2000). Generally, the varieties recognized by Tryon were found ‘good’ taxa; seven of them Thomson (2000) suggests to be treated as species. Concurring the tradition, two main taxa occur in Europe: P. aquilinum s. str. and P. latiusculum (Desv.) Hieron. ex R.E.Fr. Later (Thomson 2004) he found more appropriate to treat these taxa as subspecies: ssp. aquilinum and ssp. pinetorum (C.N.Page & R.R.Mill) J.A.Thomson (= ssp. latiusculum sensu auct.). The work of Dr. John Thomson (Sidney, Australia) and his collaborators, covering the whole genus Pteridium worldwide, continues intensively.

In the following, my observations – with traditional methods – on the variability of bracken in Finland are given. Observations have been made intensively in the field and on my own collections though the collections of the Botanical Museum of the University of Helsinki (H) have been consulted, too. Below, a comparison table is given of our dominant bracken, P. aquilinum ssp. pinetorum and the stoutest of our brackens which I now call P. aquilinum ssp. ×subaquilinum nom. prov. (= P. aquilinum ssp. aquilinum sensu Harmaja 1990). I have the impression that the latter is a hybrid between ssp. aquilinum and ssp. pinetorum though not being intermediate in its characters but more resembling the first-named parent. The nomenclature here follows the suggestions of Thomson (2004). It is to be emphasized that the characters given relate to "typical" plants which are truly dissimilar in their supposedly pure appearances. Intermediate appearing stands also occur; at least partly they may represent backcrosses between ssp. ×subaquilinum nom. prov. and ssp. pinetorum. Notes on additional variation among the Finnish brackens ("3rd taxon" and "4th morph") are also given below. Typical P. aquilinum ssp. aquilinum, as it is found in e. g. central and western Europe, does not seem to occur in Finland. Though having been the object of my interest and much field work for a fairly long time, these opinions may be considered preliminary, presented in the hope that the different kinds of the bracken of northern Europe will be subjected to a thorough, molecular-based analysis.

If you want to get acquainted with alive P. aquilinum ssp. ×subaquilinum (nom. prov.), please visit the Botanical Garden of the University of Helsinki. Two dense stands are housed there: one below the NW corner of the main building, the other – labelled as to species – being situated in the systematic compartment towards the southern edge of the garden!

A third taxon
(Figs. 7-9)

While our ssp. ×subaquilinum nom. prov. and ssp. pinetorum are very different at first sight, the present plant is rather similar to the latter. This tentative "third Finnish main taxon" differs from ssp. pinetorum: (i) the petiole is remarkably short, (ii) leaves are generally somewhat smaller, (iii) the position of the leaf (petiole and lamina) is oblique in relation to the ground, (iv) the shape of the lamina is somewhat less broadly triangular, the pinnae getting smaller upwards fairly gradually, (v) the ultimate segments tend to be relatively shorter, and slightly more obtuse, (vi) the leaf texture is more hygrophilic, the lamina being thinner and softer (likewise as compared with ssp. ×subaquilinum nom. prov.), (vii) the hyaline hairs of the lamina are more numerous, the margin of the false indusium being ciliate, (viii) this false indusium, or the very margin of the leaf segments is slightly narrower and often only indistinctly recurved, and (ix) plants of this kind tend to grow in small groups or sparse stands in fairly undisturbed, half-shady woods showing some rich forest character. The withered, dead leaves possess a greyish-brown colour; those of ssp. pinetorum are 'pure' brown. The present plant resembles the recently described "P. pinetorum C.N.Page & R.R.Mill ssp. sibiricum Gureyeva & C.N.Page" of Siberia and the north of European Russia (Gureyeva & Page 2005). However, in the latter taxon (i) the basal pinna is broader absolutely and as compared with the second lowest one, and (ii) the pinnae and pinnulae are disposed more obliquely (at a smaller angle) at their rachides and rachillae.

A fourth morph
(Figs. 10, 11)

Lastly, odd-looking plants ("4th morph") are found: the leaf is more or less small, obliquely erect, and the lamina is thin and composed of somewhat hairy narrow pinnae which only very gradually get smaller upwards. Such ’Dryopteris-like’ plants not necessarily represent a taxon of their own but may be some kind of abnormalities, or modifications of the taxa described above, especially of the "3rd taxon" (in stands of ssp. pinetorum or the "3rd taxon", solitary leaves that for some reason develop later in the season, sometimes possess this appearance). The "fourth morph" is often met with at margins of woods (like the plant of Fig. 11) or forest tracks (like the plants in Fig. 10). Page (1997) depicts this kind of leaves from Britain and regards them as "juvenile frond forms", or "sporelings" of P. aquilinum s. str. (ssp. aquilinum).

Some minor differences, too, exist between ssp. ×subaquilinum nom. prov. and ssp. pinetorum. The lamina of the former has more numerous hyaline hairs, its colour is usually slightly darker (however, not often so pronouncedly as in Fig. 5), its autumn colour is often somewhat less yellow-tinged, and also when withered and dead its leaves display a slightly darker, more grey-tinged colour. The former species may have proportionally a greater amount of the slimy juice that the petiole contains especially in the springtime. The habitats of ssp. pinetorum tend to be slightly drier and poorer in nutrients (already noted in the Finnish text in Harmaja 1990).

The tall bracken of Finland was previously considered by me to represent the true P. aquilinum s. str. (ssp. aquilinum) though not appearing fully identical with the latter (Harmaja 1990 and the previous versions of this web article). The first-named plant, now called by me as P. aquilinum ssp. ×subaquilinum nom. prov., differs from the extralimital, more southern ssp. aquilinum: (i) the pinnae are broader, (ii) the pinnae are more densely disposed along the rachis, thus partly covering each others, (iii) the young fiddleheads are more densely beset with brown hairs (see Fig. 2), and (iv) the cold resistance of the frond may be better. It is interesting that ssp. ×subaquilinum nom. prov. is not intermediate in its characters but resembles more one of the supposed parents, the extralimital ssp. aquilinum. Moreover, the density of brown indumentum of young shoot tips appears to be a feature that has not been directly heredited from either of the parents.

P. aquilinum ssp. pinetorum is no doubt indigenous in Finland but it does favour moderate and even fairly strong human influence through which it gains well-lit habitats. To the contrary, the Finnish occurrence pattern of ssp.×subaquilinum nom. prov. with a more southern main distribution, would suggest that it might be introduced (of fairly recent origin, favoured by the warming of the climate?). The Finnish bracken taxa do not seem to differ significantly in their resistance against the autumn frost and  in the time of frond withering.

P. aquilinum ssp. pinetorum is common and abundant in southern and central Finland and even reaches the middle boreal zone while ssp. ×subaquilinum nom. prov. is more southern. All three of our main taxa are essentially sympatric and widely distributed in southern Finland in the hemiboreal and southern boreal bioclimatical zones.

In 2001, a deviating form of ssp. pinetorum was encountered in Muijala (Lohja): two small stands in very dry heath forest with Pinus sylvestris and Calluna vulgaris comprised plants that were otherwise rather typical but the margin of the false indusium was rather densely hairy.

Gureyeva, I. I. & Page, C. N. 2005: Towards the problem of the bracken taxonomy in Siberia. – Taxonomy notes on the data of the Krylov Herbarium
of Tomsk State University 95: 18-29 [Russian, with very short English summary].
Harmaja, H. 1990: Suomessa on kahdenlaista sananjalkaa (Summary: Two races of bracken (Pteridium aquilinum) in Finland). – Lutukka 6: 15-16.
Page, C. N. 1997: The ferns of Britain and Ireland.2^nd ed. – Cambridge University Press. Cambridge.
Thomson, J. A. 2000: Morphological and genomic diversity in the genus Pteridium (Dennstaedtiaceae). – Ann. Bot. 85 (Suppl. B): 77-99.
Thomson, J. A. 2004: Towards a taxonomic revision of Pteridium (Dennstaedtiaceae). – Telopea 10(4): 793-803.
Tryon, R. M. 1941: A revision of the genus Pteridium. – Rhodora 43: 1-31, 37-67.
Wolf, P. G., Sheffield, E., Thomson, J. A. & Sinclair, R. B. 1995: Bracken taxa in Britain: a molecular analysis. – In: Smith, R. T. & Taylor, J. A. (eds.), Bracken: an environmental issue. Contributions to an international conference. International Bracken Group special publication 2: 16-20. University of Leeds, Leeds.

Fig. 2. Pteridium aquilinum ssp ×subaquilinum nom. prov., late spring aspect (Finland, Lohja, Gunnarla, 1990). – Photo: Harri Harmaja. – The tips of the young leaves ('croziers', or 'fiddleheads') are densely covered with brown hairs which make the plants conspicuous among the pale green vegetation.

Fig. 3. Pteridium aquilinum ssp. ×subaquilinum nom. prov. in Mid-June aspect when still proceeding with growth (Finland, Lohja, Paavola, 1990; the same stand as in Fig. 1). – Photo: Harri Harmaja.

Fig. 4. Pteridium aquilinum ssp. pinetorum, full-grown leaves in late summer aspect (Finland, Lohja, Paavola, 1990). – Photo: Harri Harmaja. – The leaf lamina is characteristically in a horizontal position. The leaf may reach to about the height of an adult´s legs. The leaves are clearly lower than those of  ssp. ×subaquilinum nom. prov. (compare Figs. 3 and 4!). – The dense stand of the latter taxon in Figs. 1 and 3 can be seen in the upper right corner.

Fig. 5. Pteridium aquilinum ssp. ×subaquilinum nom. prov. (on the left) and P. aquilinum ssp. pinetorum, fresh full-grown leaves from near-by stands, stretched and spread out onto one plane (Finland, Vantaa, Keimola, 2000). – Photo: Harri Harmaja.

Fig. 6. Pteridium aquilinum ssp. ×subaquilinum nom. prov. (two leaves on the left) and P. aquilinum ssp. pinetorum (on the right), tips of young leaves (Finland, 1990, from four different stands in two localities, the left leaf in each taxon originating from Lohja and the right one from Lammi). – Photo: Tuuli Timonen. – The plants of the former taxon are robust, the leaves are covered with numerous long, brown hairs at this stage, and the lamina spreads out gradually towards the height of summer ('vernation slow'); in the latter, the plants are less stout, the young leaf tips are rather white-tomentose with only scattered brown hairs, and the lamina reaches its full size fairly soon after having emerged above the ground.

Fig. 7. Pteridium sp. ("third taxon") in situ (Finland, Helsinki, Länsi-Pasila, 2003). – Photo: Harri Harmaja.

Fig. 8. Pteridium sp. ("third taxon") in situ (Finland, Helsinki, Pirkkola, 2001). – Photo: Harri Harmaja.

Fig. 9. Pteridium sp. ("third taxon") in situ (Finland, Helsinki, Pirkkola, 2001). – Photo: Harri Harmaja.

Fig. 10. Pteridium sp. ("4th morph") in situ (Finland, Espoo, Kalajärvi, 2003). – Photo: Harri Harmaja.

Fig. 11. Pteridium sp. ("4th morph") (Finland, Vantaa, Länsisalmi, 1990). – Image: Harri Harmaja (scanned from dried specimen).