Dicksonia L'Hér.

Timothy M. A. Utteridge and Laura V. S. Jennings (2022). Trees of New Guinea. Kew Publishing. Royal Botanic Gardens, Kew

Distribution

A genus of 30 species found in the New World from south-eastern Brazil to as far north as Mexico, throughout the larger islands of Malesia and from Australia and New Zealand to Samoa; seven endemic species in New Guinea, some variable and not well understood.

Morphology General Habit

Large tree ferns to 10 m with a usually unbranched, erect trunk (caudex), adventitious buds and basal shoots may occur regularly; leaves and petiole bases densely hairy especially when young, characteristically persisting with age, hairs vary from soft and matted to bristly and spreading

Morphology Leaves

Fronds few to many (3–20), spirally arranged, borne at the trunk apex forming a rosette, usually bipinnate-pinnatifid to 4-pinnate, usually large, up to 5 m long, rarely less than 2 m; petioles (stipes) closely arranged, with numerous vascular bundles arranged in 3 arcs (hippocampiform), stipe bases persistent but rarely exposed and visible, mostly hidden by hair and adventitious roots; pinnae numerous (10-25 pairs), rachis raised above, lower pinnae either basally tapering or not (1/10 to ½ the length of the largest pinnae), fertile segments weakly to pronouncedly more deeply lobed than sterile ones, lobes with simple or branched veins

Morphology Reproductive morphology Sori

Sori 0.8–2 mm wide, borne on a receptacle at end of veins or acroscopic vein branches, protected by a “bivalved indusium”, formed from a reflexed marginal lamina lobe (“outer valve”) and a thinner cup-shaped inner indusium (“inner valve”)

Morphology Reproductive morphology Sporangia

Sporangia distinctly stalked, with complete oblique (=helicogyrate) annulus

Morphology Reproductive morphology Spores

Spores trilete, with baculate perine deposited evenly or in reticulate fashion with varying mesh size, appearing smooth, pitted or bumpy at low magnification.

Ecology

Dicksonia species are found in forest habitats from 1000–3000 m elevation. Some species from temperate areas may become dominant in the understory locally.

Recognition

The arrangement of hairs on the petiole is of great diagnostic importance but may be severely altered or lost during specimen preparation; the presence of irritating hair tips should be noted. Further important characters for identification are the length ratios of petiole vs. blade and longest medial pinna vs. basal pinna. Some species have hemidimorphic fronds, with skeletonised fertile pinnae and regular sterile pinnae on the same frond (not yet recorded in New Guinea). Dicksonia species could be confused with Cyatheaceae, but members of that family have mainly scales on the petioles and sori sitting on the back of a vein or in the fork of a branched vein, positioned at a specific distance between segment margin and segment midvein. Indusia of the Cyatheaceae are very diverse in shape and size, sometimes missing but never bivalved as in Dicksonia.