Pellaea calidirupium Brownsey & Lovis

Rhizomes long-creeping, to 90 mm long (in herbarium specimens), 1.0–2.0 mm in diameter, scaly. Rhizome scales appressed, narrowly ovate, 1–2 mm long, 0.3–0.5 mm wide, with black occluded midribs and open orange-brown cells on the margins. Fronds dimorphic; sterile 35–270 mm long; fertile 55–410 mm or rarely to 460 mm long. Stipes 30–230 mm long, dark red-brown, brittle, bearing appressed orange-brown scales; stipe scales mostly short and hair-like but some narrowly ovate with filiform apices, up to 3 mm long and 0.5 mm wide. Rachises dark red-brown throughout, bearing appressed scales. Fertile laminae 1-pinnate, elliptic or narrowly elliptic or linear, 30–240 mm or rarely to 330 mm long, 14–60 mm wide, dark glossy green on adaxial surface, paler green abaxially, coriaceous; hair-like scales on costae and both lamina surfaces. Primary pinnae on sterile fronds in 2–8 pairs, ± round or oblong with obtuse apices and truncate bases, the terminal pinna sometimes greatly enlarged; those on fertile fronds in 2–22 pairs, ± triangular or slightly falcate with acute or acuminate apices, bases truncate or slightly cordate, the terminal pinna only slightly enlarged. Fertile pinnae not overlapping, the longest near the middle, 8–40 mm long, 4–17 mm wide, widely spaced proximally and often distinctly stalked, close together distally and almost sessile; margins minutely toothed. Sori continuous along acroscopic and basiscopic pinna margins, often confluent at apices. Mean exospore diameter 35.9–40.5 μm; perispores echinate.

Pellaea calidirupium is distinguished from most populations of P. rotundifolia by the appressed scales on the stipes and rachises, the more obviously dimorphic fronds, generally shorter laminae (usually 30–240 mm long cf. 100–600 mm long), fewer pairs of pinnae on the fertile fronds (2–22 cf. 8–40), a sometimes enlarged terminal pinna, widely spaced proximal pinnae often borne on distinct stalks, and sori that are continuous around the pinna margins and often confluent at the apices. The fronds usually have an irregular and untidy appearance compared to the neat rows of pinnae in P. rotundifolia. The fertile pinnae are characteristically triangular, or slightly falcate, with tapering apices, in contrast to the rounded or oblong pinnae with obtuse to acute apices abruptly narrowed to a short, sharp point in P. rotundifolia.

Pellaea calidirupium is less easily distinguished morphologically from populations of P. rotundifolia on the Kermadec Islands. The latter plants have more or less appressed scales on the stipes and rachises, so the distinction between the two species relies on the remaining characters outlined above. Nevertheless, on the basis of molecular evidence, P. calidirupium is clearly distinct from P. rotundifolia on the Kermadecs and the North and South Islands (Brownsey et al. 2020).

Pellaea calidirupium grows on coastal cliffs, lava fields, dry rocky outcrops, bluffs, scree and roadside banks, either in the open, in crevices, under rock overhangs, under low scrub, or in rough grassland. It is often associated with Discaria toumatou, Rubus rubiginosa, Thymus vulgaris and species of Sophora and small-leaved Coprosma. It grows on greywacke, basalt, rhyolite, scoria, lava, schist, and rarely on limestone.

Together with Anogramma leptophylla, Asplenium flabellifolium, A. subglandulosum, Cheilanthes distans and C. sieberi, Pellaea calidirupium forms a characteristic assemblage of ferns that inhabit dry rocky areas, especially in the eastern South Island.

There is evidence that Pellaea calidirupium hybridises with P. rotundifolia in areas where the two species are contiguous (Brownsey & Lovis 1990), and that the hybrids have aborted spores (e.g. CHR 147559, 385976, 386678, 387589).

n = 58 (Brownlie 1961 – as P. falcata; Brownsey & Lovis 1990). However, in Australia P. calidirupium has been reported as diploid with a single count of n = c. 29 from Kaputar National Park (Tindale & Roy 2002), and as triploid and tetraploid in populations from Queensland (Kokubugata et al. 2007). If this is confirmed, the status of some Australian plants may need to be reconsidered.

There is a striking disjunction in the distribution of P. calidirupium in the North Island. Populations extend from the Three Kings Islands through eastern Northland to Mayor Island, but are absent along the whole Bay of Plenty and east coast, reoccurring again only in the very south of the North Island. Given the apparent suitability of habitat along the eastern coast, further investigation is needed to determine if the absence is real. There are no obvious morphological differences between the two groups, and spore measurements suggest that they are uniformly tetraploid (Brownsey et al. 2020).