Lepidium rekohuense, Lange & Heenan & Houliston & Rolfe & Mitchell, 2013

Lange, P. J. de, Heenan, P. B., Houliston, G. J., Rolfe, J. R. & Mitchell, A. D., 2013, New Lepidium (Brassicaceae) from New Zealand, PhytoKeys 24, pp. 1-147 : 98-103

Lepidium rekohuense sp. nov.

A L. oleraceo habitu renascenti, habenti periodo distincto rosulato, ramis sparse papillato-pilosis, floribus 2-staminatis, pedicellis minute pilosis, siliculis orbiculatis, minute alatis et emarginatis et serie DNA differt. A speciebus Lepidii ceteris Insularum Chathamicarum (L. panniformo et L. oligodonto) caulibus minute papillato-pilosis, floribus semper 2-staminatis, foliis serratis et siliculis orbiculatis (raro obovatis) differt. A L. panniformo praeterea habitu decumbenti et serie DNA recedit.

Holotype.

Chatham Islands ( Figs 86 -88 ): Chatham (Rekohu) Island, Kaiangaroa, Kaiangaroa Point, 11 January 2006, P. J. de Lange CH405, J. W. D. Sawyer & A. Baird, Collection over three sheets comprising four pieces (one sterile) taken from the same plant. (AK 295129-AK 295131! Isotypes: BM!, CANB!, CHR!

Etymology. The epithet ' rekohuense ' is derived from ‘Rekohu’, the Moriori name for Chatham Island which is said to mean 'land of misty skies’ ( King 1989). This name was chosen to reflect the endemic status of this species on the Chatham Islands group.

Description ( Figs 89 -92 ): Tap-rooted, pungent-smelling, decumbent, summer-green, perennial herb forming densely leafy masses up to 2 m diam., and arising from stout, semi-circular, dark reddish-grey (when exposed) rootstock 100-500 mm diam. Tap root woody, up to 1.5 m long, deeply descending. Plants dying down to rootstock and/or previous seasons stem nodes, over winter or in times of adversity. Stems decumbent, widely spreading, up to 2 m long and 30 mm diam., woody, ± spherical in cross-section, prominently ridged and/or grooved (especially when dry), dark reddish-green to dark green, usually scarred throughout with numerous old leaf bases; stems heavily branched in upper ⅔, branches and branchlets numerous, prostrate, widely spreading, very leafy; basal portion of stems, ± glabrous, otherwise finely and sparsely papillate-hairy, especially along leaf decurrencies, and within stem grooves, hairs very short 0.01-0.3 mm long, white, glandular-pustulate, rather sticky when fresh. Leaves glabrous, firmly fleshy to succulent, dark green to green, at senescence turning yellow. Rosette leaves persistent at fruiting; petioles distinct up to 50 × 3 mm, slightly concave in cross-section, fleshy; lamina narrowly spathulate to spathulate-oblong, up to 30.0 × 13.3 mm, margins usually denticulate, crenulate, if denticulate then with 10-18(-26) pairs of blunt teeth running to and including apex, base broadly attentuate. Middle stems leaves persistent at fruiting; petiole distinct up to 15 × 2 mm, mostly flat in cross-section, sometimes slightly concave, fleshy; lamina elliptic, narrowly elliptic to oblong, 18.86-26.18(-35.00) × 9.64-16.20 (-18.00) mm; margins sharply and regularly serrate-dentate with 10-16(-22) pairs of teeth running to and including the apex, lamina base broadly cuneate to cuneate. Upper stem leaves with or without a distinct petiole, petiole if present 2.14-5.60 mm, flat; lamina 9.46-10.58 (-17.00) × 2.03-3.48(-6.14) mm, narrowly oblanceolate, oblanceolate to obdeltoid, apex often tridentate, base cuneate to narrowly cuneate; lamina margins deeply dentate, incised, or otherwise entire except for the upper ⅓ which is prominently toothed; teeth if present in 2-6 pairs running to and including the apex. Racemes (10-)26(-60) mm long, elongat ing up to 90 mm at fruiting, terminal and axillary; rachis and pedicels finely and sparsely covered in retrorse to patent, very short, 0.05-0.8 mm long, ± clavate, eglandular-glandular, hairs; pedicels, erecto-patent to patent,1.04-1.27(-2.38) mm, 2.34-5.00(-6.02) mm long at fruiting. Flower buds dark green, apex bearing a conspicuous, caducous, crest of white, eglandular, antrorse hairs up to 0.9 mm long. Flowers sweetly fragrant, 1.4-1.8(-2.0) mm diam. Sepals, broadly ovate to oval, c.0.6-1.0 × 0.6-1.2 mm, apex broadly obtuse, centrally green with a white margin, deeply concave, adaxially weakly keeled, adaxial midrib invested in conspicuous, caducous, white, eglandular, antrorse, hispid hairs, hairs sometimes scattered across rest of adaxial surface; abaxial surface glabrous. Petals white, 0.3-0.8(-1.0) × 0.2-0.8 mm, erecto-patent or patent, clawed; limb broadly obovate, apex obtuse, retuse or distinctly emarginate. Stamens 2, equal. Anthers c.0.16 mm long. Pollen bright yellow. Nectaries 4, subulate, 0.40 mm long. Silicles cartilaginous when fresh, coriaceous when dry, orbicular to obovate, (2.8)-3.3(-4.1) × (2.2-)3.3-3.4 (-4.0), narrowly winged, apex shallowly, minutely, notched, base cordate, valves dark green to green maturing straw-yellow, glabrous; style 0.8 (-1.0) mm long, free from the narrow wing, equal to or slightly exceeding the notch; stigma 0.2-0.4 mm diam. Seeds 2, 1.20-1.38 × 0.80-1.10 mm, ovoid to suborbicular, red-brown, dark red-brown or brownish black, not winged. FL. Nov-Feb. FR. Jan-Apr.

Representative Specimens.

Chatham Islands: n.l., n.d., [H. H.] Travers 105, (MEL 301452); Rekohu, Kaingaroa Point, 3 March 1985, D. R. Given 14017, (AK 225198, CHR 417647); Rekohu, Kaingaroa Point, 21 February 1996, P. J. de Lange CH 80 & G. M. Crowcroft, AK 230459; Rekohu, Kaingaroa Point, 15 July 2002, P. J. de Lange CH332 & A. Baird, (AK 259130); Rekohu, Kaiangaroa, Kaiangaroa Point, 13 December 2005, A. Baird s.n., (AK 295132); Rekohu, Wharekauri Farm Station, Cape Young, 13 January 2006, P. J. de Lange CH424 & J. W. D. Sawyer, (AK 295153); Rekohu, Waitangi Village, near Council Buildings (naturalised), 19 September 2007, P. J. de Lange CH975 & P. B. Heenan, (AK 300991); Rabbit Island, 14 February 2006, P. J. de Lange CH676 & P. B. Heenan, (AK 296754). Forty Fours (Motuhara) 27 January 2005, R. M. Bellingham s.n., (AK 290290). Cultivated (New Zealand): Lincoln, ex Kaiangaroa, Landcare Research experimental nursery, December 2008, P. B. Heenan s.n., (CHR 609795).

Distribution

( Fig. 63 ). Endemic. Chatham Islands where it is known only from Rekohu and Rabbit Island, and the Forty Fours (Motuhara). Lepidium rekohuense has also been collected once as a casual in a car park in the main settlement of Waitangi. This occurrence of a single plant along with that of Lepidium panniforme collected in the same site probably accords with the use of nearby accommodation by PdL and PBH in 2006, during which time fruiting herbarium specimens of both species were processed in that general area.

Recognition.

Healthy specimens of Lepidium rekohuense can form patches up to 2 m in diameter, which is the largest of the New Zealand endemic Lepidium species ( Fig. 89 ). Within the Lepidium oleraceum group, Lepidium rekohuense is morphologically most similar to Lepidium oblitum and Lepidium oligodontum . From these species by is easily separated by the flowers which consistently have two rather than 2-4 ( Lepidium oblitum ) or 2-4-6 ( Lepidium oligodontum ) stamens, by its much larger overall stature (up to 2 m diam.), by the sparsely papillate- hairy upper branch stems, and by the presence of retrorse to patent, very short, ± clavate, eglandular-glandular hairs on the inflorescence rachis and pedicels. The silicles of Lepidium rekohuense are orbicular (rarely obovate) and consistently, though minutely, notched ( Fig. 92 ), while those of Lepidium oligodontum , orbicular to suborbicular and not or scarcely notched.

Ecology.

Lepidium rekohuense is currently known from salt-marsh and meadow at Kaiangaroa, from steep, eroded basaltic tuff erosion gullies and cliff faces at Cape Young and on Rabbit Island, and from the crevices and ledges of greywacke rock outcrops of the Forty Fours (Motuhara). At Kaiangaroa, Lepidium rekohuense is a seasonally conspicuous member of the salt marsh and meadow vegetation that has developed behind the cobble beach and shallow shelving schist shore platform in and around Kaiangaroa Point. Here, plants grow in a variety of situations ranging from fully exposed and eroded habitats to low windswept thickets dominated by Hebe chathamica , and Hebe chathamica × Hebe dieffenbachii hybrids. In the salt marsh plants are usually found growing within dense Sarcocornia quinqueflora (Bunge ex Unq.-Sternb.) A.J.Scott. subsp. quinqueflora, Samolus repens var. repens, and Selliera radicans Cav. turf. In this habitat, plants are often lost during storm surges or during the winter months, and it would seem, from the presence of seedlings and young plants along drainage channels and in and around eroded sections of salt marsh, that these storm events are necessary to exhume and disperse buried seed. At the back of the salt marsh, where the salt meadows are dominated by taller plants such as Apodasmia aff. similis, Ficinia nodosa (Rottb.) Goetgh. Muasya et D.A.Simpson, and occasional Myosotidium hortensium , Lepidium rekohuense plants are also present, and here they often grow intermixed with Apium prostratum subsp. denticulatum, Selliera , Samolus , and Leptinella potentillina . Higher up, where thickets of Hebe chathamica and hybrids form the dominant cover, Lepidium rekohuense is less common, in part because they are often easily missed as they grow with Apium prostratum subsp. denticulatum threaded through Hebe chathamica . Lepidium rekohuense is also occasionally found growing on and around the small schist rock stacks around Kaiangaroa Point.

At Kaiangaroa the highly exposed and dynamic habitat means that many Lepidium rekohuense plants, especially the younger plants are often lost through coastal erosion and from storm surges. However, in favourable sites, plants are remarkably resilient and long-lived once established. For example, mature plants first observed in 1996 are still present at the time of writing (2012) 16 years later, making this species easily the longest lived member of the Lepidium oleraceum complex in New Zealand. The key to this species success at Kaiangaroa seems to be its remarkable tap root, which, once established, firmly anchors the plant into the substrate such that coastal erosion often leaves mature plants exposed, festooned in driftwood and kelp, while the surrounding salt-marsh turf has been destroyed.

The habitat occupied at Cape Young and on Rabbit Island is markedly different. Here the species grows at the apex of steeply descending, erosion gullies that have developed within the easily eroded basaltic tuff. In these sites it is often the only plant present though, in a few places on Cape Young, it grows with Lepidium flexicaule , with which it occasionally hybridises. On Rabbit Island, large plants grew at the head of an erosion gully under a sparse canopy of the introduced tree mallow ( Malva arborea ).

Little is known about its habitats on the remote Forty Fours (Motuhara). From the limited information available (P. N. Johnson pers. comm.) it seems that the species is very uncommon there, and that it grows mainly within crevices and on ledges on the cliff faces of those rock stacks.

Conservation status.

The most recent census data that we have (July 2007) recorded 114 adult plants of Lepidium rekohuense from just three accessible sites; two on Rekohu (Kaiangaroa and Cape Young) and one on Rabbit Island. The status is uncertain of the species on the Forty Fours, privately owned land from which the Department of Conservation has not been granted visiting rights. Nevertheless, observations made in 2005 by a private landing party of geologists, entomologists and ornithologists suggest that there are probably fewer than 10 plants on the larger of the two main islets making up the Forty Fours. Using the New Zealand Threat Classification System ( Townsend et al. 2008), Lepidium rekohuense is rated "Threatened/Nationally Critical" using criterion A(1) because there are <250 adult plants known from the wild. To this threat rating we recommend appending the qualifiers ‘CD’ (Conservation Dependent - due to need for ongoing management of the Kaiangaroa population), ‘IE’ (Island Endemic - because Lepidium rekohuense is naturally confined to the Chatham archipelago). It is worth noting that, without management, the largest population of Lepidium rekohuense known to the Department of Conservation, that at Kaiangaroa, would probably now be extinct. There, intensive management has built the population up from an apparent low of six plants in 1996 to more than 100 in January 2006. At Cape Young the sole accessible plant seen in January 2006 had disappeared by January 2007 but others observed further down the cliffs in sites inaccessible to human traffic are apparently still present (A. Baird pers. comm.). On Rabbit, eight plants were observed in February 2006 and these are assumed to be still present. Observations at Kaiangaroa suggest that, aside from the losses caused by the naturally dynamic conditions of that exposed coastal shore platform and wetland, predation from the caterpillars of the moth Epyaxa rosearia Doubleday, 1843 (identified by J. Dugdale pers comm.) is the main threat facing that population. Currently the caterpillars’ of this moth are managed by regular applications of derris dust, without which most (sometimes all) adult Lepidium rekohuense plants can be severely damaged, affecting especially flowering and seed set, and killing seedlings and young plants. At Cape Young, Lepidium rekohuense may be threatened by hybridisation with Lepidium flexicaule though this requires further study. So far only one putative and not completely convincing example of this hybrid has been observed (see Lepidium flexicaule above).

Figures

Figure 63. Distribution of Lepidium oblitum, Lepidium oligodontum, Lepidium panniforme and Lepidium rekohuense.

Figure 86. Holotype of Lepidium rekohuense de Lange et Heenan (sheet A).

Figure 88. Holotype of Lepidium rekohuense de Lange et Heenan (sheet C).

Figure 89. Wild plant of Lepidium rekohuense showing decumbent branches and inflorescences, plant growing amongst Chatham Schist boulders at Kaiangaroa, Rekohu (boulders are 1 - 2 m diameter)

Figure 92. Mature silicle of Lepidium rekohuense from holotype (AK 297694). Scale bar = 1 mm.