Lepidium oligodontum, 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 : 84-92

Lepidium oligodontum sp. nov.

A L. oleraceo habitu renascenti, habenti periodo distincto rosulato (folia rosularum anguste spathulata, cuneata, obdeltoidea vel obovata - raro ellipto-lanceolata), habitu plantae maturae decumbenti ramulis extensis foliis integris vel leviter dentatis (dentes solum distales), floribus 2-4(-6)-staminatis, siliculis manifeste turgidis plerumque late orbiculatis leniter alatis emarginatis vel integris ad apicem et serie DNA differt.

Holotype.

Chatham Islands ( Fig. 77 ): Chatham (Rekohu) Island, Western Reef, 15 January 2006, P. J. de Lange CH378 & J. W. D. Sawyer, AK294925! Isotypes CANB!, CHR!

Eytmology.

The name ' oligodontum ' from the Latin meaning 'few teeth’ refers to the leaves of this species which may be deeply toothed, sparingly toothed near the apex or entire, sometimes with all three conditions on the same plant. Plants with deeply toothed leaves are uncommon, whilst those with sparingly toothed leaves are more usual.

Description

( Figs 78 -80 ). Tap-rooted, decumbent, rather flaccid, pungent-smelling, summer-green, succulent, perennial herb forming sparse to densely leafy, circular ± flat masses up to 1 m diam., and arising from stout, semi-circular, greyish-white or reddish-grey (when exposed) rootstock 6.0-30.0 mm diam. Tap root woody, up to 0.2 m long, deeply descending. Plants dying down to rootstock and/or previous seasons stem nodes, at fruit set or soon thereafter. Stems decumbent, widely spreading, up to 0.8 m long and 3.20-4.56(-5.20) mm diam., ± woody throughout, ± square to somewhat angular-spheroidal in cross-section and prominently ridged on angles (this especially conspicuous when dry), dark reddish-green to dark green when fresh, drying dull grey; stems usually heavily branched from base, branches and branchlets numerous, prostrate, widely spreading, and usually very leafy; basal portion of stems, glabrous. Leaves glabrous, succulent, dark green, green to yellow-green at senescence turning yellow. Rosette leaves 5-10(-14), mostly present in autumn - early spring but not persisting (very rarely so) at fruiting; petioles distinct up to 70 × 2 mm, flat or slightly concave in cross-section, succulent; lamina narrowly spathulate, cuneiform, obdeltoid, obovate or rarely elliptic-lanceolate, up to 60 × 22 mm, margins entire, or sparingly dentate in upper ⅓, if teeth present then in 1-3(-5) pairs running to and including the usually tridentate apex, basal teeth often asymmetric, base narrowly cuneate, cuneate to attenuate. Middle stem leaves persistent or not at fruiting; petiole usually distinct (rarely not) up to 14.0 × 1.13 mm, mostly flat in cross-section, sometimes slightly concave, succulent; lamina spathulate, cuneiform, linear-cuneiform, oblanceolate, narrowly ovate, to narrowly obovate, or rarely orbicular, 10.6-22.8(-33.2) × 5.4-9.2 (-17.5) mm; margins entire, or sparingly dentate with 1-2-3(-5) pairs of teeth in upper ⅓ apex usually tridentate, basal teeth often asymmetric, lamina base narrowly attenuate, attenuate, cuneate or rarely acute. Upper stem leaves usually without a distinct petiole, petiole if present 1.0-3.6 mm, flat or slightly concave; lamina narrowly cuneiform, oblanceolate, or narrowly obdeltoid, 7.6-10.8(-11.9) × 2.7-3.1(-3.5) mm; margins entire or weakly dentate to deeply incised, if dentate or incised then with 1(-2) often asymmetrical teeth present in the upper ⅓, apex entire or tridentate, lamina base cuneate to narrowly cuneate. Racemes (5.0)-9.7(-28.9) mm long, usually congested, elongating up to 60 mm at fruiting, terminal and axillary; rachis and pedicels glabrous; pedicels, erecto-patent to patent,0.82-1.00(-2.08) mm, 2.1-3.5(-6.1) mm long at fruiting. Flower buds dark green to green, apex glabrous. Flowers sweetly fragrant, 1.2-1.5(-2.2) mm diam. Sepals 4, saccate, pale to dark green with a broad white, ± undulose margin, pale to dark green with a broad white, ± undulose margin, deeply concave, adaxially weakly keeled or not; lateral sepals 0.9-1.4 × 0.8-1.2 mm, broadly ovate to oblong, ± overlapping at base, apex rounded to obtuse, adaxial surface mostly glabrous sometimes diffusely papillate, abaxial surface often hairy, hairs patent, weakly flexuous, 0.1-0.4 mm long, eglandular, mostly shedding at anthesis except near base; median sepals 1.0-1.4 × 0.9-1.2 mm, broadly ovate to oblong, apex rounded to obtuse, adaxial surface glabrous, abaxial surface usually glabrous, rarely sporting a small tuft of patent, eglandular, flexuous hairs 0.1-0.2 mm long. Petals usually present (occasionally absent) usually equal to or slightly overtopping sepals (rarely> sepals), white, 0.8-1.6(-1.9) × 0.6-1.2(-1.8) mm, patent, clawed; limb broadly obovate, apex obtuse, rarely retuse. Stamens 2-4(-6), equal. Anthers c.0.14 mm long. Pollen bright yellow. Nectaries 2, subulate, 0.32 mm long. Silicles somewhat fleshy and distinctly turgid when fresh, on drying collapsing to form a coarse reticulum, broadly orbicular to orbicular, (3.0)-3.5(-3.8) × (2.8-)3.0-3.2(-3.8), margin slightly winged, sometimes more so toward apex, apex not or very slightly notched, base cordate (rarely truncate), valves green maturing yellow-green, glabrous; style 0.2-0.8(1.2) mm long, free from the narrow wing, equal to or slightly exceeding the notch; stigma 0.4-0.6 mm diam. Seeds 2, ovoid to suborbicular, light red-brown or red-brown, not winged, 0.84-0.98 × 0.80-1.00 mm. FL. Dec-Mar. FR. Dec-Apr.

Representative Specimens.

Chatham Islands: Rangitatahi (Sisters) 29 January 2005, R. M. Bellingham s.n., (AK 290289); Rekohu, Point Somes, 10 January 2006, P. J. de Lange CH393, (AK 294941); Rekohu, Rock Stacks, Point Somes, 10 January 2006, P. J. de Lange CH390, (AK 294938); Rekohu, 'the Clears’ near Owenga, April 1979, M. A. Ringer s.n., (AK 150010); Rekohu, between Point Gap and Tuku, November 1980, A. M. Ringer s.n., (AK 170636); Rekohu, Coastal cliffs south of Tuku River, November 1987, G. A. S. Taylor s.n., (AK 235255); Rekohu, Otauwae Point, 12 January 2006, P. J. de Lange CH417, (AK 295634, CANB, CHR); Rekohu, South Moriori Creek, 12 February 2006, P. J. de Lange CH801, (AK 299967); Rekohu, Point Gap, 12 February 2006, P. J. de Lange CH802, (AK 299966); Rekohu, Te Wakaru Island, 13 January 2006, P. J. de Lange CH431 & J. W. D. Sawyer, (AK 295160, CHR, MEL); Rekohu, unnamed point south-west of Ocean Bay, 14 January 2006, P. J. de Lange CH441 & J. W. D. Sawyer, (AK 295120); Western Reef 15 January 2006, P. J. de Lange CH380 & J. W. D. Sawyer, (AK 294927); 15 January 2006, P. J. de Lange CH379 & J. W. D. Sawyer, (AK 294926); Star Keys (Motuhope), 15 December 2005, A. Baird s.n., (AK 295162); Rabbit Island, 14 February 2006, P. J. de Lange CH675 & P. B. Heenan, (AK 296753, CHR); Rangatira (South-East Island), 1 January 1970, B. G. Hamlin 1703, (WELT SP042706, WELT SP049930A, WELT SP049930B); Tarakoikoia (The Pyramid) 27 February 1993, G. A. S. Taylor s.n., (AK 228962). Antipodes Islands: Antipode Island, January 1909, B. C. Aston s.n., (WELT SP027627); Antipode Island, Anchorage Bay, 7 November 1995, G. A. S. Taylor s.n., (AK 233772).

Distribution

( Fig. 63 ). Endemic. New Zealand, Chatham and Antipodes islands. On the Chatham Islands, it is known from Rekohu and South West (Rangatira) Island, and from all of the outlying islets and vegetated rock stacks except Mangere, Little Mangere and the Forty-fours. It is apparently absent from Rangiauria (Pitt Island). On the Antipodes Islands, Lepidium oligodontum is known only from Antipodes Island, where it is has a very restricted occurrence at the northern end of that island ( Godley 1989).

Recognition.

Lepidium oligodontum is recognised by the combination of having a decumbent rather than erect growth habit ( Fig. 78 ), glabrous stems, branchlets and pedicels, leaves which are either sparingly toothed or entire ( Fig. 79 ), flowers with 2-4(-6) stamens, and silicles mostly broadly orbicular to orbicular, slightly winged, mostly cordate-based, and that are distinctly swollen (turgid) when fresh and whose apices are slightly notched ( Fig. 80 ). Of the New Zealand species, Lepidium oligodontum is unusual for the variability of stamen number. In some populations plants are found that have either two or four stamens, whilst others may show a range from two to four (rarely six). Some populations consistently have four stamens whilst others show a grade from two to six.

Ecology.

Lepidium oligodontum is a strictly coastal, island endemic inhabiting richly manured, frequently disturbed habitats. On Rekohu it is extremely uncommon and now virtually confined to coastal cliffs, near-shore rock stacks, and, very occasionally, coastal turf communities, especially in places frequented by New Zealand fur seals ( Arctocephalus forsteri Lesson 1828). In these habitats it is usually associated with Disphyma papillatum , Apium prostratum subsp. denticulatum P.S.Short, Crassula moschata G.Forst., Dichondra spp., Hebe chathamica , Leptinella potentillina F.Muell., and Puccinellia chathamica . In a few sites, such as at Point Somes and Ocean Bay, Lepidium oligodontum grows intermingled with Lepidium flexicaule and hybrids between both species have occasionally been collected where they co-exist. Habitats occupied by Lepidium oligodontum vary on the outer islands, islets and rock stacks of the Chatham archipelago. It is an occasional associate of coastal turf and cliff vegetation on islands such as South East (Rangatira). On the other outer islands (e.g., The Sisters (Rangitatahi), Star Keys), islets (e.g., The Pyramid (Tarakoikoia)), reefs and rock stacks Lepidium oligodontum is a prominent species of the distinctive guano-dependent vegetation that has developed in the presence of New Zealand fur seals, and sea birds such as the albatrosses ( Diomedea sanfordi Murphy, 1917, Thalassarche eremita (Murphy, 1930), Thalassarche bulleri ), northern giant petrel ( Macronectes halli (Mathews, 1912)), Chatham Island fumar prion ( Pachyptila crassirostris pyramidalis C.A.Fleming, 1939), and white-fronted tern ( Sterna striata Gmelin, 1789) ( Aikman and Miskelly 2004). In these guano-enriched habitats, Lepidium oligodontum is usually found growing in close association with the woody shrub, Leptinella featherstonii F.Muell., and herb, Senecio radiolatus subsp. radiolatus (see de Lange and Sawyer 2008). On Rabbit Island, Lepidium oligodontum was found growing at the entrance to petrel ( Pterodroma spp.) and shearwater ( Puffinus spp.) burrows, sometimes in association with Lepidium oblitum and Lepidium rekohuense , and other coastal turf plants such as Disphyma papillatum . Detailed information on the habits of Lepidium oligodontum on the Antipodes is lacking, although herbarium specimens and people’s observations (G. A. Taylor and S. P. Courtney pers. comm.) note that it is seemingly restricted to a few coastal headlands where it grows at the top of steep cliffs in sites kept free of taller vegetation by wind and salt burn. Oddly, despite the abundance of sea birds and seals on the Antipodes, Lepidium oligodontum was not observed growing anywhere near sites frequented by these animals, leading us to infer that the habitat is similar to that occupied along the southern coastal cliffs of Rekohu, e.g., Moriori Creek and Otauwae Point.

Lepidium oligodontum , more than any other member of the Lepidium oleraceum complex, has a very seasonal growth pattern, with most vegetative growth occurring from late winter to summer. Lepidium oligodontum plants are scarcely visible during autumn and early winter because growth virtually ceases and most of the foliage, branches, branchlets and inflorescences wither away. During this time of ‘dormancy’, plants persist as minute leafy shoots clustered around the rootstock apex, or as a single leafy rosette.

It has also been observed that the annual growth cycle of Lepidium oligodontum seems to be dictated, perhaps more than any other New Zealand Lepidium , by the onset and spread of the reproductive phase of the oomycete Albugo candida (Pers.) Kuntze. The spore-bearing pustules of Albudo candida appear on wild and cultivated plants within weeks of the initiation of spring growth, and by late January/February have usually erupted across most of the stem leaves, flowers and fruits. Further vegetative growth from the host plant is either completely retarded or aborted. Observations of wild population of Lepidium oligodontum suggest that these seasonal eruptions of Albugo candida have a tremendous impact on this species’ vigour, and we have found that the intensity of these Albudo candida infections make it impossible to maintain Lepidium oligodontum in cultivation. It is not clear if the severity of these attacks in Lepidium oligodontum populations is natural or a consequence of other, as yet undetermined, external factors, or even if the strain of Albudo candida infecting Lepidium oligodontum is endemic to it (E. H. McKenzie pers. comm.). Whether indigenous, endemic or naturalized, this oomycete is now, at least, a critical constraint on the growing season, flowering and fruiting of this species, especially in what appear to be ecologically suboptimal sites. Field observations suggest that Lepidium oligodontum is more abundant and the plants clearly thriving in sites where nutrient levels remain high, such as on the guano-enriched Pyramid (Tarakoikoia), or within the seal colonies of the Star Keys and Western Reef ( de Lange and Sawyer 2008). Despite the presence of Albudo candida , their growth season is longer and the plants more persistent than at any of the other Chatham Islands’ Lepidium oligodontum sites that we have been able to visit.

Conservation Status.

Lepidium oligodontum as Lepidium aff. oleraceum (b) (AK 208579; Antipodes - Chatham Islands) was listed in Appendix 2 of de Lange et al. (2009, p. 89) as "Threatened/Nationally Critical DP, EF, RR". With its formal description and recognition, a reassessment of that conservation listing is now appropriate.

Aside from a few, small, isolated islands, islets and rock stacks in the Chatham archipelago and possibly Antipodes Island, trend data gathered by the New Zealand Department of Conservation over the last 15 years confirm that Lepidium oligodontum ,though naturally prone to extreme population fluctuations, is now in final stages of a major terminal decline on Rekohu (A. Baird unpubl. data).

Based on our knowledge of this species’ ecology, its loss from Rekohu appears to be the result of an initial loss of habitat as key sea bird and seal populations on the larger islands of the group went extinct over the last 600 or so years ( King 1989). Habitat loss has accelerated since the early 19th century following the settlement of Rekohu by Maori and Europeans whose impacts on the remaining seal and sea bird breeding grounds have been severe ( Wills-Johnson 2008; King 2008). Land clearance and ongoing modification of the main Chatham Islands has also facilitated the spread of competing plants that were previously unknown there and which had been either deliberately or inadvertently introduced ( de Lange et al. 2008; de Lange et al. 2011).

Another factor in the decline of this species is the oomycete Albugo candida ,discussed above. It remains unclear whether Albugo candida truly poses a serious threat to indigenous New Zealand Brassicaceae , and in particular Lepidium oleraceum s.l. ( Armstrong 2007). Nevertheless, our field observations and the remarks made by collectors on herbarium specimens note that Albugo candida strongly retards the growth of Lepidium oligodontum and that it continues to be a major factor in the decline and loss of many of the smaller populations on Rekohu. Lepidium oligodontum seems to be buffered from the severity of Albugo candida outbreaks only on less modified outer islands, islets and rock stacks of the Chatham Island archipelago and, presumably, the Antipodes Islands, where there are intact, functional guano- and marine mammal-based ecosystems ( de Lange and Sawyer 2008). Furthermore, Albugo candida has prevented Lepidium oligodontum from being successfully cultivated beyond a single season or translocated to new sites (A. Baird, S. Benham, G. Davidson, J. Santos, T. Silbery, and R. Smith pers. comm.).

Aside from Albugo candida , some of the Rekohu populations of Lepidium oligodontum are also subjected to a diverse range of threats. These include browse by feral sheep ( Ovis aries Linnaeus, 1758), cattle ( Bos primigenius taurus (Linnaeus, 1758)) and possum ( Trichosurus vulpecula (Kerr, 1792)). Possums, in particular, have been found avidly browsing Lepidium oligodontum plants growing on near-shore rock stacks at Point Somes and Te Koparuparu Bay on Rekohu.

Following extensive surveys of the Chatham Islands by the Department of Conservation during the summer of 2005/2006, 14 functional subpopulations were recognized, down from 28 known in 1996 (a decline rate of 50 % over 10 years). These gave a total population for that island group of 1321 adults, of which 748 (57%) were confined to one site, Western Reef. These figures exclude population data for The Sisters and The Pyramid (Tarakoikoia), which are privately owned and inaccessible to the Department of Conservation. To date, the few reports of Lepidium oligodontum from these islands suggest that it is “common” (R. Emberson and P. Schofield pers. comm.). Monitoring at Te Wakaru Island, which has an intact guano and marine mammal-based ecosystem, shows that the population there fluctuates greatly from year to year. Therefore, trend data needs to be gathered over several years before any evidence of decline can be distinguished from natural population “boom/bust” cycles. Little is known about the species’ status on the Antipodes. In part because these remote islands are infrequently visited, and then usually by ornithologists at times when our data for the Chatham Islands suggests Lepidium oligodontum was already undergoing its seasonal decline. For example, an ornithological party considered Lepidium oligodontum to be very uncommon there in 1995, seeing perhaps only a few tens of plants (G. A. S. Taylor pers. comm.). These were restricted to one site at Reef Point, where, because of their close proximity to a former castaway hut and the unusual habitat they occupied, it was though that the few plants seen may have been introduced there (G. A. S. Taylor pers. comm.). This contrasts somewhat to the observations published by Godley (1989) (a botanist) who visited the islands in 1969. Godley recorded Lepidium oligodontum (as Lepidium oleraceum ) as being 'common there [at Reef Point] on coastal rocks and occasionally on cliffs’, though he then went on to state that 'it was not seen by any of the 1969 party at other possible localities’. Nevertheless, based on what we now know of the Lepidium oleraceum group, the possibility that Lepidium oligodontum is introduced to the Antipodes seems unlikely, and based on the historical observations summarised by Godley (1989, p. 546) it would seem that, if anything, observers were witnessing different stages of the ‘boom/bust’ cycles observed elsewhere on the Chatham Islands. Nevertheless, as the Antipodes Islands are so infrequently visited, it is important that future visits focus on gathering better data and setting up long-term monitoring of that island group’s Lepidium oligodontum population.

In summary, we consider that Lepidium oligodontum is more appropriately assessed as 'Threatened/Nationally Vulnerable’ (criteria B(1/1) of Townsend et al. (2008). This is based on a population size that is likely to be between 1000 and 5000 mature individuals spread across the Chatham and Antipodes island groups (noting that our best available data (field season 2005/2006) recorded 1321 plants from those accessible locations on the Chatham Islands; that outside those areas on other islands in that group, such as The Sisters (Rangitatahi), Star Keys and The Pyramid (Tarakoikoia) there are likely to be at least as many individuals; and that there no reliable population data for the Antipodes Islands). Further, it would seem that, aside from the ongoing decline of what are effectively non-functional, residual populations on Rekohu, the species is secure on the outlying islands, islets and rock stacks of the Chatham Islands group where the ornithocoprophilous ecosystem remains intact and functional. Accepting the absence of hard data for the Antipodes Islands, observations by field parties suggests that Lepidium oligodontum , while less abundant there, is probably also secure due to the intact nature of the ornithocoprophilous ecosystem there.

To this recommended threat listing we suggest appending the following qualifiers ‘DP’ (Data Poor - to reflect uncertainty over the status of the species on the Antipodes Islands), ‘EF’ (Extreme Fluctuations - to reflect the wide population fluctuations experienced throughout a normal growing season), and ‘RR’ (Range Restricted - because Lepidium oligodontum is naturally confined to a geographically small part of the New Zealand Botanical Region).

Figures

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

Figure 77. Holotype of Lepidium oligodontum de Lange et Heenan.

Figure 78. Wild plant of Lepidium oligodontum on Te Wakaru Island, showing decumbent growth habit.

Figure 79. (A) (from left to right) variation in rosette and basal-stem leaves from a single plant of Lepidium oligodontum; (B) (from left to right) leaf variation within populations from The Sisters (Rangitatahi), Western Reef, Point Somes, (Rekohu), Moriori Creek (Rekohu), Star Keys (Motuhope), Rangatira and Antipodes Island. Scale bars = 20 mm.

Figure 80. Mature silicle of Lepidium oligodontum. AK 295634. Scale bar = 1 mm.