Lepidium oblitum flowers

New Endemic Plants for the Chatham Islands

As progress toward the preparation of a Chatham Islands Flora continues, the number of endemic vascular plants accepted for the Chatham Islands has now increased from 38 to 42 with the formal recognition of three new scurvy grasses and one new hook sedge (oft known as bastard grass) from the islands (de Lange et al. 2013a; de Lange et al. 2013b).

Two of the three endemic scurvy grasses (Lepidium panniforme, and L. rekohuense) were first determined as distinct in the mid-1990s, while the third (L. oblitum) was discovered in 2006 during field work on Mangere Island. A fourth newly described scurvy grass, L. oligodontum is known only from the Chatham and Antipodes Islands.

Lepidium panniforme

Lepidium panniforme – a mature flowering and fruiting plant photographed in February 2006 on the track side, Mangere Island. Image: Peter J. de Lange

Lepidium panniforme (the species name means ‘like a shredded rag’ and alludes to the deeply serrated, lacerated foliage – which looks like a frayed rag) is an oddity. It is known only from Mangere and Little Mangere Island and, as far as we can tell, has never been collected from anywhere else in the Chatham Islands. Virtually nothing is known about its status on Little Mangere where its presence is known only from a few photographs taken by the late Don Merton. On Mangere Island it was previously confused with Cook’s scurvy grass (Lepidium oleraceum) which also grows there. Using DNA data and morphology we now know that L. panniforme is most closely related to L. oblitum and the extinct New Zealand, Wellington endemic L. obtusatum.

On Mangere Island L. panniforme is mostly found growing along track sides and in a few cliff habitats. Ironically, the re-vegetation of the island potentially threatens it, through closure of the open habitat it needs. Its management will require the maintenance of open ground, at least until the regenerating forest canopy opens out as burrowing sea birds return to the island.

Lepidium oblitum

Lepidium oblitum fruiting plant photographed on the northern cliffs of the main summit plateau of Mangere Island. Image: Graeme Taylor

Lepidium oblitum is so far known only from Mangere and nearby Rabbit Island. Unlike L. panniforme, this species was not initially recognised as distinct. Specimens collected from these islands in 2006 were initially thought to be hybrids between Cook’s scurvy grass and L. panniforme. Those plants raised from seed collected from Mangere Island, and subsequent DNA data obtained from them showed that they were not hybrids but another new species. The species name ‘oblitum’ alludes to the ‘accidental’ discovery of this ‘overlooked’ species. Of the two endemics on Mangere, L. oblitum is the most common. However, it too is potentially at risk from the reforestation of that island. On Rabbit Island it is very uncommon, being found only along the eastern facing side of the summit ridge.

Lepidium rekohuense

Lepidium rekohuense growing on shell and cobble beach, Kaingaroa Point. Image: Peter J. de Lange

The last endemic, L. rekohuense (the species name is taken from the Moriori name for Chatham Island ‘Rekohu’), is the species island visitors see growing on the margins of the salt marsh and cobble beach near Kaingaroa Point. It is also known from the cliffs below Cape Young, from Rabbit Island, and the remote Forty-fours (Motuhara).

This species also once grow on the shores of Te Whanga where it was collected by the botanist Henry Travers in the 1860s.

Lepidium rekohuense is the largest and longest-lived species of scurvy grass in the New Zealand archipelago. Thriving plants may get up to 2 m diameter and one plant is believed to be older than 17 years – in contrast, Cook’s scurvy grass rarely lives for longer than three years (up to five in cultivation).

The only population the Department of Conservation has any ability to manage is the one at Kaingaroa – the others are too difficult to safely access. Further, all populations occur on private land, and their management requires the good will and cooperation of landowners and the Department. Through careful management the Kaingaroa population has steadily grown from the five plants found there in 1996 but it’s still a chancy affair. Major storms can bury or dislodge plants, though it’s a delicate balance, as storms are also necessary to maintain the species’ habitat at Kaingaroa. A further complication is a native moth (Epyaxa rosearia) that routinely denudes foliage, destroys flower heads and seed capsules, and in severe infestations can even kill adult plants. The caterpillars of this moth posed the Department of Conservation major management problems because, until the species was identified, the Department was reluctant to spray all the Lepidium plants for fear that the moth might be a new species endemic to it. Eventually after many failed attempts to raise the caterpillars to adulthood, New Zealand entomologist John Dugdale was consulted and he was able to identify the caterpillars. Even though the moth is widespread and indigenous to all of New Zealand, it is still not fully controlled at Kaingaroa, because its association with the Lepidium is unusual and so worth maintaining. Currently about half of the Kaingaroa plants are derris dusted several times during the growing season. This is a simple management technique that seems sufficient to keep the moth at bay and maintain a healthy population.

Nevertheless, Lepidium rekohuense will survive at Kaingaroa only through the continued goodwill of the private owners, by ensuring that tour groups visiting that area don’t damage the plants, and through on-going hand weeding, derris dusting and monitoring by the Department of Conservation.

Lepidium oligodontum

Lepidium oligodontum – Te Wakaru Island. Jan 2006. Image: Peter de Lange

Lepidium oligodontum, though not endemic, deserves some mention too. On the Chatham Islands this species seems tied to areas where an intact, functional seal colony and/or sea bird nesting ground is maintained. Thus it is still common on the more remote rock stacks, islets and islands of the Chathams group. For example 700 plants of it were noted on Western Reef in 2006 (de Lange & Sawyer 2008). Outside these areas it is fast going extinct, and this despite numerous attempts to manage populations or establish new ones. It seems that without the regular heavy dollops of seal scat, urine and bird guano, this nitrogen and phosphate–loving plant just can’t thrive. When you add to this the fact that all populations thus far seen are infected with a water mould – Albugo candida (often called ‘white rust’) – and that in sites where plants are not routinely sprinkled with animal faeces, this water mould seems capable of killing plants. It’s a real concern. So far all attempts to control the water mould have failed, and this is why it’s so important that the few functional seal / seabird breeding / nesting grounds are left intact – not only for these animals but for the plants like Lepidium oligodontum that have evolved with them. Outside the Chathams, Lepidium oligodontum is also known from the Antipodes islands, where it seems to be very uncommon.

Uncinia auceps

Uncinia auceps – fruiting mature plant growing in Tuku Nature Reserve. Image: Peter B. Heenan

The last endemic, Uncinia auceps is a species of bastard or hook grass that was discovered in May 2008. This plant is not really a grass but a sedge – i.e. one of those plants with triangular shaped, emergent leaves, and razor-sharp leaf margins. Previously three species of bastard grass had been accepted for the Chatham Islands, U. rupestris, U. uncinata and U. zotovii. Uncinia auceps had previously been confused with U. uncinata which it closely resembles. What mostly distinguishes it from U. uncinata is the unique way the fruiting inflorescence elongates at maturity and then trails across the ground – sometimes up to 2 m from the parent plant! Thus fruiting plants are not evident unless you look very carefully or pull one up – only then can the long trailing seed heads be seen. Like all Uncinia, the seeds of this species are enclosed in a papery capsule furnished with a nasty hook. It is the hook that catches in your clothing, socks or the hair on your legs – earning sedges of this genus the name ‘bastard grass’ or the politer ‘hook grass’.

The paper describing Uncinia auceps suggests that only this species occurs on the islands (de Lange et al. 2013b). However, it may not be that simple – there is now some evidence that U. uncinata is also on the islands after all. Further field work is needed.

Uncinia auceps with trailing inflorescences

Uncinia auceps – fruiting plant, Nikau Bush, showing the long trailing inflorescences which are often only evident when plants are carefully inspected or lifted up from the ground. Image: Peter B. Heenan

Uncinia auceps takes it species name from the Latin for a ‘bird catcher’. Ornithologists had noted how the hooked fruits of this species are a major problem for some ground nesting birds and burrowing petrels on Rangatira (South-East Island). Extrapolating from that observation, and with the knowledge that the Chatham’s bird fauna was once rich in small flightless rails, suggested to the authors that the fruit heads of Uncinia auceps, which greatly elongate and flop or trail across the ground was an evolutionary trait that increased the likelihood of Uncinia auceps fruits becoming entangled in the feathers, and so dispersed by these flightless birds.  Obviously, we will never know whether it’s true or not.

All of these endemics are threatened to some degree – the Lepidium species especially – all the three endemics are rated as ‘Nationally Critical’, and, L. oligodontum (which is not endemic) is rated ‘Nationally Vulnerable’ (de Lange et al. 2013c). Uncinia auceps despite its abundance seems tied to forest systems, and it does not long thrive outside these. So it has a lower threat rating of ‘Declining’ to reflect its ongoing loss from outside fenced forest systems (see de Lange et al. 2013c).