The ongoing decline of the enigmatic Chatham Island linen flax

Linum (Linaceae) is a genus of about 200 species found mostly in the northern hemisphere though it extends into the subtropics and has representatives on most of the major landmasses of the world. Universally known by the vernacular ‘flax’ some species, notably ‘linen flax’ Linum usitatissimum produce fibres that are used for the production of cordage and linen fabric; while the seeds are the source of linseed oil (Mabberly 2017). New Zealand has five species, of which only Linum monogynum, known to Maori as ‘rauhuia’ is indigenous and endemic. There are two varieties of rauhuia, L. monogynum var. monogynum which is widespread in the North, South and Stewart Islands (Webb et al. 1988), and var. chathamicum, which is endemic to the Chatham Islands (Cockayne 1902; de Lange et al. 2011).

Chatham Island linen flax (Linum monogynum var. chathamicum) is a small, sparingly branched shrubby plant, rarely exceeding 80 cm in height. The upper third to two-thirds of the whip-like, erect or floppy stems are covered in grey, spreading, entire, lanceolate leaves. At the apices of these stems, in spring through summer are borne clusters of 10-15 mm long tubular flowers, which when fully expanded are white with a bluish cast, though the buds just prior to flowering are distinctly coloured sky-blue. It was on account of the blue-tinged, or as Leonard Cockayne put it ‘broadly-striped or flaked with pale blue’ flowers that Cockayne (1902) described the Chatham Islands plant as a distinct endemic variety. The New Zealand plant, var. monogynum Cockayne noted has white flowers.

When preparing a checklist of the Chatham Island plants (de Lange et al. 2011) it was discovered that Linum monogynum var. chathamicum didn’t seem to have a common name. In the absence of one the name ‘Chatham Island linen flax’ was conjured up, and as an afterthought the Maori name ‘rauhuia’ added as an alternative, though I have never heard that name used on the Chatham Islands only in New Zealand, and even then, infrequently. The addition of ‘linen’ to ‘flax’ seemed necessary though as by itself ‘flax’ is potentially confusing especially as Linum have no semblance to, nor are they remotely related to the widespread flax (Phormium tenax) found throughout New Zealand, the Chathams and Norfolk Island. That plant, a member of the monocotyledonous Asphodelaceae, and known to Maori as harakeke and Moriori as harapepe, was in turn bestowed the name ‘flax’ by early European voyagers because when they saw the way Maori used it’s fibres for cordage, clothing and other items, they were reminded of the way the northern hemisphere linen flax (Linum usitatissimum) was used for the same purpose. So ‘Chatham Island linen flax’ seemed a ‘quick fix’ way out of this confusion, though I have since found in my discussion with islanders that this is not the case, confusion remains. So, from a Chatham Islands perspective, short of people learning the scientific name for the plant, I feel that someone on the islands needs to get creative and think up a more sensible common name for the plant.

Possibly though the lack of a vernacular name for Linum monogynum var. chathamicum reflects the fact that it may never have been a common plant on the Chatham Islands anyway – so ‘out of sight then out of mind’ I suggest. On this issue of past abundance Cockayne (1902) though recording it as an associate of the vegetation of exposed limestone cliffs and rock outcrops throughout the islands gave no indication of its abundance at the time. Certainly, by the time I became familiar with this variety it was already regarded as seriously threatened. I well recollect a very long day spent with Amanda Baird, Ian Atkinson, Gillian Crowcroft and Richard Suggate during February 1996 traipsing over to Ocean Bay and then scouring up and down Te Moko Creek searching for a population reported ten years previously by the late David Given; only after much searching did, we eventually find a single plant. That was all that was left then. We soon learned that as a rule, this seems to be the pattern, Chatham Island linen flax seems to be a very uncommon, biologically sparse member of the Chatham Islands flora. Infact the only places I have ever seen more than a handful of plants are on limestone bluffs along the coast of Te Whanga south of Airport Road, Rekohu / Wharekauri (Chatham Island), and on coastal cliffs just north of Waipaua Stream mouth, Rangahaute / Rangiuria (Pitt Island).

A further impediment to our understanding of Linum monogynum var. chathamicum has been Cockayne’s original description of it. Cockayne never particularly cared for traditional taxonomy (Cockayne 1926; Thomson 2019; A.D. Thomson in. litt.), and with respect to his descriptions of new plants from Chatham Islands (Cockayne1902) he distinguished his new Linum variety using the bare minimum then acceptable, stating thus “petals broadly striped or flaked with pale blue. The type has white flowers”. This brief description has without any doubt been the cause of much taxonomic debate ever since. Here I provide a brief summary starting with Auckland Museum botanist Thomas Cheeseman, who unusually for him, as he typically disregarded Cockayne’s taxonomic discoveries (much to Cockayne’s chagrin (Thomson 1990; A.D. Thomson in litt.)), accepted the variety. In doing so though he noted that it also occurred on the ‘mainland [sic] as well as on the Chatham Islands’ (Cheeseman 1925). Despite this uncharacteristic vote of support from Cockayne’s main taxonomic critic, the Linum was then not accepted by Harry Allan, which is even more surprising as Allan was very much Cockayne’s protégé. Yet in his treatment of the indigenous Linum for his Flora of New Zealand (Allan 1961; p. 240) Allan considered var. chathamicum as a synonym of L. monogynum, further noting that he had ‘observed plants with variously blue-tinged flowers near Fielding and on the Wellington coast, but always with the introduced L. marginale [a name then widely used in New Zealand for the plant now referred to L. bienne] nearby’. This statement has the inference that hybridisation between Linum monogynum and bluish-flowered L. bienne might be the explanation for the bluish tinge described by Cockayne (1902) for the flowers of the Chatham plant. If so, it was rather speculative as hybridisation, between Linum bienne, with 2n = 30 chromosomes, and L. monogynum, with 2n = 84 chromosomes would be rather unlikely. If it did happen, then the resulting progeny would be sterile, because the hybrid would have 2n = 57 chromosomes. Inferred hybridism or not I strongly suspect that the comment by Allan (1961) stemmed more from his understanding of the range of variation in Linum monogynum in New Zealand (it is a very variable species) and his lack of familiarity with the Chatham Island plants. Allan was also correct in that blue-tinged flowers are present in New Zealand populations of Linum monogynum. for example, I have seen them on Aorangi Island, in the Poor Knights group (de Lange & Cameron 1999). So blue-tinged flowers are not a reliable way to separate out the two varieties.

Blue tinged flowers or not, the stance as to whether Linum monogynum var. chathamicum was a valid taxon or not has vacillated over the last three decades. For example, Given (1996) uncritically accepted the variety in his summary of the endemic plants of the Chatham Islands. Others regarding the extent of variation in Linum monogynum as a whole, felt the species needed a modern taxonomic revision. Until that was done, they preferred to treat var. chathamicum as a named unit within a morphologically variable L. monogynum (see de Lange et al. 1999; de Lange et al. 2004; de Lange et al 2009). Whatever its status though on one thing all were agreed, the Chatham Islands plant was (and still is) seriously at risk of extinction.

This remained the situation until in 2010 a molecular study of the origins and divergence of the Chatham Island endemic plant flora (Heenan et al. 2010) discovered, on the basis of DNA sequence data obtained from the Internal Transcribed Spacer (ITS) region Linum monogynum var. chathamicum diverged by 0.7% from two New Zealand samples of L. monogynum var. monogynum. On this basis, at least for now, L. monogynum var. chathamicum has been cautiously reinstated (see de Lange & Rolfe 2010; de Lange et al. 2013; de Lange et al. 2017). There the matter still stands, as there has been no further advance on the taxonomic situation.

Meanwhile the conservation plight of Linum monogynum var. chathamicum seems to be going from bad to really bad. As far as I can tell there has not been a population census undertaken for the plant since 2008. So, what little is known about the conservation status of Chatham Island linen flax is now 11 years old. The most recent records I am aware of from the islands is of plants reported by botanists visiting Mang’re / Mangere (2013) and Hokorereoro / Rangatira / South East Island (2015). At both locations there are only a ‘handful’ of plants. In September and November 2018, I searched for Linum at Blind Jim’s, a well-known location for this plant where upwards of 50 plants grew on the limestone cliff tops at least since 1990. Worryingly my searches found none, and even more concerning is that I don’t know why this is. The habitat is still there, nothing obvious has changed.

What may have tipped the balance though is the spread of an introduced rust, linum rust (Melampsora lini). This rust was first detected in New Zealand on Linum monogynum plants growing around Wellington in 1920 and 1921 (Cunningham 1924). The rust is believed to have naturally dispersed from Australia where it was first detected in 1889 as an accidental introduction associated with linen flax seed imported from Europe (McAlpine 1906). Intriguingly Cunningham (1924) recorded linum rust from both varieties of Linum monogynum, var. chathamicum and var. monogynum. The initial rust samples were collected from York Bay, by E.H. Atkinson, a keen gardener who lived there. So, I suspect but cannot prove that these Linum plants were cultivated (var. chathamicum, least ways in the strict sense, had to be as we have already shown is a Chatham Island endemic so it is unlikely to have been naturally occurring around Wellington). Possibly Atkinson’s Linum monogynum var. chathamicum had been given to him by Cockayne who by then resided in Wellington (Thomson 2019) and who as a keen gardener probably grew it.

Melampsora lini seriously impacts on Linum monogynum and is especially severe on var. chathamicum, plants of which soon succumb and die when infected by it. In New Zealand all attempts to cultivate Chatham Island linen flax have failed, plants always get the rust and once infected within a matter of months the afflicted plants die. Unfortunately, the same happens on the Chatham Islands where the Department of Conservation has tried to grow plants as part of an ex-situ conservation plan. Work which had to be abandoned about 2005 because of the constant outbreaks of rust killing all the plants. At that time the rust had not yet been reported from wild plants but by 2008 plants at Blind Jim’s had it. That was also the last time I saw Linum there. Interestingly the sole Mang’re / Mangere Island observation of Linum monogynum var. chathamicum posted on iNaturalist NZ by Department of Conservation Botanist Dr Catherine Beard (see https://inaturalist.nz/observations/3634320) depicts a plant also infected by Melampsora lini.

We are of course fully aware that the major upheavals of the Chatham Islands landscape over the last 150 or so years of farming and other ecosystem-changing land use activities has had a profound impact on the natural biota of the islands. However, from a plant perspective, with the exception of the Chatham Island scurvy grasses (Lepidium spp.) and button daisy (Leptinella featherstonii), plants that are tied to the ornithocoprophilous ecosystem (now only seen intact on the outer predator free islands and islets of the Chatham group) most Chatham Island endemic plants respond well to basic predator control and fencing. Only a few, such as Barkers tree koromiko (Hebe (Veronica) barkeri), rautini (Brachyglottis huntii) and Chatham Island toetoe (Austroderia turbaria) require hands on management. In the case of Chatham Island linen flax, it had hitherto been assumed it was simply a very uncommon plant of secure cliff habitats. If linum rust is truly the cause of the loss of plants at Blind Jim’s and elsewhere then this needs urgent investigation, especially as there is no ex-situ reservoir for this plant.

Further research into the decline of this plant is needed. In the meantime, reporting occurrences, and ideally the number of plants seen of Linum monogynum var. chathamicum to the Department of Conservation at their Te One Office will help researchers to gain a better appreciation of what is currently happening in the wild.

Acknowledgements

I would like to thank the late Dr Andy (Andrew) Thomson for his insightful and voluminous correspondence about Leonard Cockayne and in particular, his honesty in answering my questions about Cockayne’s taxonomic contributions and relationships with leading New Zealand taxonomists of that time. I also thank Amanda Baird for introducing me to Linum monogynum var. chathamicum, Bridget Gibb for arguing about the need to do more to prevent its extinction, and to Gillian Crowcroft, Richard Suggate and the late Ian Atkinson for company in the field during my first visit to the islands in 1996. Finally, a big thank you to Dr(s) Jerry Cooper and Eric McKenzie for answering my questions about linum rust (Melampsora lini); Jerry also for use of his image of Melampsora lini, John Barkla for permission to use his Linum monogynum var. monogynum image, and Dr Catherine Beard for allowing me to use her image of rust-infected Linum monogynum var. chathamicum for this article.

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