By Peter de Lange, Principal Science Advisor
Rātā moehau is now New Zealand’s most threatened tree, but DNA data is providing a glimmer of hope for the species’ survival.
The late John Bartlett discovered rātā moehau/Bartlett’s rātā in 1975 at Te Paki, a biodiversity hotspot perched at the tip of the North Island. Bartlett was looking for liverworts but instead found a completely new species of rātā.
By 1990 ten trees had been found across two Te Paki sites—Radar Bush and Kohuronaki. A year later, a number of colleagues and I stumbled upon another population in the Muriwhenua Incorporation’s Unuwhao Forest near Spirits Bay.
34 wild trees were documented by 1992, which still isn’t many, but it was much better than the 10 we previously believed existed. Sadly, in 2007 DOC staff found that seven trees at Unuwhao had died from possum browse and several more were seriously defoliated. New Zealand’s tree rātā are self-incompatible (individuals that breed with close genetic relatives will not produce good seed), and there were fears for the loss of genetic diversity. Genetic variation was already very limited; therefore it was crucial that any losses were avoided.
To manage the species effectively, we needed to understand the genetics of all living rātā moehau, including plants that had been propagated and grown in cultivation from those early discoveries.
Following consultation with Ngāti Kuri, Te Aupouri and the Muriwhenua Trust, DOC staff undertook a field survey and DNA sampling of rātā moehau in April 2015. Our purpose was to determine the status of the species in the wild, collect material for DNA analysis, and collect seed for the New Zealand Seed Bank. The DNA profiles of wild plants would be compared with the DNA of cultivated specimens.
DNA samples were obtained from helicopter by human sling operator Brad Lett. Aside from the known trees at Radar Bush and Kohuronaki, our survey found only one tree in the original Unuwhao Forest stand of 16, but we discovered three more in a nearby catchment. Samples were sent to Landcare Research, where Dr Gary Houliston and his team profiled them using two DNA fingerprinting techniques.
The DNA results are alarming. Of the 14 trees in the wild, there are only five distinct genotypes and three of them are known only from Unuwhao. Further, there is no Unuwhao stock in cultivation; all cultivated plants derive from two trees: one from Radar Bush and the other from Kohuronaki. Rātā moehau is now one of the most threatened plants in the world.
Although the results are far worse than we’d anticipated, the DNA data provides some hope for saving the species. We now know where the immediate priorities are, and we are working closely with iwi, Muriwhenua Incorporation and Ngā Whenua Rāhui to protect the trees at Unuwhao.
Saving the species will involve other partners initially (such as botanic gardens and Project Crimson) to raise genetically viable plants in cultivation.
The process will involve bringing together the five lineages and hand pollinating these to produce more genetically fit plants to restore to the wild.
Oratia Native Plant Nurseries sell this species (that’s where my one seedling came from). I know the immediate concern is to manage the remaining species in the wild and build up the genetic diversity there, but would it make sense for places like Oratia to be given access to a couple of seedlings of mixed genetic background so that they can breed more genetically diverse plants for general use?
Is this plant also known as White Rata?
In short – no – white rata is a vernacular used for two vine rata in New Zealand – Metrosideros perforata (also known as akatea – see http://www.nzpcn.org.nz/flora_details.aspx?ID=984) and M. albiflora – see http://www.nzpcn.org.nz/flora_details.aspx?ID=979. These names were coined long before Metrosideros bartlettii was discovered (it was discovered in 1975 and named in 1985).
So people tend to call Metrosideros bartlettii “Bartlett’s Rata” after the Physics / Science Teacher John Bartlett who found it, or more recently by the Ngati Kuri name ‘rata moehau’ – a name that iwi created for this species when it was discovered.
Hope that helps,
Peter J. de Lange
Has the DNA told us what other metrosideros species these are derived from or are most closely related to? And when you say that they are not ‘self compatible’, does that mean within the species? Would they hybridise with other metrosideros, and would hybrid plants be robust?
Genetically Metrosideros bartlettii is not closely related to other Metrosideros it sits at the bottom of the Metrosideros subg. Metrosideros phylogenetic tree as sister to southern rata (M. umbellata). It is not closely related to southern rata either. So we conclude its a very distinct species whose closest relatives have long since died out. Nevertheless it is most unwise to say that hybrids cannot form with this species and our other tree rata and pohutukawa (hybrids are very common, indeed a feature of the New Zealand Flora and we get strange hybrid crosses commonly appearing between quite distant relatives, consider Celmisia hybrids with Olearia as one such example). There is also some field evidence that suggests M. bartlettii may hybridise with pohutukawa (M. excelsa), and less convincing evidence that it may hybridise with northern rata (M. robusta). In an earlier study Auckland University refuted these claims but I am not convinced they sampled the right trees at Te Paki. Significantly though such hybrids have not been produced in cultivation (and some people have tried).
So I speculate that hybrids with other tree rata and pohutukawa if they occurred would be scarce. I cannot say if they would be more ‘robust’ (I assume you mean genetically fitter). I just don’t know – and while hybridism has been used to resurrect some highly threatened plant species from near extinction, for M. bartlettii its a last option that I hope we don’t need to use. We can still save this species if we act now.
Self incompatible means that the trees, in isolation will produce some viable seed through in breeding (the tree crossing with itself) but that this seed is not very ‘fit’ (genetically). All our Metrosideros can, and will produced seed by in breeding – but its now well known that seedlings raised from ‘selfed’ Metrosideros seed are of ill thrift, grow more slowly and often die before reaching maturity. In gardens where we pamper our plants progeny raised this way will mostly flourish but we now know that if you plant them out into the wild they mostly die as they cannot compete well with other plants and natural stresses like drought and insect attack. The problem of in breeding is that if you have a stand of trees that derive all from the same parent then in effect they are still going to be in breeding – we are seeing this with M. bartlettii – trees on Public Conservation Land are all derived from each other, they are producing poor quality seed and natural seedlings are scarce and of ill-thrift. Seed collected from these trees has very low viability.
I hope that explains the matters you have raised,
Peter J. de Lange
I have a Bartlett’s rata in my garden. It is thriving! It came to me via a DOC person around 20 years ago, but I am not sure where he got it. Is it worth you having a look at?
That’s nice to know – where do you live please? If you don’t wish to be public please email me at email@example.com. I suspect your tree came from one of two sites – Radar Bush or (more likely) Kohuronaki – these are the two lineages found in cultivation.
Peter J. de Lange
Iwi, (Ngati Kuri, Te Aupouri and the Muriwhenua Trust Inc) and the Department of Conservation are working on this right now. First priority is possum control on iwi / Muriwhenua Trust Inc. land.
Peter J.de Lange
Hi Peter, I got two “Bartlett’s rātā” seedlings from DOC Kerikeri c. 1995 when they had a quite a number of them in their nursery. I planted them at my house in Whangarei. One died at c. 1.5m but the other is still doing well, at 4.5m. Happy to help with material if you need it. Cheers Jack Craw
Thanks for letting me know Jack. In our research we found that there are lots of plants in cultivation but so far all of them have been found to descend from either one tree in Kohuronaki or one tree at Radar Bush. Sadly they alone will not secure this species (because there is not enough genetic variation in those two lineages and isolated trees will be forced to self and seedlings raised from ‘selfed’ seed will not thrive without considerable pampering) – but treasure the one you have.
Peter J. de Lange
Do we need to get more protection for the sites via pest control, or tenure, or fencing, or other measures?