Tuesday, 12 November 2013

A new classification for the southern beeches.

ResearchBlogging.org
In New Zealand, the forest we typically identify with—the “bush”— is the lowland mixed conifer-angiosperm forest, with a canopy usually of angiosperm trees like tawa (Beilschmiedia tawa) or kamahi (Weinmannia racemosa) and large emergent conifers of the southern families Podocarpaceae (e.g., rimu, Dacrydium cupressinum) and Araucariaceae (i.e., kauri, Agathis australis).  But in most montane parts of the South Island (Te Wai Pounamu) and on often drier ridges and hill country of the North Island (Te Ika A Māui), a very different type of forest is dominant.  This forest comprises a uniform canopy of often a single species of southern beech.  Usually the forest doesn’t have a dense understory, giving an open and well-lit appearance to the interior.  This is just as much an iconic New Zealand forest as the “bush”, and one that’s familiar to many trampers.
Southern beech forest, Orongorongo Valley, near Wellington
Similar forests are found in the southern part of South America, so that travelers there from New Zealand often feel it’s just like home.  Southern beech forest also occurs in Australia, New Guinea and New Caledonia.  It’s known from fossils in Antarctica too, going back to the Cretaceous, as well as in many of the places where it still occurs today.

The southern beeches were originally classified in the genus Fagus, along with their northern namesakes, but by 1850 their differences had been recognised and they were transferred to the genus Nothofagus (the name means southern beech [correction, 21 Nov 2013: it means "false beech"; southern beech would be Notofagus.  H/T Rosi]).  As Nothofagus, the southern beeches have been important trees in New Zealand ecology, conservation, forestry, and biogeography.  Whole books have been written about them.  Nothofagus is currently reckoned to have about 40 species.  In 1962, a Russian botanist, Lyudmila Kuprianova, went a step further and proposed a new family, Nothofagaceae, for the southern beeches.  This took rather a while to be accepted.

Many botanists have wrestled with the relationships of the species within Nothofagus, using sometimes single or few characteristics, other times multiple ones.  The advents of (1) cladistic thinking (using explicit evolutionary trees) and (2) molecular characteristics from DNA sequencing have been of major help to this enterprise, because DNA has provided a wealth of new characters that are independent of the morphological ones and because the analysis and interpretation are out in the open for everyone to evaluate.  Pretty quickly, the understanding of relationships in the southern beeches has converged on a single well-supported arrangement, which was arranged into a classification by Australian botanists Bob Hill and Jenny Read, who recognised one genus (Nothofagus) with four subgenera.
Southern beech forest near Eastbourne, Wellington.
Beyond the southern beeches, DNA sequence data were also telling us a lot about the relationships of southern beeches to the oaks, beeches, chestnuts and she-oaks and it became pretty clear that Kuprianova was correct in isolating them in their own family.  It turns out that the ancestor of the beech order (Order Fagales) first divided into two species: one that was the common ancestor of the northern sweet chestnuts, beeches & oaks, she-oaks, myrtles, and more—seven families in all—and the other that was the common ancestor of just Nothofagus.  If you were to divide Fagales into two suborders, one would have seven families and many genera, the other would have just one family, and that family would have just one genus.  Thus Nothofagus and Nothofagaceae have different ranks (their place in the hierarchical classification) but identical circumscriptions (the species they contain); that redundancy means we're not using the available hierarchy of ranks to full advantage.

What’s more, the current classification of all the southern beeches in one genus Nothofagus can be a bit misleading.  Most biologists agree that it’s absolutely essential that every genus or family should contain closest relatives.  In other words, a species shouldn’t be more closely related to a member of another genus than it is to a species that’s classified in its own genus.  Nothofagus doesn’t break that rule: every species of Nothofagus is more closely related to every other species than it is to any species that’s not placed in Nothofagus.  So far, so good.

But it’s easy to assume that our New Zealand species—black, hard, red, mountain and silver beeches—might be each other’s nearest relatives, and often people are surprised to find that’s not the case.  In fact, hard, black, mountain, and red beeches are related, but silver beech’s nearest relative is in Australia.  Wouldn’t it be better if their classification and their scientific names could reflect that?

This week two New Zealand botanists, Peter Heenan and Rob Smissen from Landcare Research, have revisited the classification of the southern beeches (Heenan & Smissen 2013).  They brought together everything that’s been published so far, from both morphology and molecular systematics, and added some new data and analyses of their own.  Their findings are pretty much the same as several previous reports, but they can now place greater levels of confidence in the groups they recognise.  They comprehensively discuss alternative classifications and alternative criteria and come down with what I think is the most sensible classification. 

Nothfagaceae now contains four genera.
  • Nothofagus comprises just five species from temperate South America.  The rest of the family is no longer classified as Nothofagus.
  • Lophozonia is a reinstated genus, containing seven species from South America, New Zealand, and Australia.  
  • Fuscospora has six species and a very similar distribution; it’s a newly recognised genus, although like the others it has been treated as a subgenus in the past.  Additionally in Fuscospora, this paper promotes mountain beech to species rank as F. cliffortioides.  I look forward to reading the evidence for that change, because it was previously treated just as a variety of black beech.
  • Finally, Trisyngyne is the largest genus (25 species) and found today in the tropics: New Caledonia, Papua New Guinea and extending into Indonesia.  
These genera are strongly supported by both molecular, morphological, and chemical characteristics, and they have symbiotic fungi and parasitic fungi and insects that also seem to recognise their relationships.
Red beech, Fuscospora fusca.
Black beech, Fuscospora solandri.
Mountain beech, Fuscospora cliffortioides, near Cass, Canterbury.
Hard beech, Fuscospora truncata
Silver beech, Lophozonia menziesii.
When we use these new names for the New Zealand plants, we see immediately that we have two natural groups represented here: Fuscospora and Lophozonia.  New Zealand no longer has any species of Nothofagus.
Red beech, Fuscospora fusca, Tunnel Gully near Wellington.
A final word of a more general nature.  Some people will want to reject this change, perhaps because they feel nostalgic about the name Nothofagus, or perhaps because they feel name changes are disruptive.  But taxonomy is science and there are scientific criteria involved.  Like climate change, evolution, and vaccination, you can’t simply reject sound science because you don’t like it.  
Silver beech, Lophozonia menziesii, Haast Pass.
It’s very rare in science for there to be two equally well-supported positions such that users are free to choose whichever one they prefer.  Rather, scientists make decisions after critically considering the evidence.  It's true a classification is a human construct, but it's based on facts about evolutionary history.  Those facts are hard-won data from the field, herbarium, and genetics lab.  If your opinion contradicts those facts, then you're at risk of denying the science.

In this case however, both the old classification and the new one do pass the most important test, that of classifying related species together, so we can't rule out one or the other on that ground.  The question here is, "what's the appropriate rank for these four well-supported groups?"  But is one answer better than the other?  Heenan and Smissen argue strongly and in detail that there are good reasons to prefer their new scheme over the old one.  For instance, they show that the newly-recognised genera are at least as old, diverse, and distinct as established genera in the other families of the order, that the new names are more informative about relationships among the southern beeches, and that a redundant grouping has now been eliminated.  They conclude, and I agree, that these benefits far outweigh the temporary disruption of having new names to learn.
An Australian beech, Lophozonia moorei, growing at Eastwoodhill.

Heenan, P.B.; Smissen, R.D. (2013). Revised circumscription of Nothofagus and recognition of the segregate genera Fuscospora, Lophozonia, and Trisyngyne (Nothofagaceae) Phytotaxa, 146 DOI: 10.11646/phytotaxa.146.1.1

5 comments:

  1. Excellent blog. Heartiest the greetings of Lithuania

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  2. Thank you FP! It's nice to know you're reading it so far away.

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  3. Great stuff Phil. I wonder what this means for New Caledonian species.

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  4. They're all in Trisyngyne, and form a well-supported clade nested among the species of that genus from Papua New Guinea. Perhaps that implies dispersal.

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  5. Great work, Phil, and it has an inherent logic. Just bit hard for 71 year old to remember the new names. Ian James, Okarito

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