The main diversity among genus Araucaria is hosted in New Caledonia, where 14 species, all endemic, are described out of a total of 20 extant species.[1] These New Caledonian species are mainly found as dispersed populations in open areas, where competition is less intense.[2]

Araucaria humboldtensis growing in altitude shrubland, on the slopes of Mont Humboldt, New Caledonia

New Caledonia, considered as the smallest of the most significant biodiversity hotspots in the world,[3] hosts a unique flora of which 75.1% is endemic.[4] Species of Araucaria trees can be found in every habitat that New Caledonia possesses.[2] However, almost all of them are growing on ultramafic substrate, characterized by low fertility (low N, P, K levels) and high levels of heavy metals (nickel, cobalt, etc.).[5]

Phylogenic history

Context

An alternative to the view that the island is a museum for plant relicts is the possibility that the diversity results from a recent radiation, subsequent to the island's emergence.[6] New Caledonia was part of the supercontinent Gondwana, and separated from Australia 80 Ma.[6] However, geological evidence suggests that New Caledonia was submerged during Paleocene (ca. 65 Ma) and Eocene (until ca. 37 Ma).[7] The actual New Caledonian biota may then result from a total recolonization since the Oligocene.[8]

Araucaria columnaris growing on calcareous substrate, in the South of New Caledonia

The first Araucaria species were mentioned by Guillaumin in 1948: A. balansae, A. cookie, A. montana, A. muelleri and A. rulei.[9] In 1949, J. T. Bucholz added three new species: A. bernieri, A. biramuluta and A. humboldtensis. In 1972, D. J. de Laubenfels described five other species: A. luxurians, separated from A. cookie renamed A. columnaris, A. laubenfelsii, A. nemorosa, A. scopulorum and A. schmidii,[10] and divided these thirteen species in two groups.[2] The first includes species following the Massart model (plagiotropic branches, partial reiterations) while the second includes species following the Rauh model (orthotropic branches and no reiterations) with bigger leaves.[11] Veillon described how the morphology of New Caledonian Araucaria conforms to these models and noticed that species following the Rauh model have bigger leaves. He also designed a key to help in field identification, based on characters fixed in adults, so ecological factors impacting tree morphology won't interfere with identification.[2]

New Caledonian Araucaria species belong to the Eutacta section, one of the four sections defined by Wilde and Eames in 1952.[12] This section also includes A. heterophylla from Norfolk Island, and A. cunninghamii hosted by both Australia and New Guinea.[12]

Rejection of the Gondwanan hypothesis

In 1998, first genetic analyses based on RBCL gene sequence validated the recognition of the four sections within genus Araucaria.[13] Among Eutacta section, New Caledonian species formed a monophyletic group where A. cunninghamii (Papua New Guinea) was derived first, then A. heterophylla (Norfolk Island). The New Caledonian species revealed a strong homology for rcbL sequences (from 99.5 to 100%), where 10 out of 13 species are identical for this gene sequence.[13] This strong homology and A. heterophylla as a sister group of New Caledonian species (the Norfolk Island being relatively young, less than 3 million years old) are first elements suggesting a recent differentiation of Araucaria trees in New Caledonia. This hypothesis comes in opposition of an older Gondwanan origin.

Gaudeul et al. attempted in 2012 to better describe the evolutionary relationships and diversification of New Caledonian species by using AFLP markers and by performing Bayesian, genetic distances and cladistics analyses.[14] Ecological, morphological and geographical parameters were also considered in the study, which ended supporting a recent diversification of the genus in New Caledonia. Moreover, another genetic group was created: coastal species. However, no evidence were shown for more environment implication in driving speciation, which may be the result of both adaptation and allopatry. The concept of cryptic species, rare among such an iconic group, is mentioned, regarding several divergent populations.[14]

In the meantime, Escapa and Catalano published a phylogenetic analysis using parsimony.[15] For the first time in the family Araucariaceae, genetic (19 plastid, 2 nuclear and 2 mitochondrial genomic regions) and morphologic data (52 discrete and 10 continuous characters) were combined, confirming a strong monophyly for the 4 sections existing. However, relationships among New Caledonian species remain difficult to elucidate even if their relatively recent origin is confirmed by phylogeny based on combination of plastid and nuclear data, and the use of molecular clock.[6] While this study seems to corroborate with overwater dispersal consecutively to the submersion of New Caledonia, New Caledonian Araucaria species seems too old to originate from one single dispersal from Australia to Norfolk Island, and then to New Caledonia.

Cryptic diversity

Araucaria muelleri population, Yaté, New Caledonia

The complexity of species identification and delimitation in New Caledonian Araucaria is illustrated by Rusham et al. (2016),[16] underlying a cryptic diversity present between two relative species (A. rulei and A. muelleri). This work leads to the identification of a fourteenth endemic species, Araucaria goroensis,[1] confirming the difficulty to distinguish species among the Monkey Puzzle genus in the territory. This new species was initially confused with A. muelleri, but it appears to be more closely related to A. rulei. Distinctions with the latter hold in larger leaves, microsporophylls without a shouldered base and shorter female cone bracts.[1]

The origin of such a diversity among the genus Araucaria in New Caledonia still remain unclear, but appears to be relatively recent. Also speciation may be still be processed as suggested by the cryptic diversity observed among the genus.[14]

Group Species IUCN status Leaf length (mm) Leaf width (mm) Architectural model Substrate Habitat Altitude (m)
Coastal A. columnaris LC 5-7 4-5 Massart Calcareous Calcareous platforms < 50
A. nemorosa CR 6-10 1.5-3 Massart Ultramafic Maquis < 100
A. luxurians EN 5-7 4-5 Massart Ultramafic Humid evergreen forest < 300
Small-leaved A. bernieri VU 2-3.5 1.5-2.5 Massart Ultramafic Humid evergreen forest, deep valleys 100 - 800
A. schmidii VU 7-10 1.5-2 Massart Acidic Montane cloud forest > 1400
A. scopulorum EN 3-4 2.5-3 Massart Ultramafic Maquis 50 - 800
A. subulata NT 4-6 2-2.5 Massart Ultramafic Humid evergreen forest, deep valleys 100 - 1100
Large-leaved A. biramulata VU 7-9 5-6 Massart Ultramafic Humid evergreen forest, deep valleys 300 - 1000
A. goroensis EN 26-33 1-16 Rauh Ultramafic Maquis 150 - 550
A. humboldtensis EN 5-6 4-5 Massart Ultramafic Maquis of high altitude 600 - 1500
A. laubenfelsii NT 12-20 8-10 Massart Ultramafic Evergreen forest or maquis 700 - 1200
A. montana VU 11-14 7-8 Massart Ultramafic and acidic Dense humid forest or maquis 400 - 1200
A. muelleri EN 30-35 15-20 Rauh Ultramafic Dense humid forest or maquis 150 - 1200
A. rulei EN 20-25 11-14 Rauh Ultramafic Dense humid forest or maquis 400 - 800

References

  1. 1 2 3 Mill RR., Ruhsam M., Thomas PI., Gardner MF. and Hollingsworth PM. 2017. Araucaria goroensis (Araucariaceae), a new Monkey Puzzle from New Caledonia, and nomenclatural notes on Araucaria muelleri. Edinburgh Journal of Botany 1–17.
  2. 1 2 3 4 Veillon JM. 1980. Architecture des espèces néo-Calédoniennes du genre Araucaria. Candollea 35: 609-640.
  3. ↑ Brummitt N., Lughadha EN. 2003. Biodiversity: where's hot and where's not. Conservation Biology 17: 1442-1448.
  4. ↑ FLORICAL vers. 22.IV.2016. Morat P., Jaffré T., Tronchet F., Munzinger J., Pillon Y., Veillon J.-M. and Chalopin M. 2012. Le référentiel taxonomique FLORICAL et les caractéristiques de la flore vasculaire indigène de la Nouvelle-Calédonie. Adansonia sér. 3 34(2): 177-219. Munzinger J., Morat P., Jaffré T., Gâteblé G., Pillon Y., Tronchet F., Veillon J.-M., and M. Chalopin. 2016. FLORICAL: Checklist of the vascular indigenous flora of New Caledonia. vers. 22.IV.2016. http://www.botanique.nc/herbier/florical Archived 2017-04-01 at the Wayback Machine
  5. ↑ Jaffré T., Munzinger J. and Lowry PP. 2010. Threats to the conifer species found on New Caledonia’s ultramafic massifs and proposal for urgently needed measures to improve their protection. Biodiversity and Conservation 19: 1485-1502.
  6. 1 2 3 Kranitz ML, Biffin E, Clark A, Hollingsworth ML, Ruhsam M, Gardner MF, et al. 2014. Evolutionary Diversification of New Caledonian Araucaria. PLoS ONE 9(10)
  7. ↑ Pelletier B. 2006. Geology of the New Caledonia region and its implications for the study of the New Caledonian biodiversity. In C. E. Payri and B. Richer de Forges [eds.], Compendium of marine species of New Caledonia. Documents Scientifiques et Techniques, vol. II 7, 2nd ed. IRD, Nouméa, New Caledonia.
  8. ↑ Grandcolas P., Murienne J., Robillard T., Dessuter-Grancolas L., Jourdan H., Guilbert E. and Deharveng L. 2008. New Caledonia: A very old Darwinian island? Philosophical Transactions of the Royal Society of London, B, Biological Sciences 363: 3309-3317.
  9. ↑ Guillaumin A. 1948. Flore analytique et synoptique de la Nouvelle Calédonie et dépendances. Office de la Recherche Scientifique, Paris.
  10. ↑ De Laubenfels DJ. 1972. Gymnospermes. In: Aubréville, A. & Leroy, J-F. (eds) Flore de la Nouvelle-Calédonie et Dépendances, vol.4, pp. 1–167. Paris : Muséum national d’Histoire Naturelle.
  11. ↑ Kranitz ML. 2005. Systematics and evolution of New Caledonian Araucaria. Thesis, University of Edinburgh.
  12. 1 2 Wilde MH, AJ Eames. 1952. The ovule and “seed” of Araucaria bidwillii with discussion of the taxonomy of the genus. II. Taxonomy. Ann Bot 16: 27-47.
  13. 1 2 Setoguchi H., Osawa T.A., Pintaud J.-C., Jaffré T., and Veillon J-M. 1998. Phylogenetic relationships within Araucariaceae based on rbcL gene sequences. Am. J. Bot. 85: 1507-1516.
  14. 1 2 3 Gaudeul M., Rouhan G., Gardner MF. and Hollingsworth PM. 2012. AFLP markers provide insights into the evolutionary relationships and diversification of New Caledonian Araucaria species (Araucariaceae). American Journal of Botany 99: 68-81.
  15. ↑ Escapa IH. and Catalano SA. 2013. Phylogenetic Analysis of Araucariaceae: Integrating Molecules, Morphology, and Fossils. International Journal of Plant Sciences 174: 1153-1170.
  16. ↑ Ruhsam M., Rai HS., Mathews S., Ross TG., Graham SW., Raubeson LA., Mei W., Thomas P.I., Gardner M.F., Ennos R.A., et al. 2015. Does complete plastid genome sequencing improve species discrimination and phylogenetic resolution in Araucaria? Molecular Ecology Resources 15: 1067-1078.
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