Parasitaxus usta (corail) description

Parasitaxus usta

(Viellard) de Laub. 1972

Common names

Corail (Schmid 1981), cèdre rabougri (Thomas 2010).

Taxonomic notes

The sole species in Parasitaxus de Laub. 1972. Type: New Caledonia, Grande Terre, Province Sud, mountains around Poila, E. Viellard 1267 (holo P). Synonymy: Dacrydium ustum Vieillard 1861; Podocarpus ustus (Vieill.) Brogn. et Gris 1866; Nageia usta (Vieill.) Kuntze 1891 (Farjon 1998). Two studies covering the Podocarpaceae, a molecular analysis by Sinclair et al. (2002) and a combined molecular and morphological analysis by Biffin et al. (2011), have placed Parasitaxus sister to a clade containing Lagarostrobos and Manoao, two monotypic genera of New Zealand. Conversely, studies of cuticle micromorphology suggest a close relationship with Lepidothamnus, particularly with L. fonkii (Stockey et al. 1995); molecular studies are unsettled on the placement of Lepidothamnus, however.

Description

Erect monoecious shrubs to 150 cm tall, usually multistemmed, with numerous, contorted, spreading and ascending branches. Bark thin with large lenticels, on large stems breaking into small gray scales. Twigs purple, covered in reddish scale leaves, 2-3 mm thick. Leaves scale-like, imbricate, decurrent, 2-3 × 1-2 mm, separated on thicker twigs, broad-lanceolate or triangular, apex appressed or free, obtuse or acute. Stomata on both sides of leaves, abaxially more conspicuous. Pollen cones usually on same branch systems as seed cones, terminal, solitary, ovoid, 3-3.5 × 1.5-2 mm. Seed cones numerous on erect twigs, mostly terminal, comprised of 3-6 spreading, narrow, incurved red bracts; the distal 1-2 bracts subtend short pedunculate fertile scales with a single terminal inverted ovule. Seeds ovoid-oblong with an apical crest within a hard white epimatium; 3-4 mm diameter (Farjon 2010).

Distribution and Ecology

New Caledonia: Pouebo, Bleue River, found at 400-1100 m elevation (Silba 1986). Based on data from 19 collection localities, its climate preferences include a mean annual temperature of 20.2°C, with an average minimum in the coldest month of 13.3°C, and a mean annual precipitation of 1938 mm (Biffin et al. 2011, Table S5). Zone 10 (cold hardiness limit between -1°C and +4.4°C) (Bannister and Neuner 2001).

This map shows herbarium records of Parasitaxus usta. Click on an icon for further information. Distribution data from GBIF (2020.03.30), edited to remove duplicates.

The IUCN reports that this taxon is "Vulnerable" to extinction in the wild (Thomas 2010). Based on results of a 2009 assessment, it has a limited geographic range that is severely fragmented with continuing decline in both the extent/quality of habitat and the number of mature plants, which include no more than 10,000 mature individuals with no more than 1,000 in any subpopulation.

De Laubenfels (1959) discovered that this species, known to science since 1861, is the only known parasitic gymnosperm. Its parasitism operates through a vesicular-arbuscular mycorrhiza, and the plant appears to be connected to its host through a root graft (Woltz et al. 1994). As such it is a singular parasitic relationship. Formerly this mode of parasitism was a great mystery, but recent studies (Feild and Brodribb 2005) have revealed much about its physiology, which differs from any of the more than 3,000 described angiosperm parasites. The foliage and shoots have chloroplasts, but lack any significant photosynthetic electron transport. The transfer of carbon (sugars) from the host plant is achieved mainly through a fungal intermediary, as in the case of mycoheterotrophs such as the nonphotosynthetic ericads pinesap (Hypopitys) and indian-pipe (Monotropa), but a direct xylem connection with the host may also occur and may facilitate nitrogen tranfer from the host. The plant also has a high stomatal conductance and low water potential, like the mistletoes. The nitrogen-scavenging ability of Parasitaxus also allows it to produce cone crops the year around, even in plants as small as 15 cm tall, despite growing in extremely poor ultramafic soils (Feild and Boddribb 2005).

Remarkable Specimens

No data as of 2023.02.22.

Ethnobotany

Despite many attempts, this plant has not been grown either experimentally or horticulturally, and remains endemic to its New Caledonia habitat. It is sacred in the traditions of the Kanaks, the native New Caledonians (Farjon 2010).

Observations

It is a very difficult plant to locate in the wild and, because it can be killed simply by stepping on the delicate shoots that are often concealed beneath forest litter, it is probably best if no one seeks to observe it unless they have a valid scientific reason -- preferably, a reason connected with the preservation and protection of this unique and very rare conifer.

Remarks

The genus name transliterates as "parasitic yew"; podocarps were formerly placed in the yew family. You will often see this species called Parasitaxus ustus. However, the Latin word for yew, taxus, is feminine; thus adjectives such as ustus, meaning "burnt", a reference to the brownish twigs and foliage, should also be feminine. This principle applies equally to names in the other genera derived from taxus, e.g. Amentotaxus, Austrotaxus, Cephalotaxus, Pseudotaxus, and Taxus.

These general remarks are taken from de Laubenfels (1959):

[Parasitaxus] has always occupied a unique position ... among conifers in general. It grows in remote, densely forested highland parts of New Caledonia. ... Its uniqueness has derived from the fleshy deep red or purple scale-leaved branches ...

Because of the special interest which P. ustus has aroused, a special effort was made during a December 1957 visit to New Caledonia to collect the elusive plant. One of its areas of occurence was visited, and several plants were seen. This report derives from the fact that one of these plants was growing out of the base of a tree of [Falcatifolium taxoides]. As might be suspected from the fleshy red nature of P. ustus, it is a parasite, certainly sometimes, probably always. The parasitic specimen was collected and has been the subject of a careful study.

[Parasitaxus] is a woody shrub. In the case of its attachment to [F. taxoides], the largest of two emerging stems had a diameter of about [12 mm] and the host had a diameter of about [8 cm]. The parasitic stem curves sharply so that the root zone is oriented upward. Most of the woody roots travel up the trunk of the host, probably for several feet; a few grow downward for several inches. The modified roots are imbedded in the bark between the cork cambium and the vascular cambium. General stimulation of the vascular cambium has occurred, producing a thickening in both wood and bark tissue. The cambium of the slightly anastomosing roots, significantly, is on the outside toward the cork cambium. No normal phloem is produced, although a tissue full of sclerids opposite the xylem may represent modified phloem tissue. Around the parasitic roots, the host tissue is somewhat disorganized and forms a sheath of abnormally large cells.

In several ways the specimen of P. ustus reported here is not typical. No other specimen has been seen growing from the trunk of another plant. Typically P. ustus grows on the open forest floor under rather shady conditions (chlorophyll is present in the leaves). Probably most individuals are root parasites. Several attempts by foresters in New Caledonia to dig up small specimens for transplanting resulted in immediate death of the specimens. It was noticed that removing the specimens involved cutting out various tree roots, but, because a parasitic attachment was not expected, it was not looked for. ...

Until now, all conifers were known to be independent trees, bushes, or trailing shrubs. Podocarpus ustus, in being parasitic, thus differs from all other conifers. In fact no gymnosperm of any kind has previously [or since] been discovered in a parasitic relationship to other plants."

Citations

de Laubenfels, David J. 1959.07.10. Parasitic conifer found in New Caledonia. Science 130:97.

Feild, T.S. and T.J. Boddribb. 2005. A unique mode of parasitism in the conifer coral tree Parasitaxus ustus (Podocarpaceae). Plant, Cell and Environment 28: 1316-1325.

Sinclair, W. T., R. R. Mill, M. F. Gardner, P. Woltz, T. Jaffre, J. Preston, M. L. Hollingsworth, A. Ponge, and M. Moller. 2002. Evolutionary relationships of the New Caledonian heterotrophic conifer, Parasitaxus usta (Podocarpaceae), inferred from chloroplast trnL-F intron/spacer and nuclear rDNA ITS2 sequences. Plant Systematics and Evolution 233:79-104.

Stockey, Ruth A., Helen Ko and Philippe Woltz. 1995. Cuticle micromorphology of Parasitaxus de Laubenfels. International Journal of Plant Sciences 156: 723-730.

Thomas, P. 2010. Parasitaxus usta. The IUCN Red List of Threatened Species 2010: e.T31002A9597883. https://dx.doi.org/10.2305/IUCN.UK.2010-3.RLTS.T31002A9597883.en, accessed on 2023.02.22.

Woltz, P.R., A. Stockey, M. Gondran and J. F. Cherrier. 1994. Interspecific parasitism in the gymnosperms: unpublished data on two endemic New Caledonian Podocarpaceae using scanning electron microscopy. Acta Bot. Gallica 141: 731-746.