Coniferales

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Coniferales

Distribution

Antarctic: present Asia-Temperate:, Taiwan (Taiwanpresent) Asia-Tropical: Malesian tropics: present continental part of Asia: present eastern half of Malesia: present elsewhere in the world: present western Malesia: present
With 12 genera the Coniferales are well represented in the Malesian tropics, while elsewhere in the world only in the rich Sino-Japanese flora is there a substantially larger concentration of conifer genera; nearby Taiwan for example has 15 genera (only four of which are shared with Malesia, viz. Nageia, Podocarpus, Taxus, and Pinus). Eight wide ranging conifer genera of Antarctic affinities are today well established throughout Malesia up to the westernmost margins and four of these extend well beyond onto the continental part of Asia (Dacrycarpus, Dacrydium, Nageia, and Podocarpus). All are strictly confined to rain-forest habitats.
Seven of these have seeds dispersed by birds, the eighth (Agathis) has small seeds with large membraneous wings. Except for the genus Phyllocladus, these genera are well represented at low and medium elevations and it is not necessary to imagine long-range dispersal between isolated mountain peaks as far as altitude is concerned. We have, however, to keep in mind that through the insular physiography of Malesia there may have been the necessity of crossing sea barriers. Unfortunately too little is known in detail about the precise distribution of land and sea in the course of the Tertiary.
Two Holarctic conifer genera (Taxus, Pinus) penetrate into western Malesia and two more Antarctic genera are at present confined to the eastern half of Malesia (Libocedrus, Araucaria). The fossil record, although incomplete, suggests that the situation was quite different as late as the Miocene.

Ecology


Anatomy

The most important wood anatomical surveys of Coniferales are by , and by . Additional data for Malesia and adjacent regions can be found in ;

Despite the seemingly homogeneous microscopic structure of the vesselless wood of conifers, there are a number of highly diagnostic wood anatomical differences at various levels of the taxo-nomic hierarchy which can be profitably used for identification and which can help in the reconstruction of a natural classification. In the Malesian representatives of the Coniferales a number of genera can be immediately recognized on unique, single or combined characters:
  • Pinus — Vertical and horizontal resin ducts present, cross field pits (i.e., pits from tracheids to ray parenchyma cells) fenestriform or pinoid, ray tracheids present. (N.B.: resin ducts and ray tracheids are absent from all other Malesian Coniferales.)
  • Agathis and Araucaria — Pits on tracheids alternate and in a closely spaced honeycomb-like pattern ('araucaroid')
  • Taxus — Tracheids with distinct spiral thickenings.
  • Libocedrus — Cross field pits strictly cupressoid (i.e., with narrow, included apertures).
  • Podocarpaceae — The distinction of Podocarpaceae from Libocedrus is fairly subtle: cross field pits in Podocarpaceae often include cupressoid types but almost invariably also other types such as taxodioid, pinoid, or piceoid pits. Most Malesian Podocarpaceae and Libocedrus have fairly common to abundant axial parenchyma in their wood, a feature absent from the other conifers. However, Phyllocladus and at least some temperate species of Dacrydium lack axial parenchyma. Presence or absence of parenchyma has been used as an important character for sectional delimitation in Podocarpus sensu lato by KAEISER, l.c., but some of her observations have been contradicted in a more detailed study by . The evidence available from the literature at present suggests that the wood anatomical variation pattern within the Podocarpaceae does not coincide with generic delimitation; further studies of well-authenticated samples are needed to assess the taxonomic significance of the wood anatomical variation in this family.


Leaf anatomy can also play a useful role in identification and classification of the Coniferales, as exemplified in the study by on the systematics of Podocarpus sensu lato and by , also including the two anatomically distinct Malesian species Pinus merkusii and P. kesiya. The extensive leaf anatomical literature on conifers is summarized in . — P. BAAS.

Taxonomy

In spite of generalized impressions sometimes advanced about the decline and decrease of the Gymnosperms through the enormous development of the Angiosperms in the Cretaceous and their rapidly accelerated development in the Tertiary, it must be realized that this impression is confusing as far as Coniferales are concerned.

It is of course a truism that the Gymnosperms are completely outnumbered in genera and species by the Angiosperms, the latter occupying terrain earlier beset by Gymnosperms. It must be realized, however, that possibly the almost entirely woody Gymnosperms did never have the potential for producing such immense numbers of genera and species as now found among the Angiosperms. This statement is also valid for the Coniferales.

The Coniferales were only part of the Cretaceous richness in Gymnosperms and whereas many Gymnosperm groups became extinct or lived on with meagre remains, Coniferales — though proportionally with few genera and few species — still represent a most essential part of the world's standing timber and involve a huge biomass through their sociability and their morphology: usually a large size and the little tapering of their cylindric boles.

The 'decline idea' is thus not valid for the Coniferales and this is further validated by their extremely wide ecological capacity, as they thrive from the Arctic to the Antarctic, in all major parts of the globe, in the lowland, the hills and the mountains, and in the tropics from the seashore almost to the alpine zone, a colossal range, among the Angiosperms shared or approached by only very few families, e.g. Ericaceae and Fagaceae.

This universal presence is also due to their most diverse ecological capacities. Coniferales are represented on the permafrost of the taiga as well as in hot semi-deserts, on all sorts of soils, from mineral-rich to mineral-poor, even in peat-swamps, enabling them to stand all sorts of environmental conditions.

In the forest vegetation they show not seldom a high power of competition, often leading to dominance or codominance, often coupled with longevity.

Some are agressive and tend to fill gaps in the vegetation by possessing nomad ecology.

Seed is mostly produced in ample quantity; seedlings may be shade-tolerant or -intolerant. A number of species are distinctly fire-resistant.

From these facts can be concluded that Coniferales are not just 'on the decline', but that they form still a most successful super-order of the Gymnosperms.

Notes

A remark must be made about the references in the synonymy of the species. I have omitted in many cases the mention of names without description or notes which occur in so many local plant lists and casual enumerations. This was made especially urgent by the fact that the names used in these lists, e.g. of Agathis and Podocarpus, are often wrong according to my classification. To account for all these 'non ' or 'sensu ' names would have caused an unnecessarily complicated synonymy. If collectors' numbers were cited in these local lists, proper identity of these records can easily be checked by means of the , which was issued separately by the Rijksherbarium, Leiden.

Phytochemo

Chemical characters of Coniferales were summarized twice in 'Chemotaxonomie der Pflanzen' (). Here rather extensive bibliographies can be found for all families of Gymnospermae, Cycadopsida, Coniferopsida, Taxopsida and Chlamydospermae.

General characters of Coniferales are: cuticular waxes of the so-called estolide-type; lignin which usually lacks the syringyl component; seeds which store predominantly starch or oils with unusual fatty acids, i.e. bi-tetra-unsaturated C18- and C20-acids with an isolated double bond in 5-position; accumulation of cyclitols such as pinitol, sequoyitol and (or) 0-methylmucoinositol in leaves, bark and wood; storage of shikimic and (or) quinic acid in leaves; accumulation of lignans (phenylpropanoid dimers) and (or) agatharesinol-type norlignans in wood, bark, traumatic resins and leaves (here sometimes as glycosides); production and exudation after injury of oleo-resins or gum-resins.

Oleo-resins and gum-resins are deposited in schizogenic canals and cavities which seem to be lacking only in some representatives of Taxaceae. Oleo-resins are mixtures of essential oils and resins; turpentine is the essential oil produced by distillation of oleo-resins obtained from several species of Pinus. Gum-resins are mixtures of essential oil, resin and mucilage; Araucaria is the main producer of gum-resins among Coniferales. The predominant constituents of the essential oils of most Coniferales are mono- and sesquiterpenoids; some members of Podocarpaceae and other families produce appreciable amounts of steam-volatile diterpene hydrocarbons and hence yield diterpene-rich essential oils. The resins of Coniferales are mainly composed of diterpenoids; often diterpenic acids predominate. Colophony or rosin is the resin part of pine oleo-resins and Manila copal is the hard oleo-resin from Agathis dammara. Amber or succinite is the fossil resin of pines and Kauri copal is fossilized Agathis resin.

Tannins are ubiquitous in Coniferales. They are represented in the taxon exclusively by the so-called condensed tannins and their building stones, the catechins and proanthocyanidins. Galli-and ellagitannins are totally lacking.

Other classes of compounds which seem to be totally absent from Coniferales are iridoid compounds, cardenolides and steroidal saponins. The same seems to be true of triterpenoids of the ursane, oleanane and lupane classes, and hence of corresponding saponins.

Triterpenoids are represented in Coniferales by lanostane-type tetracyclic, onocerane-type tetra- and pentacyclic, and hopane-type pentacyclic compounds. Steroids are represented by the ubiquitous phytosterols; moreover, the frequent occurrence of phytoecdysones in rather high concentrations is somewhat typical of the taxon. In the products of steroid and triterpenoid metabolism Coniferales strongly resemble Pteridophytes.

Polyphenolic compounds other than lignans and tannins are accumulated by all conifers but, besides the general occurrence of flavonoids as a group, most classes of compounds and many individual compounds are restricted to taxa of lower levels such as infrageneric, generic and suprageneric entities. The same is true of alkaloids and several other classes of chemical constituents. Some examples to illustrate the situation follow.

Agathisflavone-, amentoflavone-, cupressuflavone-, robustaflavone- and hinokiflavone-type biflavones seem to be nearly ubiquitous in leaves of Gymnosperms, but are lacking in Pinaceae which yielded hitherto only one biflavonoid, the flavone-flavonol dimer abiesin.

Cephalotaxin-type alkaloids occur in all species of Cephalotaxus.

All members of the genus Taxus (but not the other representatives of Taxaceae) produce taxane-type diterpenoids which are often esterified with the so-called Wintersteiner acid, which is a dimethylamino derivative of a hydroxydihydrocinnamic acid; the resulting nitrogen-containing constituents, such as the taxines and related compounds, are the 'Taxus alkaloids'; they are accompanied in Taxus by the cyanogenic glucoside taxiphyllin. The latter too seems not to occur in other genera of Taxaceae, but is present in Metasequoia and in some species of Juniperus.

In Podocarpaceae several tendencies concerning secondary metabolism are recognizable: essential oils with appreciable amounts of diterpene hydrocarbons, ferruginol- and totarol-type phenolic diterpenes, bitter and biologically highly active mono- and bisnorditerpenoid lactones such as nagilactone, and accumulation of large amounts of phytoecdysones such as the maki-strones and the podecdysones are examples of such family-characteristic tendencies. At the same time the family is the only representative of conifers which makes use of anthocyanins to advert its diaspores: red to pink fleshy parts of Dacrydium, Phyllocladus, and Podocarpus diaspores contain an array of anthocyanins; anthocyanins may also be present in young leaves and strobili; the latter feature is not restricted to Podocarpaceae, however.

Antibiotically active carvacrol and thymol derivatives and tropolone-type mono- and sesquiter-penic compounds are present in the wood of many Cupressaceae, including Libocedrus s.l.

Exudates of many species of Araucaria contain larger amounts of mucilage than most other conifers; they are true gum-resins; ANDERSON and MUNRO observed 20-80% of mucilage in Araucaria-exudates with 10-20% of uronic acids, 50-70% galactose and up to 7% of the rather unusual sugar acofriose (3-0-methylrhamnose) as building stones. Acofriose is also present in mucilages of Cycadaceae.

An array of low-molecular phenolic compounds, such as hydroxyacetophenones, stilbenes, di-hydrostilbenes and phenylpropanoids (monolignols) is known from Pinaceae; they occur free and as glycosides and often have a taxon-characteristic distribution, and hence can be useful as taxo-nomic characters. Pinosylvin and its monomethyl ether have been interpreted as phytoalexins of Pinus because their synthesis is induced in the softwood after infection; normally these antifungal compounds are present in Pinus only in hardwoods and in barks.

Flavonoid patterns were taxonomically exploited by many phytochemists; flavonoids yielded characters applicable at all levels of the taxonomic hierarchy. Just one example: C-glycoflavones have not yet been traced in Araucariaceae, Cephalotaxaceae, Cupressaceae and Taxaceae, and seem to be restricted in Pinaceae to Abies, Keteleeria, Tsuga and Larix; moreover, they were detected in Podocarpaceae in some species of Podocarpus.

In recent times detailed analyses of essential oils were performed during biosystematic studies of a number of American conifers; the results proved to be rather promising; in many instances a better understanding of complex population structures was made possible by such investigations.

As a whole Coniferales are chemically well characterized by the general presence of several classes of chemical constituents and by the total absence of others. Moreover, secondary metabolites yielded a large number of characters applicable at different levels of the taxonomic hierarchy. — R. HEGNAUER.