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Agathis

Kauri, Dammar  R. A. Salisbury
Araucariaceae

 

Evergreen, often giant trees, or rarely shrubs, most commonly becoming emergent over surrounding species with age. Trunk cylindrical to slightly fluted, sometimes forked, often straight and with little taper throughout the massive bole, which may be free of branches for as little as 1-2 m or as much as 15 m or more. Bark fairly thin, remaining relatively smooth in most species, flaking continuously to produce varied patterns of pockmarks, rarely becoming ridged and furrowed with age (Agathis ovate), exuding when wounded a sparse to copious white, yellow, or reddish resin that dries brown. Crown dense, conical and regular in youth, with closely spaced crisscross pairs of horizontal to upwardly angled branches (or apparent whorls of four to six), becoming more open and irregular, and egg-shaped, dome-shaped, or flat-topped with age, with slender to stout, horizontal to upwardly angled branches curved up at the ends and bearing crisscross pairs of horizontal branchlets. Branchlets all elongate, without distinction into long and short shoots, remaining (various shades of) green for several years, hairless, scarcely marked by to prominently grooved between attached leaf bases. Resting buds well developed, more or less spherical, tightly wrapped by a few crisscross pairs of specialized but green and soft-textured bud scales. Leaves attached in crisscross pairs but arranged in two (sometimes untidy) flat rows on horizontal shoots by bending and twisting of the petioles and of the branchlets between the leaves, the two leaves in a pair sometimes somewhat displaced in position from each other along the twig. Leaves of seedlings and juvenile trees generally similar to those of adults in shape, structure, and arrangement. Leaf shapes quite variable within trees and species, not needlelike, broad, flattened top to bottom, and variously egg-shaped or elliptical, with pointed to rounded tips and rounded to wedge-shaped bases on a wide and thick, short, flattened petiole.

Plants monoecious. Pollen cones single on short stalks in the axils of otherwise ordinary foliage leaves. Each pollen cone more or less cylindrical, with (none or) two to six (to eight) crisscross pairs of spoon-shaped bracts at the base (the lowest pairs sometimes leaflike) and with numerous (often hundreds of), spirally arranged pollen scales. Each scale with 2-8(-14) elongate pollen sacs attached to the lower edge of the rounded or faceted external face of the scale and extending inward parallel to the scale stalk. Pollen grains medium to large (40-65 µm in diameter), nearly spherical, without air bladders or apparent germination structures, minutely and irregularly roughened over the whole surface. Seed cones single and upright at the tips of specialized, thick, leafy reproductive shoots in the axils of foliage leaves, sometimes two or three of these in a near whorl around the supporting shoot. Each seed cone nearly spherical (hence the scientific name, Greek for “a ball of string”) to noticeably longer than wide or a little wider than long, with many (often a few hundred), tightly packed seed scales intimately united with their accompanying bracts, maturing in two seasons and shattering to release the seeds. Each seed scale (and bract) fan-shaped, thin, the exposed face somewhat thickened, horizontally elongately diamond-shaped, sometimes with a more or less well developed projecting triangular tip, the base with a notch or indentation on one or both sides, the whole scale shed at maturity. Seed one per scale, attached to a bump on, but not apparently embedded in, the upper surface of the scale near its middle, the micropyle pointed back toward the cone axis. Seed body roughly a flattened egg shape, with one very large wing (larger than the body) attached on one side in the upper half and a much reduced, rudimentary wing on the other. Resin canals absent but sometimes with individual resin parenchyma cells that may look like resin-filled tracheids. Cotyledons two, each with several parallel veins and resembling the foliage leaves. Chromosome base number x = 13.

Wood soft, light to moderately heavy, with nearly white to light brown sapwood more or less sharply contrasting with creamy, yellowish, or pinkish brown to rich reddish brown heartwood, commonly shiny or speckled, or both. Grain very even and fine to moderately coarse, with somewhat cryptic growth rings marked by a gradual transition to smaller, thicker-walled latewood tracheids.

Stomates confined to and almost completely covering the lower surface of the leaf, with various orientations within their many interrupted and somewhat irregular lines. Each stomate tucked beneath the four or five (to six) inner subsidiary cells and surrounded by a prominent, steep, nearly complete, (usually) sunken Florin ring. Without a single well-marked midvein and midrib but with numerous, closely spaced, parallel veins produced by repeated branching near the leaf base from two veins in the petiole. Veins in the leaf blade alternating with single resin canals (or vertical pairs in Agathis borneensis and Agathis ovata, at least), not noticeably raised above or beneath and not disrupting the nearly continuous single (or partially double or triple) palisade layer beneath the upper epidermis. Leaf tissue commonly with a variable density and distribution of compact and branched sclereids and with small strands of transfusion tissue paralleling and near the upper side of the veins.

Fifteen species distributed discontinuously from Sumatra (Indonesia), Malaya (Malaysia) and Luzon (Philippines) across northern and central Malesia and through Melanesia to southern Queensland (Australia), northernmost New Zealand, and Fiji.

Like some of the giant conifers of the northern hemisphere, for instance, Douglas fir (Pseudotsuga menziesii), redwood (Sequoia sempervirens), and giant sequoia (Sequoiadendron giganteum), kauris are often relatively fast growing. Combined with their tolerance of growing in single-species stands and the considerable value of the timber and resin that they yield, this has led to serious efforts at plantation culture. Natural stands in many regions were severely depleted of old-growth kauris. As well, the commercial exploitation of dammar resins by wounding of the thin bark damages the trees over time and leads to deformation and decline. The rapid growth of kauri trees allows them to keep up with hardwoods in young stands, but their great dominance is achieved primarily through longevity. They outlast their original hardwood neighbors and one or more successor generations, growing all the while. Despite the superior quality of such old-growth trees for timber production, plantations can yield commercially viable harvest within reasonable rotation times, especially in those tropical regions where there are few other conifers suitable for plantation culture. Some species are cultivated for ornament in the tropics and warm temperate regions, but there has been no cultivar selection for horticultural purposes in the genus.

Considering the substantial economic importance of kauri trees, both as sources of timber and of commercial resins, there is surprisingly little consensus on an appropriate classification of the species and of their relationships within the genus. The two most recent overall treatments of Agathis differ one-and-a-half-fold in the number of species recognized (13 versus 21), and their groupings of species display only a little overlap. Two less recent formal treatments for only a portion of the range of the genus recognize additional species and accept conflicting assignments to the same number (three) of groups as the more recent treatments. On the other had, two more recent studies of epidermal structures delineate yet another grouping of species or none at all. The few DNA studies conducted as of 2007 include only a fraction of the known morphological and geographic variation and do not identify any convincing taxonomic structure within the genus. One of the problems is that most of the species, no matter how many one cares to recognize, from an intergrading mass while those with any really distinctive features, like the faceted bosses of the pollen scales in Agathis labillardieri and Agathis microstachya or the prominent triangular projections of the seed scales of Agathis australis, Agathis endertii and Agathis ovate, from groupings of too few species to be of much practical significance (other than facilitating identification).

Taking up the issue of how many species to recognize, one problem is that, while the upper montane populations in western Malesia differ collectively from the surrounding lowland and lower montane populations, they also differ to varying but lesser degrees from each other and from the upper montane populations of Sulawesi, the Philippines, and the Moluccas. Most are treated here as belonging to the same species as the north-central Malesian dammars, but other taxonomists separate several into their own, more local species. Likewise, the number of species occurring in Melanesia and the relationships to one another are unsettled. For example, the five species accepted for New Caledonia in most contemporary literature are usually considered to be endemic, and relationships beyond the island have not been entertained. Yet the trees usually called Agathis corbassonii are very similar to those of Agathis macrophylla in Vanuatu, little more than 250 km away, just about the length of New Caledonia itself. Since they also have a similar ecology, rather different from that of other New Caledonian species of Agathis, the New Caledonian trees could well be treated as being part of Agathis macrophylla, the earliest name for the combined species.

The whole genus is rife with such uncertainties, in large part because of both complications due to political fragmentation of the region and the difficulties of obtaining good specimens from these towering trees with both pollen and seed cones linked to substantial foliage samples. The same difficulties impede the assembly of comprehensive samples for DNA studies. The known fossil record of Agathis is too sparse to be of much help and is confined to Australia and New Zealand. While a few Mesozoic fossils were assigned to the genus, these lack fully diagnostic features that would confirm their identity. Instead, the earliest secure specimens date from the mid-Eocene of southern Australia, about 40 million years ago. The pollen is not readily separable from that of some araucarias and has contributed little to understanding the history and distribution of the genus, except during the last 12,000 years, for which period the progressive restriction of New Zealand kauri (Agathis australis) to the northern tip of North Island has been documented.

 

References

  • Farjon, A. (2010). A Handbook of the World's Conifers. Koninklijke Brill, Leiden.
  • Eckenwalder, J.E. (2009) Conifers of the World: The Complete Reference. Timber Press, Portland.
  • IUCN Red List of Threatened Species, International Union for Conservation of Nature and Natural Resources. Cambridge, UK /Gland, Switzerland

Copyright © Aljos Farjon, James E. Eckenwalder, IUCN, Conifers Garden. All rights reserved.


 

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