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Tasmanian cedar, D.Don  1841

Athrotaxis - Tasmanian cedar description


Small to moderately large evergreen trees with thin, shallowly furrowed, fibrous bark peeling in long, thin strips on a straight or contorted trunk. Densely branched with stiff, ascending branches. Crown roundly conical at first, remaining so (Athrotaxis cupressoides) or becoming broadly dome- to ball-shaped with age (Athrotaxis selaginoides) and remaining dense where not broken up in the vicissitudes of time. Branchlets cylindrical, upright, branching from all sides or roughly two-ranked. Resting buds unspecialized, consisting solely of arrested immature foliage leaves. Leaves in a dense spiral, forwardly directed, overlapping, scalelike to broadly awl-shaped or clawlike, with a more or less well developed keel along the lower midline, tight against the twig or spreading.

Plants monoecious. Pollen cones numerous, single at the ends of ordinary branchlets, with 25-35 tightly spirally arranged pollen scales, each scale with two pollen sacs. Pollen grains small (25-30 µm in diameter) spherical, minutely bumpy and with a small germination papilla. Seed cones numerous, single at the ends of ordinary branchlets, maturing in a single season, with 15-25 spirally arranged seed scales. Bract intimately fused with and more massive than the seed-bearing portion, each scale with three to six inverted seeds. Seeds small, oval, with two wings derived from the seed coat extending the whole length of and as wide as or slightly wider than the body and giving an overall heart-shaped outline. Cotyledons two, each with one vein. Chromosome base number x = 11.

Wood soft and weak, fairly light, with yellowish sapwood sharply contrasting with pink to reddish brown heartwood. Grain very evenly and moderately fine, with strongly delimited growth rings marked by a gradual transition to flatter, thick-walled latewood tracheids. Resin canals absent but with sparse to moderate scattered individual resin-filled parenchyma cells, especially within or near the latewood.

Stomates irregularly oriented, confined to small patches at the base of the leaf underside and collected in two broad bands extending the length of the upper side. Each stomate sunken beneath the (four or) five or six (or seven) subsidiary cells, which are often shared between adjacent stomates and topped by a low Florin ring. Leaf cross section with a central midvein flanked by transfusion tissue extending out to the sides. Resin canal prominent, following the midvein beneath and sometimes flanked by additional, discontinuous canals or cavities. Photosynthetic tissue with a thin palisade layer on the lower (outer) surface and a mass of spongy mesophyll, both surrounded by a thin, single-layered hypodermis (except thicker at the leaf corners and absent beneath the stomates) directly beneath the epidermis.

Two species in Tasmania, Australia. (Athrotaxis x laxifolia, these trees are a hybrid of Athrotaxis cupressoides and Athrotaxis selaginoides). Neither of the living species is of particular economic importance, but both are greatly reduced in range within their nonagricultural montane habitats because they do not regenerate well after the fires that became more prevalent with European colonization since the 1800s. Both species, however, can produce new plants from root sprouts, an uncommon trait among conifers, and both have the potential for a long life span and are therefore useful in dendrochronological studies. While both species are cultivated to a limited extent in botanical gardens in wet, cool, temperate regions, they are not in general cultivation, and no cultivar selection has taken place. The name Athrotaxis, Greek for “crowed arrangement”, refers to the organs of the seed cones, but these are no more crowded than those of other conifer genera, including Cryptomeria, which David Don described at the same time as he described this genus.

Athrotaxis is the only living genus of southern hemisphere Cupressaceae with a spiral adult leaf arrangement. Based on DNA sequences it appears to be most closely related to the northern hemisphere Taiwania. These two genera fall between the basal Cunninghamia and all other genera of the family in molecular phylogenies. Locally common hybrids between the two species occur where their ranges overlap ecologically. These used to be treated as a third, intermediate species, but DNA studies confirm their hybrid origin.

Although restricted to Tasmania today, Tertiary fossils are also found in mainland eastern Australia, New Zealand, and perhaps in South America, providing a Tertiary distribution similar to that of Fitzroya or Libocedrus. The oldest fossils of the genus date from the Cretaceous. There are also numerous fossils in the northern hemisphere that are similar to Athrotaxis, particularly in the Cretaceous. These species belong to related extinct genera and link Athrotaxis to a Cunninghamia-like ancestor common to all extant Cupressaceae.




Attribution from: Conifers Garden