Eucalyptus microtheca F.Muell., J. Proc. Linn. Soc., Bot. 3: 87 (1859).
E. barklyensis L.A.S.Johnson & K.D.Hill, Telopea 5: 753 (1994). T: Northern Territory: 18 km from Alroy Downs homestead towards Alexandria Downs, 6 Aug. 1984, K.D.Hill, 1020, L.A.S.Johnson & D.Benson; holo: NSW; iso: CANB.
E. helenae L.A.S.Johnson & K.D.Hill, Telopea 5: 759 (1994). T: Northern Territory: T: Barkly Tableland: 26.2 km from Brunette Downs homestead on Rockhampton Downs Road, 5 Aug. 1984, K.D.Hill 1017, L.A.S.Johnson & D.Benson; holo: NSW; iso: CANB, DNA, PERTH.
Tree to 10 m tall, rarely more, or sometimes a mallee. Forming a lignotuber.
Bark rough, whitish grey to dark grey, box-type, often deeply fissured, coarsely flaky or becoming tessellated, the branches commonly rough-barked though this feature is variable.
Branchlets lacking oil glands in the pith; usually non-glaucous, rarely glaucous.
Juvenile growth (coppice or field seedlings to 50 cm): stems rounded to square in cross-section, usually non-glaucous; juvenile leaves always petiolate, opposite for a variable number of nodes then alternate, narrowly lanceolate, 5–15 cm long, (0.3)0.5–2.5 cm wide, apex rounded or pointed, bluish green to green.
Adult leaves alternate, petioles 0.6–2 cm long; blade lanceolate to falcate, 5–19.5 cm long, 0.6–3(4.5) cm wide, base tapering to petiole, apex pointed or rounded, margin entire, concolorous, dull, green to blue-green or rarely glaucous, side-veins acute or at a wider angle to the mid-rib, reticulation very dense, intramarginal vein present, oil glands sparse, intersectional.
Inflorescences terminal, peduncles slender, 0.2–0.9(1.2) cm long; buds in 7s but umbels are sometimes irregular due to short internode elongation within the cluster, pedicels 0.1–0.3 cm long. Mature buds pyriform to obovoid or globular, 0.3–0.4 cm long, 0.2–0.3 cm wide, non-glaucous or glaucous, scar present (outer operculum lost early), operculum rounded and apiculate, rarely conical, stamens irregularly flexed, all fertile, anthers adnate, cuboid, dehiscing by lateral slits, style long, straight, stigma blunt, locules 3 or 4, the placentae each with 4 vertical ovule rows. Flowers white.
Fruit on pedicel 0.1–0.3 cm long or, rarely, subsessile, truncate-globose to obconical, usually contracted at the top, 0.2–0.4 cm long, 0.3–0.7 cm wide, glaucous or non-glaucous, rim narrow, rarely flared, disc descending vertically, narrow, valves 3 or 4, near rim level or barely exserted.
Seeds dark brown, 1–2 mm long, flattened-ovoid, dorsal surface smooth, hilum ventral.
Cultivated seedling (measured at ca node 10): cotyledons reniform; stems rounded or square in cross-section; leaves always petiolate, opposite for ca 4 to 10 nodes then becoming alternate, lanceolate to narrowly so, 6–10.5 cm long, 0.5–1.5 cm wide, base tapering, apex rounded or pointed, blue-green to grey-green or green.
A tree of widespread distribution in northern Australia, found fringing seasonally dry watercourses, swamps, lakes and low-lying areas subject to temporary inundation. It occurs east from the Ord River in the Kimberley region of Western Australia through the Northern Territory north from about Newcastle Waters and including the Barkley Tableland (but see below), to the Roper River and Macarthur River, extending into the Gulf country of Queensland as far east as western Cape York and as far south as near Mount Isa and Chillagoe.Eucalyptus microtheca usually has rough bark over the whole trunk extending to the branches and a crown of dull green, grey-green or sometimes glaucous leaves and small fragile fruit with valves at rim level when dehisced.
Eucalyptus microtheca is most closely related to the widespread E. coolabah which is found in similar but drier habitats to the south and south-east, and to E. victrix which occurs in still drier habitats from central Australia west to the Pilbara. E. coolabah has rough bark on part or all of the trunk, having whitish smooth bark on the branches, and has conspicuously exserted valves in the otherwise shallower hemispherical-obconical fruit. E. victrix has smooth powdery white bark and similar fruit to E. microtheca with valves at rim level or slightly exserted. It is also closely related to E. tectifica, both species having rough bark to the small branches. E. tectifica and E. microtheca are separated by the shape and dimensions of their juvenile leaves, E. microtheca usually being narrowly lanceolate and E. tectifica normally ovate to broadly lanceolate. The fruit of E. tectifica also tend to be slightly wider than those of E. microtheca but some overlap in dimensions does occur. Another closely related member of this group is E. acroleuca from Cape York Peninsula. It differs by having basal rough bark with a fairly regular "cut-off" about halfway up the trunk and conspicuous smooth bark on the upper trunk and branches, and very small fruit (0.2–0.3 cm wide only) with enclosed valves.
Several localized forms of species
in this group have been described – E. cyanoclada, a blue-leaved more
or less glaucous
form occurring around Lake Woods and Newcastle Waters in the Northern Territory;
E. barklyensis with smooth smaller branches, thicker leaves and more
spreading form than E. microtheca, restricted to the Barkly Tableland
in the Northern Territory; and E. helenae, described from the
south-western Barkly Tableland, differing in having marginally larger fruit
with flared top a broader disc,
smooth branches and thicker and sometimes wider leaves than E. microtheca.
In EUCLID we take a broader view of E. microtheca, including E.
cyanoclada, E. barklyensis, E. helenae within it. The
taxonomy of the group is complicated by E. microtheca, E.
coolabah and E. victrix having
adjoining distributions and showing intergradation in form. To make reliable
identification collectors will need to note the extent of rough bark
and ensure collection of mature fruit.
Eucalyptus microtheca and related species
in late spring and early summer, quickly ripen their fruit
and shed seed
over the following few months. The fruit
are on slender pedicels
and are soon lost from the trees. Even with mature fruit
and the most careful observation of bark
and leaves there will still be many individuals, especially on the Barkly
Tableland, that don't fit neatly into any particular coolibah species.
Populations of coolibahs may contain individuals all of which display distally
(like E. microtheca, E. barklyensis and E. helenae) but
which vary in bark persistence, from having some scrappy basal rough bark
with the upper trunk and branches being smooth-barked, to being fully
rough-barked to the smaller branches, and with all possibilities in between.
Because of the variability in bark persistence and the seeming consistency
of "microtheca-like" fruit shape we have chosen to synonymize
E. barklyensis and E. helenae with E. microtheca rather
than E. coolabah. However it
would not surprise us if there were coolibah trees in this area that matched
E. coolabah in fruit
shape just to complicate the picture.
Eucalyptus microtheca belongs in Eucalyptus subgenus Symphyomyrtus section Adnataria (the boxes) because the buds have two opercula, ovules are in four rows, seeds are flattened-ovoid, cotyledons are reniform, and anthers are rigid on the staminal filaments. Within section Adnataria, E. microtheca is part of a subgroup of box species with mostly tropical distribution, series Aquilonares subseries Protrusae, having inflorescences terminal on the branchlets, adult leaves very densely reticulate and fruit that are small, fragile and have exserted valves.
Flowering has been recorded in September, November, December and January.
Origin of Name
Eucalyptus microtheca: from Greek, micros, meaning small, and thece, a box, referring to the small fruit.