Brachychiton (PROSEA)
Introduction |
Brachychiton Schott & Endl.
- Protologue: Melet. Bot.: 34 (1832).
- Family: Sterculiaceae
- Chromosome number: x= 20; for all 16 Australian species examined: 2n= 40
Vernacular names
- Brachychiton, flame tree (En).
Origin and geographic distribution
Brachychiton comprises about 30 species and is essentially Australian, but 2 species occur in Papua New Guinea, one of which is endemic. Many species have very restricted areas of distribution.
Uses
In Australia the wood of several Brachychiton species (e.g. B. acerifolius (Cunn. ex G. Don) MacArthur and B. discolor F. v. Mueller) is applied where a very light wood is advantageous, e.g. for boxes, sporting goods, blockboard, matches, toys and novelties. It is also used for the production of wood-wool board.
In Australia some species are much valued as fodder plants, particularly B. populneus (Schott & Endl.) R. Br. and B. rupestris (Mitchell ex Lindley) K. Schumann. These and other species are also planted as ornamental and shade trees.
Production and international trade
There are no records of trade of Brachychiton from the Malesian area; in Australia it is sold sporadically, and has been used by the plywood industry for core stock.
Properties
Brachychiton yields a lightweight hardwood with a density of 275-400 kg/m3at 15% moisture content. Heartwood and sapwood white to cream; grain of the Australian B. discolor fairly straight, its texture coarse and its wood lustrous with mottled appearance. Growth rings indistinct; vessels medium-sized to large, solitary and in radial multiples of 2-4, tyloses sparse but often with gum-like deposits; parenchyma abundant, apotracheal diffuse to diffuse-in-aggregates, paratracheal vasicentric to sometimes aliform, or in wide irregularly spaced apotracheal bands; rays medium-sized to broad, conspicuous on radial surface; faint ripple marks due to storied parenchyma and vessels, visible with a hand lens.
The wood of the Australian B. acerifolius and B. discolor needs slow and careful seasoning and preliminary drying in the log before sawing. The wood is weak and very soft, and is easy to work with all tools, but cannot be planed to a smooth surface. The wood is non-durable. The sapwood is susceptible to Lyctus . The sapwood is permeable, the heartwood is moderately resistant to impregnation. The wood is highly susceptible to blue stain and sap-stain.
See also the tables on microscopic wood anatomy and wood properties.
Botany
Deciduous, monoecious shrubs or small to medium-sized trees up to 35 m tall; bole cylindrical, sometimes slightly swollen towards the base, branchless for up to 15 m, up to 75 cm in diameter; bark surface finely fissured, grey or brown. Leaves alternate, simple, shallowly 3(-5)-lobed, hairy below, petiolate; stipules caducous. Flowers in an axillary or cauliflorous, paniculate inflorescence, unisexual, pinkish-red to brownish-red; perianth campanulate, 5-lobed; androgynophore present. Male flower with 15-30 stamens and rudimentary ovaries. Female flower with 5 free superior ovaries with many ovules, 5 styles and stigmas and up to 30 staminodes. Fruiting head consisting of up to 5 woody follicles dehiscing along the inner suture, with brown stellate hairs outside, many-seeded. Seed loosely enclosed by a hirsute exotesta, initially covered with a yellow or orange pulpy layer, endosperm abundant, cotyledons flat and thin. Seedling with the cotyledons emergent or not.
Leaves of young trees have more lobes and are more deeply lobed than those on mature trees described above. Trees are deciduous during the dry season. In Papua New Guinea B. carruthersii is deciduous from June to October, flowers from January to May and fruits ripen in 3-4 months; B. velutinosus flowers from May to September.
Brachychiton has previously been included in Sterculia . The characteristics of the fruit, seed-coat and embryo, however, strongly support the retention of Brachychiton as a separate genus. Ripe fruits of Brachychiton are brown to black, whereas those of Sterculia are pink, scarlet or vermillion. Brachychiton seeds abscise before the fruit dehisces and, unlike Sterculia , do not remain attached to the pericarp by their funicle.
Ecology
In Papua New Guinea Brachychiton is mainly found in coastal lowland and on river flats, but also occurs on slopes and ridge crests up to 500 m altitude. The soils range from fine sands to friable clays. It is often found in thickets and semi-deciduous, liana-rich forest together with Adenanthera , Alstonia , Bombax , Buchanania , Cleistanthus , Clerodendrum , Diospyros , Erythrina , Ficus , Gardenia , Garuga , Gyrocarpus , Harpullia or Pandanus species.
Genetic resources and breeding
Both B. carruthersii and B. velutinosus have restricted areas of distribution and seem to be at risk of genetic erosion.
Prospects
The lightweight Brachychiton wood has potential for special applications, but limited supplies restrict its use.
Literature
125, 128, 173, 214, 304, 348, 393, 536, 1037, 1185, 1186, 1187.