Eucalypts

From Tropical Topics newsletter, No. 71, December 2001, produced by Stella Martin at the Queensland Environmental Protection Agency. Download the PDF to read the whole issue.

Eucalypts, melaleucas and acacias dominate the dry tropical woodlands. The first two are particularly common in areas where there is a reliable annual wet season, eucalypts favouring the drier situations and melaleucas preferring those which are seasonally flooded. Acacias predominate further south, in arid areas where drought may last for years.

Ubiquitous eucalypts

Eucalypt trees are synonymous with Australia, although a number of species also grow on islands to the north where they would have spread at times of lower sea levels.

Buds

The name Eucalyptus comes from two Greek words, eu , which means 'well' and kalyptus which means 'covered'. This is a reference to the unusual flower buds which are covered with a little cap (or operculum) which is actually formed from the fused petals and sepals. When the cap comes off, the numerous stamens unfurl around the central pistil, or female part — eucalypt flowers have no petals as such.

Gum nuts

Eucalypt fruits are also distinctive. After fertilisation, the cup-like base of the flower dries, enlarges and becomes a woody fruit, the gum nut. The roof of the ovary splits, in most species, into hard woody valves, which might poke out in the form of points, or may sink below the rim, out of sight. The various shapes of these gum nuts, along with size and shape of the buds, can help with identification.

Common eucalypts

Stringybarks (Eucalyptus tetrodonta) and woollybutt (E. miniata) are the most widespread tree species in northern tropical woodlands, occurring over Cape York Peninsula, the Top End and the Kimberley. Although Eucalyptus tetrodonta (left) is often called the Darwin stringybark it is common right across northern Australia. Woodlands dominated by this species cover almost 37 percent of Cape York Peninsula. It can form dense patches. The stringybark has a long tap root and does best in deep, sandy soils. It can grow to 30m, but its height depends on the depth of the soil. The name tetrodonta is composed of the Greek words tetra (four) and odontos (teeth) refers to the four prominent projections below the cap of the bud which later form four 'teeth' on the seed capsule. To see a recent list of reseach findings on E. tetrodonta click here .

Wooly butt

Woollybutt flowers

Darwin woollybutt (E. miniata) also grows well beyond the Northern Territory and is commonly found associated with E. tetrodonta right across the northern woodlands. Growing to about 15-25m, this tree has earned its common name from the particularly spongy and fibrous bark which grows a quarter to half-way up the trunk. Above this, the bark is smooth and white. Flowers are a stunning orange with bright yellow-tipped stamens and are responsible for the scientific name: miniatus is the Latin word for 'flame'. The barrel-shaped, ribbed gum nuts, grow to 6cm in length and almost as wide. They are popular in flower arrangements. Native bees often nest in branches which have been hollowed out by termites, providing 'sugarbag' — a traditional source of honey. To see a recent list of research findings on Eucalyptus miniata click here .

Bark

Eucalypts are commonly referred to as gum trees, but these trees with typically smooth barks are only part of the story. Other eucalypts have quite different barks.

In gum trees , the bark, or outer, dead layer of tissue is shed annually, exposing the new, inner bark below. This is often highly coloured and may be covered with a white powder. In some species, old bark remains attached in dangling strips.

Rough-barked species, by contrast, retain the outer dead bark allowing it to accumulate year after year. As the girth of the trunk increases, this bark splits longitudinally. The different patterns created depend on the varying nature and lengths of the fibres composing the bark.

Stringybarks, Ironbarks and Bloodwood

Stringybarks (top right) have a thick, spongy, fibrous bark which can be pulled off in long strips.

Ironbarks (middle right) have a solid, thick, heavy bark with deep longitudinal furrows. It is often impregnated with sap (gum) from the tree, known as kino, which makes it very hard. It is generally very dark — grey or black.

Bloodwood (Corymbia sp) bark (bottom right) is often tessellated, or chequered. This is because the short fibres in the bark break into small plates which may be hard and woody, spongy or loose and flaky. The trunk may be patterned with different coloured plates. A blood-red sap (kino) leaks from wounds in the bark. To see a recent list of research findings on the Corymbia species click here.

Box eucalypts have a firm short-fibred bark with narrow longitudinal lines or tessellations. It is relatively thin and may wear off, leaving a mottled appearance. These trees are named after the European box trees due to the similar pattern of interlocking grains in the timber.

Separated eucalypts

In 1995 all the bloodwoods were separated from the Eucalyptus genus into their own genus, Corymbia . There are now 550 recognised Eucalyptus species and 130 species of Corymbia. Bloodwoods are better represented in northern Australia than in the south. They are present, and sometimes dominant, in the northern eucalypt woodlands.

Deciduous eucalypts

Although they are known as evergreen trees, in northern Australia a number of eucalypt species respond to the seasonally dry conditions by dropping their leaves. This is unusual in eucalypts. However, it enables certain eucalypt species to compete effectively in shallow soils. On deeper soils evergreen species dominate. The degree to which a tree loses its leaves may be related to the situation in which it grows. For instance, poplar gum (E. platyphylla), found in eastern Queensland, may drop all its leaves in a dry situation but just some of them in a well-watered position.