Scribbly gum

Scribbly gum

What is a scribbly gum?

Scribbly gums are a type of eucalypt tree known for their white or silvery grey bark covered in scribbles.

They can also be spotted by their low, stretching growth habit and leaves which appear silvery grey from afar. When blossoming, scribbly gums can also be identified by their clusters of cream-coloured flowers. Scribbly gums are a low to medium tree, sometimes with more than one trunk, and can grow up to 15 m high.

The scientific name for the scribbly gum that grows around the Sydney region is Eucalyptus haemastoma. This comes from the Greek words haima, meaning blood, and stoma, meaning mouth, referring to the reddish disc of the fruit. It is one of three species of scribbly gums.

Local Darug Peoples of the Sydney basin refer to eucalypt trees as yarra.

How are the scribbles formed?

The scribbles found on the bark of the scribbly gum are the feeding trails of scribbly gum moth larvae. Each scribble is the feeding trail of one larvae.

In late autumn, an adult moth will lay its eggs between layers of old and new bark of the scribbly gum tree. When the moth larvae hatch, they must live in the bark through the winter, feeding on the bark tissue as they move around. As old bark sheds off the tree, these feeding burrows are revealed and what we see as scribbles. In early summer, the larvae come to the surface of the bark and pupate in cracks of the bark or down in the leaf litter. By the next autumn, the new adult moth has hatched from its cocoon.

Although it has been known since the 1930s that the scribbles are the work of the scribbly gum moth, it was only in 2012 that scientists were able to track the growth of the scribbly gum moth larvae and see how changes in its development related to specific patterns in the scribbles they made. The 2012 study also revealed that there are actually 14 species of moth that make these scribbles, as opposed to what was previously thought to be just one species - Ogmograptis scribula.

Where are scribbly gums found?

In the Sydney region, scribbly gums grow in dry woodlands on shallow sandy soils which have been formed by underlying sandstone rock. They are most often found on ridgetops.

The scribbly gum which grows in the Sydney region is Eucalyptus haemastoma. It grows from Royal National Park in the south, north to Lake Macquarie and west to the Blue Mountains. 

Whilst there is some cross-over of species, other species of scribbly gums are found in NSW in places such as Jervis Bay National Park on the south coast, Koonyum Range in Mount Jerusalem National Park in the northern rivers and Wambool Nature Reserve near Bathurst in the central west.

How are scribbly gums used by Aboriginal Peoples?

Eucalypt timber is hard and suitable for making tools, weapons and vessels. Carrying vessels such as coolamons can be cut from the trunk of scribbly gum trees. The cut is usually made on the south side of the tree to minimise stress to the tree. Scar trees provide evidence of where coolamons have been cut in the past.

What eats scribbly gums?

The blossoms of scribbly gums attract many native animals such as possums, grey-headed flying foxes, birds, and bees, who come to feed on the nectar.

What role do scribbly gums play in the ecosystem?

Each part of the scribbly gum tree plays an important role in the ecosystem. For example, the roots of the scribbly gum help control erosion of the loose sandy soils in which they grow. The leaves and flowers are an important food source for a range of invertebrates, mammals and birds.

Mature scribbly gums are especially important as they provide critical habitat in the form of tree hollows and hollow logs. Hollows are used as shelter by native mammals such as possums and birds such as owls and parrots. Scribbly gums can live for well over 100 years so it is important to protect them to provide a long lasting source of hollows.

What adaptations do scribbly gums have?

Scribbly gums are a type of 'sclerophyll' plant meaning they are well adapted for growth in dry conditions and poor soils. Scribbly gums have hard leaves which do not require a lot of water to grow. Their leaves are covered in a thick waxy layer which reduces the amount of water that is lost through the surface of their leaves.  Mature leaves hang vertically, limiting their exposure to direct sunlight and reducing the amount of water lost due to heat.

Inside the leaves of the scribbly gum are oil glands which produce a toxic oil that makes most animals sick when eaten. It is important to ward off herbivores as replacing eaten leaves is difficult when there isn’t much water or nutrients available in the environment. Humans make use of the toxic properties of eucalyptus oil in a range of household products such as surface cleaners or antiseptic solutions, although not specifically distilled from scribbly gum leaves.

As part of the Australian landscape, scribbly gums are also well adapted to fire. Their pale bark reflects heat. The woody capsules protect their seeds from becoming heat damaged.

After fire, leaves resprout from special ‘back up’ buds called 'epicormic buds' found beneath the bark of the trunk. As well as this, the scribbly gum drops its seeds to the ground after fire events so that they can germinate in ash-rich soil that is clear from other plants which would otherwise compete for sunlight and water. 

Scribbly gums can survive large sections of their trunks being burnt and are often seen in bushland with black hollow sections that have been burnt out. The tree continues to survive and thrive using its unburnt section of trunk for support and to obtain nutrients.

What threats do scribbly gums face?

One of the greatest threats to scribbly gums is land clearing as they tend to grow in areas like ridgetops which are preferred for housing developments and roads.

Climate change is also a potential threat for the ongoing survival of scribbly gums due to changes in temperatures and rainfall patterns.

We can help scribbly gum populations by protecting their habitats and by planting more through bush regeneration efforts.

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