Scribbly gum

Scribbly gum

What are scribbly gums?

The scribbly gum is a tree species that is endemic to the Sydney region. This means that it is only found in the Sydney region and nowhere else.

Scribbly gums can be easily spotted due to their white or silvery grey bark with prominent scribbles. Their leaves have a silvery grey appearance from afar. When blossoming, scribbly gums can also be identified by their clusters of cream-coloured flowers. Scribbly gums grow up to 15 m high.

The scientific name for the scribbly gum is Eucalyptus haemastoma. This comes from the Greek words haima, meaning blood, and stoma, meaning mouth, referring to the reddish disc of the fruit.

How are the scribbles formed?

The scribbles found on the bark of the scribbly gum are the feeding trails of scribbly gum moth 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?

Scribbly gums grow in dry woodlands on shallow sandy soils which have been formed by underlying sandstone rock. 

These woodlands are found across the Sydney region from Lake Macquarie in the north, to Royal National Park in the south.

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 they grow in. The leaves and flowers on the other hand are an important food source for a range of 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 a range of 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.

As part of the Australian landscape, scribbly gums are also well adapted to fire. Their thick bark and woody capsules to protect themselves and 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. 

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 the habitats they grow in, and by planting more through bush regeneration efforts.

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