Grass trees

Last updated: Friday 27 May, 2016

(Xanthorrhoea) 

Grass trees epitomize the Australian bush: they’re beautiful, ancient, hardy, thrive in nutrient-poor soils and respond to wildfire by flowering profusely. They're iconic plants, recognisable even to budding  botanists. 

<em>Xanthorroea australis</em> amongst heath at Friendly Beaches Reserve (Tas). Photo Wayne Lawler/EcoPix.
Xanthorroea australis amongst heath at Friendly Beaches Reserve (Tas). Photo Wayne Lawler/EcoPix.
All 66  species of grass tree are endemic to (only found in) Australia. They're all perennial, flowering plants. The smallest species grows to about 1m, others reach 6m tall. Most species are extremely slow growing. Studies of some of the taller species found that trunk height increases at about 0.8cm to 6cm per year, but this varies with local environmental conditions1. In any event, grass trees are often very long-lived; some are estimated to be 350 to 450 years old!2

Xanthorrhoea are monocots (meaning they only have one cotyledon, which is the leaf attached to the embryo within the seed). Some form a ‘trunk’ from old leaf bases stacked on top of each other and stuck together by a naturally occurring resin. Some species have a branched trunk, others, like X. gracilis, don't form an above-ground trunk at all.

<em>Xanthorroea australis</em> at Friendly Beaches Reserve (Tas). Photo Wayne Lawler/EcoPix.
Xanthorroea australis at Friendly Beaches Reserve (Tas). Photo Wayne Lawler/EcoPix.
Grass tree leaves are narrow, linear and stiff. Where present, old leaves often hang down and form a ‘skirt’ around the base of the trunk. The length of the skirt is a good indication of the time since the last fire – the longer the skirt, the longer the duration without fire.

Grass trees are even more interesting below the soil surface! They have a root system, where microbes called mycorrhiza surround the roots in a symbiotic relationship, which helps the plant take up nutrients.

Grass trees at JC Griffin Reserve in Victoria. Photo Matthew Newton.
Grass trees at JC Griffin Reserve in Victoria. Photo Matthew Newton.
Many species have an amazing ability to survive fire. A fire may burn their leaves and blacken their trunk, but the tree usually survives: the living growth-point is buried underground, protected by tightly packed leaf bases. In fact, some grass trees are stimulated by fire – in the spring after a summer bushfire, large numbers of plants can flower. However, not all species are fire-tolerant: the threatened Grey Grass Tree (X. glauca angustifolia), which is found on JC Griffin Reserve in Victoria, is killed by hot fires and needs protection in the fire-prone landscapes in which it occurs.

Grass trees may take several years to flower. Flowers form on a spear-like spike which can be up to 4m long! Flowers are arranged in a spiral up the spike, and produce a great amount of nectar, attracting a wide variety of insects, birds and mammals. Once pollinated, the flowers form a tough, pointed fruit capsule that’s typically matt-black. One flowering stem of the X. johnsonii grass tree can produce up to 10,000 seeds!3 

Lorikeets enjoying flower spikes at Carnarvon Reserve (Qld). Photo Wayne Lawler/EcoPix.
Lorikeets enjoying flower spikes at Carnarvon Reserve (Qld). Photo Wayne Lawler/EcoPix.
Xanthorrhoea comes from the Greek xanthos, which means ‘to flow’ and refers to the yellow gum or resin that flows from the stem. The grass tree is important to Aboriginal people across Australia. This resin is traditionally used as glue in spear-making and in patching up water containers. Flower spikes make fishing spear shafts and firesticks; the tough seed pods are used as cutting implements; and the flower’s nectar forms a sweet, slightly fermented drink. Grass trees are also known as 'yacca', which is likely derived from a South Australian Aboriginal language, mostly likely Kaurna. 

Where do grass trees live?

Grass trees are found in all Australian states and territories, especially on the east and west coast. The Austral Grass Tree (X. australis) is one of the most widely distributed species, growing in Tasmania, Victoria, New South Wales, Australian Capital Territory and South Australia.

Xanthorrhoea species are yet to be assessed by the IUCN Red List of Threatened Species, but under the Environment Protection and Biodiversity Conservation Act 1999, the Shiny Grass Tree (X. bracteates) is Endangered and the Sand Grass Tree (X. arenaria) is considered Vulnerable to extinction4. Both are found in north east Tasmania. The Grey Grass Tree (X. glauca angustifolia), found on JC Griffin Reserve, is listed as Threatened under the Flora and Fauna Guarantee Act in Victoria.

Threats

A grass tree experiencing die back from a pathogen. Photo Wayne Lawler/EcoPix.
A grass tree experiencing die back from a pathogen. Photo Wayne Lawler/EcoPix.
Populations of many species of grass tree  have been devastated by Phytophthora cinnomonia, or Cinnamon Fungus. This fungus rots the roots of grass trees and causes dieback. The disease caused by this introduced plant root pathogen is often difficult to detect, and, more worryingly, its impact may be considerable before detected. In Australia Cinnamon Fungus infection threatens several plants with extinction.5

Further threats include: land clearance for agriculture and urban development; inappropriate burning; overharvesting of foliage and illegal removal of the plants from their natural habitat for sale as garden plants (the species has very sensitive roots and the chance of grass trees growing after translocation is fraught).6 

What's Bush Heritage doing?

We have grass trees on Chingarrup, Kojonup, Monjebup/Monjebup North and Yarabee (WA); Carnarvon, Reedy Creek and Yourka (Qld); Burrin Burrin and Tarcutta (NSW); JC Griffin Reserve in Victoria; and, in Tasmania, on South Esk and Friendly Beaches, where the vulnerable Sand Grass Tree is found.

We’re protecting grass trees by halting land clearing, managing fire regimes, and managing total grazing pressure (some species are limited by over-grazing of seedlings).


References
1. PlantNet: Genus Xanthorrhoea 
2. PlantNet: Genus Xanthorrhoea
3. Plants Rescue: Xanthorrhoea johnsonii
4. Dept of the Environment and Energy: Species Profile and Threats
5. Dept of the Environment and Energy: Phytophthora root rot 
6. Dept of the Environment and Energy: National recovery plan for threatened Tasmanian Grass Trees
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