Kimberley Savanna Vegetation

by Tony Start, CALM WA

Woodlands

Savanna woodlands cover much of the Kimberley. They are home to many of the well-known animals of the region such as cockatoos, lorikeets, bustards, agile wallabies, large goannas and frill-necked lizards. Eucalypts, including many bloodwoods usually dominate the tree stratum but there are many other tree and tall shrub genera including Acacia, Terminalia, Gardenia, Erythrophleum, Planchona gyrocarpus, Brachychiton and Melaleuca .

Two notable species are the boab, Adansonia gregorii , which is endemic to the Kimberley and the adjacent VRD in the Northern Territory and the cypress pine, Callitris intratropica, which is the only indigenous conifer in the region.

Grassy understorey

The understorey is dominated by grasses but the species and life-forms vary from site to site. Hummock grasses including Triodia sp. grow in the harshest environments like rocky hillsides and arid sandy soils. Perennial tussock grasses, though still common and widespread on better soils, have been replaced at many sites by annual grasses as a consequence of heavy grazing and changed fire regimes. This may be one reason that some granivorous birds, especially finches, have declined generally. However, the irrigated agricultural landscape around Kununurra (in particular for sugarcane production) promotes the rank grasses needed by star finchs for breeding and is now one of the bird's most important strongholds in the tropical savannas. (See our completed research project on Star and crimson finches to find out more, link at the end of the page.)

Impact of fire

Grass makes good fuel and so fire is common in the savannas. After a fire spinifex (Triodia sp.), which is slow growing, usually takes several years to accumulate sufficient fuel to carry another one. However, other grasses can carry fires annually. Extensive, hot, late-season fires are a feature of the Kimberley savanna and in some years up to 30 per cent of the country burns.

This regime is very different to that imposed by Aboriginal people before European settlement and the consequences are poorly understood. Most perennial shrubs and trees re-sprout after being burnt but there are indications of change in the flora. For instance, cypress pine was once common and widespread. Today, many stands are degenerating. Fire has killed mature trees and is preventing seed recruitment for others. Furthermore, hot, late fires seem to promote some species such as annual sorghum that create excellent fuels. Fires in tall annual sorghum may extend scorch heights further into tree canopies.

One group of savanna shrubs and trees is particularly susceptible to fire if burnt too frequently. These species are killed by fire and depend on seed for regeneration. If the interval between consecutive fires is shorter than the time required by seedlings to mature and replace the seed store, they will be eliminated by fire. Cypress pine is an example, but most are shrubs. Many acacias are also affected. These species are generally scarce or absent from frequently burnt landscapes but they can be seen wherever rocky terrain (e.g. screes, gorges and rugged sandstone hills) ensures there are areas which escape being burnt by most fires.

Fire-sensitive plants in sandstone hills are increasingly threatened by fire. Photo: Jeremy Russell-Smith

Pindan

In the south-west Kimberley there is a characteristic vegetation type, pindan, of scattered trees, including many eucalypts that are little higher than the tall shrubs which dominate them. Typical species are Acacia eriopoda, A. tumida, A. monticola, Grevillea wickhamii and G. refracta . The understorey contains herbs and grasses. Pindan usually grows on red, sandy soils with a high clay content. Many savanna animals (e.g. agile wallabies, red-winged parrots) reach their southern limits in Western Australia in a strip of pindan that parallels the coast south along the Eighty-mile Beach.