EcoCheck: Australia's Southwest jarrah forests have lost their iconic giants

Our EcoCheck series takes the pulse of some of Australia’s best-known ecosystems to find out if they’re in good health or on the wane.

When the first European settlers travelled into the jarrah forests of Australia’s Southwest, they found a vast area of giant trees and imagined an inexhaustible forest. The timber industry took off. Within 100 years, Western Australia’s Conservator of Forests, Charles Lane Poole, noted that: “Sentiment may dictate the preservation of a few … as reminders … but whole forests of giant trees will no longer be seen.” How right this early conservation visionary proved to be.

Today only 5% of the Jarrah forest is in protected areas such as nature reserves or national parks. Although more than 45% is still natural vegetation (much due to Lane Poole’s efforts), virtually none of the forest has escaped some degree of logging, and there are now few signs of the once iconic giant jarrahs. The timber industry and other disturbances have transformed the structure of the forest – what mostly remains now is low-growing jarrah saplings and woodland trees.

What’s more, Australia’s Southwest has been in the grip of human-triggered climate change since around 1970. Along with other Mediterranean-type climate regions, this area is rapidly drying due to global warming, with rainfall declines of 15-20%. This drying trend poses a major new threat to conservationists and industry in aiming to restore these unique forests.

What is to become of them, and what should we do about it? Under the present climatic regime, the jarrah forest is unlikely ever to return to the stature seen by early colonists. We can safeguard what remains, while acknowledging profound loss in many areas.

The Jarrah Forest bioregion within Australia. Hesperian/Wikimedia Commons, CC BY-SA

Forest giants

Jarrah (Eucalyptus marginata) naturally grows in low-fertility, gravelly soils in a highly seasonal Mediterranean-type climate. Depending on the conditions, it can grow into a shrub or a mighty tree, although in areas of low fertility, giant trees require access to a water table.

The declining rainfall, particularly in autumn and early winter, has shifted the region’s creeks and streams from year-round to seasonal, and caused water tables to decline. There have been observed declines in groundwater in some catchments of half a metre per year since 1995, and the problem is forecast to get worse still over the coming decades.

So far, the result has been that most forest cover has been lost in lower-rainfall, inland margins, as well as in more intensively logged, high-rainfall zones. In 2010, an extremely dry year even by today’s standards, even typically high-rainfall zones lost forest cover in areas where soils were shallow and couldn’t retain as much water.

As these changes continue, the jarrah forest will become structurally unrecognisable. Many stands of trees will die, with jarrah persisting as a shrub or woodland tree. However, the understorey will remain incredibly diverse.

The dead and reduced trees will leave a great deal of dead wood around. Undoubtedly there will be a threat of increased bushfires to contend with.

Fire is a naturally frequent (and evolutionarily important) aspect of the Southwest’s Mediterranean climate. But a warmer, drier climate with more extremes of fire weather will make bushfires more frequent and severe, as forest fuels dry more readily and burn for longer once ignited.

More intense bushfires will put extra stress on forest ecosystems, meaning that they take longer to recover. Habitats such as wetlands and rock outcrops, which in the past have provided refuges from fire, will be less able to do so.

Thin options

Thinning of the forest has been used as a management tool to increase yield into water supply dams. This approach also has potential to more rapidly restore pre-logging or pre-fire forest structure. However, recent thinning in the jarrah forest has failed to increase stream-flow.

Thinning is increasingly ineffective. This is because in a drying climate, it would first need to reverse decline in groundwater levels. Such treatment is not now likely to be achievable without a return to a pre-1970 rainfall regime.

The area has also been previously mined for bauxite (aluminium ore), and the rehabilitation of these mine sites will need to be done in a way that recognises the new, drier water regime. Restoring these areas with trees to provide future sawlogs would have been appropriate in a stable, high-rainfall climate, but not today.

Broader forest thinning is expensive and would probably result in the loss of forest carbon. But more targeted thinning, perhaps by culling invasive plants from the forest, could still be useful for increasing water flows. Leaving the remains of the culled plants on-site would also limit carbon emissions.

Together, unsustainable logging and the shifting climate has lowered the biomass and changed the structure of the jarrah forest. The only form of timber industry with any future in this environment is the collection of coarse woody debris. But this material is worth so much more to the environment and so much less to industry that it’s questionable whether it can be justified at all.

A few remaining venerable “king jarrahs”, and elements of old-growth characteristics, are all that remain of the original forests. But the rest of the mighty jarrahs are gone, and so the focus needs to shift to the importance of this area for the great biological diversity that does remain.

We need a new, wider vision for the vegetation of the jarrah forest. It now needs to be valued as a biologically diverse environment harbouring venerable trees from a former era.

Retention as public land allows maintenance of important conservation values. Thus, best practice rehabilitation focused on the understorey, should now target a resilient jarrah forest under changing environmental conditions. Prudence would reestablish understorey in future rehabilitation and reassess previous rehabilitation. Restoration of surrounding degraded forest may also be possible. These approaches may provide a future for this ecosystem, despite inevitable change.

Meanwhile, the increasingly scarce giant jarrahs that remain are the living museums of the forest - our irreplaceable Notre Dames.

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