Dead trees in river bed

Running Down – climate transition in Margaret River

Tree die-off heralds the acceleration of the impacts of climate change in the south-west of Western Australia.

For decades, the Margaret River region has suffered declining rainfall. This summer, the south-west has experienced the driest six month period on record, and after a hot start to 2024, we are seeing mass vegetation die-off across the region. This isn’t just a drought, but the visible signs of climate change, and potentially heralds the transition to a more arid landscape.

Scientists have long been warning of climate change if we don’t drastically reduce our carbon emissions, but those warnings have been too easily ignored: the changes were too far in the future, deniable, or somewhere else. While the east coast has been suffering cyclones, catastrophic fires and floods, life in the south-west appeared to be going on as normal.

Now the consequences are unrolling before our eyes. Previously, the changes have been gradual, subtle, invisible: lurking out of sight beneath the surface. Climate research undertaken in the caves of the Leeuwin – Naturaliste Ridge indicates that the current decline in rainfall has serious implications for the future. Analysis of cave drip-water shows that not only is rainfall declining, the groundwater recharge rate has decreased, which is very bad news for ecosystems.

For rainfall to penetrate the soil and recharge the watertable, the soil first needs to be saturated: more soil dry days per year means that the groundwater recharge rate is at its lowest level for at least the last 800 years, possibly longer. In 2023, climate scientists released their interpretation of data from caves in Boranup, with a warning of potentially dire consequences for the groundwater-dependent ecosystems and communities of the south west.

Without access to groundwater, trees are left dangerously vulnerable to drought. Once the regolith, or soil profile dries out, they are reliant on rainfall for survival. If rainfall fails, groundwater-deprived trees start to die.

There are many ecosystems types around Margaret River, from coastal heath, to woodland, riparian systems, and forests. All have different rainfall requirements due to the complexity of the underlying regolith and hydrological systems. This means that some patches are coping better than others, but we are already seeing mass tree death in stands where the soil profile is shallow.

It’s true that the area has experienced drought before. And, during the cycle of ice ages over the last two million years, the Margaret River Region experienced arid periods, documented by studies like a recent pollen analysis of stalagmites in Mammoth Cave. Researchers found that during the heights of the last ice age between 28,000 and 17,000 years ago, eucalypt species were almost completely absent from the record. Instead, dry woodland species were abundant, alongside arid-zone fauna such as the now locally extinct black-flanked rock wallaby.

This is no cause for complacency. The dry periods of the ice ages were quite different to what we are facing today: then, the climate was cool/dry. Our warming climate will see the region transition to hot/dry, a very different scenario, with increased risk of bushfire further exacerbating the ecosystem impacts of a drying climate.

Human reliance on groundwater supplies is also making the situation worse for ecosystems, and in a tourism-dependent economy, ecosystem collapse spells uncertainty for the future.

Coastal heath along the iconic Cape to Cape Track has died off. Much of this may recover with the winter rain, but there is a longer-term risk that the sand dunes will be more subject to shift, a dry-climate process that helped form the karst landscape of the Leeuwin-Naturaliste Ridge.

Boodjidup creek valley – view from the Cape to Cape Track.

The beloved stands of karri (Eucalyptus diversicolor) around Margaret River and Boranup are an outlier population from the taller forests further to the south. Karri requires 900 – 1300mm of annual rainfall and sadly, this species is likely to retreat with the onset of climate change. At the northern edge of their range, the change has already begun.

Dying karri on the Cowaramup Brook at Gracetown.

Vegetation along waterways, like the Wooditjup Bilya, is particularly vulnerable to drought due to the underlying bedrock and shallow soil profile. Since 1970, rainfall has declined 20 percent, with a subsequent 70 percent decline in streamflow. In my lifetime, the Wooditjup Bilya, or the Margaret River, has gone from flowing all year, to only flowing in winter and spring. The rapids, a stretch of the Margaret River with exposed granite-gneiss bedrock, has already experienced mass death of trees and riparian vegetation.

The regional impacts of climate change are already apparent, and to minimise the damage we need to reduce carbon emissions immediately. But now that we’re seeing a transition in the regional landscape, it’s time to look at what we can do to mitigate the impacts of processes already underway: what can we do to help local species and ecosystems adapt? How do we manage water resources into a future where projected water availability will decrease, and demand increase? This isn’t just up to government regulators, but to each and every one of us.

Some of the images in this article are available for download from the Earth Sea Star Shop. Proceeds will be donated to Nature Conservation Margaret River Region.

The title of this article refers to the visionary work of paleo-botanist Mary E. White, and her 2000 book Running Down: water in a changing land.

Water – Always running down – across landscapes, through the eons and the ages, carving creases and wrinkles on the face of ancient continents. Life-giving, the essential resource for survival of Earth as a living planet – yet running down, degrading, under our selfish human stewardship.

Mary E. White, 2000.