TREES are clever. In times of drought, the red gums that live on the floodplains of the Murray-Darling Basin begin to drop their leaves—the less foliage they have the less water they need.
When the water once again spills over the banks of the river and out onto the floodplain, new leaves appear and the tree canopies become lush and green. Indeed, for decades, researchers and river managers have viewed the canopy as an indicator of the health of the Basin’s trees.
But just how much water is enough, and how long can these trees go between drinks in today’s highly altered and tightly managed river system where over-bank flooding is a much rarer event than it was even 20 years ago?
To find out, CSIRO’s Dr Tanya Doody ran a series of field studies spanning many years. To assess the health of the trees, she didn’t just look at the tree canopy or satellite images of the canopy—she waded along the river banks and into the forests and measured what was happening inside the trees.
Now, for the first time, we have quantitative data on how much water these floodplain trees need for optimum health, and how often they need it.
The results show that we cannot tell the health of the trees by looking at the canopy alone. We need to look inside.
Long adapted to getting its feet wet, the red gum is known as the ‘ecological engineer’ of the floodplain, says Doody: “Everything relies on the red gum to maintain health. The canopy is home for nesting birds; fallen limbs are habitat for snakes and small critters. It’s kind of like a lion, at the top of the food chain. It drives everything. And it’s an indicator of the health of the floodplain.”
Yanga National Park in south-west New South Wales proved the perfect spot for Doody’s field studies. “This location has been highly modified by humans. A series of barricades manages where water goes in the floodplain and we knew we had trees that were watered every 2, 5 and 10 years. It was fortuitous really.”
Over 10 years, and several different projects, Doody has measured how much water the trees were using and how they responded to drought and floods.
What she found was that after five years without water (no flooding from the river and below average rainfall) the trees started to use a lot less water. “When you look at them, the canopy is kind of similar to those trees watered every two years but their water use is 70 per cent lower. They’re adapting to the conditions. But what you can’t tell by looking at these trees is that they may only have a couple more years to live. It’s a gradual decline at first, but more rapid later.”
After 10 years without water, the trees looked “really poor” and to recover to good condition, she says, these trees need a flood every two years over the next eight years.
She concludes that the red gum should go no longer than seven years without being flooded, assuming below average rainfall.
Rainfall, while good, doesn’t have the same effect as an over-bank flood from the river because it doesn’t flush salt from the soil, she adds. With the extraction of water from the river system, the water table has risen, bringing with it saline water which the trees can’t process. “An over-bank flood is needed to dissolve the salt and push it back down to the water table.”
Rainfall is really important, however, to the black box trees generally found higher on the floodplain. Despite tolerating drought better than the red gum, the black box is in an unfortunate position, explains Doody.
“Because they’re elevated, they are unlikely to be naturally flooded again. We would need more flow in the river to get water up there but floods no longer get that big because we’re pushing the water down the river through locks and weirs and consuming it. We need some way to water them that’s not a flood. We need to pump the water to them.”
Some areas of black box are ecologically important, for example as bird breeding areas, she adds, and most are at risk under climate change and reduced flows.
Doody set out to understand what drought-stressed black box trees do when they get water. “If we know how much water it takes, and when, for them to start growing new leaves, we can translate that to gigalitres of environmental flow,” she says.
In South Australia’s Riverland, over three years a team of scientists used drip irrigation to artificially flood the trees and discover how they would respond. Solar-powered sap-flow sensors were drilled into the trunk to measure how quickly water travels up the tree.
The speed with which the trees responded took Doody by surprise. “They very quickly increased their water use rate. I could tell within one to two hours.”
The team discovered thresholds of water required to improve the inner health of the trees (their ‘ecophysiology’). The optimum amount to create an appreciable change is 100 mm/month, delivered in one ‘flood’, but amounts greater than 40 mm/month will lead to sustained improved tree condition, she says.
Doody’s research data is being used to underpin a new remote-sensing tool for monitoring the condition of floodplain trees in the Basin.
“The tool is for river red gums only at this stage, but it helps us understand the past and start predicting what will happen in the future. Floodplain managers have a tough job, knowing where to put their water allocation. The Murray–Darling Basin plan is based on visual tree assessment so, to my mind, they could be overestimating the tree condition. That’s a really big outcome of this research—you can’t just look at the tree; you need multiple lines of evidence. And that’s what this research provides.”
Read more about the research on drip irrigation and black box.