The drought and heat waves that swept eastern Australia in the run-up to the black summer bush fires of 2019-20 killed as much as 60% of the trees in some areas that escaped the fires, according to new research.
While Australian species are typically hardy to extreme conditions, the record heat and drought in 2019 pushed some common tree species beyond their thresholds, potentially threatening entire ecosystems if they do not grow back.
Working from data on setbacks provided to the civil science website Dead Tree Detective, researchers at Western Sydney University’s Hawkesbury Institute for the Environment assessed the health of 18 tree species in 15 New South Wales forests and forests.
Measurements were made during the drought and about eight to 10 months after good rainfall, and the deaths of the trees reached 60-70% in the worst affected areas. The sites studied stretched from the northern tablelands down to the southern highlands region of the state.
“There was some snow rubber up around Armidale – which is the northern extent of their range – they were hit very hard,” said Brendan Choat, an associate professor at the university and lead author of a paper to be submitted to Frontiers in Plant Science Journal.
“About 60% of them did not recover in any way.”
Red bark trees and brittle gums were among other species that suffered losses of up to 50% at the sites studied.
Choat said visits after the rain could mask the extent of deforestation, with plenty of grass returning and pioneer species returning. “But many of the larger trees among them [green] patches just sit there dead, ”he said.
The study found that “trees with a severely compromised crown immediately after drought failed to recover themselves with prolonged favorable water availability”. The loss of leaf cover indicated a fault in the trees’ internal hydraulic system that draws moisture from the roots to the crown.
Researchers are debating whether younger or older trees did worse. Choat said smaller trees with less developed root systems seemed more vulnerable as they were less able to utilize soil moisture and groundwater than larger ones.
Yet many trees aged 40 or older also perished, underscoring the severity of the conditions for plants that were “excellently adapted” to Australian conditions.
Some of the trees that survived the drought also struggled afterwards. With root loss, they were susceptible to being blown over, especially if soil was later saturated.
The widespread loss of often common species could trigger the transformation of ecosystems from forests to forests or forests to scrub areas, Choat said.
However, such trends were difficult to predict given the limited modeling of vegetation carried out in Australia compared to many areas of the Northern Hemisphere. There is also uncertainty about how the climate will change with global warming.
“Even if you have very high rainfall [and] you have more extreme drought and more intense fire burning between them, it will not be enough to maintain some of these systems, ”he said. “You’re more likely to get some big changes there.”
Philip Spark, an ecologist based in Tamworth, northern NSW, recently photographed large areas of decline near his town and country between Bundarra and Uralla.
The affected areas include the endangered Blakelys red chewing gum and the white box ecological community. “There are whole hillsides of dead trees,” Spark said. “It has definitely crossed a threshold.
“It’s clearly the worst I’ve seen [in terms of mass dieback] and I’ve been here since 1964. “
The potential change in tree species could have far-reaching effects because such shifts would change the availability and seasonality of food resources for insects, birds and other species, Spark said.
“It remains to be seen if they will regenerate down the field.”
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