The drought and heat waves that ravaged eastern Australia in the run-up to the 2019-2020 black summer bushfires killed up to 60% of trees in some areas that escaped the fires, according to new research.
While Australian species are typically hardened under extreme conditions, the record heat and drought of 2019 pushed some common tree varieties past their thresholds, potentially threatening entire ecosystems if they didn’t grow back.
Using dieback information provided to the citizen science website Dead Tree Detective, researchers at the Hawkesbury Institute for the Environment at the University of Western Sydney assessed the health of 18 tree species in 15 forests and woodlands in the New South Wales.
Measurements were taken during the drought and about eight to 10 months after good rains, revealing that tree mortality was as high as 60 to 70% in the worst affected areas. The sites examined ranged from the northern plateaus to the southern highlands region of the state.
“There were snow gums around Armidale – which is the northern extent of their range – they were hit very hard,” said Brendan Choat, associate professor at the university and senior author of a article to be submitted to the Frontiers in Revue des sciences vegetales.
“About 60% of them didn’t get over it in any way.”
Trees with stringy red bark and brittle gums were among other species that suffered losses of up to 50% at the sites studied.
Choat said visits after the rains can mask the extent of forest loss, with much grass returning and pioneer species returning. “But many of the larger trees among those [green] the patches are right there dead, ”he said.
The study found that “trees whose canopy was severely compromised immediately after the drought failed to recover even with prolonged favorable water availability.” The loss of leaf cover indicated a failure of the trees’ internal hydraulic system which draws moisture from the roots to the canopy.
Researchers wonder if younger or older trees fared worse. Choat said smaller trees, with less developed root systems, appeared more vulnerable because they were less able to tap into soil moisture and groundwater than larger ones.
Yet many trees 40 years of age or older also perished, underscoring the severity of the conditions for plants that were “superbly adapted” to Australian conditions.
Some of the trees that survived the drought also suffered afterwards. With the loss of roots, they were likely to be knocked over, especially if the soils later became saturated.
The widespread loss of often common species could trigger the transformation of ecosystems from forests to woodlands, or woodlands to scrublands, Choat said.
But such trends were difficult to predict given the limited vegetation modeling done in Australia compared to many parts of the northern hemisphere. Uncertainties also remain about how the climate will change with global warming.
“Even if you have years of very heavy precipitation [and] you have more extreme droughts and more intense fires in between, that won’t be enough to keep some of these systems going, ”he said. “You’re more likely to get big changes there.”
Philip Spark, an environmentalist based in Tamworth in northern NSW, recently photographed large areas of dieback near his town and land between Bundarra and Uralla.
Affected areas include the ecological communities of Blakely’s Red Gum and Endangered White Box. “There are whole hills of dead trees,” Spark said. “It has certainly crossed a threshold.
“It is certainly by far the worst that I have seen [in terms of mass dieback] and I have been here since 1964.
The potential change in tree species could have far-reaching effects as such changes would alter the availability and seasonality of food resources for insects, birds and other species, Spark said.
“It remains to be seen if they will regenerate on the track.”