Imma Perfetto
Cosmos science journalist
Australian plants adapted to resist and recover from fire are becoming threatened by it, as climate change worsens fire weather and drives more frequent and severe wildfires.
A recent study of the striking hairpin banksia (Banksia cunninghamii), in areas of Victoria affected by the 2019-2020 Black Summer bushfires, found the species is threatened with local extinction if fires return to these areas in less than 12 years.
Other, mountainous trees of the region are threatened by fire intervals of less than a human generation.
The paper appears in CSIRO Publishing’s Australian Journal of Botany.
“I think most foresters and forest scientists in Australia are worried about [the effects of climate change],” Director of the CSIRO’s Australian Tree Seed Centre, David Bush, who was not involved in the research, told Cosmos.
“It’s certainly true that things like this banksia could disappear, and there could be some flow on effects from that. That’s not a widespread species, but it’s still … a unique part of the Australian biota.
“But collectively, there are quite a lot of [obligate seeder] species, and to lose a significant proportion of them would be a profound change.”
Like many Banksia species, B. cunninghamii is a “serotinous obligate seeder”. This means it produces woody cone-like fruits, which respond to the heat of fire by opening and releasing seeds.
Obligate seeders do not resprout from their trunks and tend to be killed by fires, relying solely on dropped seed to regenerate.
Bush says that if the conditions are right, the seed will germinate in the months following the fire.
“The problem is that there needs to be seed present for that to happen, obviously, and various species have varying times to sexual maturity.”
If fire returns before the new seedling can reach reproductive maturity, then this cycle is disrupted.
The researchers assessed the recovery of B. cunninghamii populations across 25 locations in Victoria, while considering the severity to which they were burnt in the 2019-2020 bushfires and the time interval between the previous fire.
They found seedlings only grew at sites where B. cunninghamii had had at least 12 years to mature since the last fire.
They also found that, regardless of the preceding fire interval, new seedlings were very scarce in areas that experienced high severity fires – known as crown fires.
“If the fire is big enough to scorch the eucalypt canopy, then it will have scorched the whole crown of the banksias and damaged or incinerated, in some cases, the seed,” explains Bush.
The findings suggest B. cunninghamii is expected to persist only in locations that experience fires of moderate severity, with minimum fire intervals of 12 years. However, as little as 33% of its habitat in East Gippsland has a suitable fire history to support this, rendering its populations vulnerable to fire in the future.
But banksias aren’t the only obligate seeding plants under threat by changing fire regimes
Unlike most eucalypts, which resprout after fire, the alpine ash (Eucalyptus delegatensis) and mountain ash (Eucalyptus regnans) of southeastern Australia regenerate only from seed.
“In the case of the eucalypts, it can be … 15-20 years [to reach sexual maturity],” says Bush.
“So, if you get 2 fires that are more closely spaced than those intervals, then there is no seed to drop down post-fire, and then you’ve got that local population disappearing.
“Taller species, like the ash group, can tolerate sometimes a more intense fire because they’re taller. But then again, if they’ve only had a few years, then they won’t be that tall.”
Climate change is having other impacts too. Bush says that it’s possible that warmer, drier conditions may impact seed production. Dieback from drought, disease, and other compounding stressors is also a threat.
The authors of the B. cunninghamii study recommend multiple management approaches be used to allow seed banks to accumulate and prevent declines towards local extinctions, including managing parts of the landscape to exclude fire until at least after 2032.
“It is possible to do that. Though, of course, it’s costly and it has some environmental consequences of its if its own,” says Bush.
“You often have to do things like preventative burns and … it’s actually very hard to do those in a safe way as we get fewer and fewer days per year where it’s actually cool and the conditions are right.”
Seed banking may be needed to protect to restore populations that do not regenerate following fire.
“In the context of the ash group, quite a bit of thought and study has gone into that in the past,” says Bush.
“Various bodies have had quite big stores of seed – in the tonnes – and they’ve been used because the ashes are economically important source of timber.”
But, according to Bush: “…even though the seed stores are substantial, they aren’t substantial enough to completely reboot really extensive damage like the 2019-2020 bushfires.”
He says its also important to have smaller reserves that completely represent the genetic diversity of each species, to preserve local adaptations and resilience within the genome.
“A certain part of the population might be more resistant to hotter and drier climate … might be actually able to flower earlier,” he explains.
Seed orchards may provide an additional strategy to access this seed in the long-term.
“The idea would be to gather these genetically diverse seeds, then plant them somewhere where they can be protected from fire,” says Bush.
“That would include places like cleared farmland that are suitable for the species.”
In the event of a fire, seed could be aggressively harvested from such plants without further harming natural ecosystems.
“I’m doing a piece of work at the moment … to try and explore what sort of seed stocks would be desirable to have.”
