Rare Tasmanian eucalypt primed for climate change as others decline
Eucalypts vulnerable to climate change are in decline across Australia, but scientists say a rare one found only in southern Tasmania is actually expanding its range in part because it is so "well adapted" to warmer, drier conditions.
The endemic Risdon peppermint (Eucalyptus risdonii) is a "scraggly looking tree" with unusual blue leaves, found around Risdon Vale on Hobart's eastern shore.
Species distribution modelling by consultant Peter Harrison during his PhD predicted an increase in the extent and number of areas suitable for the species by the end of this century.
Plant scientist Rebecca Jones, of the Eucalypt Genetics group at the University of Tasmania (UTAS), said the species had proven its ability to adapt to harsh conditions.
"I'm recommending people plant it because it's actually one of the only eucalypts that climate modelling predicts will be able to expand its range under future climate change because it loves hot, dry conditions," Dr Jones told ABC Hobart.
"There's been research at UTAS on this eucalypt for 40 years or so. There are a few really cool things about it, other than it being totally beautiful."
Dr Jones said the species was unusual because it retained its blue-grey baby foliage.
"It stays blue. Most trees don't, they go from blue to green," she said.
"In fact, the early explorers thought they were two different species and described them as such.
"And you can't blame them really, they look pretty different, but they're actually the same thing.
"But Eucalyptus risdonii, it holds those juvenile leaves its whole life. So, the whole crown of the canopy are juvenile leaves, and you can see little flower buds coming out on those juvenile leaves.
"It's thought that's an adaptation to the sort of harsh conditions that it lives in in [postcode] 7016, on the north-west facing slopes."
Eucalypt trees dominate the Australian landscape.
Botanists have identified about 900 species of them and 29 are known to be native to Tasmania, including the Tasmanian blue gum (Eucalyptus globulus), mountain ash (Eucalyptus regnans) and brown stringybark (E.obliqua).
Known collectively as Tasmanian oak, these are commonly used as building timbers.
The Risdon peppermint, on the other hand, has ornamental qualities and is better equipped than some eucalypts to handle climate change.
Another Tasmanian eucalypt, the Morrisby gum (Eucalyptus morrisbyi), has suffered catastrophic declines due to rising temperatures and drying soils.
"It's kind of a scraggly tree but the foliage itself is quite beautiful," Dr Jones said.
"It's the kind of stuff you might see in foliage arrangements."
Its smaller size is also better suited than most eucalypts to home gardens.
"It doesn't really make a proper tree. It sort of makes a mallee kind of form. So, it sprouts out [like] a little low bush," she said.
"But I think that's what lends itself well to planting it in your garden, right? Because it's not going to grow up into a big tree. You can chop out the top of it and it loves that."
Dr Jones said research suggests the species' peculiarities "might be an adaptation to extreme environments".
As well as its grey-blue baby foliage, it has grey, waxy branches that may "deter things from eating it," she said.
"It can help with water conservation and things like that."
It has stout leaves, which merge across the stem, making one leaf with the stalk growing up through the middle.
"When they're dead, they actually dislodge from the stem while remaining attached to the branch and start spinning around and so when you're walking through that forest in the wind you'll hear it rattling around as it spins," Dr Jones said.
Forensic scientist Thais Ribeiro Pfeilsticker, who studied the Risdon peppermint as part of her PhD at UTAS, believes hybridisation is also assisting its spread.
Her thesis provided evidence supporting her supervisor Brad Potts's PhD in the 1980s, which hypothesised that hybridisation was contributing to the expansion of the rare tree in the range of the widespread Eucalyptus amygdalina.
She said if you went to Risdon reserve, it would be clear the tree had a restricted distribution to the "hotter slopes of the place", but there was evidence that it was expanding its footprint despite the fact eucalypts could not move their seed far.
"[My supervisor Brad Potts] came up with this idea that the pollen was actually landing on related species and another species, which is called Eucalyptus amygdalina," Ms Pfeilsticker said.
"And by doing that, it was actually expanding its distribution. So instead of like landing and crossing with another risdonii, it was crossing with a different species. And then you would start to grow populations of things that we call hybrids."
Dr Jones said it was pleasing to see this type of movement with a rare tree.
"It's currently expanding, but very slowly through the seed dispersal mechanism. It won't become more than just rare without human intervention," she said.
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