Allocasuarina woodlands in a future, warmer climate
Well there's nothing like being reminded of the ever-escalating impacts of climate change, or the associated risks if we continue to burn fossil fuels. The government released their first ever National Climate Risk Assessment on 15/9/25.
The findings of course are sobering but not surprising. We need to reduce our emissions and the sooner the better. Under a warmer climate, agricultural, environmental, social and health systems will suffer, if not verge on collapse. I'm more surprised this is the first government report given how long we have known about climate change. The report arrived in suspiciously convenient timing, as just days after this report, the government declared its ambition to cut emissions by 62-70% by 2035.
Regardless, I decided to have read myself. Of course I haven't had time to go through it all, and just focused on sections relating to the natural environment, and scrolling through the various figures and infographics.
I was drawn to an interesting corellogram on page 191:
The figure presents how resilient ecosystems are under likely future climate scenarios (more on this later). This ecosystem resilience is calculated from the Biometric Ecosystem Resilience Indicator (BERI) scores. BERI scores are measured on a scale of 0 to 1, where 1 is the highest possible capacity of ecosystems to retain biological diversity in the face of climate change and 0 is no capacity. Keep in mind BERI scores only consider plant communities, and not the vertebrate/invertebrate fauna.
Of course your eyes get drawn to the most extreme numbers. At the low end (in black) the ecosystems with the least resilience (with a shared lowest BERI score of 0.09), were the following:
- Casuarina forests, woodlands and shrublands (in Victoria)
- Melaleuca forests woodlands and shrublands (in South Australia)
- Tussock grasslands (in Victoria)
With many ecosystems not far behind on marginally better scores. Victoria especially has low scores across almost all ecosystems (probably partly due to the relatively small land area, of which has been extensively cleared for human uses).
At the top end (in bright yellow) isn't really that great either. A top score of 0.58 (Rainforest and vine thickets in Tasmania). What's also strange is that the colour doesn't match the legend provided. 0.58 should be presented as a more purplely-reddy sort of colour. Maybe they wanted to make it seem better than it really is? Hmmm.
But honestly who even cares about BERI scores? No one goes around quoting them. The average office worker (or elected official) has never stepped foot in a Casuarina woodland - let alone understand their ecology, why they are less resilient, or even just appreciate their uniqueness.
What these numbers don't show is what its like to be in one of these ecosystems. Why we should care about them. If we cannot appreciate them now, we certainly won't when they are gone. We need to know what we may lose if we don't address climate change. I was fortunate to have visited some of these Casuarina woodlands earlier this year, walking amongst remnant stands of Allocasuarina luehmannii or Buloke.
Casuarina Allocasuarina woodlands
Firstly, most of these woodlands have an overstory dominated by Allocasuarina species - not Casuarina. Whilst we do have 6 species of Casuarina, there are 59 species of Allocasuarina in Australia. The genus Allocasuarina was created in 1982 to better reflect the taxonomy of the group of plants known then as Casuarina. Subsequently, most Casuarina species were transfered into the genus of Allocasuarina. Anyway when the government is refering to 'Casuarina woodlands' they do mean both Allocasuarina and Casuarina (I checked here and here). This is all semantics anyway and is not the main point.
Allocasuarina luehmannii or more commonly 'Buloke' (pictured here) are long-lived, slow growing trees. So slow growing that even to reach a diameter at breast height of 50cm, it would take around 70-100 years! A good guide on how to grow Bulokes is here. I like their quote:
"From the time you plant seedlings until the trees reach maturity, the farm on which you plant these trees will probably change hands five times. There will be at least two bushfires, maybe a flood. The fence you build will have rotted away. The climate will be hotter and probably drier"
Bulokes are listed as 'Vulnerable' on IUCN red list mainly due to widespread clearing for agriculture. In the Wimmera alone, an estimated 97% of Bulokes have been lost. Tree-planting efforts are also hampered due to the time it takes to mature (being slow-growing) and its difficult to establish (a very small proportion will make it to maturity). It also doesn't help young seedlings/recruits of Allocasuarinas are palatable to rabbits.
Thankfully, they Bulokes are now listed under the FFG Act in Victoria (and grants them the highest protection possible under state legislation).
But what its like to walk in these woodlands?....
Another obvious difference is the foliage. They don't have leaves, instead they have photosynthethic stems, known as 'Cladodes'. The leaves are just visable in the image, as brown, minute teeth-like structures between each green section (or node). Cladodes likely evolved as a strategy to be more water efficient, in low water, drought environments.
Note: this image I took is foliage from an Allocasuarina verticillata. However the foliage is similar on Bulokes and in general representative of all Allocasuarinas.
Another unique feature is found below ground. The roots have nodules that house an actinorhizal nitrogen-fixing bacteria. The bacteria responsible are from the genus Frankia and form a symbiotic relationship with the roots. Nitrogen is one of the macronutrients all plants need to grow, and is often a limiting factor for plant growth in Australian soils. Whilst being abundant in the air, plants are unable to capture this nitrogen. Therefore plants need a water soluble form of nitrogen to be uptaken by their roots. The Frankia help fix this problem.
Allocasuarina woodland under future modelling
I went into the technical report to seek further information on Allocasuarina woodlands. In this, they aimed to determine how much of the 1990 extent of Allocasuarina woodlands would remain in the same location (in-situ) by 2050 under two climate scenarios/models. Climate is predicted to cause movements in the distribution of these woodlands, and also will likely introduce new species/movement of species into new areas. It may also lead to novel ecosystems forming. This 'community composition' (that considers what species are where) was also estimated. The two climate scenarios were;
- 2050 CAN-ESM2, RCP 8.5 model (which tends to be a hotter, drier scenario - up to 3.5 degrees warmer than 1990 by 2050) and\
- the 2050 MICROC5, RCP 8.5 model (comparably milder and moister - up to 2 degrees warmer than 1990 by 2050).
Under 2050 CAN-ESM2, only 9.12% (of the 1990 extent) of Allocasuarina woodlands will be retained in-situ. There would also be a net change in the compositional dissimilarity by 29.065%.
Under 2050 MICROC5, 28.95% (of the 1990 extent) will be retained in-situ. There would also be a net change in the community dissimilarity by 3.44%
It must be noted, these estimates only reflect climate pressure only, and exclude interactions with land use, or invasive species etc.
You can see the all the shrubs, forbs and grasses that co-exist alongside the Bulokes. Each summer Allocasuarina woodlands suffer through heatwaves. These heatwaves are predicted to only get worse (both hotter and longer) in the future. I wonder how close Allocasuarinas are to their thermal limits - given the 0.09 resilience score, I really think we are right at their limits currently.
If these trees all no longer persist under a warmer climate, the whole structural complexity is reduced to these ecosystems. Suddenly the microclimate will be hotter with less shade, and more sunlight reaching the ground species. This is one example. Buloke Mistletoe Amyema linophylla subsp. orientalis is a parasitic creeper that relies on Bulokes almost entirely. There are multitude of species that co-exist in Allocasuarina woodlands, and a multitude of interactions, and processes.
The report itself summarise this in the following way:
"These findings suggest that there will be substantial change to vegetation structural characteristics as the climate warms; consistent with the compositional dissimilarity finding that suggest most species in exposed locations will need to tolerate or adapt to the change, move to another suitable area, or perish"
The relationship between biodiversity loss, and ecosystem functioning is well established. A reduction in biodiversity is not just the loss of species. It will change the composition of communities, leading to novel commnunities, it will change the biomass, the abundance of plants and animals, and the magnitude of ecosystem processes and services.
On reflection
When I started this blog, I wanted to share my thoughts/observations and experiences of the Australian bush. I have recently realised another benefit. It is to documenting these unique ecosystems. I want to describe them in detail. If no-one know these exist, how can we expect them to be conserved. How can we expect people to care?
Climate change cannot be ignored.
Also blogs are a great medium for this. To write about the forgotten. To write about the obscure. To write about what you care about.