But there is ongoing debate about whether to prioritise native or non-native plants to fight climate change. As our recent research shows, non-native plants often grow faster compared to native plants, but they also decompose faster and this helps to accelerate the release of 150% more carbon dioxide from the soil.
Our results highlight a challenging gap in our understanding of carbon cycling in newly planted or regenerating forests.
It is relatively easy to measure plant biomass (how quickly a plant grows) and to estimate how much carbon dioxide it has removed from the atmosphere. But measuring carbon release is more difficult because it involves complex interactions between the plant, plant-eating insects and soil microorganisms.
This lack of an integrated carbon cycling model that includes species interactions makes predictions for carbon budgeting exceedingly difficult.
There is uncertainty in our climate forecasting because we don’t fully understand how the factors that influence carbon cycling – the process in which carbon is both accumulated and lost by plants and soils – differ across ecosystems.
Carbon sequestration projects typically use fast-growing plant species that accumulate carbon in their tissues rapidly. Few projects focus on what goes on in the soil.
Non-native plants often accelerate carbon cycling. They usually have less dense tissues and can grow and incorporate carbon into their tissues faster than native plants. But they also decompose more readily, increasing carbon release back to the atmosphere.
Our research, recently published in the journal Science, shows that when non-native plants arrive in a new place, they establish new interactions with soil organisms. So far, research has mostly focused on how this resetting of interactions with soil microorganisms, herbivorous insects and other organisms helps exotic plants to invade a new place quickly, often overwhelming native species.
We established 160 experimental plant communities, with different combinations of native and non-native plants. We collected and reared herbivorous insects and created identical mixtures which we added to half of the plots.
We also cultured soil microorganisms to create two different soils that we split across the plant communities. One soil contained microorganisms familiar to the plants and another was unfamiliar.
Herbivorous insects and soil microorganisms feed on live and decaying plant tissue. Their ability to grow depends on the nutritional quality of that food. We found that non-native plants provided a better food source for herbivores compared with native plants – and that resulted in more plant-eating insects in communities dominated by non-native plants.
Similarly, exotic plants also raised the abundance of soil microorganisms involved in the rapid decomposition of plant material. This synergy of multiple organisms and interactions (fast-growing plants with less dense tissues, high herbivore abundance, and increased decomposition by soil microorganisms) means that more of the plant carbon is released back into the atmosphere.
In a practical sense, these soil treatments (soils with microorganisms familiar vs. unfamiliar to the plants) mimic the difference between reforestation (replanting an area) and afforestation (planting trees to create a new forest).
Reforested areas are typically replanted with native species that occurred there before, whereas afforested areas are planted with new species. Our results suggest planting non-native trees into soils with microorganisms they have never encountered (in other words, afforestation with non-native plants) may lead to more rapid release of carbon and undermine the effort to mitigate climate change.
If you fondly remember May Gibbs’s Gumnut Baby stories about the adventures of Snugglepot and Cuddlepie, you may also remember the villainous Big Bad Banksia Men (perhaps you’re still having nightmares about them).
But banksias are nothing to be afraid of. They’re a marvellous group of Australian native trees and shrubs, with an ancient heritage and a vital role in Australian plant ecology, colonial history and bushfire regeneration.
The genus Banksia has about 173 native species. It takes its name from botanist Sir Joseph Banks, who collected specimens of four species in 1770 when he arrived in Australia on board Captain Cook’s Endeavour.
One of the four species he collected was B. integrifolia, the coastal banksia. This can be a small to medium tree about 5m to 15m tall. In the right conditions, it can be quite impressive and grow up to 35m.
It’s found naturally in coastal regions, growing on sand dunes or around coastal marshes from Queensland to Victoria. These can be quite tough environments and, while B. integrifolia tends to grow in slightly protected sites, it still copes well with sandy soils, poor soil nutrition, salt and wind.
From ancient origins
Coastal banksia – like all banksias – belong to the protea family (Proteaceae). But given the spectacular flowering proteas are of African origin, how did our Australian genera get here?
The members of the Proteaceae belong to an ancient group of flowering plants that evolved almost 100 million years ago on the southern supercontinent Gondwana. When Gondwana fragmented more than 80 million years ago, the proteas remained on the African plate, while the Australian genera remained here.
The spikes of woody fruits on the Australian banksia, sometimes called cones, are made up of several hundred flowers. The flower spikes are beautiful structures, soft and brush-like. But with B. integrifolia, they are pale green, similar to the foliage, and can be hard to see within the canopy at a distance.
Up close, these fruit spikes can look quite spooky, almost sinister, especially when wasps have caused extensive gall formation. Galls are swellings that develop on plant tissues as a result of fungal and insect damage, a bit like a benign tumour.
Maybe this is what led May Gibbs to cast them as the baddies in her Gumnut Baby stories. While the galls may look unsightly, they rarely do serious harm to banksias.
Given the fruit spikes of coastal banksia look like brushes, it’s not surprising Indigenous people once used them as paint brushes.
The flowers are very rich in nectar, which attracts insects and birds. If you run your hand along the flower spike you, like generations of Aboriginal people before you, can enjoy the sweet taste if you lick the nectar off your hand. You can also soak the flowers in water and collect a sweet syrup.
In the garden, B. integrifolia is wonderfully attractive to native insects, birds and ringtail possums. It’s easy to establish and, until it grows more than a few metres high, can be successfully moved and transplanted.
Unlike many other banksia species, coastal banksias don’t need fire to release their seed. For many Australian species, the woody fruits remain solid and sealed, and it’s only when fire comes through that they burn, dry, crack open and release their seed.
This can happen with B. integrifolia too, but in a garden setting the fruits will mature, dry and crack open and release the seeds, which germinate readily. This makes propagating coastal banksia easy work.
In touch with its roots
Perhaps one of the more important, but less obvious, attributes of B. integrifolia are its roots. These are a special type of root possessed by members of the protea family.
The roots form a dense, branched cluster, a bit like the head of a toothbrush, that can be 2-5cm across. They greatly increase the absorbing surface area of the roots, as each root possesses thousands of very fine root hairs.
Proteoid roots can be very handy in sandy and other poor soils, where water drains quickly and nutrients are scarce.
These roots, also described as cluster roots, are often visible in a garden bed just at the interface of the soil with the humus or mulch layer above it. They’re very light brown, almost white, in colour.
B. integrifolia, like other banksias, also has the ability to take in nitrogen and enrich the soil, which can be very handy in soils low in nitrogen. It’s like a natural living and decorative fertiliser.
Proteoid roots are unfortunately very well suited to the presence of Phytophthora cinnamomii (the cinnamon fungus). It causes dieback in many native plant species, but can be particularly virulent for banksias.
But B. Integrifolia is one of the more resistant species to the fungus. Promising experiments have been done on grafting susceptible species onto the roots of B. integrifolia to improve their rates of survival.
This could be important, as banksias have a role in bushfire regeneration in many parts of Australia, so the occurrence of the fungus can compromise fire recovery.
As bushfires blackened forests last summer, one tree species was protected by a specialist team of firefighters: the Wollemi pine.
These trees have a deeply ancient lineage dating back to when dinosaurs walked Gondwana 100 million years ago. Back then, rainforests – including Wollemi pines (or their cousins) – covered what became Australia.
So when a handful of Wollemi pines were discovered alive in 1994 on the brink of extinction, it caused a frenzy of interest that has barely died down among plant enthusiasts.
Today, fewer than 100 mature pines are left in the wild. But their exact location is one of the best kept secrets in Australian plant conservation, to protect them from pathogens such as the root-rotting phytophthora that might hitch a ride on human visitors.
But while rare in nature, our ongoing research with citizen scientists is finding Wollemi pines grow in backyards all over the world, in a range of environments, and this information can inform how we can protect them in the wild.
From Gondwana to the garden
The Wollemi pine is considered the iconic poster-child for plant conservation. It’s an unusual-looking plant – each wild tree has many trunks covered in bark resembling bubbling chocolate and branches of lime or grey-green fern-like leaves. And in the wild, they grow to more than 40 metres tall.
The species is a member of the southern conifer family Araucariaceae, and its cousins include the monkey puzzle tree and the Norfolk Island pine. While considered a rainforest tree, many remaining in the wild exist between rainforest and dry eucalypt woodland, on the ledges of a sandstone gorge.
One of the first strategies was cultivation. Horticultural scientists at the Australian Botanic Garden Mount Annan (Sydney) worked out how to propagate the species so it could be grown and enjoyed in gardens, reducing the risk of illegal visitation in the wild.
After the Australian Botanic Garden established a basic “insurance population” of plants propagated from the wild trees, some of the first cultivated Wollemi pines were distributed to botanic gardens in Australia and overseas, including in the UK’s Royal Botanic Gardens Kew.
In 2005, Wollemi pines were auctioned to the public at a Sothebys Auction. Since then, they’ve been exported to many nurseries around the world, and now grow in many public and private gardens.
I spy a Wollemi pine
When plants are very rare in the wild, or are very restricted in their distributions, conservation away from the site (ex situ) can play an important role in their survival.
This includes seed banking, translocation (establishing new populations of rare plants in new locations) and cultivation for the nursery trade.
Enter our I Spy A Wollemi Pine project. Fifteen years after the Wollemi pine became available for sale, our study asks people to report where Wollemi pines are growing in gardens across the world.
So far, results from the online survey have revealed the species grows across 27 different countries, from Australia to Russia, and the UK to Peru.
The tallest trees so far – stretching to 7 metres tall (though dwarfed by their wild counterparts) – have been reported from the UK. To date, 987 people have contributed data about Wollemi pines.
What we can learn
Reading comments from survey participants – from “Has survived minus 10 degrees” to “I just love it” – has been a source of interest and joy for us researchers.
When the survey is finished, we’ll analyse the responses to understand what influences the growth of this species, such as different climates and soils.
Knowing how Wollemi pines grow in other parts of the world will provide gardening tips for home growers, but more importantly it will inform future conservation efforts in the wild in the face of climate change.
For example, this research will provide information on what environments the Wollemi pine can tolerate. We’re discovering the hottest, coldest, wettest and driest places on earth this species can survive in.
This information can help us find places to establish new populations of Wollemi pines. It may also provide clues on the evolutionary history of this species and how it managed to survive multiple ice ages and other dramatic climate changes in deep history.
Conservation with cultivation
Conserving Wollemi pines in backyards is not quite the same as Wollemi pines in the wild – in the same way its important to have pandas in the wild, and not just in zoos. But using cultivation for conservation does mean these species have much greater distribution today than they have ever had in the past.
In fact, this isn’t the first time a rare tree has ended up in gardens. The dawn redwood, thought to be extinct in the wild, was rediscovered in China in the 1940s and can now be found in gardens across the world.
And the internet is a great place to foster conservation. In online forums, people share every stage of their Wollemi babies’ growth, from seed germination to pine cone production.
This love and connection to Wollemi pines might even help address “plant blindness”: the propensity for people to see, recognise and focus on animals rather than plants, despite plants being central to providing us with food, the air we breathe and our climate.
So, as more species are threatened with extinction every day, everyone’s actions – even in their own backyards or online – can make a difference.
If you have a Wollemi pine in your backyard, or know of a Wollemi pine in a park or garden, and would like to get involved in our citizen science survey, please click here.
No other event in our lifetimes has brought such sudden, drastic loss to Australia’s biodiversity as the last bushfire season. Governments, researchers and conservationists have committed to the long road to recovery. But in those vast burnt landscapes, where do we start?
We are among the wildlife experts advising the federal government on bushfire recovery. Our role is to help determine the actions needed to stave off extinctions and help nature recover in the months and years ahead.
Our first step was to systematically determine which plant and animal species and ecosystems needed help most urgently. So let’s take a closer look at how we went about it.
Sorting through the smoke
One way to work out how badly a species is affected by fire is to look at how much of its distribution – or the area in which it lives – was burnt.
This is done by overlapping fire maps with maps or records showing the species’ range. The greater the overlap, the higher the potential fire impact. But there are several complicating factors to consider:
1. Susceptibility: Species vary in how susceptible they are to fire. For instance, animals that move quickly – such as red-necked wallabies and the white-throated needletail – can escape an approaching fire. So too can animals that burrow deeply into the ground, such as wombats.
Less mobile animals, or those that live in vegetation, are more likely to die. We also considered post-fire recovery factors such as a species’ vulnerability to predators and reproductive rate.
2. What we know: The quality of data on where species occur is patchy. For example, there are thousands of records for most of Australia’s 830 or so bird species. But there are very few reliable records for many of Australia’s 25,000-odd plant species and 320,000-odd invertebrate species.
So while we can estimate with some confidence how much of a crimson rosella’s distribution burned, the fire overlaps for less well-known species are much less certain.
3. The history of threats: The impact of fires on a region depends on the extent of other threats, such as drought and the region’s fire history. The time that elapses between fires can influence whether populations have recovered since the last fire.
For instance, some plants reproduce only from seed rather than resprouting. Fires in quick succession can kill regrowing plants before they’ve matured enough to produce seed. If that happens, species can become locally extinct.
4. Fire severity: Some areas burn more intensely than others. High severity fires tend to kill more animals. They also incinerate vegetation and can scorch seeds lying in the soil.
Many Australian plant species are exquisitely adapted to regenerate and resprout after fire. But if a fire is intense enough, even these plants may not bounce back.
5. Already threatened?: Many species affected by these bushfires were already in trouble. For some, other threats had already diminished their numbers. Others were highly vulnerable because they were found only in very limited areas.
The bushfires brought many already threatened species closer to extinction. And other species previously considered secure are now threatened.
With these issues in mind, and with contributions from many other experts, we compiled lists of plant, invertebrate and vertebrate species worst-affected by the 2019-20 fires. A similar assessment was undertaken for threatened ecosystems.
Some 471 plant, 213 invertebrate and 92 vertebrate species have been identified as a priority for interventions. Most had more than half their distribution burnt. Many have had more than 80% affected; some had 100% burnt.
Priority invertebrates include land snails, freshwater crayfish, spiders, millipedes, beetles, dragonflies, grasshoppers, butterflies and bees. Many species had very small ranges.
For example, the inelegantly named Banksia montana mealybug – a tiny insect – existed only in the foliage of a few individuals of a single plant species in Western Australia’s Stirling Range, all of which were consumed by the recent fires.
Some priority plants, such as the Monga waratah, have persisted in Australia since their evolution prior to the break-up of the Gondwanan supercontinent about 140 million years ago. More than 50% of its current range burned, much at high severity. During recovery it is vulnerable to diseases such as phytophthora root rot.
Some priority vertebrates have tiny distributions, such as the Mt Kaputar rock skink that lives only on rocky outcrops of Mt Kaputar near Narrabri, New South Wales. Others had large distributions that were extensively burnt, such as the yellow-bellied glider.
The priority lists include iconic species such as the koala, and species largely unknown to the public, such as the stocky galaxias, a fish that lives only in an alpine stream near Cooma in NSW.
What’s being done
A federal government scheme is now allocating grants to projects that aim to help these species and ecosystems recover.
Affected species need immediate and longer-term actions to help them avoid extinction and recover. Critical actions common to all fire-affected species are:
careful management of burnt areas so their recovery isn’t compromised by compounding pressures
protecting unburnt areas from further fire and other threats, so they can support population recovery
rapid surveys to identify where populations have survived. This is also the first step in ongoing monitoring to track recovery and the response to interventions.
Targeted control of feral predators, herbivores and weeds is also essential to the recovery of many priority species.
In some rare cases, plants or animals may need to be moved to areas where populations were reduced or wiped out. Captive breeding or seed collection can support this. Such restocking doesn’t just help recovery, it also spreads the risk of population loss in case of future fires.
Long road back
The COVID-19 pandemic has led to some challenges in implementing recovery actions. Like all of us, state agency staff, NGOs, academics and volunteer groups must abide by public health orders, which have in some cases limited what can be done and where.
But the restrictions may also have an upside. For instance, fewer vehicles on the roads might reduce roadkill of recovering wildlife.
As states ease restrictions, more groups will be able to continue the recovery process.
As well as action on the ground, much planning and policy response is still required. Many fire-affected species must be added to threatened species lists to ensure they’re legally protected, and so remain the focus of conservation effort.
Fire management methods must be reviewed to reduce the chance of future catastrophic fires, and to make sure the protection of biodiversity assets is considered in fire management planning and suppression.
Last bushfire season inflicted deep wounds on our biodiversity. We need to deal with that injury. We must also learn from it, so we can respond swiftly and effectively to future ecological disasters.
Many species experts and state/territory agency representatives contributed to the analyses of priority species. Staff from the Department of Agriculture, Water and the Environment (especially the Environmental Resources Information Network (Geospatial and Information Analytics Branch), the Protected Species and Communities Branch and the Threatened Species Commissioner’s Office) and Expert Panel members also contributed significantly to this work.
Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.
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If carbon dioxide levels were to double, how much increase in plant growth would this cause? How much of the world’s deserts would disappear due to plants’ increased drought tolerance in a high carbon dioxide environment?
Compared to pre-industrial levels, the concentration of carbon dioxide (CO₂) in the atmosphere will have doubled in about 20 to 30 years, depending on how much CO₂ we emit over the coming years. More CO₂ generally leads to higher rates of photosynthesis and less water consumption in plants.
At first sight, it seems more CO₂ can only be beneficial to plants, but things are a lot more complex than that.
Some plants do grow faster under elevated levels of atmospheric CO₂, but this happens mostly in crops and young trees, and generally not in mature forests.
Even if plants grew twice as fast under doubled CO₂ levels, it would not mean they strip twice as much CO₂ from the atmosphere. Plants take carbon from the atmosphere as they grow, but that carbon is going straight back via natural decomposition when plants die or when they are harvested and consumed.
The most important aspect is how long the carbon stays locked away from the atmosphere – and this is where we have to make a clear distinction between increased carbon flux (faster growth) or an increasing carbon pool (actual carbon sequestration). Your bank account is a useful analogy to illustrate this difference: fluxes are transfers, pools are balances.
Of the almost 10 billion tonnes (gigatonnes, or Gt) of carbon we emit every year through the burning of fossil fuels, only about half accumulates in the atmosphere. Around a quarter ends up in the ocean (about 2.4 Gt), and the remainder (about 3 Gt) is thought to be taken up by terrestrial plants.
While the ocean and the atmospheric sinks are relatively easy to quantify, the terrestrial sink isn’t. In fact, the 3 Gt can be thought of more as an unaccounted residual. Ultimately, the emitted carbon needs to go somewhere, and if it isn’t the ocean or the atmosphere, it must be the land.
So yes, the terrestrial system takes up a substantial proportion of the carbon we emit, but the attribution of this sink to elevated levels of CO₂ is difficult. This is because many other factors may contribute to the land carbon sink: rising temperature, increased use of fertilisers and atmospheric nitrogen deposition, changed land management (including land abandonment), and changes in species composition.
Current estimates assign about a quarter of this land sink to elevated levels of CO₂, but estimates are very uncertain.
In summary, rising CO₂ leads to faster plant growth – sometimes. And this increased growth only partly contributes to sequestering carbon from the atmosphere. The important questions are how long this carbon is locked away from the atmosphere, and how much longer the currently observed land sink will continue.
The second part of the question refers to a side-effect of rising levels of CO₂ in the air: the fact that it enables plants to save water.
Plants regulate the exchange of carbon dioxide and water vapour by opening or closing small pores, called stomata, on the surface of their leaves. Under higher concentrations of CO₂, they can reduce the opening of these pores, and that in turn means they lose less water.
This alleviates drought stress in already dry areas. But again, the issue is more complex because CO₂ is not the only parameter that changes. Dry areas also get warmer, which means that more water evaporates and this often compensates for the water-saving effect.
Overall, rising CO₂ has contributed to some degree to the greening of Earth, but it is likely that this trend will not continue under the much more complex combination of global change drivers, particularly in arid regions.
Big development isn’t the only challenge for urban tree cover. During the period covered by our newly published study, the inner city lost a further 8,000 street trees to a variety of causes – vandals, establishment failures of young trees, drought, smaller developments and vehicle damage.
Trees add beauty and character to our streets, and (so far) they’re not a political wedge issue in the ongoing culture war that is Australian climate policy. In short, they’re a very good idea, at just the right time.
Next time you’re walking past a large construction site, look for empty tree pits – the square holes in footpaths where trees have been removed. Maybe you’ve already seen these and wondered what all the construction means for our trees. Well, now we know.
Our study puts a number on the impact of major development on city trees. In the City of Melbourne – that’s just the innermost suburbs and the CBD – major developments cost our streets about 2,000 trees from 2008-2017.
Using council databases and a mapping tool, we tracked removals of trees within ten metres of hundreds of major developments. We found much higher rates of tree removal around major development sites than in control sites that weren’t developed.
The silver lining in this story is that the city council’s tree-protection policy seems to be quite effective at saving our bigger trees. The vast majority of removals we saw were of trees with trunks less than 30cm thick. Only one in 20 of the trees lost was a large mature tree over 60cm thick.
This may partly reflect the fact that the council charges developers for not only tree replacement but also the dollar equivalent of lost amenity and ecological values. It gets very expensive to remove a large tree once you factor in all the valuable services it provides. When a tree is a metre thick, costs can exceed $100,000 – and that’s if there are no alternatives to removal.
The protection of bigger trees means Melbourne retained canopy fairly well, despite losing over 2,000 trees. Only 8% of city-wide canopy losses during our study period happened near major development sites. This modest loss is still serious, as removals are having more of an impact on future canopy growth than current cover.
While Melbourne-centric, there are lessons in this study for cities everywhere. Robust policies to protect and retain trees backed up by clear financial incentives are valuable, as even well-resourced councils with strong policy face an uphill battle when development gets intense.
Our findings highlight that retaining and establishing young trees is especially difficult. This is troubling given these are the trees that must deliver the canopy that will in future shelter the streets in which we live and work.
Improved investments in how young trees are planted and how long we look after them can help. For example, in a promising local study, researchers showed that trees planted in a way that catches rainwater run-off from roads grow twice as fast, provided planting design avoids waterlogging.
Finally, in the context of rapid development, buildings themselves can play a positive role. Green roofs, green walls and rain gardens are just a few of the ways developments can help our cities deal with both heat and flooding.
The solutions are out there, and urban greening is rising in profile. Recent commitments in Melbourne, Canberra and Adelaide are promising. Our study findings are a reminder that, even for the willing, we’ll have to take two steps forward, because there’s inevitably going to be one step back.
At the current rate of warming, the number of days above 40℃ in cities including Melbourne and Brisbane, will double by 2050 – even if we manage to limit future temperature rises to 2℃.
Trees can help cool your home. Two medium-sized trees (8-10m tall) to the north or northwest of a house can lower the temperature inside by several degrees, saving you hundreds of dollars in power costs each year.
Green roofs and walls can reduce urban temperatures, but are costly to install and maintain. Climbing plants, such as vines on a pergola, can provide great shade, too.
Trees also suck up carbon dioxide and extend the life of the paint on your external walls.
2. Keep your street trees alive
Climate change poses a real threat to many street trees. But it’s in everyone’s interests to keep trees on your nature strip alive.
Adequate tree canopy cover is the least costly, most sustainable way of cooling our cities. Trees cool the surrounding air when their leaves transpire and the water evaporates. Shade from trees can also triple the lifespan of bitumen, which can save governments millions each year in road resurfacing.
Tree roots also soak up water after storms, which will become more extreme in a warming climate. In fact, estimates suggest trees can hold up to 40% of the rainwater that hits them.
This shows state laws fail to recognise the value of trees, and we’re losing them when we need them most.
Infrastructure works such as level crossing removals have removed trees in places such as the Gandolfo Gardens in Melbourne’s inner north, despite community and political opposition. Some of these trees were more than a century old.
So what can you do to help? Ask your local council if they keep a register of important trees of your suburb, and whether those trees are protected by local planning schemes. Depending on the council, you can even nominate a tree for protection and significant status.
But once a development has been approved, it’s usually too late to save even special trees.
3. Green our rural areas
Outside cities, we must preserve remnant vegetation and revegetate less productive agricultural land. This will provide shade and moderate increasingly strong winds, caused by climate change.
It’s important to have a fire-smart garden. It might seem counter-intuitive to plant trees around the house to fortify your fire defences, but some plants actually help reduce the spread of fire – through their less flammable leaves and summer green foliage – and screen your house from embers.
Depending on where you live, suitable trees to plant include crepe myrtle, the hybrid flame tree, Persian ironwood, some fruit trees and even some native eucalypts.
If you’re in a bushfire-prone area, landscape your garden by strategically planting trees, making sure their canopies don’t overhang the house. Also ensure shrubs do not grow under trees, as they might feed fire up into the canopy.
And in bad fire conditions, rake your garden to put distance between fuel and your home.
But it is heartening to remember that bushfire can be a boon to some plants and animals. We’re already seeing fresh green shoots as plants and trees resprout. Beetles and other insects are making short work of animal carcasses; they will soon be followed by the birds which feed on them.
Australia’s worsening fire regimes are challenging even these tolerant species. But let’s take a look at exactly how life is returning to our forests now, and what to expect in coming months.
The science of resprouting
Of course, bushfires kill innumerous trees – but many do survive. Most of us are familiar with the image of bright green sprouts shooting from the trunks and branches of trees such as eucalypts. But how do they revive so quickly?
The secret is a protected “bud bank” which lies behind thick bark, protected from the flames. These “epicormic” buds produce leaves, which enables the tree to photosynthesise – create sugar from the sun so the tree can survive.
Under normal conditions, hormones from shoots higher in the tree suppress these buds. But when the tree loses canopy leaves due to fire, drought or insect attack, the hormone levels drop, allowing the buds to sprout.
This summer’s fires left in their wake a mass of decaying animal carcasses, logs and tree trunks. While such a loss can be devastating for many species – particularly those that were already vulnerable – many insects thrive in these conditions.
For example, flies lay eggs in the animal carcasses; when the maggots hatch, the rotting flesh provides an ample food source. This process helps break down the animal’s body – reducing bacteria, disease and bad smells. Flies are important decomposers and their increased numbers also provide food for birds, reptiles and other species.
Similarly, beetles such as the grey furrowed rosechafer, whose grubs feed on decaying logs and tree trunks, add nutrients to the soil when they defecate which helps plants grow again.
Insects also benefit from the mass of new leaves on trunks and branches. For example, native psyllids – an insect similar to aphids – feed on the sap from leaves and so thrive on the fresh growth.
Then come the birds
Once insects start to move back into an area from forested areas nearby, the birds that eat them will follow.
An increase in psyllids encourages honeyeaters – such as bell miners and noisy miners – to return. These birds are considered pests.
A CSIRO study after bushfires in Victoria’s East Gippsland in 1983 found several native bird species – flame and scarlet robins, the buff-rumped thornbill and superb fairy-wren – increased quickly to levels greater than before fire. As shrubs in the understorey regrow, other species will move in, slowly increasing biodiversity.
Since the recent bushfire in woodland near Moonbi in New South Wales, numerous bird species have returned. On a visit over this past weekend, I observed currawongs landing in the canopy, saw fairy wrens darting in and out of foliage sprouting from the ground, and heard peep wrens in tufts of foliage on bark and high branches.
Honeyeaters moved between burnt and intact trees on the edge of the blackened forest and butterflies visited new plants flowering after recent rain.
Weeds can help
Weeds usually benefit when fire opens up the tree canopy and lets in light. While this has a downside – preventing native plants from regenerating – weeds can also provide cover for native animal species.
A study I co-authored in 2018 found highly invasive Lantana camara provided habitat for small mammals such as the brown rat in some forests. Mammal numbers in areas where lantana was present were greater than where it was absent.
Lantana often grows quickly after fire due to the increase in light and its ability to suppress other plant growth.
Is there hope for threatened species?
Generalist species – those that thrive in a variety of environments – can adapt to burnt forest. But specialist species need particular features of an ecosystem to survive, and are far less resilient.
It requires large fires to create a specific habitat: big dead trees provide hollows for shelter and nesting, and insects feeding on burnt wood and carcasses provide a food source.
But for the Leadbeater’s possum to benefit from the fire regime, bushfires should be infrequent – perhaps every 75 years – allowing time for the forest to grow back. If fires are too frequent, larger trees will not have time to establish and hollows will not be created, causing the species’ numbers to decline.
Similarly in NSW, at least 50% and up to 80% of the habitat of threatened species such as the vulnerable rufous scrub-bird was burnt in the recent fires, an environmental department analysis found.
Only time will tell whether biodiversity in these areas is forever damaged, or will return to its former state.
Large fires may benefit some native species but they also provide food and shelter for predatory species, such as feral cats and foxes. The newly open forest leaves many native mammals exposed, changing the foodweb, or feeding relationships, in an ecosystem.
This means we may see a change in the types of birds, reptiles and mammals found in forests after the fires. And if these areas don’t eventually return to their pre-fire state, these environments may be changed forever – and extinctions will be imminent.
The alarming rate of carbon dioxide flowing into our atmosphere is affecting plant life in interesting ways – but perhaps not in the way you’d expect.
Despite large losses of vegetation to land clearing, drought and wildfires, carbon dioxide is absorbed and stored in vegetation and soils at a growing rate.
This is called the “land carbon sink”, a term describing how vegetation and soils around the world absorb more carbon dioxide from photosynthesis than they release. And over the past 50 years, the sink (the difference between uptake and release of carbon dioxide by those plants) has been increasing, absorbing at least a quarter of human emissions in an average year.
So, to put it simply, humans are producing more carbon dioxide. This carbon dioxide is causing more plant growth, and a higher capacity to suck up carbon dioxide. This process is called the “carbon dioxide fertilisation effect” – a phenomenon when carbon emissions boost photosynthesis and, in turn, plant growth.
What we didn’t know until our study is just how much the carbon dioxide fertilisation effect contributes to the increase in global photosynthesis on land.
But don’t get confused, our discovery doesn’t mean emitting carbon dioxide is a good thing and we should pump out more carbon dioxide, or that land-based ecosystems are removing more carbon dioxide emissions than we previously thought (we already know how much this is from scientific measurements).
And it definitely doesn’t mean mean we should, as climate sceptics have done, use the concept of carbon dioxide fertilisation to downplay the severity of climate change.
Rather, our findings provide a new and clearer explanation of what causes vegetation around the world to absorb more carbon than it releases.
What’s more, we highlight the capacity of vegetation to absorb a proportion of human emissions, slowing the rate of climate change. This underscores the urgency to protect and restore terrestrial ecosystems like forests, savannas and grasslands and secure their carbon stocks.
And while more carbon dioxide in the atmosphere does allow landscapes to absorb more carbon dioxide, almost half (44%) of our emissions remain in the atmosphere.
More carbon dioxide makes plants more efficient
Since the beginning of the last century, photosynthesis on a global scale has increased in nearly constant proportion to the rise in atmospheric carbon dioxide. Both are now around 30% higher than in the 19th century, before industrialisation began to generate significant emissions.
So how does more carbon dioxide lead to more plant growth anyway?
Higher concentrations of carbon dioxide make plants more productive because photosynthesis relies on using the sun’s energy to synthesise sugar out of carbon dioxide and water. Plants and ecosystems use the sugar both as an energy source and as the basic building block for growth.
When the concentration of carbon dioxide in the air outside a plant leaf goes up, it can be taken up faster, super-charging the rate of photosynthesis.
More carbon dioxide also means water savings for plants. More carbon dioxide available means pores on the surface of plant leaves regulating evaporation (called the stomata) can close slightly. They still absorb the same amount or more of carbon dioxide, but lose less water.
The resulting water savings can benefit vegetation in semi-arid landscapes that dominate much of Australia.
We saw this happen in a 2013 study, which analysed satellite data measuring changes in the overall greenness of Australia. It showed more leaf area in places where the amount of rain hadn’t changed over time. This suggests water efficiency of plants increases in a carbon dioxide-richer world.
Young forests help to capture carbon dioxide
In other research published recently, we mapped the carbon uptake of forests of different ages around the world. We showed forests regrowing on abandoned agricultural land occupy a larger area, and draw down even more carbon dioxide than old-growth forests, globally. But why?
In a mature forest, the death of old trees balances the amount of new wood grown each year. The old trees lose their wood to the soil and, eventually, to the atmosphere through decomposition.
A regrowing forest, on the other hand, is still accumulating wood, and that means it can act as a considerable sink for carbon until tree mortality and decomposition catch up with the rate of growth.
This age effect is superimposed on the carbon dioxide fertilisation effect, making young forests potentially very strong sinks.
In fact, globally, we found such regrowing forests are responsible for around 60% of the total carbon dioxide removal by forests overall. Their expansion by reforestation should be encouraged.
Forests are important to society for so many reasons – biodiversity, mental health, recreation, water resources. By absorbing emissions they are also part of our available arsenal to combat climate change. It’s vital we protect them.
On Friday I flew in a helicopter over the fire-ravaged Kosciuszko National Park. I was devastated by what I saw. Cherished wildlife species are at grave risk of extinction: those populations the bushfires haven’t already wiped out are threatened by thousands of feral horses trampling the land.
Australia’s plants and ecosystems did not evolve to withstand trampling by hard-hooved animals, or their intensive grazing. Unfortunately, the New South Wales government has allowed the population of feral horses in the park to grow exponentially in recent years to around 20,000.
I flew over the northern part of the park with members of the Invasive Species Council, who were conducting an urgent inspection of the damage. Thousands of hectares were completely incinerated by bushfires: not a green leaf was visible over vast areas. A cataclysm has befallen the western face of the mountains and tablelands around Kiandra and Mount Selwyn.
Further north and east of Kiandra the fires were less intense and burnt patchily. On Nungar Plain the grassland and peat wetlands were only lightly burnt, and the first green shoots were already visible along the wetlands of the valley floor.
At first, I wondered if the fires may have spared two animals which live in tunnels in the vegetation on the sub-alpine high plains: the alpine she-oak skink and broad-toothed rat (which, despite the name is a cute, hamster-like creature).
But not only was their understory habitat burnt, a dozen feral horses were trampling the peat wetlands and eating the first regrowth.
On the unburnt or partially burnt plains a few ridges over, 100 or more horses were mowing down the surviving vegetation.
Precarious wildlife refuges
Next we flew over a small stream that holds the last remaining population of a native fish species, the stocky galaxias. A small waterfall is all that divides the species from the stream below, and the jaws of the exotic trout which live there.
The aftermath of the fires means the last refuge of the stocky galaxias is likely to become even more degraded.
Over the years, feral horses have carved terraces of trails into the land causing erosion and muddying the stream bank. As more horses congregate on unburnt patches of vegetation after the fires, more eroded sediment will settle on the stream bed and fill the spaces between rocks where the fish shelter. Ash runoff entering the stream may clog the gills of the fish, potentially suffocating them.
Many key wetland habitats of the southern and northern corroboree frogs have also been burnt. These striking yellow and black frogs nest in wetland vegetation.
We hovered over a key wetland for the northern corroboree frog that had not been burnt, deep in the alpine forest. A group of feral horses stood in it. They had created muddy wallows, trampled vegetation and worn tracks that will drain the wetland if their numbers are not immediately controlled.
Flying down the upper Murrumbidgee River’s Long Plain, I saw large numbers of feral horses gathered in yet more wetlands. Displaced by the fires to the south and west, they were already trampling the mossy and heathy wetlands that store and filter water in the headwaters.
The Murrumbidgee River is a key water source for south-east Australia. The horses stir up sediment and defecate in the water. They create channels which drain and dry the wetlands, exposing them to fire.
One-third of Kosciuszko National Park has been burnt out and at the time of writing the fires remain active. Feral horses are badly compounding the damage.
If we don’t immediately reduce feral horse numbers, the consequences for Kosciuszko National Park and its unique Australian flora and fauna will be horrendous.
Responsible managers limit the numbers of livestock on their lands and control feral animals. The NSW government must repeal its 2018 legislation protecting feral horses in Kosciuszko National Park, and undertake a responsible control program similar to those of the Australian Capital Territory and Victorian governments.
Without an emergency cull of feral horses in Kosciuszko National Park, burnt vegetation may not fully recover and threatened species will march further towards extinction.