Every autumn we are treated to one of nature’s finest seasonal annual transitions: leaf colour change and fall.
Most of the autumn leaf-shedding trees in Australia are not native, and some are declared weeds. Nevertheless, Australia has a spectacular display of trees, from the buttery tresses of Ginkgo biloba to the translucent oaks, elms and maples.
Autumn colour changes are celebrated worldwide and, when the time is right, autumn leaves reconnect us to nature, driving “leaf-peeping” tourist economies worldwide.
However, recent temperature trends and extremes have changed the growing conditions experienced by trees and are placing autumn displays, such as Canberra’s, at risk.
This year, Canberra, like the rest of Australia, endured its hottest summer on record. In NSW and the ACT, the mean temperature in January was 6°C warmer than the long-term average. So far, autumn is following suit.
These extremes can interrupt the ideal synchronisation of seasonal changes in temperature and day length, subduing leaf colours.
In addition, hotter summer temperatures scorch leaves and, when combined with this and the previous years’ low autumn rainfall, cause trees to shed leaves prematurely, dulling their autumn leaf displays.
The subtlety of change
We learnt in childhood autumn colour change follows the arrival of cooler temperatures. Later we learnt the specifics: seasonal changes in day length and temperature drive the depletion of green chlorophyll in leaves. Temperature can also affect the rate at which it fades.
In the absence of chlorophyll, yellows and oranges generated by antioxidants in the leaf (carotenoids) as well as red through to purples pigments (anthocyanins), synthesised from stored sugars, emerge. Temperature plays a role here too – intensifying colours as overnight temperatures fall.
We’ve also come to understand the role of a leaf’s environment. Anthocyanin production is affected by light intensity, which explains why sunny autumns produce such rich colours and why the canopies of our favourite trees blush red at their edges while glowing golden in their interior.
However, early signs show this year’s autumn tones will be muted. After the record-breaking heat of summer and prolonged heat of March, many trees are shrouded in scorched, faded canopies. The ground is littered with blackened leaves.
Of course, we’ve seen it before.
During the Millennium Drought, urban trees sporadically shed their leaves often without a hint of colour change. Fortunately, that was reversed at the drought’s end.
But we’re kidding ourselves if we believe this last summer was normal or recent temperature trends are just natural variability. If this is a sign of seasons future, we need to prepare to lose some of autumn’s beauty.
Long-term and experimental data show that the sensitivity of autumn colour change to warmer temperatures varies widely between species. While large-scale meta-analyses point to a delay in the arrival of autumn colours of one day per degree of warming, individual genera may be far more sensitve. Colour change in Fagus is delayed by 6-8 days per degree.
Warming temperatures, then, mean the cohesive leaf-colour changes we’re accustomed to will break down at landscape scales.
In addition, as warm weather extends the growing season and deep-rooted trees deplete soil moisture reservoirs, individual trees are driven by stress rather than seasonal temperature change and cut their losses. They shed leaves at the peripheries of their canopies.
The remainder wait – bronzed by summer, but still mostly green – for the right environmental cue.
For years, careful species selection and selective breeding enhanced autumn colour displays. This rich tapestry is now unravelling as hotter summers, longer autumns and drought affect each species differently.
Paradoxes and indirect effects
It seems logical warmer temperatures would mean shorter and less severe frost seasons. Paradoxically, observations suggest otherwise – the arrival of frost is unchanged or, worse, occurring earlier.
When not preceded by gradually cooling overnight temperatures, frosts can induce sudden, unceremonious leaf loss. If warm autumn temperatures fail to initiate colour change, autumn displays can be short-circuited entirely.
At the centre of many urban-tree plantings, our long association with elms faces a threat. Loved for the contrast their clear yellow seasonal display creates against pale autumn skies, elm canopies have been ravaged by leaf beetles this year. Stress has made trees susceptible to leaf-eating insects, and our current season delivered an expanse of stressed, and now skeletal, trees.
This dulled image of autumn is far from universal. Climates differ between locations. So too will the climate changes we’ve engineered and their impact on autumn displays.
Increased concentration of anthocyanins associated with warmer summers has, for example, created spectacular leaf displays in Britain’s cooler climates.
Of course, we’ll continue to experience radiant autumn displays too.
In years of plentiful rain, our trees will retain their canopies and then, in the clear skies of autumn, dazzle us with seasonal celebrations. However, that too may be tempered by the increased risk of colour-sapping pathogens, such as poplar rust, favoured by warm, moist conditions. And there are also negative consequences for autumn colour associated with elevated carbon dioxide concentrations.
Of course, we need to keep it in perspective – the dulling of autumn’s luminescence is far from the worst climate change impacts. Nevetheless, in weakening our link with nature, the human psyche is suffering another self-inflicted cut as collective action on climate change stalls.
Most citizen science initiatives ask people to record living things, like frogs, wombats, or feral animals. But dead things can also be hugely informative for science. We have just launched a new citizen science project, The Dead Tree Detective, which aims to record where and when trees have died in Australia.
The current drought across southeastern Australia has been so severe that native trees have begun to perish, and we need people to send in photographs tracking what has died. These records will be valuable for scientists trying to understand and predict how native forests and woodlands are vulnerable to climate extremes.
Understanding where trees are most at risk is becoming urgent because it’s increasingly clear that climate change is already underway. On average, temperatures across Australia have risen more than 1℃ since 1910, and winter rainfall in southern Australia has declined. Further increases in temperature, and increasing time spent in drought, are forecast.
How our native plants cope with these changes will affect (among other things) biodiversity, water supplies, fire risk, and carbon storage. Unfortunately, how climate change is likely to affect Australian vegetation is a complex problem, and one we don’t yet have a good handle on.
All plants have a preferred average climate where they grow best (their “climatic niche”). Many Australian tree species have small climatic niches.
It’s been estimated an increase of 2℃ would see 40% of eucalypt species stranded in climate conditions to which they are not adapted.
But what happens if species move out of their climatic niche? It’s possible there will be a gradual migration across the landscape as plants move to keep up with the climate.
It’s also possible that plants will generally grow better, if carbon dioxide rises and frosts become less common (although this is a complicated and disputed claim.
However, a third possibility is that increasing climate extremes will lead to mass tree deaths, with severe consequences.
There are examples of all three possibilities in the scientific literature, but reports of widespread tree death are becoming increasingly commonplace.
Many scientists, including ourselves, are now trying to identify the circumstances under which we may see trees die from climate stress. Quantifying these thresholds is going to be key for working out where vegetation may be headed.
The water transport system
Australian plants must deal with the most variable rainfall in the world. Only trees adapted to prolonged drought can survive. However, drought severity is forecast to increase, and rising heat extremes will exacerbate drought stress past their tolerance.
To explain why droughts overwhelm trees, we need to look at the water transport system that keeps them alive. Essentially, trees draw water from the soil through their roots and up to their leaves. Plants do not have a pump (like our hearts) to move water – instead, water is pulled up under tension using energy from sunlight. Our research illustrates how this transport system breaks down during droughts.
In hot weather, more moisture evaporates from trees’ leaves, putting more pressure on their water transport system. This evaporation can actually be useful, because it keeps the trees’ leaves cool during heatwaves. However if there is not enough water available, leaf temperatures can become lethally high, scorching the tree canopy.
We’ve also identified how drought tolerance varies among native tree species. Species growing in low-rainfall areas are better equipped to handle drought, showing they are finely tuned to their climate niche and suggesting many species will be vulnerable if climate change increases drought severity.
Based on all of these data, we hope to be able to predict where and when trees will be vulnerable to death from drought and heat stress. The problem lies in testing our predictions – and that’s where citizen science comes in. Satellite remote sensing can help us track overall greenness of ecosystems, but it can’t detect individual tree death. Observation on the ground is needed.
However, there is no system in place to record tree death from drought in Australia. For example, during the Millennium Drought, the most severe and extended drought for a century in southern Australia, there are almost no records of native tree death (other than along the rivers, where over-extraction of water was also an issue). Were there no deaths? Or were they simply not recorded?
The current drought gripping the southeast has not been as long as the Millennium Drought, but it does appear to be more intense, with some places receiving almost no rain for two years. We’ve also had a summer of repeated heatwaves, which will have intensified the stress.
We’re hearing anecdotal reports of tree death in the news and on twitter. We’re aiming to capture these anecdotal reports, and back them up with information including photographs, locations, numbers and species of trees affected, on the Dead Tree Detective.
We encourage anyone who sees dead trees around them to hop online and contribute. The Detective also allows people to record tree deaths from other causes – and trees that have come back to life again (sometimes dead isn’t dead). It can be depressing to see trees die – but recording their deaths for science helps to ensure they won’t have died in vain.
Australia is one of the world’s most highly urbanised nations – 90% of Australians live in cities and towns, with development concentrated along the coast. This poses a major threat to native wildlife such as the koala, which can easily fall victim to urban development as our cities grow. Huge infrastructure projects are planned for Australian cities in the coming few years.
The need to house more people – the Australian population is projected to increase to as much as 49.2 million by 2066 – is driving ever more urban development, much of it concentrated in our biggest cities on the east coast. This is bad news for the koala population, unless the species’ needs are considered as part of planning approvals and the creation of urban green spaces. The good news is that koalas can learn to live the “green city life” as long as they are provided with enough suitable gum trees in urban green spaces.
Indeed, our newly published research, which analysed stress levels in wild koalas according to their habitat, reveals that koalas are the most stressed in rural and rural-urban fringe zones. This appears to be due to factors such as large bushfires, heatwave events, dog attacks, vehicle collision and human-led reduction of prime eucalyptus habitats. Koalas living in urban landscapes are less stressed as long as the city includes suitable green habitats.
In other words, wild animals including the koala can adapt to co-exist with human populations. Their ability to do so depends on us giving them the space, time and freedom to make that adaptation. This means ensuring they can carry out, without undue pressures, the biological and physiological functions on which their survival depends.
Wildlife species that lack access to suitable green habitats in cities are at higher risk of death and local extinction. Having to move between fragmented patches of habitat increases the risks. Land clearing and habitat destruction for infrastructure projects and other urban development are compounding the major threats to koalas, such as being hit by vehicles or attacked by dogs.
How does human pressure cause stress in wildlife?
Animals cope with stressful situations in their lives through very basic life-history adjustments and ecological mechanisms. These include changes in physiology and behaviour in response to stresses in their environment.
We can help make the environment more suitable for wildlife species by ensuring their basic needs for food, water and shelter are met. If animals are deprived of any of these necessities, they will show signs of stress.
So by subjecting wildlife to extrinsic stressors such as habitat clearance, climate change and pollution we are making it even more difficult for these animals to manage stress in their daily lives.
Basically any unwanted change to an animal’s environment that prevents it from performing its basic life-history functions, such as foraging and social behaviour, will cause stress.
So what can be done?
The koalas are telling us it’s a major problem when urban design is not green enough. Innovative solutions are needed!
Cities can do much more for wildlife conservation. Creating safe green spaces for wildlife is critical. Not just koalas but other wildlife such as birds, small mammals, reptiles and frogs can benefit immensely from urban green spaces.
Even in suburbs with plenty of green space, problems still arise because urban planning typically designs this space around access for human recreation and not for the wildlife that was living there before the housing development moved in.
Urban planning should always incorporate the planning of green spaces that are safe for wildlife. Providing wildlife crossings is part of the solution. Another important element is educational programs to alert drivers to the need to look out for koalas.
Measures like this can minimise impacts on wildlife that faces the many challenges of adjusting to city life.
Giant eucalypts play an irreplaceable part in many of Australia’s ecosystems. These towering elders develop hollows, which make them nature’s high-rises, housing everything from endangered squirrel-gliders to lace monitors. Over 300 species of vertebrates in Australia depend on hollows in large old trees.
These “skyscraper trees” can take more than 190 years to grow big enough to play this nesting and denning role, yet developers are cutting them down at an astounding speed. In other places, such as Victoria’s Central Highlands Mountain Ash forests, the history of logging and fire mean that less than 1.2% of the original old-growth forest remains (that supports the highest density of large old hollow trees). And it’s not much better in other parts of our country.
David Lindenmayer explains how these trees form, the role they play – and how very hard they are to replace.
Trees do a lot more for us than you probably think. Their roots prevent soil from eroding, their canopies provide shade and their leaves decompose into nutrients for crops, which feed livestock. Trees provide homes for a diverse range of wildlife and tree crops, such as coffee, rubber, and hardwoods, support countless livelihoods and entire economies. Trees also mark boundaries and hold immense spiritual, cultural and social value for smallholder communities around the world.
In the 1980s, charities proposed planting more trees to halt “desertification” in the Sahara Desert. This involved “afforestation” – planting trees where they had not grown for a while and “reforestation” – replacing recently lost tree cover.
Saving face or saving forests?
Businesses can offset their environmental impact by planting trees or supporting other forms of habitat restoration, so as to “pay off” the damage they cause locally. As climate change escalates, trees are in vogue for their potential to soak up the carbon dioxide we keep putting in the atmosphere.
The United Nations (UN) has even adopted a scheme for offering local communities and governments some sort of financial payout for saving trees from deforestation. This “economy of repair” has been adopted by some of the largest companies in their commitments to corporate social responsibility. One such programme is the Green Belt Movement – a Kenyan conservation NGO started by the late professor and Nobel Prize recipient Wangari Maathai.
Maathai’s original mission was to empower local people, particularly women, to overcome inequality through leading forest restoration and resisting the expanding Sahara Desert. Despite the involvement of charities and businesses, research has suggested that in programmes like these, it is farmers and local people, not companies, which make the biggest contributions to planting new trees. Since local people also inherit responsibility for them, it’s important that projects devised by outside parties are planned and executed wisely, and in the community’s interest.
While some may argue that tree planting is a win-win for the environment whoever does it, offsetting is just another way of corporate greenwashing. Environmental damage in one place cannot somehow be fixed by repairing habitats elsewhere, sometimes on the other side of the world.
Here are some of the ways in which indiscriminate tree planting can cause more harm than good.
Plantations are not forests
Diverse forests are often cleared for agricultural production or industrial use, and replaced by uniform stands of the same species selected because of their ability to grow fast.
Tropical forests in some cases take up to 65 years to regrow and their diversity cannot be replicated by a monoculture of reforested plots.
Reforestation and afforestation schemes must decide which species are appropriate to plant – native or exotic, multi-purpose or fast growing, naturally regenerating forests or managed plantations. Sometimes the wrong species are selected and Eucalyptus (Eucalyptus globulus) is one such poor choice.
Eucalyptus is usually chosen because it is fast growing and economically valuable. Yet, it is exotic to many places it is now planted and requires lots of water, which drains the water table and competes with native crops.
In Europe, replacing broad-leafed native oak trees with faster growing conifers has meant that forest cover on the continent is 10% greater than it was before the industrial revolution. However, the new trees are not as good at trapping carbon but do trap heat more efficiently, contributing to global warming. Clearly, tree planting without due caution can do more harm than good.
Trees need care – lots of it
Tree species take a long time to grow and need continual care. However, tree planting schemes usually “plant and go” –- meaning they do not put resources into managing the trees after they are placed into the ground. Young trees are particularly vulnerable to disease and competition for light and nutrients and if not cared for, will eventually die.
Trees are political
Trees planted by states or private donors may choose sites without consulting local communities, ignoring any of their customary land rights and management regimes. This locally-owned land may be in fallow or have different economic, cultural or spiritual uses.
Blundering into planting in these places may exacerbate tensions over land tenure, spreading disinterest in tree care and stewardship. Dispossessed locals may move to existing forests and clear land for food production. Tenure rights over trees are also not always owned by whole households either, but divided between gender. Planting trees and asking questions later may sow tensions over land ownership for long after the project departs.
It’s no surprise that trees are on the green economy agenda, but this does not necessarily mean that planting them is “green” or helpful for social harmony. Allowing trees to regrow naturally is not always effective either, as trees are unlikely to survive on their own. Community involvement is therefore crucial.
This means real consultation over site and species selection, property rights over the trees, their products, and the land they grow in and who takes on the labour to keep the trees alive after they are planted. If companies are serious about planting trees then they need to care about the communities that live with them and not just their own reputations.
Sign up to the Beating Around the Bush newsletter here, and suggest a plant we should cover at firstname.lastname@example.org.
Grass trees (genus Xanthorrhoea) look like they were imagined by Dr Seuss. An unmistakable tuft of wiry, grass-like leaves atop a blackened, fire-charred trunk. Of all the wonderfully unique plants in Australia, surely grass trees rank among the most iconic.
The common name grass tree is a misnomer: Xanthorrhoea are not grasses, nor are they trees. Actually, they are distantly related to lilies. Xanthorrhoea translates to “yellow flow”, the genus named in reference to the ample resin produced at the bases of their leaves.
All 28 species of grass tree are native only to Australia. Xanthorrhoea started diversifying around 24-35 million years ago – shortly after the Eocene/Oligocene mass extinctions – so they have had quite some time to adapt to Australian conditions.
Wander through remnant heathland or dry sclerophyll forest, particularly throughout the eastern and south-western regions of Australia, and you’ll likely find a grass tree.
Perfectly adapted to their environment
Xanthorrhoea are perfectly adapted to the Australian environment, and in turn, the environment has adapted to Xanthorrhoea. Let’s start the story from when a grass tree begins as a seed.
After germination, Xanthorrhoea seedlings develop roots that pull the growing tip of the plant up to 12cm below the soil surface, protecting the young plant from damage. These roots quickly bond with fungi that help supply water and minerals.
Once the tip of the young plant emerges above ground, it is protected from damage by moist, tightly packed leaf bases, although shoots may develop if it is damaged. The leaves of Xanthorrhoea are tough, but they lack prickles or spines to deter passing herbivores. Instead, they produce toxic chemicals with anaesthetising effects.
All Xanthorrhoea are perennial; some species are estimated to live for over 600 years. Most grow slowly (0.8–6 cm in height per year), but increase their rate of growth in response to season and rainfall. The most “tree-like” species grow “trunks” up to 6 metres tall, while trunkless species grow from subterranean stems.
Grass trees don’t shed their old leaves. The bases of their leaves are packed tightly around their stem, and are held together by a strong, water-proof resin.
As the old leaves accumulate, they form a thick bushy “skirt” around the trunk. This skirt is excellent habitat for native mammals. It’s also highly flammable. However, in a bushfire, the tightly-packed leaf bases shield the stem from heat, and allow grass trees to survive the passage of fire.
Xanthorrhoea can recover quickly after a fire thanks to reserves of starch stored in their stem. By examining the size of a grass tree’s skirt, we can estimate when a fire last occurred.
It can take over 20 years before a grass tree produces its first flowers. When they do flower it can be spectacular, producing a spike and scape up to four metres long advertising hundreds of nectar-rich, creamy-white flowers to all manner of fauna. Flowering is not dependent on fire, but it stimulates the process. The ability of grass trees to resprout after fire and quickly produce flowers makes them a vital life-line for fauna living in recently-burnt landscapes.
Grass trees provide food for birds, insects, and mammals, which feast on the nectar, pollen, and seeds. Beetle larvae living within the flower spikes are a delicacy for cockatoos. Invertebrates such as green carpenter bees build nests inside the hollowed out scapes of flowers. Small native mammals become more abundant where grass trees are found, for the dense, unburnt skirt of leaves around the trunk provides shelter and sites for nesting.
Indigenous use of grass trees
For Indigenous people living where grass trees grow, they were (and remain) a resource of great importance.
The resin secreted by the leaf-bases was used as an adhesive to attach tool heads to handles and could be used as a sealant for water containers. This valuable and versatile resin was an important item of trade.
The base of the flowering stem was used as the base of composite spear shafts, and when dried was used to generate fire by hand-drill friction. The flowers themselves could be soaked in water to dissolve the nectar, making a sweet drink that could be fermented to create a lightly alcoholic beverage.
When young, the leaves of subspecies Xanthorrhoea australis arise from an underground stem which is seasonally surrounded by sweet, succulent roots that can be eaten. The soft leaf bases also were eaten, and the seeds were collected and ground into flour. Edible insect larvae residing at the base of grass tree stems could be collected. Honey could be collected from flower stems containing the hives of carpenter bees.
European settlers were quick to clue onto the usefulness of the resin , using it in the production of medicines, as a glue and varnish, and burning it as incense in churches. It was even used as a coating on metal surfaces and telephone poles, and used in the production of wine, soap, perfume and gramophone records.
Resin can easily be collected from around the trunk of plants, but early settlers used more destructive methods, removing whole plants on an industrial scale. The resin was exported worldwide; during 1928-29, exported resin was valued at over £25,000 (equivalent to A$2 million today!).
We still have much to learn about grass trees. Current research indicates an extract from one subspecies can be used as a cheap, environmentally-friendly agent to synthesise silver nanoparticles that are useful for their antibacterial properties.
Threats to grass trees
Many of the oldest grass trees have been lost to land clearing, illegal collection, and changes to fire regimes. It’s vital we care for those remaining. Grass trees are particularly sensitive to Phytophthora cinnamomi, a widespread plant pathogen that is difficult to detect and control, and kills plants by restricting movement of water and nutrients through the vascular tissue.
Growing native plants can be a wonderful way to contribute to the conservation of genetic diversity, and attract native fauna into your garden. Grass trees certainly make an interesting conversation plant!
It’s hard to spread the idiot fruit
They can easily be grown at home, provided they’re sourced from a reputable supplier. The best way is to grow from seed, but patience is required as growth can be slow. Despite being relatively hardy, grass trees do not like being moved once large or established, so translocation of plants is not advised. In my opinion, the best way to see grass trees in their true splendour is to visit them in their natural habitat.