This story is from Cosmos Magazine Issue 93, published in December 2021. Subscribe to read more here.
Photography by Ian Connellan
The drive to Currency Creek Arboretum, an hour south of Adelaide, meanders through the picturesque Mt Lofty Ranges. It’s a landscape of quaint sandstone cottages on gently rolling hills and lush pastures scattered with vast spreading gum trees. The charming scenery feels like some kind of “Australian pastoral” in a tableau made famous by Hans Heysen.
The gum trees are the crowning glory – ubiquitous and distinctive stalwarts of Australian woodlands across the continent. Ninety per cent of Australian forests are dominated by eucalypts, and 98% of eucalypts are endemic to Australia. For the most part, we lump them together under a single moniker, barely paying any attention to which of the 900-odd species and subspecies any particular tree might be.
To the untrained eye, gum trees present a uniform forest of dull grey-green that does not change from one season to the next: “never-greens” rather than evergreens. And yet they are not so much all the same as all very different. It is their individuality as much as their uniformity that makes them so hard to distinguish.
I assume that most of the trees we pass in the paddocks are river red gums (Eucalyptus camaldulensis), with their broad, arching canopies and twisted trunks. But the taller narrow ones might equally be South Australian blue gums (E. leucoxylon). The oversized white pillars at the intersections of river and road are probably statuesque mountain white gums (E. dalyrympleana), but their canopy is too high to see from the car. The stringybarks and manna gums blur into one another as we speed along and I have no way of knowing if the dense clusters of low-growing gums in the distance are cup gums (E. cosmophylla) or grey box (E. microcarpa), or something else entirely.
Heysen painted these gums because he was worried they were being cut down for firewood, undervalued and underappreciated. His work invested them with a cultural capital that matched their ecological value. But there is no future for these lone survivors. They are monuments; the last trees standing from the vanished forest ecosystems that once blanketed these hills.
As we wind south, the landscape opens out into patchworked paddocks of golden grasses with distant views of the Coorong and the sea. For Dean Nicolle, this cleared landscape was ideal. The mild Mediterranean climate, low alkaline soil, moderate rainfall and lack of woody vegetation provided a blank canvas for his life’s work. And it’s here, at Currency Creek, over 32 hectares and the last three decades, that Nicolle has planted 10,600 individual trees, representing eight Angophora species, 100 Corymbia and fully 872 Eucalyptus species or subspecies: some 97% of all known members of the Eucalypteae tribe.
It’s a taxonomic tapestry compressing 45 million years of evolution from across the continent into a single point in time and space. By attempting to grow every Australian eucalypt species in the one place, under uniform conditions, Nicolle has not only created the largest single collection of living eucalypts in the world, but a remarkable natural experiment.
Nicolle’s passion for eucalypts began when he was about eight years old. His dad gave him a copy of Ivan Holliday’s A Gardener’s Guide to Eucalypts and he read it from cover to cover, memorising every page.
“I loved that book,” he says. “All the pictures of all the different trees just fascinated me.”
As a teenager, Nicolle tracked down unusual eucalypts to plant on his parents’ four-hectare block on the edge of Adelaide’s southern suburbs. By the time he was 16, he’d decided on a plan.
“I’d like to be a eucalyptologist,” he told a newspaper at the time. “I now have about 170 species. My aim is to grow every eucalypt tree there is.”
It wasn’t long before he started running out of room.
In 1991, 17-year-old Nicolle went looking for one of the rarest species – Ramel’s mallee (E. rameliana) – a plant he’d only ever seen depicted in a watercolour on the final page of Stan Kelly’s classic book Eucalypts.
This species had been collected by the explorer Ernest Giles on one of his journeys across the stony Gibson Desert between Adelaide and Western Australia. Giles delivered the specimen to the director of the Melbourne Botanic Gardens, Ferdinand von Mueller, who described it in Latin as trans montes, coming from the other side of the remote Alfred and Marie Range, although which side and how far depends, I suppose, on where you come from.
Before the trip, Nicolle and his dad trained by carrying 35 kg backpacks full of bricks up the nearby hills, and his parents hired a four-wheel-drive car and loaded it with camping equipment and supplies. In the days before GPS, walking into the desert meant keeping to a strict compass course and carrying a plastic jerrycan of water each. Nicolle and his dad walked for four days west across desert country while his mother and younger brother waited for them on the dusty track known as the Gunbarrel Highway. Every morning and evening they’d call in on a two-way radio, trying to work how far they had walked and how far they still had to go.
They never found the elusive E. rameliana, (it was found later, in some abundance, 800km further west in the red sandy dunes). But Nicolle’s proved useful all the same. In preparation for their trip, Nicolle had asked to see the original specimen of E. rameliana in the Melbourne Herbarium, only to find that it was on loan to a botanist in Canberra. This led to an introduction to Ian Brooker, one of Australia’s leading experts in eucalypts, who became a mentor and colleague, encouraging Nicolle to study botany at university.
His undergraduate studies prompted Nicolle to take a more systematic approach. He’d grow four of each different species, subspecies or population. And so, in the early 1990s, he planted out the first block of the Currency Creek Arboretum. Every year since, a new block has been planted, telling the story of another collecting year – trees from the north, west, south and east.
We map genomes now as we once mapped continents, in search of theoretical rewards. Genomic megastudies promise to predict variation best suited for a changing climate, or to identify new traits useful for silviculture. In species as malleable and developmentally idiosyncratic as eucalypts, though, I wonder if the proof is not in the genotypic ingredients, but the phenotypic pudding. For all its complexity, genetic analysis is quick, relative to field work. Put a tree in the ground and wait 10, 20, 100 years. It’s the kind of study that takes time, not tech.
Genetics is not just about predicting the future, but revealing the past. Genes record the history of species, when they appeared, split and diversified. Unmistakeable fossil “gumnuts” have been found in South America, suggesting a Gondwanan ancestry of some 60 million years. But recent molecular studies using samples from Currency Creek suggest that most eucalypt diversification occurred in just the last two million years.
Deciding which trees are separate species and which are clusters of variable forms is difficult. The numbers are constantly being revised, but by some recent accounts there are as many as 1,013 distinct species and subspecies within the tribe known as Eucalypteae in Australia. Most – 891 to be precise – are classified as “true” eucalypts in the genus Eucalyptus; 109 are currently classified as Corymbia (bloodwoods, ghost gums and spotted gums); and a further 13 are species of Angophora – nicknamed “apples”, perhaps by homesick European colonists. That our continent is dominated by so many species of a single tribe of trees is remarkable and contrasts with the vast monodominant conifer and beech forests of the northern hemisphere.
I ask Nicolle how many eucalypt species might be considered widespread. “Only a dozen or so, I’d say,” he replies. “The river red gums, the coolibahs and ghost gums, for example. But most species only have a distribution of about 50km or so.”
Flicking through Nicolle’s guide to South Australian eucalypts, the patterns of the distribution maps spin past like a flipboard animation. For the bulk of the species, the distributions maps are bare – pinprick ranges so small they are hardly visible. In any one location in Australia, there might only be half a dozen varieties growing naturally – Nicolle says only three species are native to the location of the arboretum. What could have made the eucalypts so extraordinarily diverse when Australia has so few mountains and rivers to divide populations and generate the conditions of geographic, and hence reproductive, isolation necessary for speciation?
The last few million years were characterised by the Pleistocene climate oscillations, which had a huge impact on the evolution of the Australian biota. As the climate bounced through extremes of dry and wet, the forests alternately contracted and expanded. On each contraction the forests and their associated wildlife were isolated from one another, creating the conditions for speciation – as well as driving waves of extinction, particularly among megafaunal species. It is climate – increasing aridity and recurrent drought – rather than rivers and mountains, that is thought to have been the major driver of the remarkably recent eucalypt radiation.
Paul Rymer started working with the Currency Creek Arboretum following a citizen science program. Rymer – an evolutionary ecologist at Western Sydney University’s Hawkesbury Institute for the Environment – and his colleagues were using genetic and physiological models to predict vulnerability to drought and to map die-back, but lacked the field observations to validate their predictions.
Prior to the devastating droughts of 2019-20, Belinda Medlyn set up the Dead Tree Detective to encourage people from across Australia to record and photograph the details of tree die-back events in their local environment. Nicolle got in touch.
Rymer and his colleagues realised that Nicolle’s experience of the drought at the arboretum had much more to tell them than just individual trees struggling.
“The issue with understanding susceptibility to drought,” Rymer says, “is being able to separate out each tree’s inherent sensitivity from their exposure to environmental impacts where the trees grow. Drought conditions vary, even across small areas, and so does the genetic composition of the trees … At Currency Creek you can separate these factors out, because all the trees of different species and different source populations have been planted in the same place. So we can measure the genetic differences among species side-by-side.”
Having thrived and diversified under climate instability, increasing aridity and fire frequency, eucalypts are probably better placed than humans to face an uncertain climate future. Their sclerophyllous leaves, regenerative habits and woody seed capsules have already adapted for the difficult times ahead.
The Currency Creek trees illustrate how important drought adaptations are. Perhaps unsurprisingly, the species that did best during the drought were those grown from seeds collected in hotter, drier climates than Currency Creek’s – particularly semi-arid mallee species. The ones that fared worst were from cooler and wetter climates.
And it’s not just variation between species that matters: many species still contain a mix of ancestral variants from the older wetter forests from which modern drought-adaptations have evolved. There’s a lot of variation within and between species, creating sister species, subspecies and provenances whose relationships and boundaries are still fluid.
As a result of this fluidity, trees from different populations of the same species vary greatly in their response to drought. The trees in the arboretum show that “climate origin” matters on an individual level. Even within the same species, trees that were the offspring of individuals from drier microclimates performed better than those from wetter microclimates. Tapping into this potential adaptability might allow us to save forests by introducing more resilient individuals of the same species.
Understanding drought resilience is more than just genetic variation. How that variation manifests in the tree’s phenotype is crucial. But mapping the functional traits – such as wood density, leaf size, shape and toughness – of widely dispersed tree species is onerous and time-consuming work. By planting so many species in the one area, Nicolle has done a lot of that leg-work for researchers.
“Instead of just looking at a dozen species,” says Rymer, “we’re looking at the drought response of almost all eucalypt species, and undertaking detailed investigation of the functional traits and genomics of 260 species.
“If we can maximise the genetic variation in the trees we plant, we can increase the likelihood of them surviving.”
Each of the four trees that Nicolle plants has been grown from seed collected in the wild, from a single capsule – the same “gumnut” fruit of a mother tree. How do you chose a single individual to represent a species?
“I’m looking for a representative tree,” says Nicolle. “One that has the key characteristics of the species, not an odd or damaged one. But there’s a pragmatic element too. The capsules are often high up, so I have to be able to reach them with the equipment we have – a pole pruner or a throw-rope.”
He’s also on the lookout for anomalies – trees that don’t quite fit the description, in case they’re a new species or if the definition of the species group needs to be expanded.
From each selected tree, Nicolle takes a cutting of leaves with flowers or fruit and presses them between papers, numbered for lodgement in that state’s herbarium. The fruit capsule or gumnut is sealed in a paper bag, labelled with the same number. Once dried, the seeds are put in an airtight jar. Moisture triggers germination for most; a few alpine species need a few weeks in the fridge to simulate winter.
Some trees are more difficult to obtain seed from than others. Most eucalypts store their seeds in their woody capsules, sometimes for years, ready to shed after fire or rain. But the Angophora and many Corymbia flower, fruit, then shed their seeds at one particular (and not always the same) time of year. A seed collector’s timing has to be perfect.
They may have different pollen parents, but all of Nicolle’s seedlings are at least half-siblings. They are as similar as it is possible to be with sexual reproduction in the wild. Eight are raised and four planted under identical conditions. The other four are “vouchered”: preserved as specimens for future reference. The juveniles are all painted by the artist who germinates and raises them.
“The juvenile leaves are highly distinctive in each species,” Nicolle says. “Sometimes they’re the only discriminating feature. You’re not always going to have a mature specimen to identify, with flowers and fruit. We can’t wait five years for them to mature – we need to be able to identify them as seedlings too.”
Despite the close kinship of each quartet of trees, as I walk along the neatly ordered rows I’m struck by the variation between siblings, even though they have all grown from the same capsule of seeds, in the same soil, same environment and with the same treatment. In one species, one individual has grown tall and straight, another has branched low and shrub-like, the third is thin and scraggly, while the final one has fallen sideways and grown along the ground.
I’ve never really appreciated the true diversity of eucalypts until now, seeing them all side by side. Between the blocks of trees the quality of sound and light changes. The uniformity of planting contrasts with the natural idiosyncrasy of eucalypt growth. These trees, some five metres high, are still infants and adolescents, but they are starting to rebel.
Even their characteristic gumnuts are remarkably varied – not just round or oval, warty or smooth, angular or elliptical, but also spiky, elongated, ridged, some reminiscent of rockets or sputniks. The fat rubbery leaves of the Western Australian square-fruited mallee (E. tetraptera) look like they are from a plastic toy; from a distance, the tree looks more like a frangipani or magnolia. By contrast, the broombrush mallee (E. angustissima) has fine needle-like foliage and tiny clustered buds along stems that look more like a conifer or sheoak. I find myself searching for the distinctive gumnut that identifies each one as a eucalypt even when they superficially resemble a bottlebrush, a tea-tree, or a grevillea. Evidence of convergent evolution in practice.
Such diversity in a single genus seems reminiscent of oceanic island biota, where a single founding organism diversifies into a vast range of species. Australia can be called the smallest continent or the largest island, but biologically it shares characteristics of both. It carries the legacy of its Gondwanan heritage, but its long isolation has afforded its biota an extended and solitary evolutionary history, with a continental scale of diverse habitats creating one of the most megadiverse countries in the world. Such blessings also carry their curses. When exposed to colonising species, Australia’s continental-scale biodiversity faces extinction akin to other isolated islands, but amplified by orders of magnitude.
Justin Borevitz, from the Australian National University, is working on landscape genomic modelling. He’d like to see Nicolle’s experiment replicated across multiple sites in Australia, using seeds from the arboretum to create the “next gen” Currency Creek. While the original arboretum compares species in a single environmental location, taking the offspring from this seedbank into new climate zones would capture the interaction between environment and both individual- and species-level variation.
It’s an idea that would magnify the power of the experimental design, with potential to factor out environmental and genetic variation on a massive cross-species scale. But not everyone has Nicolle’s vision, dedication and commitment.
“We’ve got the seeds,” Borevitz says. “But we need the land to plant them on – a philanthropist or someone who has farmland they’d be happy to donate to science and put to a new use.”
It’s not just about a scientific resource. Such arboretums are living seedbanks for future adaptations from which more resilient strains could be selected to suit local conditions. Borevitz argues that we need to move away from our restricted image of a designated set of species that belong in one place. With the climate changing so rapidly, we can’t afford to be locked into local provenance.
“It’s about climate-adjusted provenancing,” says Borevitz. “Not just trees that are locally adapted, but trees that will be future adapted.”
Trees are incredibly long-lived, once they survive the perils of infant mortality. Alpine species can live for 500 years and many of the larger eucalypts also measure their lifespan by centuries. The ecological value of older, hollow-bearing trees is immense – very often these old trees form the structural framework for much of a forest’s biodiversity. This is not about planning for 2030 or 2050, it’s about planning for survival in 2300 or 2500.
For the eucalypts, long generational turnover combined with the relatively recent burst of radiating species means that we are witness to evolution in action, in slow motion. From Borevitz’s perspective it’s not even clear what constitutes a “species”.
How local is local? Borevitz explains that pollen travels long distances; there’s evidence of genetic mixing in a 500km radius around a tree – larger than the entire distribution area of some species. There may only be a 1–2% divergence between some eucalypt species – similar to the amount of diversity between the parents of any individual. Hybridisation is common.
“Some eucalypts are like dogs,” Borevitz says. “They’re all wolves underneath, despite the wide range of breeds. The box ironbarks, for example, are incredibly closely related.”
Given that recent studies have suggested it takes 21–31 million years for eucalypts to develop reproductive isolation, it’s not surprising that hybridisation is common. Reticulate evolution – where species separate, then recombine – seems to have been a hallmark of some species and may explain why species boundaries have proven hard to define. Combining molecular with morphological and ecological research may provide the most promising way forward.
This deeper view of eucalypt evolution leaves us not so much with a list of clear-cut species but an array of genera, subgenera, sections, tribes and intermating species. Perhaps there aren’t a thousand species of eucalypts, but one hundred reproductive groups.
What happens when you cross the pollen of one species with another that lives at a far distance? Nicolle used to remove seedlings that grew between the rows of trees – surplus to requirement. Now he keeps them.
“We need to be looking at the hybrids – what’s growing between the rows in the arboretum,” says Borevitz. “Starting with hybrids might well be the best option.”
“Hopeful monsters” he calls them. They may well be where the future lies.
There’s a tendency to depict Nicolle as a maverick, a bit obsessive, a gum “nut” as it were. He doesn’t work in a university, a herbarium or a museum. But he is no character from a Murray Bail novel. He works as an arborist, ecologist and botanist, as well as being an independent scholar. He has a doctorate from Flinders University, has named and described over 40 new eucalypt species and subspecies, and has published a body of scientific papers as well as authoritative books on eucalypt diversity.
Freed from the constraints of grants, promotions and teaching, Nicolle pursues his scientific vision within the seasonal cycle of collecting, growing, planting, watering, slashing and weeding. You can see it in the types of papers he contributes to, and the types of books he and his partner Annett Börner produce. They are looking for answers in the field rather than ones grounded in theoretical debates. It reminds me that science is not all about flash laboratories, citation indices and institutional authority – it’s a way of thinking about the world, organising and interrogating data, and asking the right questions.
In this age of professional specialists and career academics, there’s a tendency to dismiss collectors as hobbyists. But collections and collecting are the foundation stone of biological science. There’s a reason that museums were the source of much early science, with their accumulation of material, objects and evidence in sufficient quantity to begin to see patterns and similarities and differences. It’s something we all do, from children sorting buttons and blocks by shape to Alfred Russell Wallace proposing a theory of natural selection on the basis of his collections in south-east Asia.
The Currency Creek Arboretum is sometimes described as a zoo for plants, but that doesn’t capture its scope. These 32 hectares are an experiment in time and space, conducted across the entire breadth of a genus and tribe in one place under standardised conditions. I can’t imagine a university or museum research project ever being able to fund and conduct an experiment of such long-term vision and magnitude.
I wonder what the arboretum will look like in a hundred years, as the trees grow larger, stretch and lean, creating light and dark. This is, after all, far longer than a single lifetime’s work. Many of these trees are likely to live for another two hundred years.
“What happens if a fire burnt through here?” someone on a tour of the arboretum once asked Nicolle. He didn’t skip a beat.
“That would be a great opportunity to study the diversity in eucalypt regeneration and recovery,” he said, then paused. “But I hope it doesn’t.”
This story is part of the New Ways of Seeing series, enabled by a grant from the CAL cultural fund.