Patterns Of Time

HUMANS ARE NOT the only species to record the passing of time, but we must be the only ones who have learnt to torture ourselves about it. Age is perceived differently in the plant world, writes author and poet Sumana Roy in her memoir, How I Became a Tree. On a quest to slow everything down so that she might finally ‘live to tree time’, Roy observes that wrinkles and lines have ‘become embarrassing to humans’, whereas ‘the age of trees was to be found in similar lines, in circles denoting lived years, in the girth of time that gave aged trees a kind of sober dignity’.

Roy is not alone in her respect for these ‘circles denoting lived years’, otherwise known as tree-rings. They’re revered by many as the timepieces of the natural world. Climate scientist Jo.lle Gergis, for instance, admires them as the ‘sentinels of deep time’, echoing the great polymath Leonardo da Vinci who noted in the 15th century that a new ring forms on a tree each year, its growth influenced by the climate. Since then, the art of dating trees by their ring patterns has evolved into the sophisticated modern science known as dendrochronology. Across the globe, international databases of tree-ring research are proliferating. Cross-matching between them yields important insights into long-range weather patterns and significant climate events of the past.

Now faced with a climate emergency, gaining a long-term perspective on meteorological change has become crucial to preparing for its impacts. It’s for this reason that one particular branch of tree-ring research is having a moment: dendroclimatology – the use of tree-rings to determine past climate conditions. To learn more, I arrange to speak with Dr Jonathan Palmer, a scientist in the School of Biological, Earth & Environmental Sciences at the University of New South Wales. When we connect via Zoom, Palmer – a specialist in using treerings for the reconstruction and analysis of climate variability and change, as well as radiocarbon dating – is quick to remind me that tree-rings, in fact, contribute to a wide range of fields.

‘Tree-rings are more than one-dimensional,’ he says. The climate component is important but, equally, ‘tree-ring researchers are a diverse bunch of scientists, hydraulics and water engineers, archaeologists and even historians.’ There are dendrochronologists dedicated to certifying the age of historic buildings, while others use tree-rings to date artworks and even shipwrecks. Tree-rings have been used by archaeologists since the founder of modern dendrochronology, Andrew Ellicott (AE) Douglass, solved the riddle of the age of the pre-Columbian Anasazi ruins built into the cliffs of the Chaco Canyon in the American Southwest. He analysed the beams of the trees that the old pueblos used in their roof structures, dating the civilisation back to the year 500.

Dialling in from New Zealand in between collecting wood samples on a field trip, Palmer drops that he is working with buried sub-fossil kauri trees (or Agathis australis, a New Zealand conifer), using their rings to help refine the radiocarbon calibration dating curve. My ears prick at the mention of kauris. These legendary trees, sacred to local Māori, are forest giants who can live for over 2000 years. Even older are the sub-fossilised ‘swamp kauri’, some of which toppled into peat bogs in the period after the last Ice Age, and other kauri known to have lived as early as 50,000 years before present. Today, the logs remain buried below ground in patches across New Zealand. For scientists interested in radiocarbon dating, like Palmer, the ancient stores of carbon in the swamp kauri act as a kind of cross-reference to establish the age of radiocarbon samples, which don’t correspond with calendar years.

In the early 2000s, Palmer was among a group of scientists who worked with a combination of samples from living kauri trees (a light, handheld instrument known as an increment borer was used to extract slivers or ‘cores’ from the tree’s trunk, with minimal impact) and sub-fossil material to establish their sensitivity to the El Ni.o-Southern Oscillation. ENSO, for short, is considered the heartbeat of the Pacific’s climate system. Its pulse is the trade winds that blow west along the Equator, giving rise to the variability in sea surface temperatures that underlie many of the droughts, floods and cyclones on the Australian continent.

Researchers found that wide rings in some kauri reflected cool, dry El Ni.o spells, while narrow rings pointed to periods of wet and rainy La Ni.a. A continuous picture of El Ni.o activity was built up for the past 3722 years, showing that five of the ten strongest El Ni.o events in the past 400 years have taken place since 1982. The revelation is incredibly valuable to understanding the region’s climate history, and for predicting the likely impact of global warming on the strength and frequency of future El Ni.o and La Ni.a events.

More recently, Palmer and his colleagues used 176 drought-sensitive tree-ring chronologies and one coral series to reconstruct the last 500 years of climate, year-by-year, for the whole of the Australian eastern seaboard. The resulting Australian–New Zealand Drought Atlas (ANZDA) furthers our understanding of past weather patterns far beyond the existing drought record, which only extends to around 1900. It’s also fun to watch. Uploaded as a short video on YouTube, the climate data is sped up in a timelapse that visualises the shifting intensities of drought and rainfall from 1500 to 2012, all in less than two minutes.

Not only does the atlas open a portal into the region’s volatile climate past, it also shows just how far scientists have progressed in disproving the notion that the southern hemisphere is not well suited to dendroclimatic research. Where the northern hemisphere boasts clearly defined seasons and many long-lived tree species, trees found on the Australian mainland typically don’t produce the type of tree-ring growth prized by tree-ring researchers. The comparisons stymied local research well into the latter part of the 20th century.

‘I grew up with eucalypts around me. In some ways, that personal connection with a landscape can make you more inclined to stick with a problematic topic,’ says Dr Matthew Brookhouse, a dendrochronologist based at the Australian National University in Canberra, and one of a small number of scientists dedicated to the study of tree-rings in eucalypts. Speaking via phone on a wintry afternoon, he suggests there was a colonial mindset at play when North American tree-ring scientists first began to travel in search of other landscapes to survey. Initially, their approach was focused on assessing whether the local trees formed clear rings.

Sadly, the ubiquitous gum tree disappointed on that front; as well as being further hampered by their susceptibility to insect attack and relatively short life span. What eucalypts do have in their favour is a sensitivity to temperature. Knowing that cold weather represents an important stressor on eucalypts, Brookhouse focused his efforts on the Victorian Alps, where he studied the snow gums (Eucalyptus pauciflora) growing at high elevation at the alpine tree line. Trees in comfortable growing conditions with easy access to nutrients and water aren’t typically chosen for climate reconstruction work. Instead, scientists seek out trees growing in extreme environments. It’s this stress that produces the type of ring growth most revealing of past climate events.

‘The picture that we have of snow gums now is different to what it was back then,’ says Brookhouse, referring to the 1970s when researchers mostly dismissed their potential. Many dendrochronologists are drawn to old species whose rings yield millennia-long records, but for Brookhouse, the discovery that species like snow gum are suitable for climate reconstruction confirms there is real value in working with younger trees like eucalypts.

For Palmer, producing drought and climate atlases for Australia loops us into ‘a bigger global picture of climate patterns, and they’re all driven by tree-rings’. It’s important to realise that ‘radiocarbon dating, archaeological dating and climate dating have all relied on tree-ring patterns’. Australian researchers have lodged more than three and a half thousand sites on the International Tree-Ring Data Bank – the world’s largest public archive of tree-ring data. The richness of all this replicating, cross-matching and sharing of data means that, according to Palmer, ‘it’s a whole orchestra contributing to the sound, rather than just a single note or a single instrument’.

Circling back to the appeal of tree-rings, Brookhouse offers that much of their power lies in how they compress time. ‘From tree-rings you gain an insight into the plant’s stresses, and through reading the rings we can comprehend it, they make the process understandable,’ he reflects. ‘Tree-rings really offer that window into plants, when for a lot of us plants are other – they operate on another level to the animal world.’

As we discuss the otherness of plants, the words of Sumana Roy and her desire to ‘live to tree time’ bounce around in my mind. I think back to the weeks leading up to my fortieth birthday, when Sydney was ravaged by another wave of COVID and I spent much time at home looking out at the tree just beyond my apartment window. I felt I was being robbed of time I could never get back. The challenge was to shift my mindset, to appreciate time as ‘an obese creature’, as Roy writes. To see ‘that history, whether it was reflected in lines or folds, loose bark or skin, new colours or pigmentation, was a beautiful thing’. The history recorded by trees is not only beautiful; it is revealing, too, as their rings corroborate the truth of a changing climate. The next chapter is already being recorded by nature’s most prolific scribes, and the rings do not lie.

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Image top: Julien De Marchi / Alamy Stock Photo