Searching for Australia’s tallest trees: Karris

By Karen McGhee 2 July 2014
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Deep in the ancient forests of south-west Western Australia are thought to lurk colossal karri trees,taller than any yet recorded

THERE WAS URGENCY in the order crackling down the two-way from the normally laid-back Guy Badger, perched on a branch more than 70m up. “Start packing everything up; we’re coming down…now!”

It was our third day in south-west Western Australia’s remarkable karri forests, looking for the tallest specimens of one of the world’s tallest tree species. Guy’s orange safety vest was visible in the crown above the massive  trunk he’d just scaled with fellow arborists Brett ‘Brown’ Moir and Greg Robbins. But his 2m-tall frame was partially obscured by distance and the crown’s thick chaos of bright-green leaves.

Photographer Bill Hatcher and I were stunned – it’d taken more than two hours of work at the base of this botanical giant to set up an access line and the ropes for a single-rope technique (SRT) ascent of her trunk. Guy and his colleagues had previously named the tree Clare.

They’d been talking for months of scaling this “perfect karri specimen”, with her long, smooth trunk – coloured creamy yellow and even pinkish in the right light – and topped by a bubble-shaped crown, which appeared to clear the surrounding canopy by many metres. Now they were coming down, without even unravelling their massive tape measure (still the most precise way to gauge a tall tree’s height when she’s surrounded by dense undergrowth). Clearly, something was wrong…

Tall flowering trees

CLARE WAS OUR sixth big tree in two days. We’d been searching because no-one really knows how big karris get. You hear pub talk in old forestry towns about the species being capable of growing more than 90m, which means 200 tonnes of wood reaching higher than a 20-storey building, but no living specimen of this size has been found – yet.

Guy, an arborist for 15 years who also has experience with tall trees in Canada and New Zealand, has an enduring passion for karris. He hopes that documenting their height will gain the species (Eucalyptus diversicolor) global recognition and help ensure its protection. Ironically, he grew up with a fear of heights in the WA Wheatbelt, where remnant mallees of less than 5m are the tallest native vegetation.

Guy’s now based at Busselton, south of Perth, and one of his major contracts, with the State Department of Environment and Conservation (DEC), is to maintain the health and safety of big trees around tourist sites in the south-west. This includes three massive karris with steel climbing steps circling their trunks to platform lookouts. Two were originally set up early last century as part of a forestry tower network to watch for bushfires.

Now tourist attractions, the best known of the three is the ‘Gloucester Tree’, near Pemberton. But the tallest is Warren National Park’s ‘Dave Evans Bicentennial Tree’, with a viewing platform above 60m – about the same height as the top of the Sydney Opera House and the world’s highest tree lookout. It offers public access to a dizzying perspective from one of the planet’s biggest life forms, providing spectacular views across WA’s southern forests.

The tallest trees currently alive on the planet are pines: giant coast redwoods (Sequoia sepervirens) that can live for more than 1000 years. They occur naturally only in a narrow strip in northern California, and the tallest living individual – ‘Hyperion’ – stands just over 115m tall in the state’s Redwood National Park.

The tallest flowering tree species, however, is the mountain ash (E. regnans), which grows in Victoria and Tasmania. Its status was confirmed in 2008 with a reading of almost 100m, taken using an airborne laser (LIDAR), on a tree called ‘Centurion’, south-west of Hobart (AG 99). Guy believes, however, there are likely to be karris almost as tall, and if they could be found they’d confirm WA’s tallest species of tree is also the world’s second tallest flowering tree.

Karri is a prized hardwood thought to live for 300 years in the right conditions, and in past centuries it’s been logged extensively. It’s the tallest of a suite of remarkable eucalypt species – including jarrah (E. marginata), marri (Corymbia calophylla) and three different types of tingle (E. jacksonii, E. guilfoylei and E. brevistylis) – that grow only in Australia’s far south-west and nowhere else in the world. Karri forests now cover less than 200,000ha, about one-fifth of which is virgin growth, and most of it is managed by DEC.

University of Western Australia (UWA) ecophysiologist Dr Stephen Burgess describes these forests as a natural wonder. “There’s nothing like them for thousands of miles and anyone who spends more than two seconds in them would notice that they are unique,” he says. “There’s a lot of red dirt and dry, arid ecosystems [beyond them] and I don’t want to downplay the importance of those, but…karri forests are a unique resource.”

From a global perspective, south-west WA’s tall forests show up as a tiny, albeit significant, green smudge on NASA satellite imagery depicting the planet’s tallest vegetation. The same satellite pictures show clearly that the nearest stand of tall trees is more than 3000km across the continent in south-eastern Victoria and Tasmania.

That, of course, is where the mountain ash is found, but it’s also where several other eucalypt species reach astounding heights. These include the Tasmanian blue gum (E. globulus), now cultivated across Australia for timber because of its fast-growing habit, and for which there are historical records of felled trees that were higher than 100m. Also in Tasmania is the alpine ash (E. delegatensis). That species, too, is said to top 90m in favourable conditions.

Why Eucalypts grow to tall heights

WHAT MAKES EUCALYPTS so successful in the tall-tree stakes? Stephen says their “hydraulic architecture” is important. In tall flowering trees, water and nutrients are transported in long, wide, permanently opened tubes of dead ‘xylem’ cells running from the roots to the leaves. Water travels up the tree along a negative pressure gradient. Several physical forces interplay to essentially suck water against gravity and up to the leaves. These include transpiration – which occurs as water evaporates from leaf pores – and surface tension created along the linings of cell walls.

In tall eucalypts, Stephen explains, the pressure can get so great that air from the canopy is sucked through pores in the xylem cell walls. “And if there’s enough suction it can pull in a little bubble that becomes a nucleation site where the water can spontaneously boil…because of the pressure it’s under.”

This can vapourise with a pop, not audible to the human ear but detectable by ultrasonic sensors. Some tall-tree climbers say they can sense all this activity going on beneath the massive trunks they scale.

“You get an adrenalin buzz from the climb,” says Guy. “And I really believe you get an energy from the tree… You feed off that energy as you go up.”

In eucalypts, xylem tubes are packed like bundles of straws through the trunk and the system is particularly efficient at moving water. “And if you’re able to shift water quickly, and it’s available, then you’re going to beat others [trees],” explains Stephen. In the cool, damp environments of Tasmania and south-eastern Victoria, this lets mountain ash and other tall eucalypts power towards the light. And it’s also undoubtedly one reason karris are able to grow so tall.

For karris, effective botanical physiology converges with the right geographic, soil and climatic conditions in WA’s unique southern forests. The area has a typical Mediterranean climate, with hot, dry summers and mild, wet winters, with annual rainfall of around 1200mm where karri forests occur.

Recently it emerged that the roots of karris may also be a particularly significant part of their physiology. Ecophysiologist Dr Tim Bleby, an adjunct senior lecturer at UWA, has begun studying karri roots that extend 50m below ground in caves beneath the Margaret River region. While it’s early days yet, these massive underground networks are expected to reveal a lot about the capabilities of karris.

“If you could turn [karris] upside down – as magnificent and majestic as they look from above, they would look equally as majestic from the other end,” says Tim.

“As much as they can grow upwards to compete for light, they can grow roots deep in search for water to get beyond the [competition] zone with other plants.” This ability to tap subterranean water reservoirs might help karris fuel the sort of growth needed to grow tall and out-compete other plants.

Bushfire approaching in the Karri forest

BILL AND I BEGAN casually to collect our gear following Guy’s instructions, and as we scanned the canopy we spied telltale tufts of smoke drifting towards the three men up the tree. The two-way spluttered again. This time Guy issued an order: “Leave everything and get out of here!”

For the past two days we’d been aware of a fire burning less than 10km away in Shannon National Park, and had even seen DEC bush brigades racing past. But our campsite was secure and the DEC knew we were in the area. There had been no evacuations, and although the smell of bushfire had been lingering for days, the danger seemed distant. But the wind had suddenly changed direction, the air around us was now filling with smoke and the smell of burning vegetation had become disturbingly close.

“It looks like the fire’s got into our valley,” Greg called down from the tree. A rush of adrenalin and sheer terror shot through my body. Bill and I paused to argue with Guy that they’d need us on the ground to help their descents, but his command was clear: “Get to your car, and if it looks like things are bad, head straight for the dam we were at yesterday – we’ll meet you there.”

It was a 45-minute hike to the site that morning, but it took 15 minutes of bushbashing to return to our car. Panicked, we sped back to our campsite where rangers assured us the fire hadn’t yet jumped the road and evacuations weren’t necessary.

That afternoon I practised climbing a 30m pine to prepare for my first ascent of a tall karri – and the next day we found, and scoped with a laser, a giant that looked certain to top 80m. But by late morning, the sky was still eerily twilight dark due to smoke, and ash was falling like snowflakes. Spooked by the previous day’s experience we decided this wasn’t the time for record books and talked of meeting up another time to search for the world’s tallest karri. Somewhere out there, Guy says, he’s sure there’s one that reaches higher than 90m, and he’s determined to find her.  

The full story can be found in Australian Geographic #110.