The higher you climb up the gigantic, millennia-old trees of Taiwan’s forests, the more layers of habitat and life emerge. On the forest floor, ferns thrive in the moist shade. Flying squirrels and owls sleep inside the hollow tree trunks. Yellow bell-shaped rhododendron flowers spring from the lower tree canopy. Higher still, dense lichen spread. Up in cloud-drenched branches, a rare, hardy orchid, Bulbophyllum ciliisepalum, can be spotted.

“In one tree, every species has their preferred location,” says Dr Rebecca Hsu, assistant researcher at the Taiwan Forestry Research Institute. “Every metre the temperature, the wind, the sun, the light is different.”

Hsu and her team have spent more than a decade mapping Taiwan’s tallest trees, scanning forests from the sky, then hiking through rugged terrain to measure the towering giants in situ.

This month her study, published in Frontiers in Forests and Global Change, revealed Taiwan’s tallest tree, a Taiwania cryptomerioides that reaches 84.1 metres, higher than the average 20-storey building.

The tree, a conifer with a straight trunk that soars into the sky, was named “Heaven Sword of the Da’an River”. It is probably 1,000 years old.

Heaven Sword joins a catalogue of huge, ancient trees around the world. The tallest living tree recorded is a coast redwood, Sequoia sempervirens, which measures 116m, at Redwood national park, California. For reference, this looms higher than Elizabeth Tower in London (Big Ben), at 96m.

Height isn’t just about numbers and records, says Chris Swanston, director of science for Save the Redwoods League, a nonprofit focused on protecting coast redwood and giant sequoia Sequoiadendron giganteum trees.

“Height is an engine for biodiversity,” he says of coast redwoods. “In a single 2,000-year-old tree you could have dozens of generations of species developing ecologically within the canopy,” adds Swanston, who was not involved in the Taiwan study. “When you’re thinking about massive trees like old growth redwoods, their branches aren’t just branches like normal trees, they’re neighbourhoods.”

Older trees are also a vital defence against the climate crisis, thanks to their ability to absorb planet-warming carbon.

Tall trees, however, are in peril. Giants that have stood for thousands of years are perishing in fierce wildfires and being thwarted by global heating. In California, it is estimated that 17.6% of all large giant sequoias have been burned since 1984, with most dying in 2020 and 2021. In Tasmania, fires in 2019 killed at least 17 of the Australian state’s largest trees.

The climate crisis poses many other threats. It is aggravating droughts, and driving the clouds that tall trees rely on for moisture higher into the sky. Extreme weather events are also becoming more frequent.

Researchers in Taiwan discovered that, over the past decade, tall trees had a death rate of about 4% to 5%, a finding that was, said Hsu, “beyond our imagination”.

Across the world, conservationists are looking for ways to save these ancient giants – thinning out forests to prevent out-of-control fires or studying the threats posed by pests such as the bark beetle. Yet researchers say there are probably more tall trees, hidden deep in forest areas, that are yet to be recorded.

Lidar – the same technology that guides driverless cars – has transformed how researchers are able to study forests and find tall trees. It generates a 3D map of a landscape by transmitting laser pulses and measuring how long it takes for light to bounce back.

It isn’t foolproof – lidar struggles to capture measurements when deployed across uneven terrain, including in the forests of Taiwan, where trees often grow on steep slopes. In Hsu’s study, 93% of lidar tree measurements that were reviewed by citizen scientists turned out to be inaccurate.

Accuracy varies hugely depending on how lidar is deployed. “The further [away] you go, you decrease resolution,” says Matheus Henrique Nunes, assistant research professor at the University of Maryland, who is studying how tropical forests respond to human pressures and climate breakdown. He was not involved in the Taiwan study.

Deploying lidar from an aircraft or drone is far less accurate than from a backpack, though it can give a big-picture overview of a forest.

Based on past lidar studies, Nunes believes there are more unrecorded tall trees in the Amazon. A 2019 airborne lidar survey identified a 88-metre tree, he says, but the survey covered just 282,750 hectares (700,000 acres), roughly 0.05% of the total Amazon area. The tree was measured by drone rather than by tape. “I would not be surprised if we had trees in the Amazon that are more similar in height to the dipterocarps of Borneo, reaching 100 metres,” adds Nunes.

Measuring trees in the steepest, most remote forests is extremely challenging. To reach Heaven Sword, researchers had to hike for days and cross a river swollen by a recent snow storm.

“The most accurate way to measure is the old-fashioned way,” says Hsu, who climbed the trees along with colleagues.

To reach the top, a fishing line is fired up from an air cannon towards the tree branches. This is then replaced with rope, which is scaled by Hsu’s team. Ascenders (mechanical rope grabs) are attached to climbers’ feet to help them haul themselves upwards.

For the first 45m, Heaven Sword is a straight trunk without any branches, says Steve Pearce, naturalist and director of the Tree Projects in Australia, who joined the team to climb and photograph Heaven Sword.

The tree has evolved as its environment has shifted over the past 1,000 years, he says. “It probably grew up in an environment where there were other tall trees around it and really raced for the sky,” says Pearce. Once those lower branches got superseded and shaded out, they died and fell off.

The top of the tree is quite skinny, Pearce says, although this isn’t a concern for climbers “as long as you stay within the strength of the column of the tree, and don’t sort of bend back”.

The tree is a gymnosperm and so does not flower, but at the time it was photographed it was producing spores and cones, which give an orange tinge to its leaves.

The top of the tree, though sparse, is healthy and still growing. More than 80 metres in the air, it is possible to hear a gentle “whirring or whooshing sound” as the wind brushes through Heaven Sword’s leaves.

Hsu says the reason the Taiwania fir can grow so tall is down to fortune – good genes and an ability to weather storms, despite being precariously rooted in steep valleys. “Nearby they have many fallen trees. So the surviving trees, they depend on their luck. I think they are lottery winners.”

Taiwan’s increasingly powerful weather events threaten to test such luck further. Typhoons have grown in strength by 35% over the past four decades, research suggests, bringing fiercer winds and rains. Most giant trees are on steep slopes in the upper reaches of rivers, making them vulnerable to floods and landslides.

Hsu’s next project is to explore the trees’ habitat, and why they tend to be found where they are. “We recently found that Taiwania seedlings grow after landslides. They prefer that new landslide area,” says Hsu. The reasons for this are unclear.

For now, she hopes her work will at least serve as a record of their presence. “I want to send these great trees’ images to the public,” says Hsu. “To let people know how lucky we are to have these beautiful trees.”

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