The world's new tallest tree?

Stephen Sillett's laboratory is dangling 90 metres above the ground. It is an intricate web of ropes and instruments strung up in the branches of a tree. And in the windy conditions that plague Tasmania's forests, it can be distinctly precarious. "You can hear big gusts come through like freight trains pounding along their way toward you," says Sillett, a forest ecologist at Humboldt State Universityin Arcata, California. "As the gust hits, then the whole tree top just lays over and there's big old blows. It's amazing." In the morning dew or rain it becomes this luminous green. It is truly beautiful The trees in question are mountain ash, the tallest flowering trees in the world. They are not quite the tallest trees of any kind: that record belongs to the coast redwoodsof the western US. But that might be because things have been skewed against the mountain ash. It turns out that humans have been cutting them down in their prime, and they may have reached even more prodigious heights in the past. If conditions improve, might they one day beat out the redwoods? Just how tall do Tasmania's mountain ash trees grow? As part of his research, Sillett and his colleagues re-measured the largest tree on the island state, in December 2013. This towering titan boasts a height of 98.8m, 3m taller than the London tower that houses Big Ben. Mountain ash also grow in south-east Australia and Victoria, but Tasmania is where they have reached their peak. The island boasts over 140 trees taller than 85m. These goliaths have been given names such as Centurion, Damocles, El Grande and Medusa. Plant a eucalyptus and it can grow 90m in 90 years The tall trunk is typically white or grey except when it is wet. "In the morning dew or rain it becomes this luminous green," says George Koch of Northern Arizona University in Flagstaff. "The outer bark gets wet and saturated, letting the green inner bark kind of shine through. It is truly beautiful." The tops of the trees are where the action is. White flowers like pom-poms dot the upper part of the crown, which can stretch 75m deep. The trees also play host to dozens of species of plants and animals, such as wedge-tailed eaglesand Leadbeater's possums. The wood decays so easily that hollows often form in their trunks and branches, and pools of water collect. These high-rise ponds provide homes to reptiles, insects and frogs, and water for mammals and birds. "There are so many things going on in the canopy," Sillett says. On the face of it, the mountain ash should be able to beat the redwoods, which top out at 115m. They grow five times faster than the redwoods, "sprinting" toward the skies. "They're the fastest-growing tree by far," says Sillett. "Plant a eucalyptus and it can grow 90m in 90 years." They just live fast and die young But they don't live anywhere near as long as the redwoods. Traditionally, a mountain ash's lifespan has been thought to be between 350 and 450 years. A recent study suggested it could be more than 500 years. Even so, they are youngsters compared to the redwoods, which reach nearly 3000 years old. "They just live fast and die young," says Koch. Historical records do indicate that mountain ash have reached greater heights than today's giants in the past. In 1881, surveyor George Cornthwaite measured a felled tree in Victoria at 114.3 metres. That is about 1m shorter than the world's tallest living tree, a coast redwood measuring 115.5m. If you believe the old record books, and measurement systems, several other trees have reached such extreme heights. The trees have a simple reason for growing tall, says Koch. They are competing for light, which they use to convert water and carbon dioxide into sugar to sustain themselves. But it is water deprivation that limits their reach for the skies. Pulling water up through the tree's trunk is a fight against gravity. So in the tallest trees, it is difficult for the upper reaches to get enough water. In a sense, the treetop is like a small plant living in a dry place, Koch says. Trees start growing more slowly once they get tall To get the most energy from sunlight, a tree needs to grow big leaves. But water stress limits the growth of leaves. The growth of leaves is driven by water pressure in their cells. But as the trees get taller, the relative water shortage decreases this pressure, so the leaves grow more slowly.