How tall can a tree grow? It might sound like an unanswerable question, but researchers working in California's redwood forests have placed the theoretical height limit at 130 metres: the height of a 35-storey skyscraper.
The intrepid researchers scaled the world's tallest trees, in Humboldt Redwoods State Park, to find out what life is like in the loftiest branches. "The only way to do it is to climb them," says George Koch of Northern Arizona University in Flagstaff, who led the team. "It is quite an experience, although a lot of work."
They discovered that despite the moistness of the ground far below, the leaves at the treetops struggle to get enough water, so they are effectively living in constant drought. The difficulty of getting water so far up into the sky is what ultimately constrains growth, suspects Koch's team.
For California redwoods (Sequoia sempervirens), the tug of gravity and the friction between the water and the vessels through which it flows mean that fluid cannot be dragged any higher than 122-130 metres, the researchers conclude in this week's Nature1.
High aspirations
Given the clement climate and nourishing soils of northern California, this may well represent the greatest possible height for any tree in the world. But even the record-breaking trees of Humboldt may not reach the calculated maximum, says Koch. "We are not saying they will grow that tall; we are saying they can," he explains.
Indeed, at 112.7 metres, the world's tallest tree falls some way short of the ultimate height. But these redwoods are around 2,000 years old, says Koch, and ancient events such as drought or lightning strikes could have stunted their past growth.
It is tough at the top, agrees Ian Woodward, a plant scientist at the University of Sheffield, UK. As well as the danger of drought, the tallest redwoods take a buffeting from the wind that could knock off their highest boughs. "In any forest you see one or two individuals sticking up above the rest; they take the brunt of the wind," he says.
However, drought remains the main factor, constraining the redwoods' height and the size and shape of their leaves, Koch's team reports. Leaves near the ground are broad and well-spaced; those at the top are small and scrunched.
Leaf cells grow by taking in water and bigger cells mean bigger leaves, explains Woodward. In the treetop there is less water around, so the cells are simply not swollen to the same extent as their low-altitude fellows and growth is stunted.
Similar growth limitations could apply to other species around the world, such as the eucalyptus trees of Tasmania in Australia, which can exceed 70 metres, Koch says. Although shorter than redwoods, they may also have a theoretical 'drought ceiling', he suggests. "They could experience the same water gradient but at a lower height."
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