NATURE has its limits, it seems. Trees cannot grow taller than 130 metres, according to a study of some of the world’s tallest giant redwoods in northern California.
Trees get bigger in order to steal precious light from their neighbours. But there is a price to pay for growing tall. As water evaporates from leaves, moisture is pulled into the tree through the roots, passing up through tubes called xylem vessels, against gravity. But the taller the tree, the higher the tension that the water in these vessels is under. If the tension is too great, air bubbles form, which in turn cut off the water supply, putting a natural limit on tree height.
That has now been confirmed by George Koch at Northern Arizona University in Flagstaff and his team, who took a series of measurements from redwoods over 100 metres tall (including the world’s tallest tree, which stands at 112.7 metres). The researchers found that leaves in the redwood canopy already receive so little water that they are effectively living under drought conditions (Nature, vol 428, p 851). Extrapolating from the minimum amount of water a drought-adapted plant needs for growth, they calculate that trees cannot rise above a maximum height of 122 to 130 metres as they would not be able to transport enough water to sustain their uppermost leaves. That fits with the tallest ever recorded tree – a 126-metre Douglas fir.
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