IT HAS been a busy week for tree huggers 鈥� discovering not only the world鈥檚 tallest tree but also what governs a tree鈥檚 ultimate height.
Until 7 September the record was held by 112.8-metre-tall giant redwood in north California, then a team at Humboldt State University found a 115.2-metre specimen in California鈥檚 Redwood National Park. There is, however, a natural limit on maximum tree height, says a team led by Quanshui Zheng of Tsinghua University in China in a paper submitted to Nature鈥榮 pre-print server.
As water evaporates from leaves, moisture is pulled into the tree via the roots, meaning trees have to work ever harder to overcome gravity and raise water to leaves at great heights (快猫短视频, 24 April 2004, p 18). Now an analysis by Zheng鈥檚 team of 22 tree species suggests why there is a limit to tree growth.
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The higher up a tree, the lower the negative pressure in mesophylls, cells in the middle of each leaf. If this pressure becomes too low, the cells collapse, 鈥渓ike when you suck through a straw very hard and the straw collapses鈥�, says Zheng. To prevent this happening mesophylls in the leaves at the top of trees are becoming so small that this is the factor which limits tree height, says Zheng鈥檚 team.
Different levels of groundwater, humidity and temperature alter the negative pressure within mesophylls, which might explain why the height of tree species varies in different regions.
