April 22, 2004 | David F. Coppedge

How Tall Can a Tree Grow?

130 meters (426 ft) seems to be the upper limit on the height of a tree, say researchers from Humboldt State, Northern Arizona University and Pepperdine University, in the April 22 issue of Nature.1  To find this out, they had to establish working stations at the tops of northern California redwoods, the tallest trees on earth (the current record holder is 369.75 feet, the height of a 36-story building).  Ian Woodward calls their in situ measurements of photosynthesis at heights of over 360 ft. a “remarkable achievement.”2  The team shot arrows over the tops of tall northern redwoods, then pulled up ropes and climbed hundreds of feet into the crown branches to take their measurements of water pressure, leaf mass, carbon dioxide exchange, and light environment.  Since the tallest trees, which are estimated to have been growing for 2000 years, have not reached the theoretical limit, they could continue growing for some time.
    Koch et al. determined that the limiting factor is ability to pump water against the competing forces of gravity and friction, which increase with height.  Transpiration through the leaves creates a suction in the woody vessels that pulls the water upward until cavitation occurs, when an embolism forms that collapses the water flow.  They found that the top leaves get smaller and denser at the top, and less photosynthesis occurs, due to the challenge of delivering water hundreds of feet off the ground.  The northern redwoods are efficient drawers of water, Woodward says:

Tall trees use considerable quantities of water.  For example, a 45-m redwood uses about 600 kg of water each day, a figure that increases substantially with height and size.  It seems surprising, therefore, that the redwoods live in a climate with an annual dry season of 3-4 months.  Offsetting such an apparent drawback, however, is the oceanic influence on local climate, which means that dry-season fog occurs for up to two weeks at a time: fog reduces transpiration, a benefit in the dry season.  Moreover, tall trees actually increase the interception and capture of fog coming in off the sea, to the tune of 34% of the annual incidence of precipitation; in their absence, the precipitation input from fog is halved.

Woodward finds it interesting that Koch’s team began their research in 1988 on the lowly radish, and now they have continued on the tallest plants in the world.  “Yet that is no real surprise,” he notes: “despite the very different packaging and longevity of the two species, their physiological processes are much the same.”
See also the BBC News writeup on this research paper.

1Koch, Sillett, Jennings, and Davis, “The limits to tree height,” Nature 428, 851 – 854 (22 April 2004); doi:10.1038/nature02417.
2Ian Woodward, “Plant science: Tall storeys,” Nature 428, 807 – 808 (22 April 2004); doi:10.1038/428807a.

Here is a good example of science as it should be done.  Excellent field work, and no storytelling about evolution.  As remarkable as today’s giant trees are, there were probably even bigger ones in the past.  Yellowstone in Wyoming and Florissant Fossil Beds in Colorado have fossilized Sequoia stumps, and redwood fossils have been found even near the Arctic circle (see 03/22/2002 entry).  Conditions in the past may have been even more conducive to their rapid growth.
    Like Ian, we should all look woodward and marvel at the trees.  Look at a tall tree nearby, whether an oak, a pine, or whatever is prominent in your neck of the woods.  Imagine if it were the only tree on earth.  Would it not be an international tourist attraction?  People would travel the world over to look at this natural marvel.  They would wonder how it could pump water from under the ground all the way up to the highest leaves.  They would admire its beauty.  It would seem almost a miracle.  The fact that trees are so plentiful should not blind us to the reality that trees indeed are some of the most elegant, handsome, remarkable, and complex entities in all of creation.  Why not take the family on a hike or picnic under the shady trees, and use it as a teachable moment?
Enjoy this gallery of the natural water pumps that enrich our world: by a lake (foxtail pine), in a canyon (cottonwood), outdoor pews (sequoia), blooms in fog (dogwood), giant’s foot (sequoia), forest blanket (lodgepole pine), Yosemite valley (pine, oak), Tehipite valley (pine, oak), redwood saddle (sequoia), mountain majesty (pine), waterfall curtain (cottonwood), giant and dwarves (sequoia).

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Categories: Amazing Facts

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