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Sequoias can top 300 feet. Photo: <a href="http://www.flickr.com/photos/henryalien/">henryalien</a>, CC <a href="http://creativecommons.org/licenses/by-nc/2.0/deed.en">some rights reserved</a>
Sequoias can top 300 feet. Photo: henryalien, CC some rights reserved

Natural Selections: Tree growth

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Trees may live for hundreds, thousands of years, but there are limits on their growth. Trees can only move so much water, and only to a certain height. Martha Foley and Dr. Curt Stager discuss the hydrology of trees.

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Why don’t trees keep growing for their entire lives? Although theories vary, Dr. Curt Stager has the most probable answer:

“There was a study in Scotland with a whole bunch of different kinds of trees, like ashes and poplars and sycamores, and they took the growing twigs off old, big, old trees. And they grafted them on to young, vigorous saplings of the same type.

“And it turned out there was actually nothing wrong with those twigs and those buds. So they haven’t answered the whole thing. There could be something wrong with the old trunk, let’s say, or the old roots. But at least that part of the tree doesn’t have anything wrong with it. So that’s not what’s causing

“it. In fact, people are speculating it has something more to do with making food—by photosynthesis; can you make enough for a big tree?—or does it have to do with the problems of getting water up the tree?”

Dr. Stager explains how complicated the watering and eating process is for trees, noting that the higher trees grow, the more strenuous the process of pushing water up the trunks of the trees from their roots is:

“When a tree gets really big—like the biggest tree in the world is a redwood, it’s over 300 feet tall—you go to the top of that and you measure how much water’s in the tissues and there’s about half as much in a typical cell at the top of one of those big trees as there is halfway down. Sothat would slow it down.

“The one thing that these kinds of trees will do is they’ll close their stomata when they’re way, way, way up there—more often than ones that are lower down—because you evaporate that water out and you’re losing water. You’re already kind of short on the water anyway—so you’re going to close your stomata more to hold it in there. Well, if you do that, you can’t breath. The trees are taking in their carbon dioxide through those little stomata, so they can’t do photosynthesis.”

Dr. Curt Stager concludes that there is a limit to tree growth (400 feet, he has heard estimated) due to the trees needing to feed and water their large, growing bodies.

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