Magazine article Natural History

How Trees Get High

Magazine article Natural History

How Trees Get High

Article excerpt

And the limit on their height is set by the force that holds water together.

(ProQuest Information and Learning: ... denotes text stops here in original.)

On a hike recently in the Montgomery Woods State Reserve, near Ukiah, California, I wandered among the area's massive coast redwoods with my friend Al Richmond. We were looking for the Mendocmo Tree, which, although it rises 367 feet above the forest floor, can still be hard to pick out from the ground. The surrounding trees are nearly that tall.

As we stood dwarfed by the grove of towering trees, I pondered a biomechamcal question that might occur to anyone who comes face to face with a life-form as majestic as the Mendocino Tree: how do trees grow so tall, and what, if anything, keeps them from growing even taller? The leading hypothesis has been that trees are limited only by their ability to get water from the ground to their highest leaves. To get to the bottom of the mystery, a group of plant physiologists went to the top: they scaled the redwoods in a grove a few miles to our north.

Water does not ordinarily run uphill. And, as Aristotle knew, it's impossible to pull water higher than about thirty feet by suction. Trees, however, can lift water well past vertical water column to draw more water from below the ground up to the top of the tallest redwood.

The weight of the water column itself puts a good deal of tension on the internal cohesive forces at its top. Imagine a narrow tube filled with water and running to the ground from a treetop 360 feet in the air. Water is free to move in the xylem, and the walls of the xylem tube provide no direct support to the water inside. The support comes instead from the water itself. Its internal cohesiveness makes the column of water act like a long suspended string, and the tension on the molecules at any point in the column must support the weight of all the water below them. Expressed as a pressure, or force per unit area, the tension on the water in the xylem is surprisingly high: for every thirty feet of tree height, the tension increases by roughly fifteen pounds per square inch. For a xylem tube 360 feet high the tension at the enough to break the water column, though, to cause problems for a tree. Photosynthesis, which takes place in leaf cells, converts carbon dioxide and water to carbohydrates and oxygen. To get the water into the cells, plants rely on osmosis, the movement of water from dilute to concentrated solutions. Such a flow can be reduced and even halted by applying a countervailing pressure. That's precisely what the tension in the water column does. With the osmotic flow reduced by the great tension in tree's lofty heights, leaf cells take up less water, which limits the amount of water available for photosynthesis.

Indeed, photosynthesis in the topmost leaves, at about 360 feet, scarcely occurs at all. By extrapolation, the investigators determined that photosynthesis would cease just above 420 feet. The finding dovetails nicely with the height of the tallest tree ever measured-a Douglas fir that towered 415 feet thirty feet, so what gives? Well, for one thing, they don't suck. Thanks to a phenomenon known as capillary action, water, even if it can't climb hills, can climb walls. Look at the surface of water in a clear vessel, and you will see at the edges that water does indeed move up the sides of the vessel. That property of water is crucial to the life of the tree.

The wooden core of a tree trunk is largely a dense array of narrow tubes, called xylem, that carry water from the roots up to the leaves. The water moves up the xylem via an entirely passive process known as transpiration, which is driven by a combination of capillary rise and evaporation through the leaves. At the tops of the open xylem tubes, the water evaporates into spaces within the tree's leaves, then exits to the atmosphere through pores in the leaves. As the water evaporates, capillary action-the electrostatic attraction between the water and the leaf cells and the inner surface of the xylem tubes-moves more water up the xylem and into the leaves. …

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