The Remarkable Lifting Power of Trees

Backyard Wisdom

by: Gilbert A Smith, ISA B. Certified Master Arborist

 photos and illustration by Lesley bruce smith

photos and illustration by Lesley bruce smith

Did you know that trees are weight lifters, and that they use that super human strength to get water to their leaves which are sometimes hundreds of feet above their roots?

Simply put, trees are like giant straws and the sun energy sucks the water up from the roots hundreds of feet to the leaves, just like we use a straw to suck water from the bottom of a glass. The unique build of a tree, and the unique character of water makes this possible. Trees are built of cells (called xylem) that are long thin tubes connected top to bottom from roots to leaf tips. Water molecules have a positive charge on one end and a negative charge on the other end, which connect, and don’t like to be broken. This cohesive power is why water will bead up, not spread out on a glass surface. At the same time that the water is attracted to itself, it is also attracted to the sides of the xylem cell walls.This attraction is called capillary action and is the reason that water will counter act gravity and soak up into a paper towel.

However, in the trees there is a “push” and a “pull”. The “push” starts at the roots where the concentration of sugar is higher inside the roots than outside, creating what is called osmotic pressure. The water that surrounds the roots rushes in to equalize the sugar concentration difference. I must point out here that the water brings with it dissolved minerals like nitrogen, phosphorus and potassium, which are essential to tree health. If the minerals are in an available form, the tree gets its vitamins as well as its water in this transaction.  In fact, osmotic pressure combined with capillary pressure is enough to counteract the downward pull of gravity sending a column of water up 3 meters into the trunk. But that’s not enough to get water 340 feet into the canopy of a giant Sequoia.
    
The “pull” starts in the leaves high above the roots. There the sun heats up the leaves and in a sense the leaves “sweat” through little openings called stomates.  For trees, perspiration is called transpiration but the effect is the same, the trees lose water as it evaporates and cools the plant. When the water is sucked out of the leaves by the sun it is just like when we create suction at the top of a straw.  Water molecules like to hold on to each other and to the sides of the cells so they, in effect reach down all the way to the roots and drag the water up with them. It sounds simple but an average tree can use 100 gallons of water on a hot day.  If that tree was 100 feet tall it will need to lift 800 pounds 100 feet every day, which is some pretty heavy lifting!. 

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