The Leaves On An Oak

Experiment has shown that if heat is allowed to accumulate in the leaf of a tree unchecked by evaporation the temperature rises at a rate of twelve degrees centigrade a minute. The results, of course, are very rapidly fatal to the tree, or indeed plant. The amount of water taken from the soil by a tree in the course of an active summer day is enormous. Average trees fifteen inches in diameter and fifty feet high possess a leaf area of about one-third of an acre—nearly fifteen thousand square feet. Now the amount of water that rises is largely dependent on the amount these leaves can use, and our average tree contains 1,230 pounds of water. It probably lifts out of the soil about 125 gallons of water a day.

An oak tree with an average of about seven hundred thousand leaves gives off 120 tons of water in a season. A single fully-grown maple or linden will evaporate as much as a barrel of water in a single day. So it seems quite probable that the volume of water constantly rising into the air from the world's vegetation is scarcely less than that carried by all the rivers of earth. By simple mathematics Forbes shows that one hundred such trees, standing on a single acre, would drain the soil of en thousand gallons every twenty-four hours, a deficit requiring about one-third of an inch of rain a day to replace.

Sap ascends a tree with startling rapidity. Actual measurements on eucalyptus and other species indicate that the rate is often thirty to forty feet an hour and in some instances fifty. Now a column of sap within a tall eucalyptus, say 330 feet high, represents a weight of ten atmospheres, or about 150 pounds to the square inch of columnar area, and to raise such a column at an average rate requires a force of about three hundred pounds a square inch. This is far greater than the force exerted by any suction pump that has ever been developed by man.

Not all trees have equal power to draw water out of the soil and raise it through trunk and branches. The degree to which a given species possesses this power is one of the characteristics determining where it can grow. The soil itself exercises a pull on the water within it, and before a tree can secure a drop of water it must counteract this opposing force. Soils differ in the tenacity of their hold on water, and as the moisture content decreases, the soil’s grasp on what is left becomes tighter. Obviously, when a tree must use a great part of its strength to wrest water from the soil it has less strength remaining to raise the water above the soil. Under these circumstances the tree will be shorter than others of the same species more fortunately situated.

By: davidbunch

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