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[81] chapter four shade trees make good air conditioners In honor of your birthday, a tree has been planted in Israel.Thursday is your day to water it. —GREETING CARD The time: about 1800. The place: northwestern Georgia. American soldiers are building a fort. The soldiers are part of one of the largest transformations of the earth in human history. The young United States engaged in a centuries-long frenzy of destroying the natural landscape and replacing it with its ideal of an artificial one. Americans cut down nearly the entire eastern deciduous forest, replacing it with farms and cities. And they made deliberate, unconcealed attempts to exterminate the native inhabitants of this land, replacing the diversity of native cultures with a uniformity of language and economy.The soldiers building the fort in Georgia are deliberately working toward these ends: they cut down every tree as they build the fort, and they consider the Cherokees, in nearby villages, to be savages—uncivilized and unworthy to inhabit the land. Sweating in the hot Georgia sun, the soldiers seek shade under canvas and log. They sit and curse the heat, hanging out their tongues in anticipation of something to drink. Nearby, Cherokee men, women, and children rest in the shade of the many large trees that are contained within the stockade fence that surrounds their village (fig. 4.1). A little creek runs through the settlement.They relax. Both the soldiers and the Cherokees are in the shade, but only the Cherokees are cool in the hot Georgia summer. The reason is that they CH004.qxd 11/12/08 10:45 AM Page 81 repose underneath the cool, living shade of the trees, while the soldiers suffer under the dead shade of logs and fabric. The roots of the trees reach far down into the moist soil and draw water through the wood and into the leaves. The water evaporates from the leaves, cooling them. This process, transpiration,makes the tree into a gigantic green air conditioner.Of course, this gigantic green air conditioner does not actually make the heat energy disappear. Energy never just disappears. Rather, the energy is in the water vapor. But the water vapor, and the heat that it contains, rises into the air, away from the surface of the earth where plants, animals, and people live. How Trees Use Water to Cool Themselves Transpiration pulls water from the soil, through the roots and trunks, and into the leaves. Microscopic streams of water are pulled through green planet [82] figure 4.1. Tsalagi, a reconstructed traditional village of the Cherokee tribe, in Tahlequah, Oklahoma. Notice that inside the walls of the village there are numerous trees. Photograph by the author. CH004.qxd 11/12/08 10:45 AM Page 82 tiny cells called xylem vessels, which I will here call “xylem pipes.” There are xylem pipes in the roots, the trunk, the branches, and the leaves of the tree. Xylem pipes are the major component of wood, and are thus very abundant in the trunk. Xylem pipes look like plumbing pipes: they are long, narrow, and empty except for water. Because water molecules can stick together (a process called cohesion), the water molecules that evaporate into the air from the leaves literally pull the water molecules that are behind them, just like a rubber band being stretched. Also like the rubber band, the microscopic streams of water become narrower as they are stretched. When water is transpiring rapidly from the leaves of a tree, the trunk of the tree actually becomes measurably narrower.1 Not all of the xylem pipes have the same diameter. Some are relatively large (about one-twentieth of an inch across); others are much smaller. Xylem pipes with large diameters can conduct more water than narrow ones. And not just more, but disproportionately more. If a large pipe has a diameter ten times greater than the diameter of small a pipe, it can conduct far more water than the sum of ten of the smaller pipes. The reason is based in the science of fluid dynamics. In theory, one of the large pipes could conduct as much water as ten thousand of the smaller pipes. This mathematical pattern does not apply precisely to wood; xylem pipes are not perfectly round, and there are occasional constrictions in the flow of water up through the pipes. Still, lightweight wood with large-diameter pipes can conduct more water, and conduct it more rapidly, than heavy wood with...


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