Green Planet: How Plants Keep the Earth Alive

Chapter Five: The Water Cycle: Plants Prevent Droughts and Floods

[93] chapter five the water cycle plants prevent droughts and floods And thou beholdest the earth blackened; then, when We send down water upon it, it quivers, and swells, and puts forth herbs of every joyous kind. —KORAN 22:5 The Water Comes Down Clouds gather ominously in the sky. A thick blanket of water vapor precipitates into torrents of rain. Cities and villages flood, washing away lives and livelihoods. The story is repeated over and over, from Johnstown, Pennsylvania, in 1889 to Honduras in 1998. People call them acts of God, as if the Almighty is responsible for causing these catastrophes . Clergy and laity unite to pray for God to bring relief to the suffering . But perhaps God had already answered their prayers—beforehand. In each of these places, the hillsides had once been clothed with trees, and those forests had helped to protect human towns and cities from floods and mudslides. But the people had cut many of them down. Often, reporters overlook the fact that part of the blame rests on the destruction of the trees. For example, a news report in the late 1990s described mudslides in Nepal without ever mentioning the local deforestation, which could be easily seen in the photograph that accompanied the report. Forests and shrublands grow profusely on mountain slopes around the world—not just the mountains with good soil and abundant rain, like the rich forests of the Appalachians and Great Smoky Mountains in eastern North America, but even in very poor and dry soils, like the White Mountains of California and Nevada.The forests and shrublands CH005.qxd 11/12/08 10:46 AM Page 93 are the protectors of the entire watershed. To visualize the many ways in which forests protect the mountains and the valleys below them, let us trace the pathways of raindrops as they fall from the sky. Some of the drops collide, at full speed, with the foliage of the trees. When, at last, they drip down to the soil, their speed is much reduced.The momentum of the raindrops is further slowed by understory wildflowers and leaf litter . By the time the drops reach the ground, they form a slow trickle, which penetrates between the soil particles and downward into deeper layers. Other raindrops drain down the branches and trunk, and percolate slowly into the ground.The roots of the trees and understory plants hold the soil, thus the small amount of water that runs off from the forest contains very little mud. When the rain stops and the dry season begins, the forest trees are able to draw on the water that has penetrated and been stored in the soil. It is not only the trees, however, that benefit from this underground water. Some of it penetrates deeply enough to replenish wells, especially those downhill from the forest. It is the groundwater that keeps the creeks and rivers running even during rainless seasons. In contrast, when raindrops collide with bare ground at full speed, they dislodge soil particles and carry them down the surface of the hill in little rivulets of not just water but also of mud. These rivulets converge into raging brown torrents and merge their contents to make muddy rivers overflow their banks. As a result, a heavy rain causes floods and mudslides from bare hillsides, instead of penetrating into the ground and replenishing underground stores.Therefore,in the dry season, the rivers have less water flow and the wells less water, if the hillside above them is barren (fig. 5.1). Half of the rain falling on bare soil runs off, in contrast to about one-fourth of the rain falling on farmland, and less than 1 percent of the rain that falls on grasslands and woodlands (table 5.1).1 We have probably all seen something similar to what I am watching from my office window as I write this manuscript: a large tract of forest has been cut down, and a heavy rain is rushing from this clearing onto the road below—and taking a lot of mud with it. Throughout human history, the destruction of forests has been followed by floods. The hills above Johnstown, Pennsylvania, had been deforested, just like most of the rest of the oak and maple forests of the green planet [94] CH005.qxd 11/12/08 10:46 AM Page 94 eastern United States. Heavy rains overwhelmed the feeble...